Octane vs Compression Ratio...

What octane your motor will need depends on the compression ratio, which heads you use (closed chamber are more efficient), the cam you select and the carb you run as well as the timing and a host of other minor things. I wouldn't begin to guess from a list of parts what octane the thing would like.

Thanks guys,

If you run 89 octane do you need a higher compression ratio? I understand that the octane rating is the ability of the fuel to withstand predetonation, but it seem that the more you compress the gas mix, the greater the heat generated and therefore the more chance of predetonation.

I'm just trying to find out what the optimum C/R would be when running mid-grade fuel in your basic stock engine.

SteveFlier

[This message has been edited by SteveFlier (edited 05-14-2001).]

I dont believe the question is "what's the optimal CR for 89 octane". It's more like how high of a CR can you run if you want to burn 89 octane. The performance potential of your engine will be somewhat limited by your compressio ratio. Higher CRs generally mean higher volumetric efficiency, which in turn offers the potential for a little more fuel/air per intake stroke, thus more power.

Higher CR will allow you to run a more aggressive cam. The detonation issue is one to pay attention to. Detonation is when your fuel/air mixture auto-ignites before your ignition fires-Like on a diesel engine. It's not just annoying pinging, but the potential to do severe damage to your engine. Many things affect detonation, piston/head materials, CR, Octane, combustion chamber shape, temperature, etc. With 89 octane and iron heads the concensus seems to be a max CR of 9:1. This is about what you get with closed chamber 4V Cleveland heads and the dished pistons from a smog-Cleveland. With aluminum heads, you may consider up to 10:1 CR. With exotic ceramic coatings, perhaps more. I've had people tell me they run 10:1 with iron heads with no detonation problems on 93 octane. Higher octane fuels burn at different rates. If you're above the detonation limit, running higher octane shouldn't hurt, but probably won't gain you a whole lot without some timing adjustments to compensate for the burn rate.

-My 2 cents

my stock 8.6:1 cj engine runs yust fine w/87 octane. as i understand it, running higher-octane gas will actually cause the car to run *less* well, as the slower burn characteristics of the higher-octane gas may cause incomplete combustion.

i'm getting parts together for a closed-chamber 4v head motor, w/a mildly hotted-up cam, flat pistons, & ~10.5:1 compression. again, i understand that i will be ok w/93 octane pump gas, because the "squish" area caused by the quench heads aids in more complete combustion, & the hotter cam will actually reduce the compression at the beginning & end of the firing cycle, due to the greater walve-opening overlap...

This discussion brings up another question. Does anybody have any independent test data on fuel additives that boost the octane rating of the gasoline?

Doug,

Can you give me a run down of the specs you'll be going to on the cam and valve combo? What pistons are you going with? Forged? Other?

Who are are having do the cam or is it off the shelf?

S

sorry if this comes thru twice - i tried to answer, but i don't see it so i'm trying again...

i used mpg heads (cam research) in colorado, for my cams. 303.762.8196

i talked w/scott; i told him i wanted a strong street-motor, this was what i was planning, please adwize for a cam. (i also gave the same info to several other cam shops & got *their* recommendations & reasoning. i liked scott's the best). ok, here's what i planned:
-64cc 4v quench heads w/keith black cast hypereutectic flat-top pistons for compression ratio of ~10.5:1
-one-piece single-groove stainless walves.
-roller rockers
-blue thunder 4v intake w/*stinger* intake & exhaust plates
-750cfm carb
-1.75" diameter-tube headers.
-hydraulic cam, not a roller-cam

scott was the only one who said not to bother w/the staggered-duration cam most folks use w/the 4v clevelands. he said the use of the *stinger* port plates will increase the flow, especially the exhaust flow, so much that a staggered-duration cam is not necessary.

here's the specs:
cam#112874:
.530 lift i & e
288 duration i & e
@ .050 - 220 i & e
110 lobe separation

double-springs - #100 lbs on seat, #300 open

scott said his desktop dyno (accurate to +/-5%) showed 460-475 hp if i used a crank windage tray. so, a minimum of 437hp... :>) i'll believe it when i see it! but, at least i know i should have a motor close to 400hp... ;~)

hope this helps,

doug s.