Too much compression for local pump gas octane?
#1530174
11/08/13 03:38 PM
11/08/13 03:38 PM
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Joined: Aug 2006
Posts: 610
long time lurker, short time p...
PorkyPig
OP
mopar
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OP
mopar
Joined: Aug 2006
Posts: 610
long time lurker, short time p...
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I'm finishing putting my engine back together and wondering if I can still get away with pump gas after the latest changes.
It's a 440 bored .055 oversize, .047” quench, ported Indy SR heads with cleaned up chambers, and a 260 at .050" solid roller on 108 advanced 2 degrees.
The car is 3800 lbs with driver, has a 4200 stall converter, 4.10s, 28" x 10" DOT meats and a 3" exhaust system.
I just ran the numbers for the compression ratio now that I have all the details and came up with right at 11:1. From what I saw in the past, it'll probably have close to 190 psi cranking pressure.
The local pump fuel is 92 octane (ROM+MON/2) w/ 10% ethanol. Is this going to work OK, or am I going to wish I'd gotten dished pistons instead of flat-tops to have lowered the compression? Maybe I just need to be extra careful about the ignition timing? It used to run fine with a little lower compression at 36 degrees total.
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Re: Too much compression for local pump gas octane?
[Re: PorkyPig]
#1530178
11/08/13 03:59 PM
11/08/13 03:59 PM
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Joined: Nov 2006
Posts: 635
mississippi
bentwheel43
mopar
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mopar
Joined: Nov 2006
Posts: 635
mississippi
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My 512 kit with flattop and 75cc Eddy heads has 12.42 it pumps 185#.On 93 pumpgas timing at 34*.
Last edited by bentwheel43; 11/08/13 04:01 PM.
Section 8 Racing We are nuts about racing.
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Re: Too much compression for local pump gas octane?
[Re: Crizila]
#1530181
11/09/13 11:24 AM
11/09/13 11:24 AM
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Joined: Nov 2003
Posts: 1,969
Chandler, AZ
Duner
top fuel
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top fuel
Joined: Nov 2003
Posts: 1,969
Chandler, AZ
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Quote:
Quote:
I'm not sure if you're on the borderline or not... I'd back off on the timing a couple of degrees and run a colder plug to start with to be safe. 
Trust me, you are on the boarder line. I think you knew this with the post. Above suggestions will help. If you don't want to change the motor, maybe water injection as an added safety. One bad load of fuel and you will be pulling the motor.
That's no lie! I recently got a tank full of what was probably 87 octane instead of 91 like it was sold as, and it wasn't good. I pumped out a third of the tank and put some 110 in it to salvage what was left. You REALLY have to pay attention or it will get ugly and expensive quickly.
For sure keep the timing in check until you know what it likes.
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Re: Too much compression for local pump gas octane?
[Re: Triggerfish]
#1530184
11/09/13 02:09 PM
11/09/13 02:09 PM
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Joined: Jan 2003
Posts: 27,421
Balt. Md
383man
Too Many Posts
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Too Many Posts
Joined: Jan 2003
Posts: 27,421
Balt. Md
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I am at 10.6 with EZ aluminum heads and a solid flat tappet thats 264 & 270 @ .050 and .585 & .592 lift. Quench is .046 and I run 37 total timing. Intake valve closes at 58 ABDC. Runs good on 92 pump with no problems. I just raced it 3 weeks ago on the 92 and thru the full exh. I am in Md close to sea level. Ron
Last edited by 383man; 11/09/13 02:09 PM.
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Re: Too much compression for local pump gas octane?
[Re: Crizila]
#1530185
11/09/13 05:57 PM
11/09/13 05:57 PM
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Joined: Jan 2003
Posts: 15,439
Val-haul-ass... eventually
BradH
Taking time off to work on my car
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Taking time off to work on my car
Joined: Jan 2003
Posts: 15,439
Val-haul-ass... eventually
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Quote:
Quote:
I'm not sure if you're on the borderline or not... I'd back off on the timing a couple of degrees and run a colder plug to start with to be safe.
Trust me, you are on the boarder line. I think you knew this with the post. Above suggestions will help. If you don't want to change the motor, maybe water injection as an added safety. One bad load of fuel and you will be pulling the motor.
The reason I wasn't (and am still not) sure is because my 452 ran on 93-octane ethanol-blended fuel with no detonation problems. It had 10.8 CR and a 266 at .050" solid flat-tappet cam.
Is adding .2 in CR, subtracting 6 degrees of duration and dropping 1 point of advertised fuel octane going to change things from a perfectly "happy" combination into an issue? I dunno... 
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Re: Too much compression for local pump gas octane?
[Re: bentwheel43]
#1530186
11/11/13 12:21 PM
11/11/13 12:21 PM
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Joined: Jan 2003
Posts: 6,840
The Swamp
Sixpak
master
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master
Joined: Jan 2003
Posts: 6,840
The Swamp
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Quote:
My 512 kit with flattop and 75cc Eddy heads has 12.42 it pumps 185#.On 93 pumpgas timing at 34*.
Say Bentwheel, a little off topic, but how's that big stroker like the Holley Street Dominator intake? Thinking of a combo similar to that with that intake in an A body.
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