SBC compression limit

[silo84]

How much compression can an AL head SBC get away with?

I have read a lot of post on the interweb and can't get a decisive answer ( I know its not that simple, but)...


So on to the set up:

383 sbc
deck clearance .020
6 in rods
forged flat top 2 valve relief piston (-4CC)
I have a set of 64 CC AL heads (blueprint heads)
NO CAM yet

Manual trans
3.08 rear gear
73 Camaro /street car/ occasional road course track day

93 octane

Anyways the problem I have is with these parts is to achieve the all important quench number of .035-.040" puts me well into the 11:1 range.

So is it possible to just use a larger head gasket(detonation?) to get a lower compression ratio (10.5)? (machine shops suggestion)

Or can I run enough duration cam to get the dynamic compression to a livable spot (8.5?) ( advertised 278 duration 110LSA) bandaid?


Or punt and get a larger chamber head? Probably the best idea but I'm cheap..

any help would be great

[Fuelie Nova]

Just curious... carb or EFI?
I have ran 11:1 on a 383 SBC using 92 octane but I was running EFI and did a ton of data logging to get my tune spot on. I also ran knock sensors just in case.
However very seldom did my knock sensors have to back out timing.
Tom

[silo84]

I have fi tech efi,

I have continued to read on the subject and it seems that the cranking pressure limit is 200 psi according to a super Chevy right up. Am I understanding this right?

[silo84]

This article

http://www.superchevy.com/how-to/81679-tuning-engine-compression/

[nokones]

I am running an all aluminum SBC AFR 245 heads with 12.25 compression and it is EFI. I use Sunoco 100 Unleaded and the motor dynoed at 646 HP at 6600 RPMS at the crank.

[silo84]

Well I decided to go for it, using the info I have gathered I have selected a cam that puts my cranking pressure in the 170's and see how goes, ill report back if its a pinging machine or not

[matt816]

Be real with yourself on your goals. A street sbc 383 does not need 11:1 nor does it need 6" rods. Don't cheat the system with a cam just so you can turn it over and maybe tune it. TBI efi systems don't like crazy overlaps. 10:1, better heads, 180cc would be great for street, all out hp chasing 200cc, 5.7 rods and a good spec'd cam with 112 LSA should idle good, great power. 14-16" vacuum, tuneable and overall more livable. But thats just me

[silo84]

I hear ya, just the way the parts came together.

This all started as a head/cam swap on a 350 that ended up needing a full rebuild. Happened upon a fresh 4 bolt roller 383 short block for cheap instead of building my 2 bolt 350 for more money.

If I get it together and it wont run well, ill get larger chamber heads to bring the compression down. and put the 64cc heads on something else.

[kimosabi]

The 200psi cranking pressure for pump gas is not an absolute, just a thumb rule. You CAN run 11:1 on pump, and run it well, but since you don't have a cam yet you're in luck, the solution is to get a taylored custom cam. Give ALL your specs to a well known custom grinder. I'd talk to Mike Jones or Chris Straub.

[silo84]

Just an update, engine is running fine on 93 octane, running 10 deg advance initial timing. have about 400 miles on the engine so far so good.

Thanks for all of the input.

[kimosabi]

Easy peasy with a 170psi cranking pressure. I'm at 210 cranking psi and run 91. 32 total.

[Skip Fix]

I'd bet you guys with 200+ cranking pressure put some race gas in you would pick up HP on a dyno pull showing you actually had too much compression-even without hearing detonation.

For the minimal HP increase(some say 4% per 1 bump in compression so a 500 hp motor 20 hp) why risk getting some bad gas at the pump and pounding the bearings?

[kimosabi]

That's a very general statement. If all engines were identical I would agree.

[kimosabi]

To add to that I just pulled another 2 degrees off, now 30 total and it runs just as good or slightly better. No reduction in response. Reduction only in rear traction but header heat increase.