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When do you have enough OCTANE?
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DRR Trophy
posted
352 sbf, AFR cyl. heads, 10.9 to 1 C.R., 34* locked timing, flattop pistons, 252/260 @ .050, 4700 stall, mix race gas with pump gas. Bracket car, no street time. Is 97 octane enough for this combination?
 
Posts: 74 | Location: Mooresville, nc | Registered: October 11, 2013Reply With QuoteReport This Post
DRR Pro
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In my opinion, yes. However, real world testing is the only way to know for sure. Take care. Tom Worthington


If it seems that bracket racing has gotten too expensive for you, maybe you are just doing it wrong.
 
Posts: 1224 | Location: Rocky Mount, NC | Registered: December 01, 1999Reply With QuoteReport This Post
DRR S/Pro
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quote:
...real world testing is the only way to know for sure.

At the risk of exposing my ignorance, how does one test for detonation with open headers and no knock sensor with accompanying software?


Illegitimi non carborundum
 
Posts: 2320 | Location: OKC, OK | Registered: February 15, 2008Reply With QuoteReport This Post
DRR Trophy
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At almost 11to1 and that short of cam cylinder pressure will need 110 octane to be safe. Detonation can be found by close spark plug inspection.


The difference between ignorance and stupidity. Ignorance is lack of knowledge. Stupidity is the inability to learn. Don't be stupid
 
Posts: 389 | Location: des moines iowa | Registered: January 10, 2020Reply With QuoteReport This Post
DRR Pro
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quote:
Originally posted by Canted Valve:
quote:
...real world testing is the only way to know for sure.

At the risk of exposing my ignorance, how does one test for detonation with open headers and no knock sensor with accompanying software?


I listen (I have mufflers) and I inspect plug condition. Oddly enough, I have heard slight detonation in the past, but the plugs still came out looking fine. Take care. Tom Worthington


If it seems that bracket racing has gotten too expensive for you, maybe you are just doing it wrong.
 
Posts: 1224 | Location: Rocky Mount, NC | Registered: December 01, 1999Reply With QuoteReport This Post
DRR S/Pro
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97 is bare minimum, maybe! I'd run 104 or so to be safe. You didn't list the cam LSA either so it's a ballpark number.
 
Posts: 2136 | Location: Tewksbury, MA,USA | Registered: November 03, 2000Reply With QuoteReport This Post
DRR Sportsman
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If it’s a bracket only car with no street time why mix race gas with pump gas? Why not just run the race gas? What are you looking to accomplish or gain by mixing?
 
Posts: 484 | Location: Going to or returning from the chipmine. | Registered: July 01, 2011Reply With QuoteReport This Post



DRR S/Pro
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quote:
What are you looking to accomplish or gain by mixing?

I'm thinking 2$/gal vs 6$/gal. But I could be wrong


Illegitimi non carborundum
 
Posts: 2320 | Location: OKC, OK | Registered: February 15, 2008Reply With QuoteReport This Post
DRR Top Comp
Picture of wideopen231
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bracket car on gas uses very little,from what I hear.So why not just run race gas and not risk hurting something just to save few bucks.What does a new piston and set of rings cost if you detonate it to point of breaking something?




America home of free. Brought to you by 2nd amendment.
 
Posts: 4156 | Location: Greensboro NC | Registered: May 24, 2011Reply With QuoteReport This Post
DRR Trophy
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10.9-1 compression, flat top, aluminum head should be okay on 97 octane fuel with that cam. Cold air intake, and reasonable coolant temps, 160 - 170 degrees max would add a little insurance. worst case scenario you may have to cut 1 or 2 degrees of timing, and I bet you won't lose any ET.
 
Posts: 20 | Location: Kanada | Registered: August 08, 2004Reply With QuoteReport This Post
DRR Pro
Picture of Eman
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Any small airports nearby? Av Gas works great and it's in the $4.50 a gallon range. Plenty of octane and always good quality.
 
Posts: 1447 | Location: E TN | Registered: February 13, 2009Reply With QuoteReport This Post
DRR Pro
Picture of CURTIS REED
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Put methanol in it. Plenty of octane, just as cheap as what you are doing and will be quicker. Problem solved. LOL

Curtis



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Posts: 2903 | Location: KIEFER, OK. | Registered: August 17, 2007Reply With QuoteReport This Post
DRR Sportsman
Picture of McK_Racing
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How about running a cranking compression test? The point is to know how much cylinder pressure you have. Lots of variables like cam duration, LSA, combustion chamber, etc., can have an effect. A rule of thumb as it applies to street engines might help here. When using 93 octane fuel, 200 psi cranking compression should be the max target, with 190 psi as the lower limit. For every octane number less than 93, compression pressure needs to be about 5 psi less to avoid detonation.

Do a compression test to get your psi number, and you should be able to use that formula to figure close to the octane required.
 
Posts: 250 | Location: Northern Hemisphere | Registered: February 23, 2008Reply With QuoteReport This Post
DRR Top Comp
Picture of wideopen231
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quote:
Originally posted by CURTIS REED:
Put methanol in it. Plenty of octane, just as cheap as what you are doing and will be quicker. Problem solved. LOL

Curtis
^^^^ this.




America home of free. Brought to you by 2nd amendment.
 
Posts: 4156 | Location: Greensboro NC | Registered: May 24, 2011Reply With QuoteReport This Post



DRR Trophy
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quote:
How about running a cranking compression test? The point is to know how much cylinder pressure you have. Lots of variables like cam duration, LSA, combustion chamber, etc., can have an effect. A rule of thumb as it applies to street engines might help here. When using 93 octane fuel, 200 psi cranking compression should be the max target, with 190 psi as the lower limit. For every octane number less than 93, compression pressure needs to be about 5 psi less to avoid detonation.

Cranking was 180 with a cold engine.
 
Posts: 74 | Location: Mooresville, nc | Registered: October 11, 2013Reply With QuoteReport This Post
DRR Sportsman
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VERY similar combination as mine, but I'm at 427" (SBF). 10.72:1 actual. Cranks 185 psi cold. 93 octane, runs perfect, zero sign of detonation but you better be on top of the tune. It's done all the time on SBF stuff; particularly 408's. If I had to do it over again, I'd run it on methanol but this is a street driver that I actually drive sometimes so methanol is out. EFI too and that helps. With the timing locked at 34, you may find that with a hot engine at low speed, it might want to rattle-which is hard on the crank & block from experience). On a standard production 351w block, there is good chance to blow the main webbing right out of the block; that is their weak point. The crankshaft can break too but that's not nearly as common. On a 8.2 deck production block, the block itself has a tendency to bust down the lifter valley, basically split in half--and many times due to detonation and RPM (or combination of the two). Turbo/supercharged/nitrous guys are well aware of it. Pic is a 351w C9 block which is supposed to be stronger than any of the other 351w blocks, that engine made maybe 600hp but it was rattled one too many times.

 
Posts: 540 | Location: central Ar | Registered: June 21, 2002Reply With QuoteReport This Post
DRR Trophy
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I agree with checking cranking compression . At 185 you should be fine with 93 pump gas My motor is 11.96 to 1 and i mix 93 with 110 50/50 cranking at 205 PSI
quote:
Originally posted by McK_Racing:
How about running a cranking compression test? The point is to know how much cylinder pressure you have. Lots of variables like cam duration, LSA, combustion chamber, etc., can have an effect. A rule of thumb as it applies to street engines might help here. When using 93 octane fuel, 200 psi cranking compression should be the max target, with 190 psi as the lower limit. For every octane number less than 93, compression pressure needs to be about 5 psi less to avoid detonation.

Do a compression test to get your psi number, and you should be able to use that formula to figure close to the octane required.
 
Posts: 8 | Location: cheboygan | Registered: January 30, 2011Reply With QuoteReport This Post
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