Two-stroke performance tuning?

[speedpro]

It has to be a whole-of-system approach to be serious from the carb to the stinger and everything in-between. With that in mind I still consider, but have no proof, that a well designed system will see an amount of air/fuel delivered from the crankcase which will exceed the volume of the cylinder. The only place for the excess to go is out the exhaust port into the expansion chamber. With everything working sweet, after the transfers close the reflected wave will jam it back through the ex port into the cylinder increasing the VE of the engine. Therefore the flow of air/fuel into the pipe towards the end of the ex open period is deliberate as it increases the amount of air/fuel in the end that is trapped within the cylinder, once it is returned.

Interesting that SS90 is now acknowledging the place for lower primary compression.

[B.Crump]

Wow! Look at what you started!

Does that deserve a KB reward for 'Most inflammatory innocent question'?

yea man and its still going i was just hoping somone had done it as in a supercharger like what you find on a car as im having a go at building a supercharger out of a turbo got to gear it up somthing wicked

[koba]

yea man and its still going i was just hoping somone had done it as in a supercharger like what you find on a car as im having a go at building a supercharger out of a turbo got to gear it up somthing wicked

Cool, I had flirted with the idea of running omthing like this on a 100cc four stroke bucket but it didn't look worth it compared to traditional bucket engines.

[SS90]

Interesting that SS90 is now acknowledging the place for lower primary compression.

Hang on Speedpro!

Don't forget, very early on, I tried to point out that you have to establish what a certain engines original crankcase volume is before you say it must be lowered.

By that I mean, you may well find an exsisting engine is (for example) 1.3:1 and if you are (unwittingly) using a dated expansion chamber design/ port layout then you infact must INCREASE the primary compression ratio.

It's really important to establish what your original crankcase volume is.
Interestingly enough, I have noticed that the gains when reducing from (as an example) 1.65:1 to 1.48:1 are quite substatual, where as, reducing from 1.48: to 1.3:1 are less noticable, and the scavenge pattern and expansion chamber design needs to be VERY accurately designed to be advantageous.

I (surmise, no hard facts to back it up) that if someone did not have the resources to VERY accurately cut,bend and weld their chamber sections as well as design their scavenge patterns, then a primary compression ratio of 1.4:1 would be a "good compromise", as like F5dave said before, the crankcase will operate as a better (more efficient) pump, substituting losses in other areas.

This (in my opinion) is why "proffesionally" built engines (to essentailly the same specs) always seem to have "Just that little bit more" than other examples.

[SS90]

Perhaps now your terminology 'short cct' is not what I would use.

The first gas entering the cylinder (esp in single transfer designs) bounces off the rear or head & escapes out the exhaust. I'd tend to call that escaping gas rather than short ccting.

Short ccting I'd say is the gas that come out of the front transfers and turns the corner hugging the cylinder between the trsf & the exhausts (hence short circuit). This tends to happen worst when the transfers are close to the exhaust. A modern setup often has a thin strip where a bridged port encroaches into the transfer clear area. (look at an RGV, RS or whatever).

Now if we consider that a crankcase as a pump, it is a pretty lousy one at low revs, worse with lower primary compression. Add to this the effect of the pipe scavenging being rev dependent; it is easy to imagine a bunch of gas short ccting when the pipe is out of tune and sucking at an out of freq range when the primary pressure is low.

I'd be surprised if the first gases would short cct as they would be spat out when the transfers open with most pressure so would be more likely to complete the loop (& then maybe escape).

But as the case pressure reduced, the velocity would reduce, making a short cct path the one of least resistance esp if the pipe was scavenging on 2nd order.

Yes, as you are aware, this is one of the reasons why correct crankcase volumes are important.

In regards to "short circuiting", when looking at the "loop" system as a whole, I (and others) would consider any mixture that DID NOT become part of the Schnürle loop as "short circuited", although I would concede that it a broad term, particularly when you look at multiple transfer layouts.

But it is true that "The mixture that first enters the cylinder through the transfers is most likely to short circuit", and like I wrote above, the best scavenge patterns are ones that are better at reducing this.

I am of the opinion that "transfer kickers" are quite helpful in this area.

[F5 Dave]

The main point of the schnurle loop & more so collide stream porting is to delay the fresh gas in the cylinder. What I'd call short cct is probably truly wasted as it is escaping way too early & mixed with the exhaust gasses.

[SS90]

The main point of the schnurle loop & more so collide stream porting is to delay the fresh gas in the cylinder. What I'd call short cct is probably truly wasted as it is escaping way too early & mixed with the exhaust gasses.

Great! 5 pages and it can be summed up in one short paragraph.

(sigh)

[speedpro]

Hang on Speedpro!

. . . . you are (unwittingly) using a dated . . . . . .

It's really important to establish what your original crankcase volume is.
. . . . . .
and the scavenge pattern and expansion chamber design needs to be VERY accurately designed to be advantageous.

I (surmise, no hard facts to back it up) that if someone did not have the resources to VERY accurately cut,bend and weld their chamber sections as well as design their scavenge patterns, then a primary compression ratio of 1.4:1 would be a "good compromise", . . . . . .

This (in my opinion) is why "proffesionally" built engines (to essentailly the same specs) always seem to have "Just that little bit more" than other examples.

I don't do anything "unwittingly" on my engines.

I ended up paying Wobbly quite a few $$s to determine optimum engine configuration for the purpose, including measuring crankcase volume, porting and scavenge patterns, squish velocity, and coming up with the laser cut patterns for the pipe. He spent a weekend with new exhaust port software designing the ports. I'd already built the short stroke crank with the long rod as suggested by him. He kept checking what reed valves I could get and what size/type of carb as the layout evolved. About this time he did Nigel's engine. Theoretically mine should make a useful amount more than his. There is nothing dated, unwitting, inaccurate, or compromised about this engine. OK maybe dated, it was done a few years ago now.

[SS90]

I don't do anything "unwittingly" on my engines.

OK maybe dated, it was done a few years ago now.

Hmm, I doubt being only 2 years old it's "dated", I am sure it's "up to scratch"

With an (obviously) well designed crank, cases,cylinder,head,expansion chamber, why isn't it assembled and raced.... what is missing?

[Buckets4Me]

Hmm, I doubt being only 2 years old it's "dated", I am sure it's "up to scratch"

With an (obviously) well designed crank, cases,cylinder,head,expansion chamber, why isn't it assembled and raced.... what is missing?

here goes again

[bucketracer]

.

why isn't it assembled and raced.... what is missing?

A wise man knows that "DO THE FUCK WHAT YOU WANT HONEY" is not permission to do what we want.

.

[speedpro]

You got it. Luckily she-who-now-holds-the-pursestrings has no idea what that motor has cost. The bottom end is together and running with my old barrel on it. Post-blowup it needed a rebore and the old crank was not so good anymore(see avatar). In between getting all the bits more or less sorted there's been a supermarket and 3 or 4 other jobs, 2 house moves Akl-Wgtn-Akl, and all the usual stuff. Having said that I pressure checked the water cooled head last weekend and it has 2 pin holes, so I am making a little progress again. Plan is to get it going in the FZR chassis reasonably soon using the current cases.

[B.Crump]

Cool, I had flirted with the idea of running omthing like this on a 100cc four stroke bucket but it didn't look worth it compared to traditional bucket engines.

yea im gona try run it low at maby 5 or 7 psi i dont wana melt piston will do a thread on it when i get it going

does anyone know how much psi it can handle? i was told 7 was pushing it on a tf125

[F5 Dave]

I read about that twin hireabusa liner had the turbo cranked up to 32psi. I would start with that & work up. . . . Unless ya yella.

SP my 100 barrel has a few leaks, but was virtually destroyed by trying to fix them. I'm going to try running it as a temp barrel, but I'll mill some holes in the jacket to get to where it leaks & devon them from the water side & cover the hole up with a plate.

[koba]

yea im gona try run it low at maby 5 or 7 psi i dont wana melt piston will do a thread on it when i get it going

does anyone know how much psi it can handle? i was told 7 was pushing it on a tf125

Don't bother, its not worth the effort to make it work on a two stroke.
If you really want a fast go-kart put an old 400 or 2 stroke 250 in it.