2 stroke design/modification spreadsheets

[Jaguar]

For anyone interested in optimizing your ride, if you can use Excel spreadsheets then check out those I offer at https://www.dragonfly75.com/moto/calculators.html

[stevo as b4]

I'm trying to optimise an old two stroke, but am still waiting for parts.

If you know your onions I do have one question I'd like to know the answer to though.

Say you have a tuned two stroke with a pipe that gives good top end power for say a band 2000rpm. Then what would be the best or most effective way to bolster up and improve the low-midrange power?

1, stuff like resonant chambers or water injection to fool the exhaust into working better at low Revs?

2, variable height exhaust port and thus compression ratios?

3, a custom mapped ignition to get the ideal timing at every speed range?

[redeem ouzzer]

[Jaguar]

Well on my 100cc street bike I designed a boost bottle for it to eliminate the power dip that happens right before the RPM at which the pipe powerband starts to happen.
And its transfer port duration is low for good low RPM power (116-118 degrees).
Yes you can use water injection to make the pipe start to kick in sooner but it won't really help low RPM power.
Having timing at a max of around 20 degrees BTDC (or more) is important for low RPP power. And around 10000 RPM it should be close to 10 degrees.
If the cylinder head has a squish band then its very important to make sure the squish clearance is correct for a squish velocity of 20-23m/s at peak power RPM. Most engines have too much clearance which causes power loss.
Increasing engine compression helps low RPM power, but it can detract from high RPM power due to pumping losses and if the engine is water cooled then it can cause power loss due to excessive engine heat.
Last, but not least, is having a pipe designed for a broad powerband which means the baffle is much longer than normal with sections of increasing angle (not using just a single cone).

[stevo as b4]

Thanks for your reply on the questions I had its appreciated.

I've had a look at your exhaust design software and I might be interested in having my existing pipe analysed for its effectiveness, as well as having a pipe designed for a larger 200cc cylinder with some tuning work.

My existing pipe runs ok, but the peak power is delivered about 1000rpm lower than std if you believe the dyno readings that is. Its also got a two stage baffle cone, which I had previously always thought to be a crap idea, as I thought it would weaken the return wave as opposed to a single taper of the angle chosen for the way you way you want it to work. Im just a noob that's read a few books and messed about with engines, but I have no experience of design or pipe simulation etc.

What you said about water injection sounds very logical too, so basically when your say 2000rpm or more below the power band it'll do nothing, but by switching it on and then off quickly at the right upper and lower rpm, you could what maybe make the engine climb onto the pipe say 3-400rpm earlier?

I need to accurately measure and check my squish clearance and compression ratio, but I need to do an all std back to back run with my new exhaust first to get a fair comparison. There's alot of messing about to be had and things to set up I expect, and I'll be letting the dyno tuner do this on this bike.

Would you be able with software be able to simulate the effects of an exhaust resonant chamber and also work out its optimum sizing too?

I ask as I wondered if say a new pipe was going to make the top end power better, could you get more midrange with a larger resonant chamber fitted? My KMX KIPS chamber including the space in the cylinder adds up to about 80cc. By fitting a KMX 200 resonant chamber it would take it to at least 140-150cc I guess. I'd like to try it on the dyno to see what the effect is and also if it screws up the fueling massively or not?

I noticed on my A/F trace that when the powervalve opens there is a big dip in the fueling trace, so I expect changes to the power valve or resonant chamber size to need some re-jetting?

Lastly is having a programmable ignition a bit like improving the reed valve flow or fitting a V-force reedblock, in that the gains you can see are across the whole rev range normally?

[Jaguar]

yes you can get the pipe powerband to come on earlier with water injection but I forgot how much.
I’m not sure about designing an exhaust resonant chamber. I assume you’d probably have to use the same formulas used to calculate a boost bottle.
A programmable ignition optimizes the power throughout the whole RPM range.
I have an analog CDI on my bike of my own design and I’ve tweaked its timing curve many times to get the best output from my bike.