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Old 06-23-2010, 06:23 PM   #1
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Compression Ratio vs Octane?

Hey guys, building a 383 stroker with 18cc dished pistons. I'm looking to buy a set of Edelbrock's 60cc heads. I've been playing around with the compression ratio calculators which is really helpful. However, I'm trying to get as much compression as I can with 92-93 octane. This is a 89 block with a stock TPI on it so it has a knock sensor on it to pull the timing. Some people have told me 9:1, some say 11:1. I was thinking 10 or 10.25:1... Altitude is low, around 300 ft.

If you guys have any insight before I buy the head gaskets that would be great. Thanks
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Old 06-23-2010, 07:43 PM   #2
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Re: Compression Ratio vs Octane?

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Originally Posted by Mr. Notorious View Post
Some people have told me 9:1, some say 11:1. I was thinking 10 or 10.25:1... Altitude is low, around 300 ft.
Cam has more bearing on what static compression you can run.
Dynamic Compression ratio ( DCR ) not static CR is what determines if it will knock or not on what octane
From this link
http://members.uia.net/pkelley2/DynamicCR.html
It appears that most gas engines make the best power with a DCR between 7.5 and 8.5 on 91 or better octane.

Download the compression calculator at bottom of the linked page. It will calc the DCR if you know the cam spec as well as static CR

Also read
http://www.kennedysdynotune.com/Dyna...ion%20Tech.htm

Last edited by vetteoz; 06-23-2010 at 07:47 PM.
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Old 06-23-2010, 08:43 PM   #3
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Re: Compression Ratio vs Octane?

Need to know what cam is in it, that is part of the puzzle. Aluminum heads? Should be ok 10:1, much more than 10.5:1 is usually the borderline if you're not paying attention to details. There are other factors such as the design of the head including the intake port and chamber, and the shape of the piston as well. Generally speaking the factory was running 9-9.3:1 with iron heads so 10-10.3 with aluminum heads puts you about in the same area. A larger cam will bleed off more cylinder pressure, usually, so that can allow you to run more static compression.

The DCR calculators are very handy for this subject.
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Old 06-24-2010, 07:55 AM   #4
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Re: Compression Ratio vs Octane?

Thanks, I haven't read into the dynamic CR very much. I just know why the cam the effects it. I haven't selected my cam just yet but I'll check out the links and see what I come up with a couple cams I have in mind. Yes they're aluminum 60cc eldelbrock performer heads with dished pistons.

Do people go through starters much more with a higher compression ratio or does no one really notice?
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Old 06-24-2010, 10:09 AM   #5
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Re: Compression Ratio vs Octane?

My advise is aim for the lower side of acceptable. You can push the limit a bit more with knock controlled EFI ignition, but chasing the maximum compression possible doesn't net you as much extra torque as you might think. In a road car it's better to play it safe and plan for less than ideal weather conditions and the possibility of bad gas.
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Old 06-28-2010, 06:54 PM   #6
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Re: Compression Ratio vs Octane?

So playing around with the dynamic compression ratio calculators, I should be looking for around 8.5:1 for 93 octane?
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Old 06-28-2010, 07:22 PM   #7
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Re: Compression Ratio vs Octane?

Quote:
Originally Posted by Mr. Notorious View Post
Thanks, I haven't read into the dynamic CR very much. I just know why the cam the effects it. I haven't selected my cam just yet but I'll check out the links and see what I come up with a couple cams I have in mind. Yes they're aluminum 60cc eldelbrock performer heads with dished pistons.

Do people go through starters much more with a higher compression ratio or does no one really notice?
If you have good cranking compression, say over 180 psi, then there's more strain on the starter than if your cranking is doggy, like a weak 150 psi. Also, the larger the bore, the more strain. If you have an automatic, you can go to the 168-tooth flexplate and matching starter, plus don't forget the starter brace for the other end of the starter. If you're running a stock T5-style bellhousing, you have to stay 153 tooth, but there are aftermarket gear reduction starters. These are light, but cost more. And if you kill one while on vacation, the nearest auto parts store isn't going to be able to help you.
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Old 06-29-2010, 06:49 AM   #8
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Re: Compression Ratio vs Octane?

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These are light, but cost more. And if you kill one while on vacation, the nearest auto parts store isn't going to be able to help you.
But since you have a manual you can get a push and get going.
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Old 06-29-2010, 07:27 AM   #9
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Re: Compression Ratio vs Octane?

I must have been much more tired than I thought.
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Old 06-29-2010, 08:47 AM   #10
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Re: Compression Ratio vs Octane?

If you get that 383 zero decked and run a typical .040-.041" gasket and those heads have a decent combustion chamber design, I think you will be fine with 92-93 octane, as your compression will only be 10 to 1.

I've run 11 to 1 with a dynamic compression calculated to be 8.3 to 1 and it ran great on 93 and probably would have run on 91. But I had a head with a decent chamber design which helped alot i'm sure.
Keep the motor cool and be careful on the timing you give it and you can get away with alot more than you think.
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Old 06-29-2010, 08:50 AM   #11
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Re: Compression Ratio vs Octane?

Quote:
Originally Posted by Mr. Notorious View Post
Do people go through starters much more with a higher compression ratio or does no one really notice?
I am running my original '90 Vette factory Denso starter ( mini reduction type ) on my 12:1 383 and it is spinning it fine 153 tooth flexplate
When the tune is right it just fires straight up , hardly cranks at all
This type
http://cgi.ebay.com/ebaymotors/NEW-C...Q5fAccessories
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Old 07-12-2010, 08:23 PM   #12
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Re: Compression Ratio vs Octane?

This is great thread now. I've been playing with the comp cams downloadable program, but it doesn't come with a DCR calculator. I'll check out some of the other programs. Thanks a ton. Some guys sent me onto some timing retards for when the rpm is under 500rpm so it's easier on the starter. I'll be running the standard T5 bellhousing. I'm not sure if this is necessary or even usable in conjunction with the electronic distributer. Maybe I'll try that starter you pointed me to. I'll look into those factory vette starters and see how available they are at local parts stores too.

Never heard of the starter brace before?

Thanks!!
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Old 07-12-2010, 10:45 PM   #13
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Re: Compression Ratio vs Octane?

Quote:
Originally Posted by Mr. Notorious View Post
. I've been playing with the comp cams downloadable program, but it doesn't come with a DCR calculator.
Put the cam specs from Comp into the DCR calc I gave you in post #2

Quote:
Originally Posted by Mr. Notorious View Post
Never heard of the starter brace before?
The old style heavy Delco starters had a brace ( looks like a piece of bent angle iron ) that goes from front of starter up to a bolt on the block above pan rail
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Old 07-12-2010, 11:45 PM   #14
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Re: Compression Ratio vs Octane?

Compression makes torque. Octane is the fuels resistance to self detonation. When the air fuel mixture is compressed, if the octane is too low, the fuel will self ignite before the plug fires. You only need enough octane to prevent this detonation. Any more octane and the fuel burns too slowly to make proper power.

For your low altitude and 92-93 octane limit, I'd try to keep the compression ratio below 9.5. Higher altitudes can get away with higher compression ratios or lower octane fuel.
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Old 07-12-2010, 11:47 PM   #15
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Re: Compression Ratio vs Octane?

Those are Edelbrock aluminum heads. 10:1 will be fine. Just choose a cam with an intake closing, at 50, of 33 or more degrees after bottom center.
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