11 to 1 compression
Racer
Joined: Dec 2001
Posts: 328
Likes: 0
From: Dallas County Texas
Thank you DJ & 1970!!! I like to think so!
you must be a VERY young guy Wayne
But you didn't dispute what I said...."contained some alcohol for octane reasons at lower cost" (not just contains TEL) for what was often marketted as "ethyl".
I wasn't aware it was hi-test only - I sure seem to remember seeing it as a big sign "ethyl's BACK!" in the Conoco in Great Falls the great, great, great grand parents used to fill up at back after we crossed the prairie in our Chevy covered wagons...1997 or so?...
(Wow...they had CARS in the FIFTIES?!??!!? NO $%&^!!! Did they have, like, CRANKS to start? Did you have to have, like, a guy with a flag 60' in front to not scare the horses?
)
Racer
Joined: Dec 2001
Posts: 328
Likes: 0
From: Dallas County Texas
That's the standard def'n of "points" - but it's not
universal. Generally it is tenths. I also doubt you get that much increase for 10:1 to 11:1. I also would not be surprised if it was not all that linear - 7:1 to 8:1 might get you that 20%, but 11.5:1 to 12.5:1 might only be 11%.
...and that would be TWO points, if that's their convention here. (I hate how ancient engineering and industrial conventions are so variable - terms like "points", "mils", "quality"....whywhywhy...it's enough to make me wish for METRICIZATION!!!)
Compression by itself is important, but I think the real key thing with compression anyhow is the greater compression allows more extreme valve timing without bad effects - the radical valve lift and duration act in concert with the higher compression.
Originally Posted by Techno
So that would mean 20% going from 10:1 to 11:1 right? 80hp?
A point would be 10.9:1 to 11:1
A point would be 10.9:1 to 11:1
Compression by itself is important, but I think the real key thing with compression anyhow is the greater compression allows more extreme valve timing without bad effects - the radical valve lift and duration act in concert with the higher compression.
Melting Slicks
Joined: Jan 2004
Posts: 2,672
Likes: 1
From: New Cumberland PA
Just joking. There was an article by Vizard dealing with compression. Going up a full point such as 10:1 to 11:1 was worth about 2%. There was a chart showing other changes. So going up 1 full point on a 300hp engine would be roughly a 6hp increase. He also wrote about to keep the engine from pinging when increasing compression.
Le Mans Master
Joined: Apr 2000
Posts: 7,562
Likes: 9
From: B'Ville NY
Cruise-In III Veteran
St. Jude Donor '05
All of this is exactly my point, it may not be worth pushing the limits of pump gas for the little (I know I was a bit generous in my 1st post 
) hp you gain with the high compression. If your wallet has enough bragging bread in it to run race fuel only, then don't stop at 11:1, you might as well go right to the race only 14:1.
) hp you gain with the high compression. If your wallet has enough bragging bread in it to run race fuel only, then don't stop at 11:1, you might as well go right to the race only 14:1.
Team Owner
Joined: Apr 1999
Posts: 21,953
Likes: 1,445
From: Reno Nevada
2024 C3 of the Year Finalist- Modified
I used thermal coated, pistons, chambers and exhaust valves to run my 383 at 11.2 That and big enough solid roller cam.
IMO - with the California 91 super unleaded your smarter to keep the static compression in the 10.7 -10.8 range with aluminum heads and then not have to worry about having to use octane boosters or retard the timing.
IMO - with the California 91 super unleaded your smarter to keep the static compression in the 10.7 -10.8 range with aluminum heads and then not have to worry about having to use octane boosters or retard the timing.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Top 10 Most Expensive Corvettes Ever Sold on Bring A Trailer
Brett Foote
10 Things Every Corvette Owner Needs (2026 Edition)
Michael S. Palmer
8 Most "Only Corvette Owners Understand" Quirks and Problems
Pouria Savadkouei
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
Le Mans Master
Joined: Oct 2002
Posts: 5,714
Likes: 6
From: Silver City NM
St. Jude Donor '05-'07
Originally Posted by mandm1200
What point was omitted? I thought you were planning on building a 302 street engine, the same old school engine that GM built. KISS. Just because you will run it at a strip doesn't change the fact that GM used it in some of their street cars.
The hp gain by increasing/decreasing compression is not something I am capable of determining. For rough estimates, there are charts that will give an approximation.
The hp gain by increasing/decreasing compression is not something I am capable of determining. For rough estimates, there are charts that will give an approximation.
#2 This isn't a thread about my car
. The original poster said "I would like to have the higher compression for the power, it is not a daily driver." Reading that I assume he isn't really worried about street use either.Compression = horsepower. No way of getting around it. Take two identical engines. One with 8.5-1 compression, one with 12-1, the higher compression engine is gonna make more horsepowerpower. Just look at the reduction in compression ratios in 1971-72 if you don't believe it. And yes, I know they were rated differently, but they STILL dropped in horsepower as the compression dropped.
Dep
Last edited by DJ Dep; Jul 19, 2005 at 05:11 PM.
Race Director
Joined: Apr 2000
Posts: 17,676
Likes: 201
From: Who says "Nothing is impossible" ? I've been doing nothing for years.
The compression ratio should be dictated by cam, heads and the gas (octane rating ) you want to run. This is as simplified as it gets, for a high performance street motor on pump gas you want to be in the 190-210 cranking psi range. Anything below you are leaving HP on the table and anything above you run the risk of pinging on pump gas.
The cranking psi is a direct result of the cam and static compression ratio, (please remember this is simplified, for my sake) the biggier the cam, the more overlap and therefore as we like to say this will "bleed off some of the compression " so we need a higher compression ratio to start with in order to keep the cranking psi within the desired range.
To make it real simple and off the top of my head cams under say 270 deg advertised would be good with 9:1 to 10:1 CR cams in the 270 to 280 woule need 10:1 t0 11:1 CR and cams over 280 would need at least 11:1 CR. These are just ballpark figures to try and illlustrate what the heil I am talking about
The cranking psi is a direct result of the cam and static compression ratio, (please remember this is simplified, for my sake) the biggier the cam, the more overlap and therefore as we like to say this will "bleed off some of the compression " so we need a higher compression ratio to start with in order to keep the cranking psi within the desired range.
To make it real simple and off the top of my head cams under say 270 deg advertised would be good with 9:1 to 10:1 CR cams in the 270 to 280 woule need 10:1 t0 11:1 CR and cams over 280 would need at least 11:1 CR. These are just ballpark figures to try and illlustrate what the heil I am talking about
Melting Slicks
Joined: Jan 2004
Posts: 2,672
Likes: 1
From: New Cumberland PA
Originally Posted by DJ Dep
#1 My car was NEVER intended for street use.
#2 This isn't a thread about my car. The original poster said "I would like to have the higher compression for the power, it is not a daily driver." Reading that I assume he isn't really worried about street use either.
Compression = horsepower. No way of getting around it. Take two identical engines. One with 8.5-1 compression, one with 12-1, the higher compression engine is gonna make more horsepowerpower. Just look at the reduction in compression ratios in 1971-72 if you don't believe it. And yes, I know they were rated differently, but they STILL dropped in horsepower as the compression dropped.
Dep
#2 This isn't a thread about my car
. The original poster said "I would like to have the higher compression for the power, it is not a daily driver." Reading that I assume he isn't really worried about street use either.Compression = horsepower. No way of getting around it. Take two identical engines. One with 8.5-1 compression, one with 12-1, the higher compression engine is gonna make more horsepowerpower. Just look at the reduction in compression ratios in 1971-72 if you don't believe it. And yes, I know they were rated differently, but they STILL dropped in horsepower as the compression dropped.
Dep
How much horsepower is gained is something I can't calculate. Going from 8.5 to 9.5 is not that big of a deal. The problem arises when the envelope is pushed to far. Running 12:1 on high octane gas is not an issue for the original poster since he is not planning on running pump gas. If I were looking for the last couple of horsepower and had a choice of roller rockers or increasing the compression (where it would not run on 92/93 octane), I would choose the rockers.
Whether you intend to drive on the street or not does matter to me. If you put a caprice engine in a vette and its still considered a caprice engine. Then you put a street engine in the car that is not driven on the street, its still a street engine.
Pro
Joined: Sep 2002
Posts: 690
Likes: 1
From: near Memphis TN
Cast my vote for pushing compression as high as you think you can get away with, or tolerate. Push it to the edge! As people point out, the law of diminishing returns applies for horsepower as you go higher in compression, but it allows you to run a bigger cam and what people don't point out is higher compression helps you much more in the low RPM throttle response and torque. I wouldn't do above 10.5 or so without a well planned out engine assembly - aluminum heads, forged pistons, etc.
Melting Slicks
Joined: Apr 1999
Posts: 2,010
Likes: 3
From: Moon Twp. PA USA
Originally Posted by DJ Dep
If you're NOT building for the street, then the price of race fuel hardly becomes an issue. 5-7 horsepower for $300 is about the WORST money-per-horsepower investment you can make. And I ain't talking imported cheap-crap budget rockers. They are even LESS cost effective because they will be breaking regularly. Some things are better NOT to skimp on.
mandm1200: Compare the 350/300 horse 1969 engine with the 350/350 horse engine. Not a lot of difference but more cam/compression and FIFTY more horsepower. Stop reading those stoopid advertiser-sponsored articles!!!
Dep
mandm1200: Compare the 350/300 horse 1969 engine with the 350/350 horse engine. Not a lot of difference but more cam/compression and FIFTY more horsepower. Stop reading those stoopid advertiser-sponsored articles!!!
Dep
Team Owner
Joined: Apr 2002
Posts: 62,198
Likes: 1
From: All humans are vermin in the eyes of Guru VA
Cruise-In IV Veteran
Cruise-In V Veteran
With the advances in metalurgy and thermal coatings 11:1 is possible.
There are places that ceramic coat the pistons and chambers to keep the heat down. Works well in high boosted engines, and nascar engines so they get that extra little bit of power. I have a good article on it.
Longer duration camshafts will also bring the compression down a bit.
Some attention to detail, and its entirely possible.
There are places that ceramic coat the pistons and chambers to keep the heat down. Works well in high boosted engines, and nascar engines so they get that extra little bit of power. I have a good article on it.
Longer duration camshafts will also bring the compression down a bit.
Some attention to detail, and its entirely possible.
Thread Starter
Safety Car
Joined: May 2005
Posts: 4,176
Likes: 7
From: Jennings LA
didn't mean to mislead
When I said it was not a daily driver I meant it may see 1500 miles a year but it is a street driven car. it is a fair weather/ weekend toy.
the reason I started this thread is because I am ordering a 383 short block and was undecided on the pistons either flat top or dished. I would hate to leave horsepower on the table,horsepower is not cheap.
the cam specs are 292/292 duration 480/480 lift 109 lobe separation. 230/230 duration at .050 compression with my 64cc iron heads will be 10.9 to 1 with flat tops or 9.6 to 1 with dished. there are several local guys running 11.3 to 1 with local pump gas 93 octane without detonation. if mine will not run hot with iron heads and 10.9 thats where I am leaning.
now I have 64cc heads on a stock 81 block and the temp runs around 185, not sure of current compression maybe 9.5 to 1. help I want to order the short block tomorrow.
the reason I started this thread is because I am ordering a 383 short block and was undecided on the pistons either flat top or dished. I would hate to leave horsepower on the table,horsepower is not cheap.
the cam specs are 292/292 duration 480/480 lift 109 lobe separation. 230/230 duration at .050 compression with my 64cc iron heads will be 10.9 to 1 with flat tops or 9.6 to 1 with dished. there are several local guys running 11.3 to 1 with local pump gas 93 octane without detonation. if mine will not run hot with iron heads and 10.9 thats where I am leaning.
now I have 64cc heads on a stock 81 block and the temp runs around 185, not sure of current compression maybe 9.5 to 1. help I want to order the short block tomorrow.
Drifting
Joined: Sep 2003
Posts: 1,325
Likes: 6
From: FRANKLIN GA
Originally Posted by gator79
When I said it was not a daily driver I meant it may see 1500 miles a year but it is a street driven car. it is a fair weather/ weekend toy.
the reason I started this thread is because I am ordering a 383 short block and was undecided on the pistons either flat top or dished. I would hate to leave horsepower on the table,horsepower is not cheap.
the cam specs are 292/292 duration 480/480 lift 109 lobe separation. 230/230 duration at .050 compression with my 64cc iron heads will be 10.9 to 1 with flat tops or 9.6 to 1 with dished. there are several local guys running 11.3 to 1 with local pump gas 93 octane without detonation. if mine will not run hot with iron heads and 10.9 thats where I am leaning.
now I have 64cc heads on a stock 81 block and the temp runs around 185, not sure of current compression maybe 9.5 to 1. help I want to order the short block tomorrow.
the reason I started this thread is because I am ordering a 383 short block and was undecided on the pistons either flat top or dished. I would hate to leave horsepower on the table,horsepower is not cheap.
the cam specs are 292/292 duration 480/480 lift 109 lobe separation. 230/230 duration at .050 compression with my 64cc iron heads will be 10.9 to 1 with flat tops or 9.6 to 1 with dished. there are several local guys running 11.3 to 1 with local pump gas 93 octane without detonation. if mine will not run hot with iron heads and 10.9 thats where I am leaning.
now I have 64cc heads on a stock 81 block and the temp runs around 185, not sure of current compression maybe 9.5 to 1. help I want to order the short block tomorrow.
No offense but that's a small cam for 11:1 in a 383. You can easily get away with 300+ duration and .520+ lift to help it breathe better. You will need a good stall in the 3000 range though and 350 to 373 gears will help also.
