Measure how well your exhaust flows cheaply and easily on the road by doing a quick test. My books are available at www.amazon.com... or Amazon in your country.
I made an O2 fitting adapter for this purpose. Steel brake line works well for this purpose. My daily driver has 6psi of back pressure before cat at max load. My track car, with custom exhaust, has less than 1 psi of back pressure.
Such a simple yet effective method of proving it, not sure why I hadn't thought of it myself to be honest. Also, here before the: 'cars need backpressure' comments come flooding in
A vacuum gauge on existing inlet manifold fittings (post throttle plate) works to find BAD exhaust restrictions. You have say 15-20" vacuum at idle then as you increase revs to steady 2500-3500 the vacuum should remain similar say 14-18'. To prove it works stuff a big rag clamped in your exhaust end and run it again the vacuum will still be 15-20" at idle but now falls to 6-12" at 3000rpm. Can do it in your driveway and no need to crawl under car and make new holes! ( turbo'd car may require you to disconnect boost pipe ,or create a leak, to inlet manifold to ensure steady vacuum, i would only do that on turbo car in driveway testing not open road!).
Yes its more a test for serious exhaust restrictions than optimising it. Thanks for the tips about using Rivnuts in exhaust, I didn't know what those things were called! I only found about this test after I have had catalytic convertor failures, one collapsed internally (car felt like it had only 40hp) and one clogged by anti freeze/engine oil (car had no power over 4500rpm).
The internal combustion engine operates on the principle of Delta P. Delta P is the difference in pressures between two points of measurement. Naturally aspirated engines only have 1 atmosphere of pressure to run on. The greater the restrictions anywhere in the system, the lower the potential Delta P.
Wonderful as always… I'm a little stumped on how I would measure that with the hose coming off let's say with no meter in the car specifically for that… Every day driver shooting down the road etc.… So in my mind I'm seeing it attached underneath the car and then coming up the side of the car in through shall ..we say the window and then holding onto the mirror laying across your lap and such and then running down the road…?🤔 great book by the way....
Something like that would work, keep in mind it's not going to be a permanent fixture in your car but only when testing. Once you know what your backpressure looks like, you take it all back out. You may wanna find a way to hold the gauge up so it's not resting in your lap but aside from that it sdhouldn't be a problem.
It's interesting to see this when so many people are getting over pressure problems and blaming their too free flowing exhausts. It seems to me that manufacturers are speccing too restrictive components on the wastegate side (or maybe something weird with variable vanes) then using too much exhaust backpressure to tame the boost rise. Is that intelligent or lazy engineering? Also which side of the DPF should we be measuring?
If you want to measure the restriction of the DPF, measure in front of it. I am willing to bet that over-pressure problems with boost is just poor wastegate control.
@@JulianEdgar Maybe my bad English don't help... Sorry...! My empirical idea was... If we can measure intake pressure, and calculate flow (like ECU does with MAF sensor measure), we can apply the same concept to the exhaust, for measuring exhaust flow... And it works like you say. But... We have a little drop in manifold pressure, if we have an increase in the exhaust flow...? More exhaust flow, implies in more free space for new A/F, not?
How far does one have to get from the exhaust with metal fittings before one can switch to plastic/rubber? Does anything need to be done to protect the gauge from the hot gas?
For temporary testing - one or two full throttle runs in lowest gear, as described - the rubber hose can go right to the exhaust barbed fitting. No heat travels to the gauge.
Just a normal rubber hose is fine for a few bursts of full power in the lowest gear. You could use a silicone hose if you wanted better heat resistance.
Hi Julian,I have a question regarding exhaust pressures.When I was a kid,my father had an 1.8 VW Jetta mk2.One day while driving on a dirt road with him,we hit a rock that ripped the exhaust off just in front of the first muffler.So all that was left was the manifold and a pipe ending about in the middle of the car.So was very loud and had basic no power.We went into town,got it welded on again and like magic the car had power again.That didn't,and still doesn't make sense to me at all because to me more flow = more power.Do you have any ideas on that?Thanks
Could be a variety of reasons but the most likely is that the engine management didn't like it eg oxygen sensor was receiving diluted airflow, MAP sensing couldn't cope with changed volumetric efficiency, etc.
@JulianEdgar Hi,thanks for the reply.The vehicle had the k-jetronic system (mechanical fuel injection).No O2 sensors as far as n know.This has been baffling me for years.lol
Every car I have ever worked with that lost power with a reduction in exhaust backpressure (after the tuned length section) was because of unhappy fuelling/ignition timing.
