What fuel for compression?

[tgerloff92]

At what point is it necessary to start running a race gas vs pump gas? In sleds we often have to go to the airport to buy our race fuel due to turbo sizes. At what compression number do you need to start running race gas or alky

 

[Joe Kidd]

It's really not at all needed. 89-93 octane will work fine in most saws, even the ported variants. Of course there will always be a exception, but it's in the very low numbers. Turbo and SC engines are much different than 2T.

 

[tgerloff92]

I understand they turn a two stroke into a different breed altogether but my question still stands, at what compression do you need to start looking at race gas if alky blends

 

[FATGUY]

@ 205 PSI I still run 91 octane, and have yet to have pre-ignition issues

 

[blsnelling]

My ported 390XP, with close to 200 PSI, runs fine on non-alky 87 octane. You don't need race gas in your saws.

 

[tgerloff92]

I'm not asking for my saws or any saws for that matter. Simply curious as to what kind of number needs better gas. Not related to porting, RPM's, or any other factors related to performance in your everyday use saw.

 

[Jimmy in NC]

A very close friend of mine is an engineer for Mahle. He has data that shows after a certain point in a small motor, such as a saw or dirt bike, you gain power by lowering octane. This is not your typical desk jockey engineer either, he's a die hard motor building gear head that would rather be greasy. The problem with higher octane fuel at very high RPM, you just don't have time to fully utilize it or burn it all. He was doing some testing with a 4 wheeler motor turning some crazy RPM's and actually picked up HP when they went from 93 to 87. It's a hard idea to grasp, but the piston is traveling faster than the flame can travel. You are going so fast, spark knock isn't an issue.

The other issue is working with static timing in a saw engine. To get the most out of higher octane / slower burn fuel, you have to advance the timing. When you advance it enough to make full use at 9500-10500rpm, you would run into issues with starting. This is why high compression motors normally have retarders on them for starting that may pull the timing back 25*. This isn't a new concept, Model A Fords had the same concept as did early Harley Davidson motorcycles.


I run 93 as I have better luck with it not giving me ethanol issues.

 

[tgerloff92]

There we go that's more what I'm looking for. Real reasons why things happen, not just well I tried that one time on one saw and had one result.

 

[mtrees]

My ported 390XP, with close to 200 PSI, runs fine on non-alky 87 octane. You don't need race gas in your saws.

My 395 is 212-215 if I recall and runs fine on 89. Brad could answer better he built it. Screams by the way Brad!!

 

[Slamm]

All my saws run great to 14,000 rpms on AV 100LL, LOL.

Sam

 

[ancy]

What does a saw run like if it needs a higher octane?? I run fresh 87 non corn base.

Sent from my XT881

 

[tgerloff92]

As well , would it not be easier to build compression on a saw with a longer stroke vs a bigger bore?

 

[UK Rich]

It has a lot to do with PCT, PCP and AFR's.

I've spent a long time building small CC big turbo engines for cars etc. Also i've found that more revs used, generally the less octane rating you can get away with.

 

[chadihman]

I switched to 100 octane because It was the closest to me without ethenol. I noticed that my saws,weed wacker and blower idled rough and I retuned the carbs. I dont think it really did anything for power. I do know the exhaust smells different. I wander if the smell is coming from unburnt gas due to slower burn of high octane?

 

[tgerloff92]

I switched to 100 octane because It was the closest to me without ethenol. I noticed that my saws,weed wacker and blower idled rough and I retuned the carbs. I dont think it really did anything for power. I do know the exhaust smells different. I wander if the smell is coming from unburnt gas due to slower burn of high octane?

Yes. These are great topics! That's what I'm looking for!

 

[Ambull]

I switched to 100 octane because It was the closest to me without ethenol. I noticed that my saws,weed wacker and blower idled rough and I retuned the carbs. I dont think it really did anything for power. I do know the exhaust smells different. I wander if the smell is coming from unburnt gas due to slower burn of high octane?

Where I really notice unburned fuel is when there is smoke coming from the muffler. Synthetic oil makes more of a difference in this than the octane rating.

 

[tolman_paul]

There is no magic compression ratio you need to increase octane to prevent detonation because the head design plays a role, squish percentage of head area as well as squish speed, ignition timing, carb tuning, ambient temperature, time in the cut, how dirty your cylinder is (cooling effectiveness) etc. You definately can raise the compression ratio high enough to cause detonation, but as to where it occurs, it depends.

I've been doing quite a bit of two stroke tuning research and in one sense getting everything set right for peak performance is much more complex than the often simple approach of x deg transfer, y deg exhaust, z deg intake, set the squish to 0.0XX" raise the compression to XXX psi and advance the ignition Y degrees. On the other hand there are formulas and software that allow you to accurately calculate the performance mods you make before whipping out the grinder and more importantly balance them all together to give you peak power over the rpm range you are tuning the engine for.

 

[GBD]

As well , would it not be easier to build compression on a saw with a longer stroke vs a bigger bore?

It would be easier to obtain a compact combustion chamber without a very wide squishband that way. It would also provide a better port time/area to duration relationship. In chainsaw/portable industrial engine applications however, the benefits of big bore to stroke relationship is preferred by the manufactorers it seems. That is compactness and lightness.

 

[tgerloff92]

and in turn revs are easier to obtain with the short stroke correct? With the bore being larger and stroke shorter the relationship between effective scavenged exhaust and intake fuel are then erratic or heavily swayed to one side or the other correct? Which is what these top brands are trying to control with strato ports to meet EPA but keep performance at tip top?

 

[Roll Tide]

Yes. These are great topics! That's what I'm looking for!

My MS261 blows 220 and my 395 XP blows 235, they both run fine on non ethinal pump gas.