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MAX Compression on Pump Gas B4 BOOOOM!!

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the following is sort of a mish-mash combination of posts i had put into a thread on the crazy azz topic of:

"How much compression can i run on pump gas and stay just shy of grenading my expensive race engine?"

long duration camshafts make a HUGE difference in octane tolerance....!!

long duration helps at the point where detonation will be the biggest problem.... accelerating from lower rpm ... where the load is the highest...

put in a big cam and there is more reversion ...and thus less complete cylinder fill at low rpm.... so there is much more compression/detonation tolerance with a long duration cam...such as the early HRC....

there are so many variables in the "whats safe" debate... proper jetting is another HUGE variable...consistant fuel quality another

so... what with things like:

outside temp

gearing

rider weight

riding sand hills or flatland

high elevation or sea level

cam duration

volumetric efficiency

and others....

volumetric efficiency is HUGE in this topic...and it is DIFFERENT for EVERY engine....

think of it like this.... if i don't get ANY air charge in a cylinder at all... how much pressure will a super high compression piston generate while compressing all that non existant mixture??

the answer is ZERO pressure!!

no charge in means you are compressing nothing.... the vacuum in the cylinder means you are compressing nothing...so its like like multiplying by zero....a zillion to one time zero still equals zero pressure

the other end of the volumetric efficiency scale: how much pressure will you see with a 14:1 piston compressing a charge that was force fed into a cylinder by a turbocharger running 80 pounds of boost??

the answer is: WAY TOO MUCKING FUCH..!! the excess pressure isn't so much a factor of the piston compression as it is the cylinder is packed with a ton of fresh pressurized charge before the piston even began to compress it... that's why blown or turbo supercharged engines dont want or need high compression... the dome of the piston would even take up space that could be packed with charge while the piston is at bottom dead center.....

so.... that's a huge reason why you need to know your Volumetric Efficiency BEFORE you ask what compression you can get away with... that is what affects your DYNAMIC COMPRESSION... which is the actual pumping compression YOUR particular engine combo is seeing during operation

no matter what, two engines of the same size..... whichever one is making more power is seeing a higher: volumetric efficiency....cylinder filling....dynamic compression....

a 12:1 engine making 55HP is seeing higher VE and pressures than a 15:1 engine making 50HP.... more charge is being lit in the 55HP setup because more charge is entering the cylinder.....

with most people not knowing anything about the detailed dynamics of their engine specifice.....if you are a manufacturer and want to tell people what is safe compression for pump gas... it had D@MN well better be safe for EVERYONE ....

splitting hairs over a point of compression when you cant control everyone's variables is like dancing in a mine field... lots of fun ...until it aint

another thing i like to tell my own kids to consider in any situation is:

"whats the worst thing that can happen?"

sometimes it's not much of a deal... put too much air in your bike tire and you pop it like a balloon.... not really a biggie.... put too little in and you wreck the tire AND bend a rim... little bit more of a problem...

if anyone has seen an engine that detonated...

cracked cylinder

broken piston

bent crank

sheared wrist pin

which also:

bends valves and trashes heads

that also plays into the "whats the worst thing that can happen if i get bad fuel or too much comp" etc... etc...

and for me personally... blowing a good perforamce thumper over a half point of compression or a couple octane points... is like playing russian roulette with something that can have dire consequences on my free time and bank account...

utterly not worth it for me....

i default to safety.... but that's my own paradigm... nobody has to buy into it...

just sharing some experience...

flat tire on road trip.... delay of trip for a bit...

so what??

bungee cord not inspected for cracks before jumping....

splat.. game over..

pick a cam/ pipe / whatever that doesnt fit your ride...

so what?? engine still runs

guess wrong on octane....

boom... game over

its all matter of what you feel is something that has an acceptable consequence....

they make pistons with LOTS of compression

they make fuels with LOTS of octane

they also make pistons that make pretty darn good power with a good safety margin...

it just comes down to comfort levels with what you are gambling with....

that's what detonation looks like...

it doesn't take long ...mere seconds or less....with the pressures that detonation creates...

sort of like using way too fast burning gunpowder behind too heavy a bullet....

gas...and gunpowder...are supposed to burn in a controlled, albeit very fast burn.... a push if you will...

detonation ignites the whole charge at one time...supernova style... and the pressure peak is enormous...

a huge component of detonation is radiant heating of the fuel charge...

you know how you can feel the radiant heat from a campfire?!

well... the fuel charge is supposed to burn evenly from center to border of combustion chamber....

the center flame front moves fast... but it acts like a sun heating the the rest of the charge... and the radiant heat speed is of course the speed of light... worlds faster than the flame front moves...

soooooo ... the heated outer gas is WAY more volatile because of this radiant heating....and capable of self ignition without waiting for the flame front.... thats where the big boom happens... when it all self ignites... insane pressures...

wider combustion chambers are more prone to detonation because there is longer time for the flame center to pre-heat the outer gas charge from radiant heat.... smaller chambers are much more resistant to deto simply because the complete burn doesn't take as long...

tighter squish does lots to control this by speeding up burn rate with more swirl... faster burn chamber... and its also better at shielding (shading) the perifery charge from the radiant flame front...

deserves more discussion....

a few topics:

1) pre-ignition Vs detonation.... the difference in the signs... and consequences....

2) what can you do to minimize both of those above.... aside from lowering compression...

3) VE... volumetric efficeincy... Dynamic (actual) compression.... and BMEP.... what are they?... how are they related?... and what do they mean to octane requirements?

4) compression of gasses.... linear or exponential??... assuming same cylinder fill, difference in psi from 10:1 to 11:1.... and diff from 13.5:1 to 14:1.... come on engineering students, crack the books and chime in... Big Country Dude, you are an engineering student :smirk:

that should make for some good tech discussion on a rainy fall weekday ... :bonk:

as far as pressure rise from 10:1 vs 11:1 compared to 13:to 14:1 , it is exponential.... lots of info on the web with just a bit of typing...

quick explanation ... if you take a vol of air and cut the space it is in by half you will get a certain pressure rise....

if everything was equal , you would double the pressure if you cut the space in half again....but you dont.... you get a much greater pressure rise...

that is why going from 13:1 to 14:1 isn't as small a comparative rise in pressure as the change in static numbers would seem.... it's big jump in pressure...

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wow... i guess everyone would be more comfortable with typical thread like:

"how do a adjust my shift lever?"

lol

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wow... i guess everyone would be more comfortable with typical thread like:

"how do a adjust my shift lever?"

lol

I read it and thought it was interesting but didn't really have anything to say. :bonk:

So there isn't a real advantage over a 13:1 and a 14:1? :smirk:

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yeah kind of an interesting read but maybe a few to many topics or subjects blended into it to choose from. Also doesnt seem to be much of anything to dispute so that puts a limiter on it for some:smirk: Any input on what fuel type, grade or octane level you would go to if you suspect pump gas isnt going to be able to control detonation?

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