Re: max comp ratio for 91/93 octane pump gas
[Re: rbkt65]
#2467965
03/18/18 02:25 AM
03/18/18 02:25 AM
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Joined: Aug 2003
Posts: 43,243 Bend,OR USA
Cab_Burge
I Win
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I Win
Joined: Aug 2003
Posts: 43,243
Bend,OR USA
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If you make sure every cylinder has between 10.5 up to 10.7 and you get the correct spark plug, ignition timing and air fuel mixture correct you should do well The next important question is which aluminum heads are you choosing? How much quench also? The compression ratio is only part of the end results along with all the other parts that affect the motors we run on the street today
Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
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Re: max comp ratio for 91/93 octane pump gas
[Re: Cab_Burge]
#2468108
03/18/18 02:26 PM
03/18/18 02:26 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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The compression ratio is only part of the end results along with all the other parts that affect the motors we run on the street today That would have been the best first reply to the OP, IMO.
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Re: max comp ratio for 91/93 octane pump gas
[Re: rbkt65]
#2468359
03/18/18 11:08 PM
03/18/18 11:08 PM
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Joined: Sep 2015
Posts: 1,096 Australia
ozymaxwedge
super stock
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super stock
Joined: Sep 2015
Posts: 1,096
Australia
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In Australia we have 91,95 and 98 at every pump. You guys dont have 98 ?
1963 Plymouth Max Wedge 1971 Barracuda
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Re: max comp ratio for 91/93 octane pump gas
[Re: rbkt65]
#2468365
03/18/18 11:18 PM
03/18/18 11:18 PM
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Joined: Jan 2003
Posts: 8,028 Tulsa OK
Bad340fish
master
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master
Joined: Jan 2003
Posts: 8,028
Tulsa OK
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I have run an aluminum headed, 360 with good quench at 11:1 with no problems, it did have a pretty big roller cam though. I have raced it on 91 and 93 with zero difference. I have also run it down the highway on 89 before with no trouble. Lots of variables but that has worked for me.
68 Barracuda Formula S 340
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Re: max comp ratio for 91/93 octane pump gas
[Re: rbkt65]
#2468483
03/19/18 09:50 AM
03/19/18 09:50 AM
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Joined: Jan 2003
Posts: 10,011 Frostbitefalls MN (Rocky&Bullw...
gregsdart
I Live Here
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I Live Here
Joined: Jan 2003
Posts: 10,011
Frostbitefalls MN (Rocky&Bullw...
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If you have already built a motor with too much compression, i understand water injection systems work great at eliminating detonation Check into that. Some heads really help reduce tendancys to detonate. Double quench chambers like magnum 5.9 heads need less timing, and can run well with more compression than other heads.
8..603 156 mph best, 2905 lbs 549, indy 572-13, alky
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Re: max comp ratio for 91/93 octane pump gas
[Re: rbkt65]
#2468568
03/19/18 01:37 PM
03/19/18 01:37 PM
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Joined: Apr 2012
Posts: 59 Florida
Sweet5ltr
member
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member
Joined: Apr 2012
Posts: 59
Florida
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Dynamic Compression Ratio - this will be a good indicator if you can run on pump fuel.
I run around 9.6:1 cr with Eddy aluminum heads and 32-34* total timing in my 470 low-deck. 259/267 @ .050 / 110 lsa camshaft.
No reason to go big on compression with pump gas, if I'm losing 3% +/- per point, i'm not going to risk detonation and pull timing as a band-aid.
In a street car, we all know the load is variable, if you've ever watched a wideband air/fuel gauge, you can see how quickly an engine can go from rich to lean, varying on throttle position, transitions, and multiple other factors; it's nearly impossible to tune a carb to be perfect in all conditions year-round. In a race car, the load is somewhat constant, simply tune for WOT - 12.5:1 +/- AFR. Make it fat, lean it out.
Detonation destroys engines, if it's not a bracket / race car, stay on the lesser end of the compression spectrum and run as much timing as needed.
Last edited by Sweet5ltr; 03/19/18 01:39 PM.
1969 Plymouth Road Runner (440 w/ Boost! RIP) now a low-deck 470 with hotchkis suspension, nascar boom tube exhaust, & big brakes.
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Re: max comp ratio for 91/93 octane pump gas
[Re: Sweet5ltr]
#2468628
03/19/18 04:01 PM
03/19/18 04:01 PM
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Joined: Jun 2008
Posts: 3,916 usa
lewtot184
master
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master
Joined: Jun 2008
Posts: 3,916
usa
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Dynamic Compression Ratio - this will be a good indicator if you can run on pump fuel.
I run around 9.6:1 cr with Eddy aluminum heads and 32-34* total timing in my 470 low-deck. 259/267 @ .050 / 110 lsa camshaft.
No reason to go big on compression with pump gas, if I'm losing 3% +/- per point, i'm not going to risk detonation and pull timing as a band-aid.
In a street car, we all know the load is variable, if you've ever watched a wideband air/fuel gauge, you can see how quickly an engine can go from rich to lean, varying on throttle position, transitions, and multiple other factors; it's nearly impossible to tune a carb to be perfect in all conditions year-round. In a race car, the load is somewhat constant, simply tune for WOT - 12.5:1 +/- AFR. Make it fat, lean it out.
Detonation destroys engines, if it's not a bracket / race car, stay on the lesser end of the compression spectrum and run as much timing as needed. makes sense approach.
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Re: max comp ratio for 91/93 octane pump gas
[Re: rbkt65]
#2468662
03/19/18 05:14 PM
03/19/18 05:14 PM
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Joined: Oct 2007
Posts: 4,209 New York
polyspheric
master
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master
Joined: Oct 2007
Posts: 4,209
New York
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High static CR increases: 1. torque, especially part throttle 2. mileage 3. vacuum
Boffin Emeritus
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Re: max comp ratio for 91/93 octane pump gas
[Re: Dave Hall]
#2468937
03/20/18 04:06 AM
03/20/18 04:06 AM
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Joined: May 2008
Posts: 5,399 Aurora, Colorado
451Mopar
master
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master
Joined: May 2008
Posts: 5,399
Aurora, Colorado
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You can buy 100 unleaded here at the pump. Just have to know where to go. What it cost there? I just paid a bit over $8/gallon for 100 unleaded here in Denver. There are a large number of variables that determine what octane fuel an engine will use. Tell us more info, what engine, what heads, what cam or application, what vehicle, transmission, stall speed, gear ratio, ect. Someone likely has already built something like you are asking about.
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