tcuda499

no way, in a car you can run 11:1 with aluminm heads on 94. 12:1 in a boat I agree is high but 104 octane should do it. Turbo Blue baby

mike tkach

iTrader: (3)

NO,BUT YOUR ENGINE WILL,try some flex fuel.

FIXX

iTrader: (2)

JET FUEL????? jet fuel is JP-5 which is kinda like kerocene and has alot of additives in it to keep it from freezing in high altitudes..

i hope you were refering av gas than that will work for a little whyle until all the chemicles does a number on all the internals..if your thinking E85 which will also work but you need to have a special carberator set up for e85 (alchol) which also eats regular & rubber fuel lines..

CafeRacerPTY

yup, aviation gasoline, my bad. im just new to all this 12:1 compression issue, kind of bummed really, and will probably use 95 with some kind of octane boost?

FIXX

iTrader: (2)

are you sure its 12:1? what pistons are you using.head cc's.shave one point of compression with the aluminum heads, also don't go above 32degree advanced on the timing or try 30* and see how it runs, they do it with blower motors and all blowers do is raise the compression by stuffing the engine full of air...octane booster is a banded if you have high compression..you may want to install a knock sensor gauge just to monitor the engine for detonation also..

post a good pic of your pistons,,would be helpful..

CafeRacerPTY

timing is 32, specs of the pistones are the following:

Bore (in) 4.530 in.
Bore (mm) 115.062mm
Piston Style Dome, with single valve relief
Piston Material Forged aluminum
Compression Distance (in) 1.395 in.
Piston Head Volume (cc) -21.00cc
Wrist Pin Style Press-fit or floating
Pin Diameter (in) 0.990 in.

would lowering the timming help?

ezstriper

are you kidding ?? av gas has the wrong additives for auto or marine use, can use blended but bottom line is comp WAY to high....you can retard the timing to the point it wont run worth a crap, you are wasting time and $$$

stevesxm

100 octane LL av gas works well in 10:1 motors as long as you jet for it. at the comp numbers he is talking about about it needs a good 103 or 105 "race fuel" and while retarding the timing some may help a bit, the more you retard the timing the higher the chamber temps get so ultimately you end up in the same place anyway. you could do that if you got stuck w/ bad gas and wanter to get home at low revs but there is no way that that is a viable long term solution.

the solution is to fix the motor mechanically or by fuel by the barrel... or pistons by the case.

rmbuilder

Cafe,

This calc will bring you +/- .1 pt for a given varience in deck height, chamber volume, and gasket volume. You should be able to radius the crowns to bring you closer to 10:1 if necessary. Try to avoid the thcker gasket approach to further reduce the CR.

Bob

bcfountain

big diffrence in a boat vs a car.the boat motor runs under 100% load all the time,unlike a car.i say use the highest octane you can find.better to be safe than sorry.after all,gas is the cheapest thing about boating.