Limits for 91 octane

[quickd100]

I'm working on a 74 440 bbm, starting out bone stock it made an Earth shattering 300.4hp&426ftlbs on my L&S Dyno. Swapping some earlier factory pistons and milled factory iron heads I'm going to go from 7.78-1 to 11.7-1. The cam I'm going to use is a flat tappet solid that's 253@.050 and with it's intake closing point the dynamic compression works out to 6.44-1. Quench isn't a factor as the piston to head clearance at best is .071. I ll be testing (2) single 4 intakes and (3) 2x4 intakes on the motor. Once I m done testing at 11.7-1 a head gasket change will drop it down to 11.13-1. Any further reduction in compression ratio will be accomplished by milling the pistons. All Dyno work will be done on race gas.
The final plan is to put this motor in a chassis for street duty. Chassis with me in it weighs 3700 will be using a 4.56 gear and a convertor that matches the torque peak. Operating temps will be 160/180 and will be using ram air induction.
The best available pump gas in my area is 91 with 10% ethanol. I don't want to start blending fuels either. What's your best guess on how high I can run the compression and keep it alive. If it breaks I m not out anything but my time.

[F-BIRD'88]

What is the answer that you want to hear?

[quickd100]

Given the parameters, how much compression can I run without detonating it to death?

[Stan Weiss]

With what Bore, stroke, rod length, CR and ICL are you coming up with 6.44:1 DCR?

WAG 10.0:1 CR.

Stan

[BradH]

Given the parameters, how much compression can I run without detonating it to death?

Similar build years ago w/ 9.8:1 CR needed a good octane booster even w/ 93 E10 pump fuel. No booster and 91 octane... maybe 9.0:1.

[quickd100]

Yes

[quickd100]

With what Bore, stroke, rod length, CR and ICL are you coming up with 6.44:1 DCR?

WAG 10.0:1 CR.

Stan

4.32 bore
3.75 stroke
Rod 6.768"
11.7-1 measured mechanical compression
Intake valve seats at 97° abdc
Altitude here is 760 above sea level

[F-BIRD'88]

What is the compression ratio number answer that you want to hear?

[Stan Weiss]

With what Bore, stroke, rod length, CR and ICL are you coming up with 6.44:1 DCR?

WAG 10.0:1 CR.

Stan

4.32 bore
3.75 stroke
Rod 6.768"
11.7-1 measured mechanical compression
Intake valve seats at 97° abdc[/size}
Altitude here is 760 above sea level

Where did that number come from? If that is in @ 110 ICL that would be 334 degrees duration. Do you have a cam card?

Stan

[Stan Weiss]

What is the compression ratio number answer that you want to hear?

The highest CR he can run with the cam he listed on 91 octane.

Stan

[David Redszus]

With what Bore, stroke, rod length, CR and ICL are you coming up with 6.44:1 DCR?

WAG 10.0:1 CR.

Stan

4.32 bore
3.75 stroke
Rod 6.768"
11.7-1 measured mechanical compression
Intake valve seats at 97° abdc
Altitude here is 760 above sea level

The above specs will result in a TCR of 6.45 which will then produce:
@TDC........176psi....605F
@-33deg.......66psi....347F

Reducing chamber volume to produce SCR of 14:1 will net a TCR of 7.55.
@TDC.........216psi.....7738F
@-33deg......71psi.......415F

Based on fuel auto ignition point, a summer blend pump gas could be used providing the inlet air temperature is controlled properly.
Mount several knock sensors and read piston tops frequently.

[cardo0]

How good is your quench? Clean all sharp edges off pistons and head chambers? How good is your cooling system/block temperature? Cylinder head temperature? Using cold air induction or hot engine compartment air?

What I'm saying you can build the engine to tolerate lower octane/lesser fuels. But static compression of 9.5:1 or less is the rule of thumb for any motor using 91 octane fuel.

Hope this can help.

[F-BIRD'88]

I'll go along with the 9.5:1 +/- guide line as a practical
Cr for this.
Got to ask why not supercharge the 7.7:1 cr engine?
More power using the existing heads.
Much more power on 91.

[BradH]

Given the parameters, how much compression can I run without detonating it to death?

Similar build years ago w/ 9.8:1 CR needed a good octane booster even w/ 93 E10 pump fuel. No booster and 91 octane... maybe 9.0:1.

FWIW, the cam in my build was an aggressive 254 at .050 sft on 108 LSA and the heads were old hand-ported OEM Chrysler iron heads with open chambers (no effective quench). Cylinder cranking pressure IIRC was 180-185. A milder version a bit lower CR and about 165 cranking pressure was noticeably less octane sensitive and would run fine on straight 93 E10.

[Powertrip]

Like Brad, on a BBM, I've never had any luck running iron heads with no quench on pump 93 at anything over 9.5:1.
I've had good luck, however, running aluminum heads with good quench at 11:1+.

If you are planning on getting real results out of your intake manifold testing, those heads are a real cork and will need more than just a bowl blend.