Unlocking the secrets of 2 stroke porting, part 2 - How to trace and measure ports. 

InstaBlaster...

Glad you like the content. Thanks for the comment.

Thank you for the comment

Me Too, lol.

Yeah I like to use what Ive got or make my own tools if I can, save a few bucks too.

Glad you found it helpful.

Thank's for the comments I do enjoy doing the vids.

@@AuMechanic How far off are you with Part 3 ?

Part 3 will be up tomorrow.

@@AuMechanic OK Thanks for letting me know. See you then. Peter.

Knowing a bit about how the parts you are buying to modify it work can help making your selections and gives you some idea on what gains to expect from them.

1 pipe per cylinder is best. That said for a fixed installation at constant RPM a 2 into 1 might do. I cant help much on the 2 into 1 design though.

Good question and in short yes Starting figures (as given by Blair) would be the carburetor flow area should be between 85% to 95% of Intake port area of a piston port engine. This is based on the specific time area of the intake port since it is not area alone that determines how much air flows through it but also time it is open for, re timing and RPM. Ill be doing more on ports other than the exhaust fairly soon.

A spacer under the barrel wont cause any difference between the stroke of crank. It only modifies the position at which that piston starts and stops in its travel form BDC to TDC

Yes use a centre line measurement for the purpose of pipe equations.

Thanks again

Yes measurements are taken at port window at the bore for maximum port window area.

Yes they do but we are just doing the exhaust to begin with for the purpose of expansion chamber calculations. I will be covering the other ports later.

Sounds great. How do people make the rolled sections of head pipe with cone software one dims are all calculated?

The software has a function to plot the diffuser and baffle cones on to sheet metal I did a video earlier on that here ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-BqYEagl6AHQ.html To do the bent header section is a bit more involved and Ive put a ref to another software to plot a curve taper pipe called "Cone Layout" Google it.

Suzuki had a combination of intake types back in the RM off road series that was a combination of piston porting with case induction reed that had a strong mid range power but not as strong in top end RPM range as standard reed.

@@AuMechanic were these single carburetor? I have seen a small power equipment engine with two carbs, but not a single carb, dual intake. That's what I hope to accomplish with a bike this summer

Single carb on the RM Yes some chainsaws have dual throat carbs on them and a few bikes too, these are commonly 2 stage carbs where one carb throat opens at lower throttle, that provides better fuel mixture control at par throttle but on saws is likely for emissions compliance since the simpler walbro single carbs they usually have are pretty rudimentary.

Good tip. There are a few methods to do it which are all ok so long as it gives and accurate cope of the port.

A free pipe calc software I wrote You can find the download site link to it in the description section under the video.

"Area" is only 2 dimensions (height and width) and "Volume" is 3 dimensions (height, width and depth) Knowing the volume you wont be able to extrapolate the width and height from it so there is no point in measuring volume.

@@AuMechanic I just watched more and have a better understanding now. Thank you for this excellent content. You make me feel I can do this with my intermediate experience. Novice question- Obviously, this would be done AFTER port matching, correct? As far as measuring exhaust port open duration, this would need to be done with a rocket key installed if you are planning to install one, right?

@@phreaktor Yes you would do it after any port work.

Yes Ive done a previous video that covers some equations for calculating the area of parts of ports.

i cant find it. have you got a link or title please?

@@aljaxon69 See video called "How to Design a Two Stroke Expansion Chamber Performance Exhaust - Step 5"

You could just measure the length from piston to flange at the top and the bottom and average out the 2 measurements.

@@AuMechanic HI, thanks for the reply. Just to clarify: is that measured down the slope (ie 50 + 55 /2 = 52.5mm from above) or exactly perpendicular to the face of the piston (44mm)?

@@davidwatsham2750 Measure down the slope of the roof and floor and average out the tow measurements.

Do you mean the small slot in the cylinder above the port ? If so that is to reduce exhaust noise by slightly reducing the pressure differential between the exhaust port and the cylinder pressure to reduce the peak of the sonic compression wave created when the port opens. I will be covering this in a later video on the reason for various port shapes.

Thank You @@AuMechanic

All that differs is the parameters used depending on the application. Some applications use similar engines to chainsaws so a few modifications can be made to suit that use of them. Just making the port wider can provide simple gains, where raising the port also changes to port timing so care needs to be taken doing that because you are changing more then just the port size but also the time it is open for and that impacts on other factors like blow down time for example.

@@AuMechanic my durations are EX: 147.5 Intake: 139 Tran: 105 BD: 22.5. The bd looks sensible but the transfer and ex look really low, looks like its tuned for low RPM? But the intake is relatively high though don't you think?

That's pretty normal for a chainsaw keeping in mind that they lean toward noise and exhaust emissions too so the exhaust timing is pretty conservative for that reason, and blow down being the diff between exhaust and transfer opening means that a low exhaust will have a low transfer timing to ensure a adequate blown down time. So what that means in short is for the purpose of best power there is some gains to be had in increasing ex/tr timing.

@@AuMechanic That's great, cheers! It does mean raising them both significantly. Would there not be a disadvantage to raising them so much that the piston doesnt get a chance to travel down much before exhaust gases leave the cylinder?

It will reduce the trapped compression ratio which if it has an integral head and barrel is a bit limiting because you can't skim the head to bring it back up. As for the exhaust gases so long as the blow down time is adequate then that should be fine keep in mind that the ports will be open for longer as well as being a little larger.

Good question but a but I bit much to cover for the comment section as it all depends on which port and which end of the ports to. it is an expansive topic I will do some more on this in ensuing porting videos.

That is true to some degree but then any engine will eventually be run at WOT at some point. For the transfer for example too slow an air speed near the cylinder end of the ports may results in poor scavenging, while too much may cause excess intake spilling out the exhaust. And for reeds you want enough air speed to give a positive opening of the reeds while on the flip side keeping the pressure differential across the reeds as low as possible. Other parts of the ports can be subject to turbulence as air speed rises that can limit bulk flow and for intakes increases in air speed can cause separation at bends where the air goes around corners fast but the fuel droplets want to go straight ahead into the port walls. It very much depends on the port and where about in the port and the engine in question and other factors which will probably take up a few videos just on those aspects.

Or the hand operating the die grinder. While the head has some bearing on the power and Ill be covering more on that soon in squish bands, there is no real magic bullet with power in 2 strokes, you need to get quite a few things all working for you to get big gains, otherwise gains can vary greatly.

The one in the video is not, it is just a small cheap 2t engine I bought to use in the videos. I do have a few chainsaws I do work but I don't get much time to use the parts to make videos as I am repairing them .

You can use one, but that will only give you height and width of the port and you need to measure the radius corners as well. And to measure the inside of the port at the cylinder your calipers may not reach so you will need to use a bore gauge of some kind of inside calipers.

Yes it is common for many to start hogging out the exhaust port to begin with but that said modifying the exhaust can be more forgiving where the transfers can really end the party if not done right, so just doing the exhaust alone is not such a bad idea for those new to it so long as they make small changes to begin with and test the results. Also important to be neat and precise about the port window too.

I'm not averse to critique so long as its not combative and they can cite sound logic for it then its fine.