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max comp ratio for 91/93 octane pump gas #2467890
03/17/18 10:36 PM
03/17/18 10:36 PM
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rbkt65 Offline OP
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for a street motor, with alum heads. here in area we have a few stations with 91 octane. don't know if shell still has 93 or not. what would be the max compression ratio with eddy rpms or e street heads? thanks in advance.

Re: max comp ratio for 91/93 octane pump gas [Re: rbkt65] #2467893
03/17/18 10:41 PM
03/17/18 10:41 PM
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Trumbull,CT.
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jim sciortino Offline
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Basically impossibe to answer since valve events play such a critical role.

Re: max comp ratio for 91/93 octane pump gas [Re: jim sciortino] #2467903
03/17/18 11:07 PM
03/17/18 11:07 PM
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Cotati, CA
Dave Hall Offline
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At some point it will still detonate on the crap they call gas nowadays. I wouldn't go past 11.0-1. If I built it to where it was even questionable I would add 110 fuel to the mix. I actually add some 110 to my Dart that has no compression issue because it runs better. I use nothing but 110 in my yard two-stroke engines because they won't run on the crap from the pump.

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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Cab_Burge Offline
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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 up scope
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 shruggy work


Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
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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Originally Posted By Cab_Burge
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.

Re: max comp ratio for 91/93 octane pump gas [Re: rbkt65] #2468337
03/18/18 10:28 PM
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lewtot184 Offline
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when I built my pump gas 440 to drive I stayed a little conservative on static compression; 9.8:1. I used RPM heads and Icon 836 pistons. with the cam I have dynamic compression is 7:1. very pump gas friendly with 93 octane and 37 degrees total timing. makes more power than I need.

Re: max comp ratio for 91/93 octane pump gas [Re: rbkt65] #2468341
03/18/18 10:35 PM
03/18/18 10:35 PM
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AndyF Offline
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Probably should stay in the 9.5 range for a normal street car. Compression can be higher but it adds risk. Lightweight car with a lot of gear out back can go higher. Heavier car with highway gears might want to go lower than 9.5. Lots of variables.......

Re: max comp ratio for 91/93 octane pump gas [Re: rbkt65] #2468359
03/18/18 11:08 PM
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ozymaxwedge Offline
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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
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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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
87 "Chrysler" Conquest
Re: max comp ratio for 91/93 octane pump gas [Re: ozymaxwedge] #2468396
03/19/18 12:05 AM
03/19/18 12:05 AM
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98 in Aus is 93 in the US.

Re: max comp ratio for 91/93 octane pump gas [Re: rbkt65] #2468481
03/19/18 09:40 AM
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lewtot184 Offline
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if the car is to be driven carbon build up has to be considered. on the average .010" of carbon in a 440 with up the static compression ratio 1/2 point. .010" ain't much.

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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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
Re: max comp ratio for 91/93 octane pump gas [Re: rbkt65] #2468528
03/19/18 11:57 AM
03/19/18 11:57 AM
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rbkt65 Offline OP
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thanks everyone. now have more to think about.

Re: max comp ratio for 91/93 octane pump gas [Re: rbkt65] #2468535
03/19/18 12:13 PM
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SILVER67 Offline
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10.96 compression in my 505.

Flat tappet
260@50
268@60

93 octane Sunaco with 10% ethonal

Timing is at 32*-33*

Bracket race it weekly
Doing just fine

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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Sweet5ltr Offline
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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.
Re: max comp ratio for 91/93 octane pump gas [Re: Sweet5ltr] #2468628
03/19/18 04:01 PM
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lewtot184 Offline
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Originally Posted By Sweet5ltr
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.

Re: max comp ratio for 91/93 octane pump gas [Re: rbkt65] #2468662
03/19/18 05:14 PM
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polyspheric Offline
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High static CR increases:
1. torque, especially part throttle
2. mileage
3. vacuum


Boffin Emeritus
Re: max comp ratio for 91/93 octane pump gas [Re: rbkt65] #2468680
03/19/18 06:00 PM
03/19/18 06:00 PM
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jwb123 Offline
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static compression has little to do with octane needed for operation, the dynamic compression is what determines the octane level, several factors involved in dynamic compression, main one is camshaft timing events. I use an engine simulation program to determine dynamic compression. Performance trends will let you change octane, calculate dynamic compression, and calculate a timing curve to keep the engine from detonation. I have built some pulling truck engines that are required to run pump gas, and used a 13 to 1 static compression piston.

Re: max comp ratio for 91/93 octane pump gas [Re: rbkt65] #2468935
03/20/18 03:53 AM
03/20/18 03:53 AM
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Cotati, CA
Dave Hall Offline
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You can buy 100 unleaded here at the pump. Just have to know where to go.

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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451Mopar Offline
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Originally Posted By Dave Hall
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.

Re: max comp ratio for 91/93 octane pump gas [Re: 451Mopar] #2468944
03/20/18 05:35 AM
03/20/18 05:35 AM
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Cotati, CA
Dave Hall Offline
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I'm not sure how much it costs. Less than $9.99 a gallon because that's as high as the pump goes. I buy Sunoco 116 at the same pumps for $9.99 a gallon. I don't buy the unleaded.

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