Why do you need vacuum advance? Port or Manifold? I give my take on which I prefer. Also, do you use lock washers? You might throw them away after watching this!
Man oh man! You’re explaining a lesson I wished that I had learned the easy way. Close to 20 years ago I built a SBC for a 1969 SS Camaro convertible. Very nice restoration. Engine had to be built to look exactly like it was off of the show room. So no HEI conversion. What the customer elected to do was a Pertronics conversion in a dual point housing. The distributor was a “rebuilt” brand from a popular parts store here in the U.S. The conversion kit was straight forward and looked very clean. Where I got into trouble as the engine builder….. When I built the engine, degreeing the cam off of top dead center from the positive stop method, I noticed that the timing reference on the balancer was off near 45 degrees from the factory timing pointer. The harmonic balancer was brand new but some stock form of aftermarket. I did not purchase this for the build it was supplied with the rest of the parts. I’ll further explain that it’s been my long term policy to lock the crankshaft down after the cam is degreed in and verified. Then I will close everything up around the front cover and install the balancer and verify timing reference. Just for this reason. Ensuring proper ignition timing. So in this case. I taped the balancer to paint a mark in line with the timing pointer and painted a nice bright white stripe on the balancer to correct. Big mistake!! So the engine is fired up. Brand new transmission from a reputable transmission shop that supplied everything surrounding. When we filled the transmission with fluid. A very loud noise began. It was the flex plate being pushed into the transmission inspection cover and the shift linkage had issues to be worked out. The car owner was standing right there and elected to take the car to the transmission shop right then and there. We managed to set timing and check for leaks but the engine did not get much running time. Maybe 5 minutes and I was not comfortable with the situation either way. A few days later I get a call at work from “the transmission guru”. He just wanted to tell me that my timing mark was off about 15 degrees. So he fixed that for me. I was busy with work and didn’t really have time to realize just what had been done. When it gets out of the shop the car owner called me to inform me that the engine was running hot. No problem, I’ll set the timing and check everything out. When time comes to set timing and I’m looking for my painted timing mark, it’s gone. That transmission guy took it upon himself to strip the painted timing reference off of the balancer and repaint the balancer. He then hastily remarked the balancer with a piece of chalk you could barely see. The explanation on how transmission guru determined top dead center??? Not worth mention the guy was screwed up in the head and just screwed me over big time. So now I’m trying to set timing based roughly off of feeling for compression and doing my best to get a mark on the balancer. It was mess everything in the way. All culminated into uncertainty. When I tried to set timing the initial was set around 8 degrees and when I looked at the total timing I was well passed 45 degrees. I thought the vacuum advance was still hooked up. It wasn’t. Where is all of this advance coming from? I was in a hand wringing situation. The car owner was very understanding. They could see and knew enough about the situation to realize that we were out in left field for referencing. They took the car home with them with the plans to disassemble the front end of the engine and reestablish TDC reference later. I took my problems to work and explained them to my boss. A very wise engine builder. He immediately asks me why I didn’t use the mill or something else to make a permanent mark on the balancer. I had the tools available to do so and didn’t. But where was all of this extra advance coming from. It’s all brand new store bought rebuilt?? The answer came a few weeks later on the engine dyno at work. Another engine. This time a 327. Same popular parts store “rebuild” dual point with an electronic conversion kit. This time the timing marks were right and not tampered with. This engine was hitting near 52 degrees with no vacuum advance. Mr. Boyd calls me into the dyno room and asks me if this is what I was seeing with my project?? Exactly. Same brand distributor from the same parts store doing the same thing. So he has me pull this distributor and he goes at disassembly. The mechanical advance plates!!! The stop pins had deep wear grooves. The plates the stop pins were mating to, worn out! He calls popular parts store and explains. Their response….. No way! For sure for certain. It was. It was the same on the distributor that had plagued me with issues. Moral of the story. Permanently mark your timing references. The uncertainty that little detail created was more than I want to ever deal with again. For those that set timing by ear. It isn’t happening when your mechanical advance is in this condition. You’re flat lost with a slow boiler that will never cool down. My apologies for the long winded this one I felt was just too important.
Easy to find TDC with a spark plug style piston stop. Turn motor each direction till piston stops, mark the balancer at those points. TDC is right in the middle of your marks. Found many spun balancers this way from 5° to 30° off
It’s all easy until it’s in a fully dressed engine compartment. Not every valve combustion chamber and valve diameter will allow for the method you described. Screw a positive stop down a spark plug hole deep enough with a valve head diameter large enough and you’re going to bend a valve. It’s always easy until details are missed or ignored. Then it’s a real problem. I’ve seen this bite the best of them. It can and has destroyed engines.
TIMING AND VACUUM ADVANCE 101 by GM Engineer. The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency. The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation. At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph). When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can returns to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean. The key difference is that centrifugal advance (in the distributor autocam via weights and springs) is purely rpm-sensitive; nothing changes it except changes in rpm. Vacuum advance, on the other hand, responds to engine load and rapidly-changing operating conditions, providing the correct degree of spark advance at any point in time based on engine load, to deal with both lean and rich mixture conditions. By today's terms, this was a relatively crude mechanical system, but it did a good job of optimizing engine efficiency, throttle response, fuel economy, and idle cooling, with absolutely ZERO effect on wide-open throttle performance, as vacuum advance is inoperative under wide-open throttle conditions. In modern cars with computerized engine controllers, all those sensors and the controller change both mixture and spark timing 50 to 100 times per second, and we don't even HAVE a distributor any more - it's all electronic. Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it. If you look at the centrifugal advance calibrations for these "ported spark, late-timed" engines, you'll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems - "ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more. What about the Harry high-school non-vacuum advance polished billet "whizbang" distributors you see in the Summit and Jeg's catalogs? They're JUNK on a street-driven car, but some people keep buying them because they're "race car" parts, so they must be "good for my car" - they're NOT. "Race cars" run at wide-open throttle, rich mixture, full load, and high rpm all the time, so they don't need a system (vacuum advance) to deal with the full range of driving conditions encountered in street operation. Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacuum advance either, so they're not alone. Vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren’t fully-deployed until they see about 15” Hg. Manifold vacuum, so those cans don’t work very well on a modified engine; with less than 15” Hg. at a rough idle, the stock can will “dither” in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. Modified engines with more cam that generate less than 15” Hg. of vacuum at idle need a vacuum advance can that’s fully-deployed at least 1”, preferably 2” of vacuum less than idle vacuum level so idle advance is solid and stable; the Echlin #VC-1810 advance can (about $10 at NAPA) provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8” of vacuum, so there is no variation in idle timing even with a stout cam. For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively.
Great explanation! Thank you Mr. Gold for the walk-through on vacuum advance with the visuals (excellent graph.) Side note for anyone interested: David Vizard has a very recent video that touches on the ignition subject and expands on a couple points, (manifold vacuum for the win). Again, thank you for helping us less experienced guys.
Not to go too far off topic but this got me thinking about the other parts of the ignition system; plug wires, distributor cap, rotor, spark plugs. Recurving the distributor would be good too. Heat range of spark plugs and adjustable vacuum diaphragm. Maybe with an engine on the test stand or dyno.
I really like the lock washer lesson! I fully agree! I think those were designed back when harmonics were not fully understood or there was a knowledge deficit on how to mitigate certain applications where harmonics came into play.” So just add a spring into the equation and maybe that bolt won’t back out because of harmonic resonance.” That’s all I can reasonably figure on.
I agree about using the knowledge from the past. If you're building a regular production engine, naturally aspirated, someone in the past already designed one, all you have to do is follow their blueprint. I remember the old "Chevy Bible" and the Chevrolet interchange manuals, they were worth their weight in gold......no pun intended.
Congratulations on the 10K subs, it is all a testament to your great content!! Back in the 80's I worked at The Carb Shop here in BC. As such, I have a Sun distributor machine as well as a Sun osilliscope/gas analyzer too!
You are 100% correct on vacuum advance. It was changed to ported for emissions purposes only. It is interesting that a more efficient engine doesn't always produce the "cleanest" emissions. The only difference between manifold and ported is at idle and deceleration. At cruise they are about the same. It is alarming how many guys don't understand how vacuum advance works or why we use it.
How can you say that he is “100% correct” when he doesn’t understand how ported vacuum actually works? I agree that using manifold vacuum is the way to go, but he thinks that having the vacuum canister connected to a ported source will ADD vacuum advance at wide open throttle. Completely wrong.
@@tonysingleton8340 it's undebatable because neither side will ever change. When changing to direct manifold vacuum, it's done as part of a series of changes designed with a purpose. If a guy has a stock emissions car, retains the stock vacuum can, keeps the factory timing specs, leaves the advance springs alone.......... I'd say that guy ought to keep the vacuum ported.
Great timing curve, mechanical and vacuum advance tutorial. I was a tune up mechanic in my youth and quickly learned to move vacuum advance from ported to manifold. The smog laws in 1966 in California moved them back to ported to lower emissions. Engines ran hotter and got poor mileage and had sluggish street performance. I've converted several friends' engines to manifold vacuum advance with great results. Some are harder to start when warm since there's more advance while cranking. Opening the throttle stops that. Street driving with this makes for cooler running and better MPG. Last note: I lived in Denver for a decade, and we could run nearly 40 degrees total there with no detonation. Higher altitude makes the difference. I'm in Texas now at 700 ft and 35 degrees is my maximum total now. Thans for the fine video.
Liked the video. I'm somewhat passionate about ignition timing and have studied it extensively over the years. FTR, I've a manifold vacuum preference too. The ported vacuum as you know was a crutch used in the early smog days and was a way to try and clean up emissions. I noticed the engines you referenced with your "good book" were all pre-smog examples. Keep up the good work.
Thanks Kevin and thanks for inspiring the latest video! Unfortunately the presentation quality wasn't the best but we got the reference info out there for anyone interested to see it. AG
Good tip on so called lock washers. When these things spread open [as they like to do], surface area under the bolt head begins to disappear and trouble has arrived. .
It’s great when you go into detail. I really learned something on that youtube about bolts. Not sure if you done one yet! But a you tube on decreeing a cam
Congratulations on reaching 10K subs! I knew that this channel would grow if you kept up the interesting content that really isn't explained elsewhere on YT.
The vacuum advance really is important in a street vehicle, they spend most of their time at something less than half throttle and at a steady state like highway use may be considerably less. People need to realize this and be truthful with themselves and the actual application of their engine. A part throttle dyno run may show the benefits of layering more vacuum advance on top of the total mechanical in such cases, there may be free torque and fuel economy to be had and better driveability. You did hint at this, but there are numbers to be had. In modern engines there are ECUs, multiple sensors, and advance tables that play the game, but really it's not that much more functionally advanced than the good old distributor. Anyway thanks for another interesting video and congratulations on breaking the magic 10k subscribers, it must feel good for you and your team.
The distributor on my 67 gto had a single advance stop on the underside of the weight plate and the weights had a different shape. For some reason the gto had a rubber bushing on the stop. Aftermarket timing kits supplied a steel bushing. Chevys that I saw had the same type. Your chart shows the total timing graphed at 35 degrees not 30. In the late 60's and early 70's the recommended total timing on both small and big block Chevys was 38-42 as I remember.......
Even emissions engines that utilize “ported” vacuum also utilize “manifold” vacuum. A thermal switch plumbed into engine coolant switches between the two depending on engine temperature. If the engine starts running hot it switches to manifold vacuum because the engine runs more efficient therefore cooler...
On another note, I never knew the factory Chevy timing curves were ridiculously slow and had too much mechanical advance and not enough initial advance. I didn't know that they were this bad until you showed the factory specs. Modifying it like you have makes a big difference in getting off the line 💪
If you think that one is bad, should see a stock TBI 350 or L31 Vortec 350 advance curve. They start out negative at WOT at low rpm. The TBIs only run about 16-20* WOT total depending on applicaton and the L31s about 23-25* WOT total. Total advance on both are not in until the 4,000+ rpm range. As the engine heats up from 180F to 210F they pull another 3-4* of timing. Vortecs also lose up to another 7* in high IAT conditions. The Vortec 350 can have NEGATIVE 10* of timing at WOT at 1,000 rpm in hot weather.
Very good, very interesting. I assumed you were going to discuss whether ported or manifold vacuum is correct.I have seen the issue argued in both directions.the one thing I might add is when you are at steady cruise, manifold vacuum will be a fuel milage improvement.
! u r right about the washers. I wondrd y they went out of shape and found loose, later. Thank you. Now, I need to go back and get rid of a few... Ha! Cheers!
Good tip I've been throwing lock washers away for years, 30 years ago my boces teacher told me the same thing, friction holds bolts tight, and there not used in engines and those bolts will stay tight forever.
Nice lesson... My experience with vac advance has been that connecting to manifold provides great cruise efficiency and smoothness, but the vac cannister cannot respond quickly enough to throttle input. It fails to pull timing out quickly enough under load. You get a brief moment of spark knock while the advance plate moves back. Connected to ported vac doesn't give such issues. I have since gone to MSD programmable, which uses a GM MAP sensor and has a fully programmable curve in addition to the run curve ie: "mechanical". My 302C has about 45 degrees total dialled in at cruise, and around 25 at idle. It's a happy engine.
I didn't know factory HEI ignitions had way more than 20 degrees of mechanical advance built into them, the aftermarket HEI ignitions on the market seem to have only 20 or so, I THINK! I have had a hard time getting info on built in mechanical advance on anything. I have been using manifold vacuum advance on my mild GM performance 350, and it been working out great, but I need access to a distributor machine for the remainder. Good video.
Thanks Mike, try this! Disconnect your vacuum advance. Set your total timing at all in 3000 RPM or more. Then check your timing at idle with the vacuum still disconnected. The difference will be your mechanical or centrifugal advance. AG
Didn't GMC put lock washers on there engines? I've done thousands of bolt ups in the oil patch and we don't even use flat washers unless the engineering tells us. Lock washers are a waste of time and sometimes we have to use them. I've flattened so many out. Good knowledge.
Thanks for the comment. In a torque application flat washers should be GR8 or hardened or they will gall, create friction that takes away from the required stretch and make the soft. AG
I added an aluminum plate to the arm of my vacuum can limiting the vacuum advance to 8 degrees. I think it originally was s 22ish degrees. With my throttle plate virtually closed, my idle was slways higher than i wanted. I was finally able to get a lower idle by redicing my vacuum advance. I think it was actually too low after the mod, so i actually opened my throttle plates slightly. I ran 13 to 14 advance, 34 total and using the stock GM weights. I used a Mr Gadket or Accell spring kit. I had heavily modified some new World Products heads, installed a mild Edelbrock performer cam and manifold. To make it pretty, i added some cast iron Sanderson block hugger manifolds and 2 1/2 dual exhaust. I spent months playing with and learning to get the timing curve right. Yup, manifold vacuum is the right way.... I don't care what other internet professionals say. It may be devicive, but it really doesnt bother me if they do it the wrong way. 😁 I had this 350 in a 73 Chevy stepside. I had it bored 30 over. I dont recall my time slips best et, but i managed 76mph in the 8th with some TA50 tires. Those didnt grip well and my converter grabbed pretty early. With so little weight in the back, this combo was so hard to launch without smoking the tires. From my et and online calcuulators, i think it was 294 HP. No race truck for sure. I never built it to race, just took it out to the track twice for curiosity. I miss my old truck. It never let me down. Did I mention happy 10,000? Yeah, but ill do it again. 😁 🎉🎉🎉🎉🎉🎉🎉🎉🎉🎉
I’m a 73 year old drag racer and have not been able to find a vacuum advance diaphragm that will last very long on my HEI distributor. I’m about to ready to order a HEI distributor from Progression Ignition that features the ability to program everything with your phone app.
the problem I have noticed with manifold vacuum is during tip in the timing gets retarded,throttle response seems better with ported vacuum but not every engine responds the same
12:24 You're one of the few that say "flame propagation" or a burn rate. So many people believe the combustion mixture explodes. As a side note: engine "ping" is a shock wave in the cylinder bouncing back and forth at the speed of sound.
I love this subject, nobody ever mentions the vacuum canisters are calibrated differently, for the application. Optimum peak combustion chamber pressure occurs at 14 degrees atdc.
I have always been a full port vacuum guy. I’ve worked on Small Block Chevy’s for 40 years now. The advantage is at idle. Engines will always idle better regardless of the cam with the distributor connected to the manifold vacuum on the carb. At WOT who cares neither port is worth a darn. At half throttle or cruise mode both ports will be putting out some vacuum so we are now back to the whole advantage of manifold port vacuum, idle. The tuned port was designed for emission compliance and hoping to get better gas mileage. That is it. Most of us are building street and muscle cars so this is really irrelevant. Go manifold vacuum port and your engine will idle so much better and be happier.
.. the huge variety of vacuum cans that were available from the factory and also Standard Motor Products. The amount of vacuum required to start the rod advancing varied widely. Some would be fully advanced with only 5” of vacuum. The one you need will not start advancing until 8 to 10. Also , the travel of the rod was almost always too much so you needed to make or buy a limiter tab that would go under the inner most screw the attaches the can to the distributor body. You cannot go wrong with the curves in the How to Hotrod Small Block Chevies. They are straight from the engineers. This stuff was figured out in the later 50s.
My street curve had 10* initial, 10* vacuum hooked to direct manifold vacuum and the centrifugal advance would begin just above idle and be all in by 3000 rpm. 12* in the distributor which is 24*#at the crank. 10 initial 24* centrifugal ---- 34 total Plus 10* vacuum Equals 44* at cruise. Very safe usually. There will always be some engines that defy conventional wisdom. They are called Infernal Combustion Engines for a reason.
We have to find for you some better audio settings. The Mic is actually shutting off as you come to the end of a word.I don't know if it's the noise blanking or the hands free activation..love the videos and the subjects. Thanks.
I haven't ever seen a reason to lockout a distributor the total advance is the only thing to worry about always has a better response from idle starts easier and still holds accurate timing at full throttle
A sbc 350 can make 400 hp. But a 383 can make 400 hp with more idle vacuum, and because of the increased stroke, make more low end torque. So if you're not limited by any racing rules, and you're changing the crank and pistons anyways, I would go with a 383, unless I already had the parts to do a 350.
Good points.@@yurimodin7333 though if changing the crank and pistons anyways, I would also get stroker clearanced 6" connecting rods so as to not use a small base circle cam.
Excellent...agreed on manifold vacuum source...working a 67 Ford 289....question...if running manifold vacuum source, do I still need to disconnect and plug vacuum source to set initial timing?...thank you in advance.
So, the vacuum advance, when manifold vacuum, actually allows the engine to act much like it has a race distributor locked at 34 degrees, but without the hard starting issue because before it fires its at 4 degrees, after it starts its at 30 and the time between the drop in pressure from WOT until max mechanical advance is about a second or two at most?
Good points whether you use ported or manifold vacuum for advancing, I found for my 340 that initial set at 22 degrees allowed the max advance go to 34 and using the manifold vacuum stalls the engine when I stomp it from stop. If I move it to ported, no more falling flat, trouble is that my centrifugal needs to advance quicker like about 1800 rpm. Can turning the set screw in the vacuum pot do that or what will it take?
Alan, I noticed that you use generic branded Bravex HEI distributors, my question is do you find that the curve is right on the money for your builds? Thanks and keep up the good informative videos. :)
Hmm,I'm running a stock l31 motor with a Eddy carb. I locked the mechanical advance to keep the timing down to 36 and hooked the vacuum advance up to ported. It feels like it's got a flat spot around 40 to 50. And I can't rev past 4000.
That is a problem Old, but I doubt that it is caused by the ported vacuum advance. One way to find out, if you know you have 36 degrees total, try removing the vacuum line and see what happens. Then you could try connecting it to a manifold vacuum source. Just a suggestion. AG
I figured out what it was there's an intake leak in the back. where the china wall meets the head. I Did hook up the Eddy 750 to the full manifold port though and I noticed the motor idles alot quieter.
The ported vacume goes away with throttle opening just like manifold vacume. The only thing different between the two is the ported vacume is zero at idle
Put a T in the hose and run a vacume gage into the the dash area and take a drive. Both sources of vacume go away at the same rate when you get on the throttle.
Absolutely correct. Only difference between the two is one is providing a vacuum at idle and one isn’t. Once the throttle plates are cracked open they both provide the same vacuum. At wide open throttle they both stop providing vacuum. Where this nonsense started about a ported source providing vacuum at WOT I’ll never know…
I VERY MUCH prefer NO vacuum advance at idle on a modified engine, ESPECIALLY with an automatic trans. I tune for a solid idle and maximum throttle response which usually involves initial timing being just short of straining the starter motor, reaching full advance as early as possible without gas ping usually being determined by the fuel being used, and the amount of mechanical advance being determined by an advance travel limiter to achieve the amount of total advance desired. When all of that is accomplished and if I want to attempt to achieve the best fuel economy possible, then I will tinker with making use of vacuum advance. If the vehicle that you're working on is for the most part factory stock and uses a factory stock type carburetor, then it very well may work best with the additional ignition timing that manifold sourced vacuum supplies at idle speed because of lean idle/low speed a/f calibration. Not looking for argument. If your methods work for you, that's good. I have been satisfied with mine for a long time.
@mickangio16 I do the same...find the best idle quality and off idle response with the best base timing setting-provides the smallest throttle blade opening and the best idle vacume number. Then I tune the mechanical advance curve up to total advance. When the base and curve is optimized then add in about 10 degrees of vacume advance and see if it likes it at cruise. Add more if it wants it.
New to your channel, but you should do a video on vacuum secondaries and how many people think if they dont feel their secondaries opening up it means its not working
My last bracket car was a 78 Camaro lightened a lot. Strip only so no street equipment. Pretty light. Std bore 350, Cam Dynamics 234* at .050, .480 lift hyd, cast flat tops, dbl hump heads ported per Vizard’s book, Performer intake, Holley 600 vac sec, T350, 3000 stall, 4.88 gears, 9” slicks. 12.63 at 105 was it’s best time on a t&t nite. The foot brake class was 13.00 and slower so I had to retard the time and limit the throttle pedal travel to slow it down to 13.00. - Vacuum secondaries CAN do the business !!
Your example for vacuum can had about 16 degree of advance. I think my can had more. My car had a misfire at cruise with the can hooked up. My whole curve may have been quicker. Where do you find specs for the different cans? My 68 has a Q-jet. A lot of HEI's have a lot of advance due to EGR.
Thanks for the question Kraschs. i have a 1968 Manual, if you can tell me the specific engine i will look it up for you. Most small blocks had 15 degrees and most BB had 12 degrees. AG
I do not agree with the lock washer episode I learned a lot about lock washers from motorcycle mechanics aluminum parts versus steel parts. Nowadays most lock washers are very cheap.
Yes indeed. And that a good thing at part throttle/cruise speed, better gas mileage. I have my advance at 45 degrees for part throttle. Under wide open throttle the addition vacuum advance goes away
How about if you have a little bit bigger cam and your vacuum is all over the place at idle and vacuum advance is hooked up to manifold kind of means the timing is jumping all over the place at idle, ported vacuum in my experience keeps the timing the same at idle!! This is confusing
Thanks for your reply I really appreciate that, it’s just a 327 in a 67 Corvette with a four speed Muncie with 331 gears with a I can’t remember the number of the cam, it’s a comp cam 280 duration it’s got a little lope, yes, I feel it works better off of ported vacuum I know the L 79 350 horse 327. I’m sure they used manifold vacuum I know I just have to play with it a little bit. Thanks for your comeback. I do enjoy your channel. Absolutely for sure. Keep up the good work.👍
They sell them split washers in grade 8, don't buy the cheap ones did not realize that until recently so will keep throwing them away unless I know they are grade 8.
So a normal engine always works better with more than initial timing? Does that mean that initial timing alone is only good for starting a cold engine ?
Thanks for the question Run. Well, initial also makes up part of total timing as well. The more initial you have, the less centrifugal advance is required. AG
@@yurimodin7333 my timing light is flashing about 11 o'clock on the balancer. I have no timing marks on the balancer but the pointer on the chain cover only reads to 24 degrees btdc. I do have a new balancer and timing tab ordered.
My car with a 4:11 gears runs 3K RPM @ 55 MPH at part throttle crusing on the highway. How would that work with a vacuum advance? I have 17 initial advance with 18 Mechanical all in by 3K. What would your advice be?
Absolutely fine with vac advance in that scenario. It allows you to run a leaner light throttle cruise mixture and helps keep engine temp in check. You wont need as much vac advance as you would if cruising at 2000 or 2500 for eg so you need an adjustment(many have this feature, but its not difficult to make a stop/limiter of some type) The big advantage is you will also get a little better mileage when cruising which saves fuel for when you want to mash the throttle👍 there is no downside to vac advance at all.
My 73 Camaro Z28 has a mild cammed 350 in it with 3:73 gears. With the turbo 400 auto transmission it turns 3000 RPM at 60 MPH. I have the timing set up so that it has 36 degrees total mechanical advance all in by about 2900 RPM. It idles with about 14 degrees of advance with the vacuum disconnected. On top of that the vacuum advance when connected to a manifold source is now limited to about 9 degrees so the total advance at part throttle cruising is 45 to 46 degree max and the engine runs perfect. I believe the rule is that you never want to exceed 50 degrees of total advance in any scenario. It is notable that when I bought the car several years ago the timing setup was all wrong. Since I took a day to make the corrections the car runs and drives so much better and the fuel economy has significantly improved. Best part is that it was practically free to do that. Has a 180 degree thermostat with a stock radiator and it never exceeds that temperature while driving on the hottest days. Only exception is if it sits idling for a long period then it might get to 190. Can has a typical stock Hayden clutch fan setup. If you were to add about 10 degrees of vacuum advance to your setup you should see some improvements over what you are running now. There really is no down side to this.
I recently saw Junkerup's video about this. I have nothing to add on the subject, but it's funny to see people rehash things like this and argue in the comments
I have a rebuilt 350 engine and im trying to install the distributor in place but even after i macht the oil pump drive with the distributor one still have a 1/4 inch gap for some reason the distributor doesn't want to drop down all the way in????
Okay i will try that also forgot to told you the distributor it was on a 70s 350 the rebuild one im working on is a 80s still the same probably???@@goldsgarage8236
You are correct Shelly, and 40-50 is fine at idle or part throttle. When the throttle opens, the vacuum drops and the advance drops out.. Hope this makes sense to you. AG
Thanks for the question Joe. Always set timing with he vacuum hose disconnected. Capping the port is ok to do but it doesn't change the timing setting. AG
Another point, always set total timing all in and let the idle timing be what may. For example, 36 degree at 3,000 RPM all in, If the distributor has 20 degrees of centrifugal that will give you 16 degrees initial at idle, however hook your vacuum back up and you will have 40-50 degrees at idle which is fine. Your engine need lots of timing at idle. AG
I THROW MY TIMING LIGHT IN THE BUSHES MR G MY VACUUM OFF MY LAUNCH STRAIGHT TO THE PING OF MY GAS. THAT'S HOW I GET YA W/ THE POSI AND THE GEARS . DIAL IN MY STREET 60' // I STOP LOSING PARTS 😎 🤙
Vacuum advance is primarily to fire lean stretched out molecules at steady state light part throttle . The fuel mixture is lean and needs more time to fire the fuel This benefits fuel economy . Where does light throttle occur typically 37mph to 55mph approx Vacuum needs to be measured and known at every 5mph How much advance , manifold vacuum at mph and get the highest reading u can ,at approx 14.5--15:1 air fuel ratio . Primarily vac advance manifold or ported can also depend on auto or manual g/box . The auto can have more vac adv because of the stall in the convertor ie helps engine thru stall and engine detonation is less likely . VS a manual =ported Is there some emissions reduced on deceleration using manifold connection ?? Many emissions distributors have both advance and retard cans
If you are going with 350 then why don't you go with a factory roller cam block with factory lifters ect! you could save 400+ dollars than using a link roller lifters and special front timing cover and that money saved can be for upgrading the crank for 383 or better heads.. Over achieve is never a bad thing!
You are 100% INCORRECT about vacuum advance…Ported vacuum does NOT add ANY timing when it is at WOT because at WOT there is NO vacuum at either port UNLESS the carb is too small in which case it creates a restriction and can pull in a little timing with PORTED and MANIFOLD vacuum…and there are times when a motor idles better with PORTED vacuum… Having said that, YES, the engineers working on STOCK motors know more than you do, but they weren’t making distributors with curves for motors with as much camshaft as we run now, so you CAN’T use a 1960’s book for tuning a 2020’s motor combo. 🤦♂️ You have to be smart enough to let the motor tell you what it wants and then make the curve do that.
I tune LOTS of carbureted motors for customers and about half of them are fairly rowdy or are low compression and over cam’d and those types of combinations typically want 20-36° INITIAL timing just so they’ll idle right…vacuum advance hooked to manifold vacuum doesn’t work well on some of those motors because if the motor won’t start with any more than say 20° timing but the motor “wants” to be at 25-30° at idle, the advance will pull in more timing and make it idle higher and then when you put the trans in gear, the RPM will drop a little and the vacuum will drop a little and then the vacuum advance will pull timing and then the RPM goes down even more all of which causes a huge RPM swing from idling in Park/Neutral to being put into gear and then people blame the converter for being too tight…on rowdy combos, it’s critical that the initial timing not move when idling to putting the trans into gear.
What? The Spring Washers? Why do you care to defend a stupid spring washer? Its a solved science that they dont work and actually end up holding less torque than without.
@@stevenrochelle2238 I didn't think twice about a washer, I was interested in the vacuum argument that many people argue about and about half are wrong
Not true. It is that plus the benefit of having your engine running cooler because it is more efficient. I especially notice the difference between connected and disconnected at idle on my 496 BBC in my 70 Chevelle.
So don't try to out smart the engineers that designed the engines. But you modify the centrifugal advance that they designed. 🙄🙄 But, your design is an improvement. 👍
Just keep ur vids honest, and SMART, like the one i just had to UN-sub from, cuz the solution for the issue was to CUT POLY LOCKS DOWN!!!...........WHAT????....DONT MAKE A SPACER TO CLEAR THE POLYS if u want stack valve covers, CUT THE POLY LOCKS.....i mean REALLY????....THIS IS UR SOLUTION???....NO!!...i had to un-sub, as i WONT participate in AWFUL band aids!!
