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Old 11-30-2003, 11:48 AM
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octane and compression

Hey folks, I was hoping to make this thread so that people could find the real deal about octane and compression in one thread. I found some bits and pieces along the search route, and a lot of the info is contradictory. For example, 1BAD posted a really in-depth work about gasoline and octane, however, this article stated that you need something like 96 octane for 9:1 100 for 10:1 and all the way like 108 for 12:1. Most of what the guys on this site say contradicts that. lluc77 talked about running btw 10.5 and 11:1 on only 91 or 92 octane gas and never having a problem.

So my question goes out to those with experience in actually running high comp motors: How do you determine what octane you need for a particular given compression ratio? I've also heard that the whole "bleed some pressure off at idle with a big cam" is a myth because at higher RPM that doesn't help.

Any ideas?

K

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Old 11-30-2003, 12:28 PM
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Re: octane and compression

Quote:
Originally posted by killerformula
Hey folks, I was hoping to make this thread so that people could find the real deal about octane and compression in one thread. I found some bits and pieces along the search route, and a lot of the info is contradictory. For example, 1BAD posted a really in-depth work about gasoline and octane, however, this article stated that you need something like 96 octane for 9:1 100 for 10:1 and all the way like 108 for 12:1. Most of what the guys on this site say contradicts that. lluc77 talked about running btw 10.5 and 11:1 on only 91 or 92 octane gas and never having a problem.

So my question goes out to those with experience in actually running high comp motors: How do you determine what octane you need for a particular given compression ratio? I've also heard that the whole "bleed some pressure off at idle with a big cam" is a myth because at higher RPM that doesn't help.

Any ideas?

K
i think those ratings would be dynamic instead of static, since given a static ratio you actually have no clue as to how much the actual mixture is being compressed.
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Old 11-30-2003, 12:35 PM
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There is no exact answer for how much compression you can run on a given octane as there are many many factors that influence it. As you mentioned cams with alot of duration tend to bleed off cylinder pressure reducing to chance for detonation but only at low rpm. Other factors to influence it such as altitude,timing, carbon and rough edges and over all design of the combustion chamber. A rough rule of thumb is that 87 octane is approximately good till 9:1 and 92 is good till 9.5 but this is by no means cast in stone and probably everyone has very different experiences with this.
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Old 11-30-2003, 12:54 PM
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I have 11:1 compression and my engine is okay with 91 octane. I had to do a lot to help it along though. My dynamic compression is 8.25:1.
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Old 11-30-2003, 01:20 PM
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Re: octane and compression

Quote:
Originally posted by killerformula
I've also heard that the whole "bleed some pressure off at idle with a big cam" is a myth because at higher RPM that doesn't help.

Any ideas?

K
Not a myth from what I have read. The reason it works is because at high RPM, things happen fast enough that detonation does not have as much time to occur do to increased piston speed. But this actually would make more sense to help stop preignition which is often confused with detonation. Preignition is premature fuel igniton usually due to hot spots and/or carbon build up. Detonation is violent uncontrolled explosions instead of a controlled fuel burn.

Maybe someone can give us some real backing information on whether or not it really works. I can tell you, it is common practice with alot of engine builders and do it yourselfers. That is what L'77 is talking about with static versus dynamic CR.

Chris
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Old 11-30-2003, 01:25 PM
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It all depends on how the cam actually builds pressure throughout the powerband. Don't bother asking the cam companies this kind of info for a particular cam. They don't train their tech with this kind of info.
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Old 11-30-2003, 01:35 PM
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Quote:
Originally posted by lluciano77
I have 11:1 compression and my engine is okay with 91 octane. I had to do a lot to help it along though. My dynamic compression is 8.25:1.
fits into the "96 octane for 9:1 100 for 10:1 and all the way like 108 for 12:1." if we use dynamic compression ratios.

Using dynamic compression ratios is the best way to "guess" at octane needed. Gasoline will only compress so much before it blows up like desiel fuel, octane is this resistance to doing so. Dynamic compression is how much the actual air fuel mixture is getting compressed.
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Old 11-30-2003, 01:55 PM
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A good measurement is cranking compression. You measure this with a compression tester for anyone who is unfamiliar. It obviously takes low RPM bleed down into account. Mine is around 215 psi., again on 91 octane.
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Old 11-30-2003, 03:33 PM
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Quote:
Originally posted by lluciano77
A good measurement is cranking compression. You measure this with a compression tester for anyone who is unfamiliar. It obviously takes low RPM bleed down into account. Mine is around 215 psi., again on 91 octane.
mine is around 120
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Old 12-01-2003, 04:54 PM
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i run 11.5 to 1 i run on 93 but i have no problems so far
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Old 12-21-2003, 12:24 PM
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Honda S2000's have 11:1 from the factory. I have 10:1 in my KLZE and I have seen both engines run 5psi of boost on 93 Octane without detonation. How do we account for that? I get 210psi in a cranking compression test. I am building a 383 stroker for my S-10 and I was wondering if I would be able to run 10.5:1 on the street with pump gas. I would assume I could if I kept the ignition timing modest. Any input?
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Old 12-21-2003, 01:06 PM
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Hondas etc. have modern computerized fuel injection and timing control to keep detonation at bay. A friend of mine has an Eclipse that has a baseline of 11:1, and is turborcharged on top of that, and it still runs on pump gas. I am a fan of Holley carbs, but you won't see a Holley able to do that on the cheap stuff.

You could probanly run 34o with that 383. Just smooth out the chambers, and the sharp edges along the valve reliefs. Get rid of the potential hot spots. Run a cam with at least 234o @ .050". Also, use a 180o thermostat. Increased stall helps too. It keeps the engine from bogging in the danger zone RPMs. So does gearing or lightening the vehicle.
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Old 12-21-2003, 05:36 PM
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I have tried premium from the pump and within a few miles the motor starts loading up, and seems to lose some of its low end power.

I usually run leaded 112 octane and within a mile of filling up after running the 94 octane pump gas it smooths right out and has that get up and go I am used to.

I am running 10.8:1 speed pros with a healthy cam (.248 duration/.525 lift @ .050), solids and roller rockers. It has a 650 dbl pump holley. Now I have run 110 octane and I am sure that is enough as it runs good with that too, its just that the only thing available locally is 112, and I get it for $2.99/gal, so what the hey
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Old 12-21-2003, 05:37 PM
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Most newer cars also have aluminum heads which have an effect on how much compression you can run on a given octane.

Cams with more overlap do tend to bleed off cylinder pressure at lower RPMs. This reduces the chance of detonation. Once you reach the upper RPM's there is typically less load on the engine (since the vehicle is already in motion), less load means less chance for detonation as well.

As you can see there are far too many variables to give an etched in stone answer to the question.

I also agree dynamic compression is the best way to determine what a given engine will need.
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Old 12-21-2003, 05:52 PM
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I think this is a very interesting subject. I'm by no means a pro engine builder or pretend to be. But I do follow things and like to be non conventionable.

I believe with proper design, you can run high mechanical rations with low octane. I believe there are too many variables to give anyone answer. Here's a guy that intriques me. I've know about him for some time. He's very unconventional. Here's the link www.theoldone.com. Due your self a favor. Read the archieves on old Hot Rod mag issues on the "Soft Head" and also some very good reading on well known racers.
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