Compression - Is 11:1 too much?
Melting Slicks
Joined: Apr 2009
Posts: 2,632
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From: Great Plains Iowa
Drifting
Joined: Jun 2015
Posts: 1,266
Likes: 156
I did an 11.5 406 a couple of years ago. Aluminum heads, 243/251@.050" with two degrees advance. It has been fine on 93 octane. I did have to slow the advance curve a bit to make it happy in overdrive but it's been fun to drive. With good heads and enough cam it can work.
Burning Brakes
Joined: Feb 2014
Posts: 1,045
Likes: 226
From: Cape Neddick Maine
2020 C3 of the Year Finalist - Modified
I run 11.0:1 on the street with 93 and don't have issues. I also don't run a huge cam to bleed off the compression either.
hang on engineering 101 coming soon
Drifting
Joined: Jun 2011
Posts: 1,704
Likes: 27
11.3:1 on a Alum headed LS engine, I run no less than 93 octane. LS are a different animal though. For a SBC I've heard 10:1 for iron headed SBC and 10.5:1 for Alum head SBC. On the Gen II SBC (LT engines), the coolant is reversed cooling the heads first, so I've heard as much as 12:1 on pump gas (93 octane). But you should also look at your DCR (dynamic compression ratio) as soon as you know your cam specs.
Last edited by htown81vette; Dec 4, 2015 at 06:02 AM.
Le Mans Master
Joined: Oct 2007
Posts: 7,453
Likes: 976
From: RI, Now Franklin/Nashville TN
I run 89 octane with 10.2:1 compression with AFR aluminum heads on my 355 L-82 with a moderate roller cam (.525/.525, duration 219/225 LSA 110) with no issues. I could easily run 11:1 with 93 octane...
Drifting
Joined: Jun 2011
Posts: 1,704
Likes: 27
I know quite a few drag racers that run "right on the edge" with their compression and what they do is during the hot summer months they run a 50/50 mix of race fuel and pump gas (highest octane available) just to make sure they don't grenade their engine during the hot weather. That seems to work for them.
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Drifting
Joined: Mar 2001
Posts: 1,396
Likes: 7
From: Indiana
I'm running 11.25:1 in my 383. I fabricated a stop to limit the amount of vacuum adv by roughly 6 degrees, as I was getting some detonation under light acceleration from cruising speeds. Mech adv is 34 and all in by 3k rpm. With the right cam, anything is possible.
Pro
Joined: Nov 2015
Posts: 600
Likes: 110
From: Mainz
I run Static Comp 10.5 :1 , Dynamic Comp 8.35 : 1 - 383 with 1970 ported and decked big valve Camel Hump iron heads. 34 degree ignition. NO Pinging. Engine is all bluprinted with extreme close tolerances. Cam: 221IN / 230EX @.50 , 112LSA , 108 Center Line -
Needs high premium Super (100 Octane - European Rating), otherwise it will "diesel" when shut off. Runs very strong! I think 11:1 is possible with the right cam that allows some "Bleeding of". Also the dynamic compression is the giving factor, that is where the cam Comes in. Everything Need to be exact and right with a high compression Ratio like this (ie Quench) I run D Cup shaped -12cc Mahle forged Pistons. Heads are flat surfaced to 62cc, intake bowls reworked and cc'd (checked for true volume) . The 1970 LT1 had a Static compression of 11:1, but the older style cams allowed some good "Bleeding of" this way the dynamic compression was still in an exceptable range especially with iron heads. (if there are spelling mistakes in there please forgive me, I am German) Thanks.
Needs high premium Super (100 Octane - European Rating), otherwise it will "diesel" when shut off. Runs very strong! I think 11:1 is possible with the right cam that allows some "Bleeding of". Also the dynamic compression is the giving factor, that is where the cam Comes in. Everything Need to be exact and right with a high compression Ratio like this (ie Quench) I run D Cup shaped -12cc Mahle forged Pistons. Heads are flat surfaced to 62cc, intake bowls reworked and cc'd (checked for true volume) . The 1970 LT1 had a Static compression of 11:1, but the older style cams allowed some good "Bleeding of" this way the dynamic compression was still in an exceptable range especially with iron heads. (if there are spelling mistakes in there please forgive me, I am German) Thanks.
Last edited by Alex66; Dec 4, 2015 at 05:22 PM.
Melting Slicks
Joined: Nov 2010
Posts: 2,216
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From: Michigan
I like to play it safe, seen too many cars sit because the owner doesn't want to buy race gas. Plus it's kinda insurance to have some room for error or bad fuel. Two of the things I don't get greedy on are compression and porting.
Team Owner
Joined: Apr 1999
Posts: 21,953
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From: Reno Nevada
2024 C3 of the Year Finalist- Modified
My 383 has 64 cc heads, - 6 cc flat top pistons, .021 head gasket, decked block for .014 piston to deck. High compression rings.
It works out for right at 11:1 Solid roller with 236/243 112 .640/.670 lift AFR 210 heads. Runs on our California 91 oct
It works out for right at 11:1 Solid roller with 236/243 112 .640/.670 lift AFR 210 heads. Runs on our California 91 oct
Team Owner
Joined: Apr 1999
Posts: 21,953
Likes: 1,445
From: Reno Nevada
2024 C3 of the Year Finalist- Modified
I should also add that it is ground 4 degree retarded compared to a comparable Comp Cams Extreme series of roller cams. You have less reversion on the intake side. It also has the 4-7 firing order swap
It works out for right at 11:1 Solid roller with 236/243 112 .640/.670 lift AFR 210 heads. Runs on our California 91 oct
It works out for right at 11:1 Solid roller with 236/243 112 .640/.670 lift AFR 210 heads. Runs on our California 91 oct
Race Director
Joined: May 2003
Posts: 10,956
Likes: 409
From: California
11:1 is fine on California 91 pump gas
I am running 11:1+ 427 Aluminum heads 650hp... not a problem..
So, everyone here says 11:1 on Aluminum heads is fine,,,, except one... guess who? the resident engineer I am sure..
I am running 11:1+ 427 Aluminum heads 650hp... not a problem..
So, everyone here says 11:1 on Aluminum heads is fine,,,, except one... guess who? the resident engineer I am sure..
Last edited by pauldana; Dec 4, 2015 at 04:05 PM.
Safety Car
Joined: Jan 2008
Posts: 3,795
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From: Aarhus, Denmark
I should also add that it is ground 4 degree retarded compared to a comparable Comp Cams Extreme series of roller cams. You have less reversion on the intake side. It also has the 4-7 firing order swap
It works out for right at 11:1 Solid roller with 236/243 112 .640/.670 lift AFR 210 heads. Runs on our California 91 oct
It works out for right at 11:1 Solid roller with 236/243 112 .640/.670 lift AFR 210 heads. Runs on our California 91 oct
Team Owner
Joined: Apr 1999
Posts: 21,953
Likes: 1,445
From: Reno Nevada
2024 C3 of the Year Finalist- Modified
Melting Slicks
Joined: Apr 2009
Posts: 2,632
Likes: 112
From: Great Plains Iowa
Modern engines with reverse flow cooling systems can tolerate higher cylinder pressures because the heads get cooled first. But for our aging C3's I feel its best to limit the compression ratios to around 10 to 10.5 to 1 to avoid detonation. I'm running a 9.95 to 1 compression ratio in my 454" powered '71 and I can barely get by with 91 octane and 31 degrees of advance.
I can remember compression ratios as high as 13.5 to 1 in the late 60's muscle cars but that was when 102 octane gasoline was available but even then an octane booster was added for racing. Many folks used 115 octane aviation gasoline which was about 5 times the cost.
I can remember compression ratios as high as 13.5 to 1 in the late 60's muscle cars but that was when 102 octane gasoline was available but even then an octane booster was added for racing. Many folks used 115 octane aviation gasoline which was about 5 times the cost.
Burning Brakes
Joined: Feb 2014
Posts: 1,045
Likes: 226
From: Cape Neddick Maine
2020 C3 of the Year Finalist - Modified
Our motors WILL run on 11:1 with aluminum heads and pump fuel. its being done by many members here and else where.....just because one or too don't think it can be done doesn't mean it cant be.
The key here is the aluminum heads. They will tolerate a full point of compression more than iron heads. Check with any other professional engine builder they will give you the same advice.
With enough cam you can even go more to help bleed off some of the compression but lets not confuse this any more, huh?
Good luck too the original poster you got some great advice from almost all on this post. I think its time to lock it up.
The key here is the aluminum heads. They will tolerate a full point of compression more than iron heads. Check with any other professional engine builder they will give you the same advice.
With enough cam you can even go more to help bleed off some of the compression but lets not confuse this any more, huh?
Good luck too the original poster you got some great advice from almost all on this post. I think its time to lock it up.