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2 stroke compression ratio / octane chart?

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Everything affects octane requirements pretty much and octane rating can generally be substituted by enriching the fuel mix ( I'm not saying you can run 87 whenever you want just run richer ) i.e. Instead of 14:1 A/F with 93 you can run maybe 12.8:1 A/F with 87-89 (just pulling random numbers out here FYI)

It really is bike specific and I wouldn't trust a general chart except maybe to get a idea on what's going on on a theoretical level

I would ask a engine builder such as Eric Gore and hope to get a response as they will have a good idea on what works and what doesn't .

Just my 2¢

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Only reason I'm asking is because my bike (ec300) is currently at 13.2:1 uncorrected, and it sputters right up top. The macdizzy chart recommends 100 octane + and I'm using 99, considering minor errors in measurement, I'm supposing I'm ever so slightly past the limit of what I can achieve... Gordon Jennings book seems to recommend about 12.5. Just want a bit more confirmation before I take another 2cc out of the bowl!

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Sputtering or other forms of misfire would not be an octane related problem.  The ONLY thing octane number indicates is the fuel's resistance to detonation.  It has no bearing on energy content, burn rate, vapor rates, etc. except as a tangental result of different chemistries in the blend in question. 

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Your maximum permissible compression ratio on a given fuel will vary depending on bore size, pipe design and port timing.  Smaller bores can run significantly higher compression than otherwise identical engines of larger bore diameters.

 

You'll know if you're running too much CR from detonation more than anything else.  Have you had the head the off lately?  What is your corrected compression ratio?  Crannking PSI?

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There was evidence of minor detonation, so I've removed 2cc from the combustion chamber taking the bike from 13.27 to 12.33. Right in the realms of what is quoted on macdizzy and a.g.bells book. Will report back if all is OK. I machined the squish clearance a while ago and have erred on the side of caution with regards to material removal... Most likely causing the problem

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On a two stroke the fuel type and squish speed also effect detonation. Leaded fuel is better at preventing detonation.

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Reducing the compression has cured the problem. This all came about from remachining the squish after using gaskets to line the ports up correctly. I got the head capacity to what is deemed top limit for pump fuel but it was too much. Add in a 125 cdi and snap crackle and pop!

Nothing on the internet really describes 2 stroke detonation well so here is my description.

At constant throttle it sounds like electrical sizzling mixed with tiny shingle stones hitting your expansion chamber. Subtle, but audible.

At full throttle it feels like electrical misfiring, it revs cleanly, then stops, sputters, and revs cleanly again, top revs vary around a 500rpm (approx) limit, never clearing up. Almost like clearing out your bike after sitting with your choke on, but only at Max revs.

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First image is as it came off the bike, there were only tiny pock marks from detonation, and head wiped clean. I'm surprised it was in such good condition as I've been riding like this for about a year!

Second image is new profile. Squish is 49% of area.

DSC_0223.JPG

DSC_0224.JPG

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Detonation has never caused any noticeable hiccups in any of my engines.  My 125 and 250 pulled just as hard as always while pinging away.  If you're noticing severe enough misifiring that it seems to be cutting out theres probably something else occurring.

 

I would describe the sound as something like a fast, slightly irregular crunching type of noise.  Unfortunately I have had my fair share of experience with it in several different engines, two and four stroke, turbo and standard.  They all sound very similar.  When you hear it and know what it is it makes sense.  Like a bunch of rapid little explosions inside the combustion chamber.  What you're hearing is each little detonation hammering on the head and piston crown like a little bomb going off.  It happens so quickly that the series of pings sounds like a crunching, rattling noise.

 

Sometimes its not bad enough to worry about.  Slight, sporadic det is acceptable if you're running on the ragged enge for performance.  Sustained det kills motors.  Two strokes will burn a hole right through the piston if the edge of the piston crown dont burn through first.

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What you're hearing is each little detonation hammering on the head and piston crown like a little bomb going off.  It happens so quickly that the series of pings sounds like a crunching, rattling noise.

 

Sometimes its not bad enough to worry about.  Slight, sporadic det is acceptable if you're running on the ragged enge for performance.  Sustained det kills motors.  Two strokes will burn a hole right through the piston if the edge of the piston crown dont burn through first.

The crunch description is especially true of two-strokes since they fire twice as often.  With four-strokes, they aren't as likely to detonate at very high RPM as they are at lower speeds, so each ping is a bit more distinct as a general rule.  As far as engine damage, almost every mechanic who worked on GM engines in the '80's or '90's has a story to tell about the piston(s) he found in one of the 3.8 or 4.1 V6's (Buick design).  They were run at the ragged fringes when they were right, and when they went wrong, it was one constant rattle fest on the freeway.  Ugly results.

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On a two stroke the fuel type and squish speed also effect detonation. Leaded fuel is better at preventing detonation.

 

Leaded fuel at 100 octane will not prevent detonation better than unleaded fuel at the same true 100 octane or higher.  In fact, in engines that experience significant carbon build up, it can actually cause detonation, as the metallic components of the fuel become embedded in the deposits and can form hot spots that lead to pre-ignition, or to secondary ignition after the actual spark.

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