It seems to me that an actual engine compression test would be the best way to determine the need for high octane race gas. But I only hear most people refer to the compression ratio of the piston. The reason I ask is because I have two 350X motors. One is bored and stroked to 500cc and has a 13.5:1 piston and an average compression reading of 278 PSI. The other motor has a 10.5:1 piston and an average compression reading of 240 PSI (both have fresh top ends and mild cams). Most people say a 10.5:1 piston does not require race gas, but I have also read that anything over 200 PSI does need race gas. Can anyone tell me if I am wasting money by using $4 per gallon gas on the motor with a 10.5:1 piston? Also do these compression readings sound like they are normal? They seem high to me - which I am reminded of everytime I kick them over!
Topic title edited by atvbbs.
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No expertise claimed, but I don't think a low-rpm, as from starter, recoil, or electric starter tells the whole story.

At high rpm's, fluid flow dynamics and ram-charging effects of cam duration and overlap come into play, as well as exhaust pulse scavenging by the plumbing. The "volumetric efficiency," an index of how well an engine "breathes," varies widely over the rpm spectrum, especially for highly-stressed engines.

The neo-static compression test checks valve and ring sealing, but I don't know if an absolute octane requirement determination can be made from the pressure readiings alone.

The true test of octane requirement on a gross scale appears in the question, does the engine ping under load at full throttle? If not, the pushalene octane number's probably o.k.; if so, higher test fuel or octane boosting additives are called for.

With your highly-modified engines, I'd buy the hightest octane fuel I could find and afford.

Tree Farmer

 

Nice post TreeFarmer, you really addressed that nicely. Compression can be used as an indication of potential octane requirements, but more of an overall "guide". There are several other variables invovled that determine ultimately the requirement. In 2 stroke motors, and 4 stroke alike, the feature called the squish band, or often in thumpers called the quench, is a main factor in finding the octane requirment.

Example, I ran a head that had 255 lbs. static cranking pressure...octane required 100...the squish was set-up in a manner that while it provided sufficient benefits, the octane requirements were low. The squish width, and clearance from the piston crown determines the mean squish velocity. It is what is sounds like, the velocity of the trapped fuel-air mixture as the piston rises to Top Dead Center and is squished between the squish band and edge of the piston crown. The MSV is measured in meters per second. Higher MSV's require more octane so that the fuel doesn't auto ignite. RPM also dictates MSV, in that higher RPM's create higher MSV's. Squish widths are varied by power-band characteristics, wider for more mid range biased motors, and narrower for high RPM motors. The bore size also influences the width to a point as well, the larger the bore the narrower the squish can be due to it's larger size. Percentages play in, typically the squish is around 30 to 40 percent of the bore size, so with a larger bore a lesser width is required to achieve that percentage.

The turbulence imparted from the squished air, tumbles the air, when ignited it provides a larger flame wall to burn, netting more power. The fuel air charge burns more completely in a shorter amount of time while the mix is still highly compressed.

I'll have to apologize for not knowing your specific application, I've simply focused on two strokes so much lately, I need to expand my knowledge some more. However, I thought that I would give you a summary of what is all invovled in determining octane requirements, simply from a bench racer(doing the math) perspective. It simply becomes a matter of pluggin numbers into formulae that then gets the MSV, the MSV and such then can be used to get an octane requirement.

I really do advocate running the highest octane that you can afford, if required for your motor's set-up. I'm not saying run 110 if you only need 92, but when the gains of higher octane fuels power potential is compared to cost over a summers riding, it really does pay off.

Hope I helped.

 

Very well said guys, man Im even getting confused You might check your compression readings again, maybe with another gauge, and at operating temp. They sound a little high for those compression ratios. Also if your running them rich, carbon deposits should be taken into account. Hope this helps.