Part Four: Compression

[Deets066]

Yeah I didn't use the decomp on my 395 one time after Randy ported it and ended up replacing parts so I'll stick with decomps on large cc high compression saws.

Husky recoils suck though, but they have better decomps than stihl

 

[Deets066]

stihl decomps blow through em and after a bit they leak. My 066 and 088 both have stout compression. No decomps for me

 

[mdavlee]

Compression ratios are very low in these saws compared to a 4 stroke. I've figured a few of them. The highest was 8.7:1 If I remember right.

 

[T.Roller]

Husky recoils suck though, but they have better decomps than stihl

Yeah guess they weren't designed for 225psi

 

[XP_Slinger]

How about when You should or should not use a pop up. Pros n cons

 

[Mastermind]

How about when You should or should not use a pop up. Pros n cons

Good topic.

If you can hit the numbers you want with a pop-up then do it. No reason not to.

Many times I want a lower exhaust than stock.......in that case a cut squish is the best way to get there.

 

[RIDE-RED 350r]

Compression... gimme some!

Obviously, compression increases will generally yield a more powerful engine, but maybe not able to spool quite as high of RPM.

Couple of terms that come to mind in this discussion, CCR and UCCR (Corrected Compression Ratio and Uncorrected Compression Ratio respectively). In the 250r enthusiast circles I frequent, these are very much things to consider when increasing compression and figuring out the minimum octane to run. UCCR is a basic compression ratio calculation that uses the full stroke of the piston for a raw compression ratio number. Obviously this is a rough figure because you are not building compression until the piston closes the exhaust port. UCCR calculation is used to determine fuel requirements most times.

These measures may not be as crucial in a work saw as it is in building a hot rodded 250r, but I think it might be worth discussing. A quick read on the topic: http://www.torqsoft.net/compression-ratio.html

I'm kind of curious myself as to how it seems so many saws are modified for more compression by various methods but I never hear talk of the need for race fuel.. I have a 394 pulling 210psi and it gives not a hint of unhappiness on Ethanol-free 91 octane. I know that raw compression numbers isn't the way to figure fuel requirements, but with the squish band and base cut, I have to think the UCCR went up??

 

[Mastermind]

From what I understand about these short stroke engines, higher octane isn't needed because there isn't time for detonation to happen. Pre-ignition could be something to worry about in the case of red hot carbon deposits though.

Somewhere I saw a video where unburnt fuel was present in the exhaust plume. The higher the octane......the longer the plume was.

The consensus was that lower octanes where making more power than higher octanes because the higher octanes burn too slow for these short stroke engines.

 

[mdavlee]

I've tested fuels a few times. 87 has always been fastest. VP 94 SEF which was designed for small engines was 2nd. I haven't done any testing with trufuel 92 yet.

 

[RIDE-RED 350r]

Yes Sir, the whole trick to the game be it saws, 250r's or Top Fuel Dragsters is to run as low of octane as the engine can run without detonation to make the most power. You can always run higher octane than the minimum required and it won't hurt a thing, but due to the nature of higher octane fuels that have higher flashpoints, slower burn rates, and remain stable to higher compression ratios, you will actually lose a little bit of power and performance. Go the other way around and dump 87 oct into an engine that requires 110 oct minimum and things will get nasty for your engine and your wallet in a hurry. I love hearing people tell me how they tried some 110 oct in their stock (fill in the blank) that requires 87 oct minimum and they could "feel the difference". LOL!

 

[rattler]

I've tested fuels a few times. 87 has always been fastest. VP 94 SEF which was designed for small engines was 2nd. I haven't done any testing with trufuel 92 yet.

Same here.

 

[mdavlee]

At 235 lbs of compression on my 372xpw it runs horrible on pump gas. It ran great on methanol with a little nitro.

 

[RIDE-RED 350r]

I've always ran Ethanol-Free 91 in my saws, stock or otherwise. Ethanol and premix don't play well together from vast amounts of reports I have read. I've never been willing to gamble with my own stuff to test those claims myself.

 

[T.Roller]

Compression... gimme some!

Obviously, compression increases will generally yield a more powerful engine, but maybe not able to spool quite as high of RPM.

Couple of terms that come to mind in this discussion, CCR and UCCR (Corrected Compression Ratio and Uncorrected Compression Ratio respectively). In the 250r enthusiast circles I frequent, these are very much things to consider when increasing compression and figuring out the minimum octane to run. UCCR is a basic compression ratio calculation that uses the full stroke of the piston for a raw compression ratio number. Obviously this is a rough figure because you are not building compression until the piston closes the exhaust port. UCCR calculation is used to determine fuel requirements most times.

These measures may not be as crucial in a work saw as it is in building a hot rodded 250r, but I think it might be worth discussing. A quick read on the topic: http://www.torqsoft.net/compression-ratio.html

I'm kind of curious myself as to how it seems so many saws are modified for more compression by various methods but I never hear talk of the need for race fuel.. I have a 394 pulling 210psi and it gives not a hint of unhappiness on Ethanol-free 91 octane. I know that raw compression numbers isn't the way to figure fuel requirements, but with the squish band and base cut, I have to think the UCCR went up??

My 395 was slower on 100ll so I switched back to 93 non e.

 

[mdavlee]

I've always ran Ethanol-Free 91 in my saws, stock or otherwise. Ethanol and premix don't play well together from vast amounts of reports I have read. I've never been willing to gamble with my own stuff to test those claims myself.

Ed heard likes running e10 in a few of his autotune. It leans them out a little more than regular pump gas. I want to try e85 in some of mine

 

[RIDE-RED 350r]

Well of everything I have read, it's not so much the octane rating of the E-fuels itself so much as it is the moisture attracting properties and the higher likelihood of separation in premix that Ethanol brings to the table that is to be concerned about.

I know one thing, the stuff goes stale and fouls a carb quicker than anything I have ever seen is measures are not taken.

Again, risks I am not willing to take with my stuff when E-free 91 is readily available to me...

But anyway, I'm dragging us off topic here.....

 

[Mastermind]

But anyway, I'm dragging us off topic here.....

Not really. Anytime compression is discussed........fuel quality is a part of that topic.

 

[mdavlee]

Well of everything I have read, it's not so much the octane rating of the E-fuels itself so much as it is the moisture attracting properties and the higher likelihood of separation in premix that Ethanol brings to the table that is to be concerned about.

I know one thing, the stuff goes stale and fouls a carb quicker than anything I have ever seen is measures are not taken.

Again, risks I am not willing to take with my stuff when E-free 91 is readily available to me...

But anyway, I'm dragging us off topic here.....

Not a big deal if you don't store them for weeks. Methanol and nitro you have to flush pretty quick after running. It's even more corrosive than e85

 

[Hedgerow]

190-215 psi is where my saws usually end up. Seems to work well.

I'll pass on the decomps, I plug em all

That seems to be a really happy range.

 

[force10powertools]

In my experience e85 is both brilliant and a nightmare. It's the only pump fuel we can get in the south west UK and as such is what almost everybody runs. Bought from a busy fuel station and used the same day i recon my 372/2165/272 etc run at their best on it, but give the fuel a couple of weeks to degrade and collect water the same saws start getting fussy, my 268 fun saw however which has considerably more compression than the previous saws seems less bothered by stale fuel and runs much the same regardless. This seems counter intuitive to me, i'd have thought it would be more noticeable in a higher compression engine.

Any ideas?