How do you Figure Piped Saws Compression?

[thompsoncustom]

I got to thinking today how do you guys come up with compression numbers on a piped saw. Since a normal compression test won't account for the added fuel and air the pipe pushes back in. Is there a standard number that people guesstimate it adds? I assume that would be dependent on the saws displacement and pipe.

 

[buzz sawyer]

Curious - why do you need a number? I don't get into piped saws, I just do normal compression tests.

 

[thompsoncustom]

Curious - why do you need a number? I don't get into piped saws, I just do normal compression tests.

I don't really need a number more would just like to have a ball park of where it is at but no it's not required.

 

[buzz sawyer]

Understood. Sorry, no ideas. Surely someone here could give a rough estimate based on experience.

 

[ericm979]

There are computer models to calculate the effective compression ratio at various rpms (the pipe of course is most effective at certain rpm). They take port timing, size and exhaust pipe shape and a bunch of other things into account.

But for checking the engine health or measuring the static compression ratio, the normal compression test works the same as with a non tuned pipe engine.

 

[Evan]

Gauge is probly way off. Just pull the cord pepe on and iff if you want numbers

Let us know what you find

 

[bwalker]

There are computer models to calculate the effective compression ratio at various rpms (the pipe of course is most effective at certain rpm). They take port timing, size and exhaust pipe shape and a bunch of other things into account.

But for checking the engine health or measuring the static compression ratio, the normal compression test works the same as with a non tuned pipe engine.

Yes, there are computer models that predict the effects of a tuned pipe.
Static compression is really only useful IMO for assessing ring and piston wear.
You really need to know the corrected compression ratio for determining things like octane requirements, and other tuning parameters. IE you can have quit high static, but fairly low corrected compression.

 

[huskihl]

Piped compression is the same as figuring for a muffler saw. It’s just a wave that resonates back to the port and shoves a little bit back in. Not enough to make any real difference in compression

 

[bwalker]

Piped compression is the same as figuring for a muffler saw. It’s just a wave that resonates back to the port and shoves a little bit back in. Not enough to make any real difference in compression

Not saw related per se, but an aggressive pipe can push a motor into detonation pretty easily via increased actual compression. This is mostly a factor on motors with high compression ratios in the first place, which saws don't have.
It's important to note that diffuser cone of the pipe sucks fresh charge up through the transfers and scavenges the cylinder via a negative pressure wave. The exhaust then hits the convergent cone which if timed right jams some of that fresh charge back into the cylinder via a positive pressure wave after the transfers have shut. With this in mind it's not a stretch to figure out that pipes can and do have a very real effect on actual cylinder compression.

 

[huskihl]

Not saw related per se, but an aggressive pipe can push a motor into detonation pretty easily via increased actual compression. This is mostly a factor on motors with high compression ratios in the first place, which saws don't have.
It's important to note that diffuser cone of the pipe sucks fresh charge up through the transfers and scavenges the cylinder via a negative pressure wave. The exhaust then hits the convergent cone which if timed right jams some of that fresh charge back into the cylinder via a positive pressure wave after the transfers have shut. With this in mind it's not a stretch to figure out that pipes can and do have a very real effect on actual cylinder compression.

Right. Never seen it make more than 5 psi difference on a saw, hence my comment

 

[bwalker]

Right. Never seen it make more than 5 psi difference on a saw, hence my comment

You won't when using a gauge because pipe only achievs this at a narrow rpm range.

 

[NSEric]

I cant get my head around how a pipe effects compression, you're gonna get the same reading with a pipe or little box muffler.
I've checked compression on dirt bikes and sleds, one without a pipe on it and the pipe made no difference in how much compression you have at the rpm you do compression test at.

 

[ballisticdoughnut]

I cant get my head around how a pipe effects compression, you're gonna get the same reading with a pipe or little box muffler.
I've checked compression on dirt bikes and sleds, one without a pipe on it and the pipe made no difference in how much compression you have at the rpm you do compression test at.

I believe what OP was asking is how does a pipe (which is actually a resonator) affect dynamic compression. Of course this only happens when the engine is “on the pipe” and as you mentioned really can’t be measured with a compression tester.

 

[Lightning Performance]

I got to thinking today how do you guys come up with compression numbers on a piped saw. Since a normal compression test won't account for the added fuel and air the pipe pushes back in. Is there a standard number that people guesstimate it adds? I assume that would be dependent on the saws displacement and pipe.

It would seem important depending in how fat the pipe is but at speed the thing can't really be measured. If it goes faster with a dished piston you likely have too much and pumping losses happen. Different rpms may or may not affect the peak power period. Just test it and go look for issues. No issues no problems. You know if lays over and pipe is wrong or come in early and it's all down low relatively speaking. Tough call there. Math will only get you so far. 200psi is probably too much. Yet 125 might not be enough depending on how your pipe works.

 

[thompsoncustom]

I believe what OP was asking is how does a pipe (which is actually a resonator) affect dynamic compression. Of course this only happens when the engine is “on the pipe” and as you mentioned really can’t be measured with a compression tester.

yes this is what I was asking. there is going to be no difference doing a standard compression test because the pipe does nothing when your pulling it over by hand. really u would have to check it around 9k rpm and up I would think.

 

[bwalker]

It would seem important depending in how fat the pipe is but at speed the thing can't really be measured. If it goes faster with a dished piston you likely have too much and pumping losses happen. Different rpms may or may not affect the peak power period. Just test it and go look for issues. No issues no problems. You know if lays over and pipe is wrong or come in early and it's all down low relatively speaking. Tough call there. Math will only get you so far. 200psi is probably too much. Yet 125 might not be enough depending on how your pipe works.

The pumping lose thing is always brought up in regards to 2 stroke compression, but it's simply not true as heat in a air cooled two stroke becomes an issue well before pumping lose Both from a charge density standpoint and from a piston reliability standpoint.

Dennis Cahoon once said that most of the gains in a ported saw came from boosting the compression and I believe him. FYI he built 500cc saws for the Stihl Timbersports series amongst others. I think he would be qualified to say.

Then there is the fact that boosting compression raised the HP and widened the torque curve on every snowmobile and motorcycle engine I had it done and this was dyno verified.

 

[Lightning Performance]

The pumping lose thing is always brought up in regards to 2 stroke compression, but it's simply not true as heat in a air cooled two stroke becomes an issue well before pumping lose Both from a charge density standpoint and from a piston reliability standpoint.

Dennis Cahoon once said that most of the gains in a ported saw came from boosting the compression and I believe him. FYI he built 500cc saws for the Stihl Timbersports series amongst others. I think he would be qualified to say.

Then there is the fact that boosting compression raised the HP and widened the torque curve on every snowmobile and motorcycle engine I had it done and this was dyno verified.

Are you following me around?

"Had it done" interesting choice of words there.

 

[bwalker]

Are you following me around?

"Had it done" interesting choice of words there.

No I am not. I responded to this thread before you did. I only responded to you after you spewed some nonsense as you often do.

Its.not very interesting at all. It's just the truth. I've never had the tooling to cut a two stroke head, but that doesn't mean I don't understand something you clearly dont.

 

[NSEric]

I messed around with dirt bikes a bit(modern yz250's)
If the compression is over 230 psi you lose some over rev on the top end, it may make more hp but it falls off quickly after peak.
The lower compression ones like 180 psi or so would rev a little higher but made less peak hp.

I doubt you can get the compression high enough in a saw to hurt the saws ability to rev high enough, so the more the better.

 

[bwalker]

I messed around with dirt bikes a bit(modern yz250's)

If the compression is over 230 psi you lose some over rev on the top end, it may make more hp but it falls off quickly after peak.

The lower compression ones like 180 psi or so would rev a little higher but made less peak hp.



I doubt you can get the compression high enough in a saw to hurt the saws ability to rev high enough, so the more the better.

IME with the latest generation YZ250 engine, which was my all time favorite 250 two, stroke 190psi seemed to be a sweet spot for broad power without the loss of over Rev. This was with a PC pipe. The thing you really have to watch out for with this engine is they are detonation prone stock. The last one I had was 2009 and it would detonate on pump fuel with only a PC pipe. With the stock pipe it was fine. You either have to go to a much larger main jet(not ideal) or race fuel to stop detonation. In 2010 IIRC Yamaha altered the head. They made less power but where much less prone to detonation.
Since we are on the subject of compression. The last Aprilia 250cc GP bike ran 12:1 compression ratio at 225psi static and made 110hp. So.much for high compression hurting HP..
And you right. With a saw more.compression is better and this BS about pumping losses is just that. The limiting factor is heat. Run as much compression as you can before reliability and heat related charge density issues start to happen.