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Author Topic:   Compression Ratio vs. Octane?
GTRocks
Gearhead

Posts: 3631
From: Lusby, MD
Registered: Jun 99

posted 02-08-2003 12:36 PM     Click Here to See the Profile for GTRocks   Click Here to Email GTRocks     Edit/Delete Message   Reply w/Quote
As I understand it, as you raise the compression ratio, you should also raise the octane of the gas you use. Since my application is strictly daily driver, and I can only get 93 octane from the pump, what's the maximum compression ratio, I'd want to go to? Also, what would that be translated to in cc's in the head?

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Moneymaker
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From: Lyons, IL, USA
Registered: May 99

posted 02-08-2003 01:09 PM     Click Here to See the Profile for Moneymaker   Click Here to Email Moneymaker     Edit/Delete Message   Reply w/Quote
The cam dictates the octane, not the mechanical compression. You can have 13 to 1 on the street with a wide profile cam and still run 93 octane safely.
GM LT-1's have 12.5 to 1 compression.

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Alex Denysenko
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D&S Induction Systems
Gearhead

Posts: 118
From: Columbia Heights Mn U.S.A.
Registered: Feb 2003

posted 02-08-2003 01:14 PM     Click Here to See the Profile for D&S Induction Systems   Click Here to Email D&S Induction Systems     Edit/Delete Message   Reply w/Quote
There is alot of grey area when it comes to compression ratio and octane but...10.5:1 is the highest you should go in most engines. I wont get into details, but some chamber designd allow you to go higher than "normal". These chamber designs swirl the air fuel mixture as the piston comes to TDC compression stroke. This swirling cools the charge helping prevent pre-ignition wich leads to detonation. Most heads with a quench area do this any way but some heads do it better. This swirling also helps mix the air fuel mixture giving a better buirn time, better buirn characteristics and more power, but thats another thread.

As far as "translated to in cc's in the head". What kind of heads are thay? And what kind of pistons you you have. Need more info...

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D&S Induction Systems
Gearhead

Posts: 118
From: Columbia Heights Mn U.S.A.
Registered: Feb 2003

posted 02-08-2003 01:18 PM     Click Here to See the Profile for D&S Induction Systems   Click Here to Email D&S Induction Systems     Edit/Delete Message   Reply w/Quote
quote:
Originally posted by Moneymaker:
The cam dictates the octane, not the mechanical compression. You can have 13 to 1 on the street with a wide profile cam and still run 93 octane safely.
GM LT-1's have 12.5 to 1 compression.


Hmmm... Interesting. Tell me you're reasoning. Are you getting at the valves being open longer wich will drop the PSI pressure? Or somthing on that order?

[This message has been edited by D&S Induction Systems (edited 02-08-2003).]

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Moneymaker
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From: Lyons, IL, USA
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posted 02-08-2003 01:30 PM     Click Here to See the Profile for Moneymaker   Click Here to Email Moneymaker     Edit/Delete Message   Reply w/Quote
Wide C/L cams will always produce less cranking compression IE; less cylinder pressure.
Less cylinder pressure = less heat.
Less heat requires less octane.
I agree that the combustion chamber has a lot to do with it, but we normally deal with quench wedge or canted valve stuff here.
On the dyno we can lose as much as 10 HP with 114 octane fuel vs 110 due to burn characteristics on 10 to 1 motors.
These engines typically have a cranking compression of 150-170 with the cams we use.

------------------
Alex Denysenko
Co-Administrator and Moderator

NHRA/IHRA/SRA member and licensed Superstock driver
MCA member# 53321
NHRA and IHRA SS/LA National Record Holder '00,'01,'02,&'03
Fleet of FoMoCo products including 88 ASC McLaren Mustang #28
Professional Manwhore
The Barry of BarrysGrrl

Quote #1: "I never met a magazine mechanic I liked."
Quote #2: "Make sure brain is in gear before engaging mouth!"
Quote #3: "If you can't run with the big dogs, stay on the porch!"
www.moneymakerracing.com

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GTRocks
Gearhead

Posts: 3631
From: Lusby, MD
Registered: Jun 99

posted 02-08-2003 05:43 PM     Click Here to See the Profile for GTRocks   Click Here to Email GTRocks     Edit/Delete Message   Reply w/Quote
I have a predominantly stock '93. It's got the hyper pistons. I ran across a GT40 head that had been milled to a 54cc chamber. Guy said the milling raised the compression ratio from 9.0 to 10.5. I was wondering whether that would be too much for me. Mine's a daily driver, so driveability is an issue for me.

I have soooo much to learn....

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steve'66
Gearhead

Posts: 8826
From: Sonoma,CA,USA
Registered: Mar 2000

posted 02-08-2003 08:14 PM     Click Here to See the Profile for steve'66   Click Here to Email steve'66     Edit/Delete Message   Reply w/Quote
You could run 10.5-1 with less total timing and/or a little more camshaft, or a custom chip, on pump gas.

SteveW

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