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What compression can I run on 91 octane?

drobertson

drobertson

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I just got a decent job, and I'll soon have funds to build my 440.

However, I can only get 91 octane fuel in my area. 93 isn't available :wtf:

Plans for the engine so far...

Steel crank,
stock ly rods,
edelbrock rpm heads,
edelbrock rpm intake,
tti headers,
fitech efi,
stock ignition.

What I'd like to know is how much compression I can run with a street/strip camshaft, without detonation issues. I'm open to recommendation for cams and pistons too... Thanks all.
 
drobertson said:
I just got a decent job, and I'll soon have funds to build my 440.

However, I can only get 91 octane fuel in my area. 93 isn't available :wtf:

Plans for the engine so far...

Steel crank,
stock ly rods,
edelbrock rpm heads,
edelbrock rpm intake,
tti headers,
fitech efi,
stock ignition.

What I'd like to know is how much compression I can run with a street/strip camshaft, without detonation issues. I'm open to recommendation for cams and pistons too... Thanks all.
Click to expand...


I'm running 11.5:1 in my 440ci R3 Small Block and have had no issues with 91 octane. My heads are Indy 360-1 CNC ported, Diamond forged pistons and a custom Bullet roller cam. 262/266 @ .050 .690/.690 lift...... 11.5:1 is about as high as i'd go with pump 91.

IMG_0695.JPG
 
The specifics are best answered by someone else, but I think it depends on how much dynamic compression is built by the cam.

I have a 10.5 static with what appears to be quite high dynamic (I measure 225 psi on compression tests at sea level) and I get detonation with the timing above about 32 total.

I think the issue is therefore choosing the correct cam and how it is set up. There are online calculators such as at Wallace racing) that may be helpful in this, but finding someone with first hand experience in this matter could also be critical. Good luck
 
Agree^^^^ I run 9.8:1 and have no issues with detonation with 89 but have a mild cam. The cams ability to bleed off compression is a big factor that would be best answered by someone like IQ that builds and dynos several combinations.
 
the static compression is a good number to know, but it is really your engines operating cylinder pressure that will dictate you required fuel.

generally 170 psi cranking is a safe number to shoot for, with Aluminum cylinder heads, you can go a little higher, but alot of other factors will start to surface, vehicle weight, gear ratio, EFI or carburated and the elevation where you will be mostly using the car.
 
mentor70 said:
the static compression is a good number to know, but it is really your engines operating cylinder pressure that will dictate you required fuel.

generally 170 psi cranking is a safe number to shoot for, with Aluminum cylinder heads, you can go a little higher, but alot of other factors will start to surface, vehicle weight, gear ratio, EFI or carburated and the elevation where you will be mostly using the car.
Click to expand...


Mine is @ 180psi cranking pressure and i've had no problems with detonation....... Like you said, the aluminum heads help
 
9.5:1. 91 Octane crap gas in California. No ping with current tune.
 
i use icon 836 pistons with a set of edelbrock rpm heads; 9.75:1 comp with clean chambers and small mech cam. this combo is very pump gas friendly; should work fine with 91 octane.
 
Here's the deal... With careful work and parts choices one can run high static ratios with pump fuel. But, IMO, this is really not worth the extra cost in most street builds. You will get precious little additional power from that extra point of static. Build the engine well by getting top quality machining, and don;t concentrate so much on the ragged edges of perofmance windows. I'd run a flat top at zero deck, the felpro 1009 gasket with it, and it will end up around 9.7:1 depending on the piston and valve relief. With a decent set of heads and cam, you will be over 1hp/in and be able to run any pump fuel.
 
My setup is much like Moper describes, flat top pistons, zero deck height, 452 cast heads and comes out at 9.8:1. My cranking pressure is 160, like I said no detonation issues with 89 octane.
 
Open vs. closed head chamber can make big difference.
 
Not chamber size in and of itself. It's the combination of the parts and the setup by the machinist all together. Either design can be made to work - or make things worse.
 
747mopar said:
My setup is much like Moper describes, flat top pistons, zero deck height, 452 cast heads and comes out at 9.8:1. My cranking pressure is 160, like I said no detonation issues with 89 octane.
Click to expand...

What are your cam specs? I'm trying to decide on whether to use open iron heads or invest into aluminum closed chambers for my 413 and what cam would be appropriate.
 
Glenwood said:
What are your cam specs? I'm trying to decide on whether to use open iron heads or invest into aluminum closed chambers for my 413 and what cam would be appropriate.
Click to expand...

Mines fairly mild and works nicely on the street but still a healthy sounding idle.
20170413_112410.jpg
 
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Aluminum heads also let you run about 1 point higher in comp. But if you build good quench in it and run the right cam to keep the cyl pressure at or below 180 you can do it. Like was said the best build is run a flattop piston at zero deck and a closed chamber head. Then use a .039 to .045 thick head gasket for good quench. Myself I run the Indy EZ aluminum heads at 10.6 comp and a solid flat tappet cam thats 264 & 270 @ .050 with .585 & .592 lift and has a 110 LSA. I have it in the eng on a 106 ICL. I actually use the D-dished pistons as my comp would have been close to 11.8 with the flattop and the 75cc EZ head so the D-dish piston has a dish in it but about half of the piston is still flat so I still have good quench. I run 92 pump I get in my area with no spark knock ever running 37 total timing. With todays technology a good eng builder can build you a nice stroker with aluminum heads and about 10.5 comp to run good on 91 octane. If you use Iron heads keep the comp around 9.5. Good luck , Ron
 
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