Two-stroke performance tuning?

[Buckets4Me]

so whats happining with these other 2 stroke engines

havent heard any news lately ????

[Skunk]

so whats happining with these other 2 stroke engines

havent heard any news lately ????

I'm taking measurements of my new engine and then I'll decide where to go with the timings. At the moment it's looking like a 3mm base gasket will be a good start. Then I can tidy the head shape a bit to bring the compression back and improve the squish. I'm also researching doing away with squish...

[TZ350]

My exhaust port timing on the AX100 is 189°. The inlet has been tidied, as have the transfers, but I don't know the timings.

If the exhaust is 189 duration then it opens 85.5 degrees ATDC.

Thats (360 - 189)/2 = 85.5 Degrees After Top Dead Center.

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[TZ350]

I'm taking measurements of my new engine and then I'll decide where to go with the timings. At the moment it's looking like a 3mm base gasket will be a good start.....

Hi Skunk, if your going to make a 3mm base gasket you could try making a super big one so it sticks out the sides, that way you can pick up some extra fin area. On my bike I found there was a lot of heat under the barrel in the area of the exhaust port. The base gasket/fin could be made from copper or aluminium.

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[TZ350]

Then I can tidy the head shape a bit to bring the compression back and improve the squish. I'm also researching doing away with squish...

My recent experiance with squish and ignition timing is that the non-squish heads required much more advance than the squish head that had an effective squish band.

I expect the difference is in the greater mean squish velocity MSV of the squish head which basically stirs the burning mixture up and that speeds the combustion process.

Optomised (using a dyno) ignition timing and a faster burn means less negative pressure on the piston BTDC. But every thing is a compromise though, and if you go with the open chamber non-squish option please let me know why you think that should be better. Thanks.

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[Skunk]

If the exhaust is 189 duration then it opens 85.5 degrees ATDC.

Thats (360 - 189)/2 = 85.5 Degrees After Top Dead Center.

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That's the old AX100 engine specs. It's last outing was at the Easter GP.

I have a TF125 in the bike now. Std except for matching and cases etc and tiding things up. A rough measure shows the exhaust at 144° - a 3mm base gasket would give 188°. I've yet to triple check my measurements...

With the non squish head I'd expect 91 to be the better fuel as I believe it burns quicker.

[TZ350]

With the non squish head I'd expect 91 to be the better fuel as I believe it burns quicker.

Looking into the burn rate of 91, 96, 98 Etc is something we plan to do too. They say you don't get "more power" from the fuel itself but we hope to make gains from being able to retard the ignition and reduce the negative work done on the piston.

From what I’ve been reading it seems that people generally think that the higher the octane the slower the fuel burns but this is mostly not true with American performance fuels.

But closer to home, it's true that Av Gas is not a fast burn fuel or even a good burn fuel, its just a reliable burn fuel for slow reving air cooled aircraft engines who's cylinder head temperatures can very a great deal.

A mix like F5 uses of leaded Av Gas and unleaded, could be a good idea though.

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[TZ350]

a 3mm base gasket would give 188°. I've yet to triple check my measurements...

When Thomas set our engines up, he used a degree wheel, found TDC and then packed the cylinder up a bit at a time until the transfer ports opened at the required point. The thickness of the packing was the amount he had to take off the top of the barrel. Then he measured how much he had to raise the exhaust and lower the inlet.

Thomas finds the inlet opening position then marks the piston skirt through the inlet port. This gives him an idea of how much he can cut off of the piston skirt and/or lower the inlet port floor. Thomas says a low wide port fills the crankcase better than a tall narrow one of the same area.

Interesting info about maintaining port velocity:- http://www.gofastnews.com/board/tech...t-volumes.html

The inlet port needs to have much the same area as the carb. And he likes to strike a compromise with port height to width, so he can cut something off of the piston skirt so that the piston clears the top of the inlet port around TDC for 30-50 degrees or so. This gives the inlet a dwell period where the inlet is fully open for a time, the more the better he says.

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[SS90]

I'm also researching doing away with squish...

That is a really good idea Skunk (in my opinion)....the TYPE (i.e useable spread) of power you are looking for on a 4 speed transmission is quite different to what you would be looking for with a close ratio (6 speed) (or,like Cagiva did with one of their Mito road 125's 7 speed!) gear box.

Using a "squish band" head will typically give a "peakier" power delivery (although it will indeed give a higher "peak power" result).... in the same way that having large exhaust port duration will.

Sure, that "squish band" head will make more power, it just places that extra power in a narrower RPM range. My opinion is that, in the case of a compromised set of available gear ratios (4 speed) this is not ideal.

My experience with non squish heads has been that, like Teezee says, non squish heads require less retard, but more so with a "squish" head the ignition map used needs to be very well controlled (to suit the pipe design as as well), whereas, the non "squish" head shows more "real world" gains using only very basic ignitions (early "twist and go" scooters are a good example) that have between 8 to 12 degrees retard from static. (the amount depends on what coil/CDI you use.....), but typically, the older stuff is either 8 or 12 degrees.

In my opinion, pretty good for an (essentially) low revving engine.

Teezee also gave some good examples in the ESE thread on how SMALL the power gain was between his "highly tuned" bucket (central spark plugged "SQUISH BAND" head, high port timing,modified inlet disc etc), compared to other GP125 engines with the same exhaust, standard heads (i.e, NO SQUISH BAND), slightly raised cylinder compression, and standard exhaust port/inlet disc etc)

I think the peak power was the same (eventually), and the less tuned ones made a wider spread of power (from memory.....)

I have seen ignition maps for modern high revving racing two stroke engines (revving to say 14,000) that range from 30 deg BTDC (at 1,500 RPM to MINUS 2 Deg ATDC (at 13500 RPM).........

These engines (naturally) have "squish band" heads.....as well as 6 speed close ratio gear boxes.........

There are road going two strokes that have curves similar to the last example as well.............



However, if, like your Bucket, you have less gear ratios available (4 speed), then some compromises must be made.

If you want to start with using a different type of head design to what others are using (in buckets anyway), you will however, find that (with careful attention to other areas of your tuning) (correct primary and cylinder compression, correct ignition (type and design) porting layout and exhaust) you will make almost the same as other "high port" tuned engines, but with a longer spread of power, and more Torque.

The figure you have posted (for the duration of the exhaust) is very much where I would suggest a 4 speed air cooled 125 would want to be (What primary gear ratio do you have?)

what ideas do you have for the duration of the transfers..... (I like 131...(just as a number)

However, like as been said before, it is silly to just throw around port timings, but with the set up you have (4 speed gear box) there are a few constants......

Oh, also (when you have the time) could you measure the distance from the top the piston to the top of the cylinder (measured when the piston has JUST closed the exhaust port) and post that as well?

If it is not too much to ask, would you also be able to post a picture of the combustion chamber of your cylinder head, as well as a picture of the crown of the piston you will be using?

I personally believe that while the intake area of an AX100 is quite complicated, you should be able to have a good tourqey engine despite this "set back"!

[SS90]

Hi Skunk, if your going to make a 3mm base gasket you could try making a super big one so it sticks out the sides, that way you can pick up some extra fin area. On my bike I found there was a lot of heat under the barrel in the area of the exhaust port. The base gasket/fin could be made from copper or aluminium.

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By all means if you think that will give you some gains (wider spacer) do it Skunk, but personally I have never seen any data to support the notion that such a design gives any tangable gains whatsoever.

There are plenty of Two stroke air cooled buckets racing in New Zealand putting out in excess of 21 Horsepower, and I am still to hear of one that has a spacer like that.

[TZ350]

we need to examine the impact the combustion chamber has on an engine.

Abnormal combustion, better known as detonation, is more likely with a slow combustion process since it allows time for an additional flame front to start.

And a combustion chamber that has a fast burn rate is essential to increase the engine's octane tolerance.

Air cooled Suzuki motor X bikes that I have had a look at, all had open combustion chambers without any squish and about 22 RWHP. Kawasaki and Yamaha much the same but with squish bands. But the Suzuki was known for the width of its power band.

The jury is still out in our camp over squish/non-squish heads for our buckets.

But it is remarkable the difference it made to the ignition timing requirements of our bikes.

We can see from the ignition timing requirements the difference in burn rates between the open and squish type combustion chambers on our bikes.

I am hoping to get my bike with the squish chamber to Henderson Yamaha's dyno so I can get a printed dyno sheet so we can compare the torqe curves to the other non squish engines.

An interesting artical on combustion chamber shape:-

http://www.highperformancepontiac.co...ber/index.html

[TerraRoot]

I'm gonna take a stab at this. don't bite.
You mention that turbulence needs to be intrduced in the combustion event, I belive that with a non squishbanded head what is suposed to happen is the scavagne loop is still going with the piston at TDC

SO.. with crap transfers that dont make much of a loop go with squish
With damn good high reving well angled multiple transfer ports it's a waste of time to squish the mixture since it's all ready moving quite fast ina loop.

do i make sense?

[SS90]

I'm gonna take a stab at this. don't bite.
You mention that turbulence needs to be intrduced in the combustion event, I belive that with a non squishbanded head what is suposed to happen is the scavagne loop is still going with the piston at TDC

SO.. with crap transfers that dont make much of a loop go with squish
With damn good high reving well angled multiple transfer ports it's a waste of time to squish the mixture since it's all ready moving quite fast ina loop.

do i make sense?

Sounds logical to me Terraroot!

Now, following along those lines, if you have a 4 speed transmission, it would be logical to suggest that "overrev" is required for high speed gear changes........ it has been established that a retarding ignition will help this, but, what is your opinion on cylinder compression ratios, (and their effect on overrev?)

skunks Ax100R is limited by that 4 speed transmission, but carb size is open....... (hmmmmmm)

Sounds like a small frame Vespa to me!

[TerraRoot]

Damn! bitten off more then i can chew! I think maybe lower (primary right?) compression ratio and I hazard a guess that the mixture just needs space to be turbulent in. The more mixture packed into the crankcase before the reed closes seems more important than the primary compression.

Also you bring up something that's being annoying me, where the heck do people get an adjustable cdi for a 6v single cylinder two stroke? 3d or 2d?

[TZ350]

A MacDizy Engine Rebuild with Photos:- http://www.macdizzy.com/1989hhopup.htm

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