Compression Ratios
Thread Starter
Drifting
Joined: Dec 2002
Posts: 1,946
Likes: 28
From: Long Beach CA
Compression Ratios
I know there are a number of factors involved, but in general, what would you consider to be the maximum static compression ratio to run each of these -- 87 octane, 89 octane, and 91/93 octane?
Team Owner
Joined: Mar 2003
Posts: 26,118
Likes: 1,874
From: Greenville, Indiana
If you narrowed the "factors" down to OEM stock, aftermarket or whatever, then you could "generalize" a little better.
Burning Brakes
Joined: Oct 2004
Posts: 1,210
Likes: 1
From: fighter pilots make movies, bomber pilots make history
Mike is right as usual, many variables.
Just for reference, the DART 400 with 200 cc heads and ALL my 50 year old 270/283 2 x 4 WCFB set-up makes 500hp dyno. Mar and Apr Auto Enthusiast articles is great primer by Jim Moore on this engine.
It has 11 : 1 ratio @36 degree full mechanical advance at 2800 rpm. I run 93 with no problems. I run fairly lean at 14-14.2 AF ratio.
If I get lazy and do not down shift and lug it a little, I hear some slight detonation. So, I am fairly close to edge.
I want to do a little blend test of 91/93 and determine my tolerance (so if 93 is unavailable how much 91 can I blend).
Thats one engine, but fairly well know parameters.
Joe
In general, maybe 427Hotrod (Jim Moorehorsepower) will chime in but I think he would agree iron heads...10-10.5 OK, Aluminum 10.6-10.9 Ok on 93 oct.
Just for reference, the DART 400 with 200 cc heads and ALL my 50 year old 270/283 2 x 4 WCFB set-up makes 500hp dyno. Mar and Apr Auto Enthusiast articles is great primer by Jim Moore on this engine.
It has 11 : 1 ratio @36 degree full mechanical advance at 2800 rpm. I run 93 with no problems. I run fairly lean at 14-14.2 AF ratio.
If I get lazy and do not down shift and lug it a little, I hear some slight detonation. So, I am fairly close to edge.
I want to do a little blend test of 91/93 and determine my tolerance (so if 93 is unavailable how much 91 can I blend).
Thats one engine, but fairly well know parameters.
Joe
In general, maybe 427Hotrod (Jim Moorehorsepower) will chime in but I think he would agree iron heads...10-10.5 OK, Aluminum 10.6-10.9 Ok on 93 oct.
Le Mans Master
Joined: Apr 2005
Posts: 6,350
Likes: 423
From: Seattle Area WA
Static compression is only part of it, you also need to know what your dynamic compression as a result of your valve timing. You could be fine at 11:1 with one cam and ping like crazy with a shorter duration cam.
Melting Slicks
Joined: Jan 2010
Posts: 2,565
Likes: 1
From: Melbourne victoria
11.1 is about as far as you would want to go with the fuels on the public market.
My muzzy has 11.1 with a set of AFR alums, square roller 240 @ 50 and a stroker crank 302 - 347 and no pinging @ 36 degrees all out.
I guess there are lots of variables but you should get away with around 11.'s of comp with a higher oct fuel. Stewy
My muzzy has 11.1 with a set of AFR alums, square roller 240 @ 50 and a stroker crank 302 - 347 and no pinging @ 36 degrees all out.
I guess there are lots of variables but you should get away with around 11.'s of comp with a higher oct fuel. Stewy
Race Director
Joined: Jan 2000
Posts: 13,015
Likes: 2,260
From: Corsicana, Tx
2020 C2 of the Year - Modified Winner
2020 Corvette of the Year (performance mods)
C2 of Year Winner (performance mods) 2019
2017 C2 of Year Finalist
There are some head chamber designs that lend themselves better to cutting detonation down...and as said....cam plays a big part.
My 555" is at 11.56 to one and runs 9's@140+ in the 1/4 on pure 93octane. I have very tight quench, but the chambers are pretty *old school* open chamber big block stuff.
Cooling system is very important also.
JIM
My 555" is at 11.56 to one and runs 9's@140+ in the 1/4 on pure 93octane. I have very tight quench, but the chambers are pretty *old school* open chamber big block stuff.
Cooling system is very important also.
JIM
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
10 Reasons the C6 Z06 is Still A Performance Benchmark After 20 Years
Joe Kucinski
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Top 10 Corvette Engines RANKED by Peak Torque (70+ Years of Muscle!)
Joe Kucinski
Corvette ZR1X Will Be Pacing the Indy 500, And Could Probably Race, Too!
Verdad Gallardo
Top 10 Corvettes Coming to Mecum Indy 2026!
Brett Foote
Team Owner
Joined: Mar 2003
Posts: 26,118
Likes: 1,874
From: Greenville, Indiana
Thats just too hilarious Bill!
Safety Car
Joined: Mar 2009
Posts: 4,191
Likes: 951
From: Corpus Christi, TX
It took three or four posts to get the concept down completely, but the factors in getting the maximum static CR right for any selected octane are ignition timing curve, quench volume, cylinder head material, combustion chamber configuration, operating water temperature, intake air temperature and cam overlap (duration of intake and exhaust valves being open at the same time).
Safety Car
Joined: May 2003
Posts: 4,797
Likes: 239
From: Ocean Township NJ
It took three or four posts to get the concept down completely, but the factors in getting the maximum static CR right for any selected octane are ignition timing curve, quench volume, cylinder head material, combustion chamber configuration, operating water temperature, intake air temperature and cam overlap (duration of intake and exhaust valves being open at the same time).
Arrrrrroooooooooooooga! You're fired! Get the hook! Not! Wrongo! Incorrectamundo! Scott had the kernel of the main determining factor when he mentioned "valve timing" above. That is only partly correct. What, specifically about valve timing is the single most critical factor relating to detonation potential? There are other valve timing issues that are important, but overlap, in and of itself, has absolutely no bearing. Who knows what engine characteristic is directly affected by valve overlap?
Apart from "valve timing", there are at least 7 other design parameters that I can think of, which have not already been mentioned, as affecting detonation potential in an engine. Can you name some of these?
Le Mans Master
Joined: Apr 2005
Posts: 6,350
Likes: 423
From: Seattle Area WA
Detonation parameters: chamber design-plug location-shape( 60's slow burn design vs fast burn and how the chamber shape relates to Location of Peak Pressure 'LPP'), compression ratio, fuel temp, fuel octane, engine timing, cylinder pressure, what did I miss?
Team Owner
Joined: Mar 2003
Posts: 26,118
Likes: 1,874
From: Greenville, Indiana
Safety Car
Joined: May 2003
Posts: 4,797
Likes: 239
From: Ocean Township NJ
It's better if you do some homework, reading, and practical application. You don't learn as well if somebody tells you the answer (unless you question/debate it).
I'll give you an easy one, since you probably already knew it, but may not recall just now:
Spark plug heat range.
I'll give you an easy one, since you probably already knew it, but may not recall just now:
Spark plug heat range.
Pro
Joined: May 2010
Posts: 560
Likes: 2
From: North Las Vegas Nevada
In addition to what's been stated...
Intake Valve Closing(IVC) position(primarily at low rpms)
Shape of piston surface. i.e. dome, flat, dish, (speed/efficiency of flame travel)
Plug gap
Strength, position, timing and frequency of ignition
Rod length
Piston/head metallurgy
combustion chamber/piston surface. i.e. edges that form hot spots or lack thereof
air charge temp as affected by cooling efficiency,coatings, head design, exhaust efficiency, velocity
air charge pressure
engine load
I believe a "rule of thumb" for aluminum heads is that they are generally good for an extra half point in C/R over identical iron heads.
Thats all ll I can think of off hand.
DT
Intake Valve Closing(IVC) position(primarily at low rpms)
Shape of piston surface. i.e. dome, flat, dish, (speed/efficiency of flame travel)
Plug gap
Strength, position, timing and frequency of ignition
Rod length
Piston/head metallurgy
combustion chamber/piston surface. i.e. edges that form hot spots or lack thereof
air charge temp as affected by cooling efficiency,coatings, head design, exhaust efficiency, velocity
air charge pressure
engine load
I believe a "rule of thumb" for aluminum heads is that they are generally good for an extra half point in C/R over identical iron heads.
Thats all ll I can think of off hand.
DT
Safety Car
Joined: May 2003
Posts: 4,797
Likes: 239
From: Ocean Township NJ
In addition to what's been stated...
Intake Valve Closing(IVC) position(primarily at low rpms)
Shape of piston surface. i.e. dome, flat, dish, (speed/efficiency of flame travel)
Plug gap
Strength, position, timing and frequency of ignition
Rod length
Piston/head metallurgy
combustion chamber/piston surface. i.e. edges that form hot spots or lack thereof
air charge temp as affected by cooling efficiency,coatings, head design, exhaust efficiency, velocity
air charge pressure
engine load
I believe a "rule of thumb" for aluminum heads is that they are generally good for an extra half point in C/R over identical iron heads.
Thats all ll I can think of off hand.
DT
Intake Valve Closing(IVC) position(primarily at low rpms)
Shape of piston surface. i.e. dome, flat, dish, (speed/efficiency of flame travel)
Plug gap
Strength, position, timing and frequency of ignition
Rod length
Piston/head metallurgy
combustion chamber/piston surface. i.e. edges that form hot spots or lack thereof
air charge temp as affected by cooling efficiency,coatings, head design, exhaust efficiency, velocity
air charge pressure
engine load
I believe a "rule of thumb" for aluminum heads is that they are generally good for an extra half point in C/R over identical iron heads.
Thats all ll I can think of off hand.
DT
I'm not sure about rod length having an effect. Explain why this should be?
Mixture was kinda mentioned in post #4, so I'll officially add it here as another very important factor. Richer mixtures cool the chambers and lessen detonation.
I can still think of at least three more factors.
Last edited by 65tripleblack; Mar 26, 2011 at 07:18 AM.