What Octane Level should I need for 10.5 to 1 Compression?

[gtx6970]

What is your cranking cylinder pressure?

Been a while since I ckd it. But Im thinking was in the 210-215 range
Its been almost 4 years since I ckd it. But I remember they were all over 200 for sure
My cam card is attached. Its real mild , car will idle at 900 with no issues
Its a solid flat tappet setup.
Just went thru the valves a few months ago.

The engine was built over 20 years ago now. And for pistons it had/has the old MP 12.5s in it. Builder cut them down to lower make it more street friendly and best he remembers its in the 11.2-11.4 range.
Carbs are box stock and it still retains the OE stock dual point prestolite dist / ignition system

 

[BSB67]

Thanks.

I don’t think I’ve ever seen an iron headed Mopar BB run above 170 psi with 93 pump gas without ping.

You’ve got some secret stuff going on with 210 psi with an iron head.

 

[cosgig]

I run a 50/50 blend of 110 racing gas and 90 octane “rec” fuel with no ethanol. My car has never seen ethanol gas so I like to keep it that way, I figure I yield about 100 octane that way, and my car runs great on it. If I slouch and it’s gets a little more rec fuel than 110, it’s starts to run like crap. It’s a 6 bbl car, so they can be finicky. I don’t really care what the cost is, I run it because it runs good, and it smells good too!

 

[Detective D]

 

[SlinktRR]

Like this thread what have you found to be a safe DCR with 91 or 93?, I'm refreshing a 451 that presently has TF240 and want to detune it for better streetability with Iron heads it's going to be a cruising car with 3.23 and going back to manifolds and it will come out to 10.3 Static and 8.8 DCR with the iron heads and I'm a bit nervous about that. Was contemplating 440 Source to make it more Detonation resistant TIA

Why on Earth would you replace trick flow heads with iron or source? Just change the cam.

 

[Hemirunner]

Like this thread what have you found to be a safe DCR with 91 or 93?, I'm refreshing a 451 that presently has TF240 and want to detune it for better streetability with Iron heads it's going to be a cruising car with 3.23 and going back to manifolds and it will come out to 10.3 Static and 8.8 DCR with the iron heads and I'm a bit nervous about that. Was contemplating 440 Source to make it more Detonation resistant TIA

It’s definitely not that simple where a certain DCR will work in all scenarios. Gear ratio and converter will greatly affect engine load. You have to look at each combo individually. Why would you go away from the TF240 heads? Just looking for something with a larger chamber to lower the static CR? What are the cam specs?

 

[m79ded]

Why on Earth would you replace trick flow heads with iron or source? Just change the cam.

Wanted to tone down the car and go back to Manifolds, tired of headers and figured too much head for manifolds. I might be wrong

 

[33 IMP]

What pistons does the motor have now? I am gonna assume a small dome, otherwise you won't get 10.3 with a 90 cc iron head.
If they are 2295 trws like mine, they are down in the hole .050 or so, and will not really have quench, even with a closed chamber head.
So..... invest in a pair of big thick head gaskets, .090 or .120 thick, drop the compression at least a whole point , and put together what you want. No quench to lose but you would have a much better shot at getting it to run on pump gas.

 

[BSB67]

I think you would be fine with the TF heads too. No real down side to try.

My heads probably flow similar to the 240TFs and use manifolds and 2 1/2” exhaust. I do have more cu.in. and cam.

 

[m79ded]

What pistons does the motor have now? I am gonna assume a small dome, otherwise you won't get 10.3 with a 90 cc iron head.
If they are 2295 trws like mine, they are down in the hole .050 or so, and will not really have quench, even with a closed chamber head.
So..... invest in a pair of big thick head gaskets, .090 or .120 thick, drop the compression at least a whole point , and put together what you want. No quench to lose but you would have a much better shot at getting it to run on pump gas.

It's a 400 block with 1.32 Ross pistons 99495 .010 Below deck 3.75 crank. Thanks for the advice

 

[m79ded]

I think you would be fine with the TF heads too. No real down side to try.

My heads probably flow similar to the 240TFs and use manifolds and 2 1/2” exhaust. I do have more cu.in. and cam.

You have a great point run it and give it a try. Thanks

 

[69hemibeep]

Hemi is a pretty friendly combustion chamber. My bone stock hemi has no issues with the 91 octane swill we have in Maricopa County AZ. My 440 needs octane booster at 9.8 to 1

 

[dart4forte]

It all depends on how much timing you run on the motor. I’ve run mid grade with 10 to 1 with no problem. Once I found the timing sweet spot I could run around with no detonation. However if you are one that constantly opens it up on the street then you’ll have to evaluate the timing.

 

[AR67GTX]

My 66 Hemi is completely stock as far as I know. I’ve scoped the chambers and looks like stock pistons. I’ve tuned my distributor to allow 16/17 initial advance and 33\34 total and it has run fine on 93 ethanol. Currently I’m running 91 non-ethanol and can’t tell of any issues. Remember 93 octane today is the octane equivalent of 98 octane back in the 60s and early 70s.

I’m usually a fan of running full manifold vacuum advance vs ported on engines to help them idle stronger and run cooler. However this is one engine that didn’t seem to really be affected much one way or the other. I had some difficulty getting the stock idle air-bypass Carters to idle down properly with manifold advance do I’ve left it plumbed to the Carter ported nipple and I use the manifold connection for a vacuum gauge when tuning.