max comp ratio for 91/93 octane pump gas

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.

Basically impossibe to answer since valve events play such a critical role.

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.

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

That would have been the best first reply to the OP, IMO.

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.

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.......

In Australia we have 91,95 and 98 at every pump. You guys dont have 98 ?

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.

98 in Aus is 93 in the US.

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.

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.

thanks everyone. now have more to think about.

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

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.

High static CR increases:
1. torque, especially part throttle
2. mileage
3. vacuum

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.

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.

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.