compression ratios

[fryedaddy]

about where is the cut off for using pump gas vs race fuel or a mix of race fuel.reason im asking is i have around 11-11.2 or 3 to 1 not really sure i dont have my build sheet,it got lost. while working on my engine my uncle got sick and passed before finishing it,and we never found them. i can feel the difference in the gas pedal when i add race fuel.it just feels smoother,it even feels like it dont labor as bad under strain.

[Stangman]

I had 11.8 to 1 on my 427/454 and never pinged on 93 octane. I had a lunati voodoo race cam the smallest one in the solid catagory. It must not have filled the cylinders at low speeds and therefore it didnt ping. Now I was young and sort of lucked out. If you are going to run anything over 11 to 1 I would ask someone on hear that could figure out what you need to not ping. I also think gas was better 20 years ago.

[justintyme73]

Well its going depend on a few things.
Aluminum or Iron heads first off
Cam specs
Car weight
Gearing
Ignition timing and curve
Altitude
Those are a few of the Big ones, Smarter people will be along to chime in but if you can give them some of that info it will help them

[plovett]

While I acknowledge its limitations, I like to use a dynamic compression calculator.  A DCR of 8.25:1 or maybe 8.5:1 at max will generally work with high octane pump gas.  The other variables Justintyme 73 mentioned are very important and should be considered, as well. 

Anyway this may help.  The download is at the bottom of the page in the link.  Use advertised duration figures, not 0.050" duration.

http://members.uia.net/pkelley2/DynamicCR.html

paulie

edit:  I have 10.5:1 compression with a cam of 279/287 advertised duration installed on a 105 ICL and it works fine with 92-93 octane pump gas.  ~3400 lb car with 4.11 gears and 3600 rpm stall.  It has aluminum heads, but I am not convinced that makes a huge difference.

[e philpott]

VP Racing makes a additive called Madditive that's a game changer , my brother runs 12.5 to 1 on pump gas and Madditive 6.70 at 99.9 mph in the 1/8th in a SBF Maverick 8.5 x 26 tires and slapper bars  , one bottle will treat 20 gallons or more depending on your needs

[Falcon67]

Based on what I've seen pump fuel do here, I would never run my 11.8:1 393C on pump fuel.  VP 110 only.  In fact, the 10:1 iron head 351C runs only VP.  Running 91 puts pepper on the plugs.  That's with a decently big hydro roller.  I have not run any DCR calcs on the combo - plugs tell me what I need to know.

[fryedaddy]

Well its going depend on a few things.
Aluminum or Iron heads first off
Cam specs
Car weight
Gearing
Ignition timing and curve
Altitude
Those are a few of the Big ones, Smarter people will be along to chime in but if you can give them some of that info it will help them

alu bbm heads,560-292 with .244 at .050,3200 weight,4.11 gears,msd pro billet-15 initial- 18 adv-total33 all in by 3000.east tn ? altitude

[TomP]

I would think you are borderline safe on the 93 octane stuff with those specs. Heavier car or smaller cam may need better fuel.

Certainly every combo is different a 9.5 to 1 BB Chev with domes can take the same octane as a 12 to 1 flattop FE. Driving style too, you can get away with regular gas if you know what not to do.