Can you run 12.5:1 compression on pump gas?

agent007kimball · Jun 6, 2004

I am looking at a 331 for sale and it will make about 12.5:1 compression. I am EFI and want to be able to run this motor on pump gas so will it be possible or will I run into some issues?

twidge · Jun 6, 2004

If you don't go WOT it'll run. If you want to drive it hard I can see that being a problem.

agent007kimball · Jun 6, 2004

So it wont run well on pump gas?...say 94 octane....will it just start knocking?...can it be tuned to run well on pump gas or is it not worth it?

agent007kimball · Jun 6, 2004

Actually it looks like its more like 12.1 - 12.3 compression....will that make it any more feasible?

giddyup306 · Jun 7, 2004

agent007kimball said:
Actually it looks like its more like 12.1 - 12.3 compression....will that make it any more feasible?

That's still pretty high for your application. There are cars that go to 13:1 stock (eg the Prius) but the computer can also compensate for detonation and retard timing. IMHO if it is a weekend only/ drag car go for it. If it is more of a daily driver I'd say go with around 10.5:1. I went with 11.5:1 on my weekend car and always have to mix fuel.

Mike

TT91 · Jun 7, 2004

run some 116 and youll be fine...or get the chambers on the heads machined to lower the comperssion ratio

perkys stang · Jun 7, 2004

Yeah I wouldn't want over 11:1 with 94... I have seen some guys try higher compression and the cars won't even start on pump gas.....

5.0guy · Jun 7, 2004

do they sell 104 octane at pumps near u?(prolly wouldn't mix it, just run that straight. I know they have 104 unleaded) that should suffice. maybe 110.

perkys stang · Jun 7, 2004

5.0guy said:
do they sell 104 octane at pumps near u?(prolly wouldn't mix it, just run that straight. I know they have 104 unleaded) that should suffice. maybe 110.

Is it realy worth the 4 to 6 bucksa gallon for a point and half compression? Somehting to think about.. How often will you be driving it..?

5.0 HOtrain · Jun 7, 2004

perkys stang said:
Yeah I wouldn't want over 11:1 with 94... I have seen some guys try higher compression and the cars won't even start on pump gas.....

My 418 is 11.25:1, runs good so far on 93 pump gas, 29* total timing.
mike.

wildstanggt · Jun 7, 2004

I have been wondering a similiar question. It sounds like 11:1 is the max for pump gas. I am looking at buying a CHP dynotec longlock 347 with 11:1 compression that makes 500 flywheel horsepower and 460 lb of torque. Can I run this on 93 octane? should I mix with 116 race gas? or should I just buy some 104 octane booster and drop that in everytime I fill up?
thanks

wildstanggt · Jun 7, 2004

could you run a 125 shot of nitrous on top of the 11:1 compression motor? would you need race gas for sure then?

ninjacoupe · Jun 7, 2004

Had 11:1 on pump gas with no issues if built for it. Cam and chamber have alot to do with it. 12:1 is really pushing it. Reverse cooling (cooling the heads then the block) would probably let you get away with it but would require more work than its worth.

stangin it · Jun 7, 2004

12:5:1 will be too much for pump gas if your going to have a some what aggressive tune and drive it daily. Personally i wouldnt spray on an 11:1 motor without a little 104 octane just to keep things safe. Just my opinion im sure people here have done both.

QDRHRSE · Jun 7, 2004

No way. Not unless the pump you go to says "trick" on it. You can get away with 11:1 with a big block but not a small block. 10:5 is really pushing it, in the summer especially. Any time the motor sees any combination of heat and load it will detonate.

perkys stang · Jun 8, 2004

5.0 HOtrain said:
My 418 is 11.25:1, runs good so far on 93 pump gas, 29* total timing.
mike.

A little over the 11:1 and you can do it with less timing but then you are defeating the purpose of the compression.. Ie less timing less power.. I just think I would rather have 11:1 and run 36 deg.. and let it scream.

Rick 91GT · Jun 8, 2004

perkys stang said:
A little over the 11:1 and you can do it with less timing but then you are defeating the purpose of the compression.. Ie less timing less power.. I just think I would rather have 11:1 and run 36 deg.. and let it scream.

Bill is right 100%, and machining the chambers to lower the compression isn't always the best thing for flame travel, quench, etc...

Since you have O2's you would need high octane unleaded gas which is $4-$6/gal

BillaMustang · Jun 8, 2004

12.5 is just too much. the cam and head type have a big effect on it. it can be done but a lot of precision measuring and machining must be performed.
i would not go over 11.0. even then it requires a lot of precision machining and measuring.
when going to high compression the quench ht. becomes very important.

beaver2489 · Jun 8, 2004

my daily driver has 10.7:1 on 91 or 92 octane @14* timing. no problems. So, I'd say 11:1 would be as high as I'd go on pump gas

5.0 HOtrain · Jun 8, 2004

perkys stang said:
A little over the 11:1 and you can do it with less timing but then you are defeating the purpose of the compression.. Ie less timing less power..

Well, it did only make 581

Going to check you're site Bill, instead of asking how the car is.
mike.

Can you run 12.5:1 compression on pump gas?

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