At what point do I need to go to a higher octane? I have a 9.5:1 ratio on my current engine, but I'm runnning propane, which is 108 octane.

Planning on building another petrol engine....

Thanks,
Joe

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Joe Hoppe

71 D100 225 Super Six 727 AT on 2008 Crown Vic CopCar frame

64 Valiant Wagon 225 904 AT 3:23 8.75"

An 8 to 1 DCR (Dynamic Compression Ratio) is about the max for 87 octane pump gas.
DD

THanks, Doc. Is that about what stock is?

Joe

_________________
Joe Hoppe

71 D100 225 Super Six 727 AT on 2008 Crown Vic CopCar frame

64 Valiant Wagon 225 904 AT 3:23 8.75"

I've gotten away with a bit more at 8.23:1 DCR but had to yard the timing back which makes for a gain and a loss (kept the mileage, but I'm sure I left some power out of it...), that engine was 10:1 SCR...

DCR is a more accurate way to check how your engine is going to possibly perform, there are several calculators online to help. SCR
is what is typically is used (stock mill is advertised in the 8.2:1+/- range, but can be anything depending on the year and vehicle).

-D.Idiot

The DCR from the factory is usually in the 7.5 to 7.8 range.
DD

So a thinner headgasket would probably be the safest and farthest you'd want to go as far as compression mods if you wanted to keep using regular?

Joe

_________________
Joe Hoppe

71 D100 225 Super Six 727 AT on 2008 Crown Vic CopCar frame

64 Valiant Wagon 225 904 AT 3:23 8.75"

Measure, calculate and set your static & DCR compression ratios... or ...
Make your "best guess" and take your chances.

The "best guess" with a factory SL6, give it more compression.
Using a stamped steel shim head gasket helps but not a lot... and you need really clean & flat surfaces to get those to seal well.

My best "guess"... mill the head to get a 50 cc chamber size and use a Fel-Pro gasket.
DD

Thanks again, Doc.

Joe

_________________
Joe Hoppe

71 D100 225 Super Six 727 AT on 2008 Crown Vic CopCar frame

64 Valiant Wagon 225 904 AT 3:23 8.75"