340 Demon

-
MOPAR487

MOPAR487

Active Member
Joined
Dec 31, 2008
Messages
36
Reaction score
0
.got a built 340 in a 71 Demon, it has about 13.5 to 1 compression ratio, if I run 91 octane fuel in it, it seems to run hot (210 deg.) on a 60 degree day, Ive got a 160 degree high per. thermostat in it. Electric fan w? stock radiator. Will running that low octane fuel in this motor make it run hot? Thanks
 
if you truely have 13.5 compression and you are running 91 octane gas...the pistons lands will be destroyed by detonation.....

please post more info on the engine....???
 
MOPAR487 said:
.got a built 340 in a 71 Demon, it has about 13.5 to 1 compression ratio, if I run 91 octane fuel in it, it seems to run hot (210 deg.) on a 60 degree day, Ive got a 160 degree high per. thermostat in it. Electric fan w? stock radiator. Will running that low octane fuel in this motor make it run hot? Thanks
Click to expand...

You mention "if I run 91 octane..." What do you normally run? I don't see how you will be able to get away with 91 on 13.5 comp? The motor must be pinging like hell.:scratch:
 
I have 11.7 compression in my X headed 340 and can’t do anything more that putt around on 91. I don’t know how you can even run 91 in a 13.5 engine….


Like was said above…. The heat really won’t matter…. The pistons will be destroying themselves anyway….


FYI.... My normal mix is 50/50 of 92 and 110.
 
To answer the question, yes running too low of an octane fuel will make your engine run hot. As the others here have said, running hot is the least of your worries! You are running hot because of detonation or "pre-ignition". Too low of an octane number means that the fuel has less resistance to fire. This means that the mixture ignites well before the piston reaches it's optimum height in the cylinder. When this happens, the crank is pushing the piston up while the explosion is pushing the piston down. Enough of this, and your piston will break into little pieces.

The additional heat is because every cylinder is firing earlier and causing a lot more friction between the components. At 13.5:1 compression, you need 110 race fuel. Other wise, you'll soon be able to fit all of your pistons in the palm of your hand!
 
Yeah, that's pretty much a no go. I know a guy that does 93 octane with a 12.5:1 hemi but he's an engine builder and has some tricky cam that I don't really understand lol. I've got a 13:1 340 but I have no plans of anything other than 110 for that lol, your kinda pushing the limits of parts breakage with that 91
 
The only reason I used 91 octane was I was moving the car from my garage to storage for the winter. (couldnt find any racing fuel) Wife wanted the garage back for the winter. I normally run 100 octane in it. Was told it has that high comp. when I bought the car. Dont know for sure. Thanks for your help.
 
To run 91 octane in a high compresion engine like that you'll need to retard the timing like ...alot.

For the short term, it'll do just for moving the car around.
 
Even 100 is low for 13.5:1! Depending on the combination of course. You might want to check it, just to be sure what you have. Just my suggestion.
 
You must log in or register to reply here.
-

Back
Top