98 vs 100 octane

[eeeyan]

I would have thought if you have a choice you would tune your car for the most readily available quality fuel you can get which i would have thought is ultimate 98. unless your in love with octane booster in which case do what u want if you build a car specifically for one type of fuel you better hope you dont run out and have to drive to far with a lesser quality fuel if u get my meaning. I'm not saying dont tune your car for 100 but considering 98 is much easier to get/more available it seems the obvious choice.

[andrew_mx83]

yes but if u love NA like i do lots of comp is the only way


the 7m i am building now will have 12:1 and will only be run on 130RON avgas

[eeeyan]

cool. hope you got deep pockets.

[Celica RA45]

well my comp was 13.5 to1 the new pistons are 14.5 to1 and still run 100

[eeeyan]

does the compression of the motor directly reflect what octane fuel you need? i didnt think it did

[Norbie]

No it does not. Cam timing, ignition timing, combustion chamber design are all important factors as well.

[DreadAngel]

^^^

Having said that though, you can't really tune a hi-comp engine on say 96RON because all you get is detonation even if you turn the wick up slightly. Where as "proper" higher octane fuel are more detonation resistant so you can advance the timing allowing you to make full advantage of the hi-comp engine =)

As for Blacktop being tuned to the ragged edge? I somewhat don't think so... Especially if you get the 4A-GE(U) then you'll know just how conservative they really are...

[oldcorollas]

how can you say it does not have a direct effect norbie?

compression with determine the limit of compression for pre-ignition and compression ignition purposes.
of course we are talking dynamic compression.

all other things being equal/fixed, compression can be linked to fuel rating. change any of the other factors and the allowable compression will change,

same goes for boosted motors.. there is a reason you can't run 60psi on 98 octane.. each fuel will have a practical boost limit, which will change depending on each motors factors, but there is always a limit, and it will always be linked to the dynamic compression.

[andrew_mx83]

i would say that YES, compression does directly affect what octane fuel you can use, but as always there are other factors as well.

[oldcorollas]

ya know.......

since octane number is actually determined by using a variable compression motor, and they wind up the compression until it knocks.. and that determines the octane ratio....

you'd think that octane of fuel would have some bearing on what compression ratio you can use... wouldn't you?

(sorry for having to point out the obvious )

[trdee]

I think the problem is that people confuse static and dynamic compression ratios (i think that's the right terminology).

your dynamic comp ratio can be altered in a great many ways. The most obvious being using lower compression pistons or a thicker head gasket when adding boost. The turbo would increase the dynamic compression ratio, but this is counteracted by your now lowered static compression ratio... thus preventing cylinder pressures getting too high and causing detonation.

At least I think that's about right

[andrew_mx83]

from what i understand static compression is what is affected by piston dish, hg thickness, head cc's etc and dynamic is affected by cam profile.... not sure what if anything else. maybe turbo boost

someone please correct me if im wrong as i would like to know what its all about

[Norbie]

how can you say it does not have a direct effect norbie?

Hard to answer that, because I said no such thing.

I was answering a question which I took to mean, does compression ratio and fuel octane have a direct relationship? And the answer to that is no, because there are many other factors to consider before you can determine what octane number is required for any given engine. You can't just say compression ratio X requires octane number Y.

If this were not the case how would you explain my 1GZ with its 11.5:1 compression running quite nicely on 98 RON fuel, while old-school Yank iron can barely approach 10:1 on the same fuel?

[oldcorollas]

ahh semantics

on a given engine, with all else unchanged, compression ratio and fuel octane have a direct relationship.

of course a different combustion chamber/cam/whatever will ALSO have an effect, but for a given engine, and a given set of mechanical... settings.... fuel octane and compression have a direct, and probably linear, relationship, since "fuel octane" is defined by the compression at which detonation occurs, in a linear relationship between the properties of the two chemicals listed previously......

of course if you change engines from something shit, to something that was actually designed, then the allowable compression ratio for a given fuel will change, but on that particular engine, there is direct relationship...
but since OP was talking about a particular engine, and if it was ok to run on 98 instead of 100.. for the particular settings of that engine (JDM blah blah...)....

edit, and speaking of old school iron... i believe TK can run much higher compression.. much higher than 10:1... on those same engines after he has improved them...so.. if you make a combustion chamber "all it can be", and you have an fuel input system that is better than a non-directional garden hose... then you will approach the kind of theoretical allowable compression for that fuel...

[Norbie]

I'm not playing semantics. In the context of the original question it seemed clear (to me) it was about engines in general, not a specific engine with different compression ratios. Look at the discussion preceding it.