ex9850

Rebuilding my 383 and contemplating an 11:1 Compression ratio. Plan on running pump gas supreme. Is anyone doing this?

toobroketoretire

You will have problems running a compression ratio that high. Much better to limit it to 10 to 1 to give some flexibility.

a striper

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.

jkippin

Aluminum heads can stand more compression ratio than steel.
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

htown81vette

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.

jb78L-82

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...

Ibanez540r

I will be running a carb'd SBC around 11:1 plus BOOST on a 224/236 cam

..she will be drinking the corn though.

htown81vette

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.

tsw71

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.

resdoggie

Lots of folks running 11 to 1 and most have alum heads and and longer duration cams. Go for it!

Kacyc3

406 Alum head 11:1 this build, last build was flat top piston 406 with 58cc heads- couldnt run more than 32 timing or it would ping.

Alex66

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.

BOOT77

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.

gkull

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

gkull

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

pauldana

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..

c3_dk

EFI engine?

gkull

I don't believe that cheap EFi units are as good as high end carbs that have been setup on flow benches and dyno test mule motors.

So I only have a 830 CFM Demon.

toobroketoretire

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.

jkippin

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.