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#250331 - 08/25/03 11:05 PM Compression ratio question  
Joined: May 2001
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chevelle502 Offline
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chevelle502  Offline
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what is the highest comp. ratio you can have and still run todays pump gas? i was watching speed channel tonight and they said that the '62 vette, 327 had 11.25:1 comp. ratio. I thought that was a little high for pump gas? Also, what is a good streetable ratio for a 350 putting out about 450 hp? Thanks in advance. Chuck!!


1988 Monte Carlo SS
Black w/T-tops
Mods: Jet stage 2 chip, 2.5" cat back with flow super 44's, K&N filter,extreme lid, chrome valve covers, custom interior from a 2003 Monte SS, 3.73 posi rear (waiting to install), re-built tranny, Elec fan setup, Msd Ignition, LED Headlights
http://chucks88ss.tripod.com/Montepics/index.album?i=1
#250332 - 08/25/03 11:18 PM Re: Compression ratio question  
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Gruvin Offline
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10.0 to 1 or less on standard high octane pump gas with Iron heads(Aluminums can get another .8 to 1 point of CR on the pump gas).

I am pushng the limits with between 10.5/11 to 1 compression and Iron heads but I am going ot be running EFI which has way more spark and fuel tuning than a carb/HEI distrib have.


My first SS! Sold Jan 2004
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87 SS bought 9-5-04: Sold 6-18-2010 - project 87 finds a new home with another undisclosed MonteCarloSS.com member!
#250333 - 08/26/03 12:33 AM Re: Compression ratio question  
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lefty o Offline
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lefty o  Offline
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mpls,mn
i tend to agree with gruvin, for the most part. the other thing to consider is your cam- bigger cams tend to bleed off some cylinder pressure donw low allowing you to get by with a little more mechanical compression ratio.


bad black '87 SS, 474hp-355
#250334 - 08/26/03 12:37 AM Re: Compression ratio question  
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TooSlow406 Online content
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TooSlow406  Online Content
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Friendswood, TX
There has to be more to it than just that. My Camaro had 10.5:1 and I had to retard the timing and run 93. If I ran any less it would spark knock. But I just saw a motor today on the dyno that had 11:1 on 93 with normal timing.


1985 Monte 406sbc, RHS ported 220s, Schoenfeld 1 3/4", 3" exhaust, Dynomax Ultra-Flo, Holley ported Strip Dominator, Dual Holley Blues, Holley HP 950, Comp roller 236/245, .570/.574
#250335 - 08/26/03 12:48 AM Re: Compression ratio question  
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littlelefty Offline
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Lockport, IL
shane- the only thing i can say about that is that is not uncommon. each motor seems to have its own set of rules. no two are the same.

#250336 - 08/26/03 12:51 AM Re: Compression ratio question  
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Barney Offline
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Barney  Offline
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Don't make the static compression ratio the limiting factor, most of the time they are over rated anyway. Here is the biggest over-estimation, a stock 350 with flat top pistons and 76cc heads. Most people say 8.5:1 when in fact it is more like 7.8:1. Use the cranking psi keep all cylinders within 10% of each other and your good. 200psi cranking is max for the street and pump gas. Like Lefty O said the cam controls the bleed off which allows a higher compression ratio late. This is appearent by the cam grinders warnings like; this cam is to be used with 11:1 or greater static compression. I read an article in Chevy High Performance that tested a 13:1 compression motor on pump gas with no problem. I was running 12:5 slugs in my motor with 72cc heads for about 11:1 static with 190-195 psi cranking on pump gas with no problem.

Barney


Barney
88 MCSS, 427CI SBC, Dart Little M, Team G Intake, PBM Strike Force Heads, 950 Holley, Schoenfeld Headers, Coan Glide W/Brake & Coan 5500 Converter. Ford 9" full spool and 5.83 gears. Ladder Bar QA1's. 32X14 Hoosier's on 15"X15" Weld's.

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#250337 - 08/26/03 12:56 AM Re: Compression ratio question  
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littlelefty Offline
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Lockport, IL
again. no two are alike. sometimes i think their is no "normal" anything that applies to all motors.

#250338 - 08/26/03 01:41 AM Re: Compression ratio question  
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chevelle502 Offline
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chevelle502  Offline
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thanks guys, every time i read about something i dont really know much aboout i figure its best to ask here!! The more i know, the better i can build my motor!! Thanks again.


1988 Monte Carlo SS
Black w/T-tops
Mods: Jet stage 2 chip, 2.5" cat back with flow super 44's, K&N filter,extreme lid, chrome valve covers, custom interior from a 2003 Monte SS, 3.73 posi rear (waiting to install), re-built tranny, Elec fan setup, Msd Ignition, LED Headlights
http://chucks88ss.tripod.com/Montepics/index.album?i=1
#250339 - 08/26/03 01:48 AM Re: Compression ratio question  
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Gruvin Offline
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Gruvin  Offline
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Texas
Quote:
Originally posted by Barney:
I was running 12:5 slugs in my motor with 72cc heads for about 11:1 static with 190-195 psi cranking on pump gas with no problem.

Barney
Good setup Barney, thats about perfect cranking pressure.

You can get away with more compression with polished chambers and piston tops.

Alot of good stuff here in this post.


My first SS! Sold Jan 2004
[Linked Image]
87 SS bought 9-5-04: Sold 6-18-2010 - project 87 finds a new home with another undisclosed MonteCarloSS.com member!
#250340 - 08/27/03 02:59 AM Re: Compression ratio question  
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TooSlow406 Online content
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Friendswood, TX
What we need is for 114 race fuel to sell for .78 cents a gallon. Then we could all run 12:1 CR easily and afford it.


1985 Monte 406sbc, RHS ported 220s, Schoenfeld 1 3/4", 3" exhaust, Dynomax Ultra-Flo, Holley ported Strip Dominator, Dual Holley Blues, Holley HP 950, Comp roller 236/245, .570/.574
#250341 - 08/27/03 12:04 PM Re: Compression ratio question  
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supernova455 Offline
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supernova455  Offline
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tampa, fl usa
I say YES to polishing the cumbustion chambers. I have noticed
a hug difference in pump gas freindlyness. My 70 455 has 10:1 from the factory. Before the rebuild (factory) it would ping and knock on 93. After the rebuild and polishing the chambers, I run 89 no problems.

Supernova455


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