340 and octane

[aaronk785]

I have a 340 in my dart. It has home ported speedmaster heads with 2316 pistons with 9.86 comp. and .035 quinch. Cam is hughes solid 237 and 242 at .050 and .537-.543 lift on a 107. Induction is eddy street tunnel ram and 2 69 340 avs carbs. Anyway, I always run 91 octane but cant buy it in my home town so sometimes I run 87 and baby it to the next town to buy fuel. So I had 87 in it and i never get on it because of fear of detonation. Well I forgot this mourning and hammered on it. Car felt way stronger with no audible detonation. Timing is at 34 total. Do you think its safe to run 87 and should i check plugs for detonation. Does this sound right or am i missing something.

 

[ChargedDart75]

I would just double check the plugs to verify no detonation, but it sounds like it might be happy with it. If you're still worried about it, try 89 and see how it does.

 

[yellow rose]

I have a 340 in my dart. It has home ported speedmaster heads with 2316 pistons with 9.86 comp. and .035 quinch. Cam is hughes solid 237 and 242 at .050 and .537-.543 lift on a 107. Induction is eddy street tunnel ram and 2 69 340 avs carbs. Anyway, I always run 91 octane but cant buy it in my home town so sometimes I run 87 and baby it to the next town to buy fuel. So I had 87 in it and i never get on it because of fear of detonation. Well I forgot this mourning and hammered on it. Car felt way stronger with no audible detonation. Timing is at 34 total. Do you think its safe to run 87 and should i check plugs for detonation. Does this sound right or am i missing something.

Keep the coolant temp in check and that CR will run on the lowest grade you can get.

 

[K.O. SWINGER]

I also run a 340 with the same heads with a little more camshaft. you're might be fine but you're probably walking The Razor's Edge. that's a big Maybe and a chance I would not be willing to take. I would invest in some good octane boost and have it on hand mixing your fuel up to at least 92. Just because you got away with it one time doesn't guarantee that you always will. Lug the motor down or get your timing wrong and pay a severe price.

 

[crewchief]

I run 91 Premium Non-Ethanol in my '70 340, timing is also at 34 w/mild cam. I don't trust anything lower than 91.

 

[aaronk785]

Temp never gets over 190. I'll check plugs and still run 91 but wont be scared of 87 either or mixing it. Motor just seems stronger on 87. I tryed to get it to ping and couldn't. Loaded it as hard as i could and no knock. I know you cant always hear detonation so I'll check plugs for sure.

 

[yellow rose]

Temp never gets over 190. I'll check plugs and still run 91 but wont be scared of 87 either or mixing it. Motor just seems stronger on 87. I tryed to get it to ping and couldn't. Loaded it as hard as i could and no knock. I know you cant always hear detonation so I'll check plugs for sure.

Right. You don’t have enough compression to run more than 87 octane. I forgot to mention you can’t have the heat range of the plug too hot either, or it would rattle.

Use the 87, save the cash the send it.

 

[Sprayin73]

With 237 duration at .050 intake you should be somewhere around 10.75:1 to 11.0:1 SC, (static compression), to make good use of that cam. So your just about a point low. Something around 216-220 at .050 would wake up your combo with that compression ratio. Over duration is actually lowering your compression ratio, DCR (Dynamic compression ratio) so what your seeing and the other members have stated is correct, 87 at the most 89 octane would be a better fit. 91-93 your loosing a lot of efficiency. Efficiency equals power and in your case torque. Your going to have to have to tune your timing to get the most out of 87 if you have set up for 91. You won’t be off much, if any, but test and tune anyway.