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

[Slap Stick]

Ozzy, look for any of the Indian Casino's in your area. A friend of mine hauls fuel from Vegas that only goes to the Casino's. Its straight gas plus 10% ethanol. It does not contain any of the CARB additives that wreak havoc on our old cars. His company goes from San Diego county, his and my old stomping grounds, to the north end of the state hauling fuel to the Casino's.

The ethanol is what wreaks havoc on engines, tanks, pumos, etc.

 

[ckessel]

Yup. Still not good but better than whats at most stations.

 

[INTMD8]

A lot of variables here. First, a wedge and a hemi are two different animals when it comes to detonation resistance. Second, intake valve closing point (which varies greatly depending on camshaft design) will affect the dynamic compression ratio and that has a huge impact on what octane fuel the engine will tolerate. An engine can safely run 93 octane at 12:1 CR if the combination is designed to do so. Or it might detonate on 93 at 10:1 CR if the DCR is too high. At this point, you really need to know a lot more about that particular engine before you can make that fuel determination.

Agreed, would be nice to know more details of the engine.

Not that it's the end all but would be curious what a cranking compression test reveals.

Feel like most don't even know their true static CR even if they were the one that built it.

 

[Hemirunner]

Agreed, would be nice to know more details of the engine.

Not that it's the end all but would be curious what a cranking compression test reveals.

Feel like most don't even know their true static CR even if they were the one that built it.

Especially critical in a hemi with a 170cc chamber.

 

[ykf7b0]

I studied the dynamic and static ratios then pushed it to the max ratio that I could get away and still be able to fill up at the pump. Big block Mopar’s need lots of compression to really run hard.

 

[33 IMP]

Agreed, would be nice to know more details of the engine.

Not that it's the end all but would be curious what a cranking compression test reveals.

Feel like most don't even know their true static CR even if they were the one that built it.

Especially critical in a hemi with a 170cc chamber.

Agree. I have been lax on checking the last three engines I have put together. I have a pretty good idea, but I don't know, exactly. I have learned my lesson, and I have the tools to know exactly, (as I always did), next time I will know for sure.

 

[m79ded]

A lot of variables here. First, a wedge and a hemi are two different animals when it comes to detonation resistance. Second, intake valve closing point (which varies greatly depending on camshaft design) will affect the dynamic compression ratio and that has a huge impact on what octane fuel the engine will tolerate. An engine can safely run 93 octane at 12:1 CR if the combination is designed to do so. Or it might detonate on 93 at 10:1 CR if the DCR is too high. At this point, you really need to know a lot more about that particular engine before you can make that fuel determination.

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

 

[BSB67]

Like this thread what have you found to be a safe DCR with 91 or 93?, I'm refreshing a 451 with Iron heads cruising car with 3.23 and going back to manifolds and it comes out to 10.3 Static and 8.8 DCR and I'm a bit nervous about that. Was contemplating 440 Source to make it more Detonation resistant TIA

I think you’ll be in trouble.

Give the cam type and specs.

 

[m79ded]

I think you’ll be in trouble.

Give the cam type and specs.

Right now SFT 242-246 @ 111 w 4adv and looking into 224-234 or 228-236 112 4adv HYDR

 

[6PKRTSE]

Especially critical in a hemi with a 170cc chamber.

Agreed. My aluminum headed Hemi is a 10.5:1 pump gas Street/Strip car. I can drive it anywhere on pump 93 with no issues. It makes more than twice the horsepower as my other big blocks cars. My Charger and Roached Runner are both 10.75:1 iron headed big blocks and occasionally will spark knock under a mild load on 93 depending on which station I purchased fuel at.

 

[BSB67]

Are you okay now with the SFT cam? What is your lash setting? What was your cranking cylinder pressure?, and what’s your elevation?

 

[m79ded]

Are you okay now with the SFT cam? What is your lash setting? What was your cranking cylinder pressure?, and what’s your elevation?

190 psi tight lash .020 and I like SFT a bit weak on vaccum for brakes, I wanted to tone it down a bit for easier driving around and I so much dispise the heat from headers. Car runs well 11:60's in a big b body but not too focused on that anymore, got a Duster for the track

 

[BSB67]

And you are running pump gas now with 190 psi on the gauge with success?

I don’t think I can help you, and retract my previous statement. I have never been able to run 190 psi with iron heads with 93 pump gas.

 

[m79ded]

And you are running pump gas now with 190 psi on the gauge with success?

I don’t think I can help you, and retract my previous statement. I have never been able to run 190 psi with iron heads with 93 pump gas.

I am so sorry for confusion I'm a DUMMY for that one, on my original post I forgot to write that I'm intending to put Iron heads to detune engine, It presently has Aluminum heads and wanted to put back Iron with manifolds. I feel like such a dufus forgetting to mention that

 

[BSB67]

Actually, You said that in your first post - I missed it.

I tried to run a 440 iron head w/ manifolds, 3.23, 222 degree cam on a 112 LSA in at +4, 10.3:1 CR at 800 ft elevation, on 93 octane and it was not even close.

 

[BSB67]

And, I’m not a fan of DCR calculators. There are three different versions out there ( or there was), and when used compared to my real world results, not great correlation.

 

[gtx6970]

Mine is approx 11.2 compression or so. Iron block and original steel heads , stock manifolds / exhaust
And I can run straight 91 octane in it. 93 and / or non ethanol gas is better. But neither are convenient to get to.
Getting AV gas is out of the question for me.

BUT, I have timing backed off a bit to get away with it though.
if I put timing up where it really wants it and runs best . I HAVE to mix it with 100-110 octane.

My base timing is currently set at 14-15 , but it likes it better at about 19-20 base ( dont remember total timing. It was dialed back almost 4 years ago now when I bought the car )

Im in southern Az and drive the car year round. And on real hot days if I get stuck in heavy or stop and go traffic it warms up to about 200-210 temp range. When that happens it will rattle on accel . So I just drive it a little easier until it cools back down.

But for the most part it stays right around thermostat temps in the 180-190 range

That said, is it as fast as it could / should be? No. But I dont care.
I dont race it. Hell, outside of an occasional romp on it just for fun and blow it out once in a while. I hardly beat on it ( damn expensive to fix if I break it )

 

[BSB67]

Mine is approx 11.2 compression or so. Iron block and original steel heads , stock manifolds / exhaust
And I can run straight 91 octane in it. 93 and / or non ethanol gas is better. But neither are convenient to get to

BUT, I have timing backed off a bit to get away with it to.
if I put timing up where it really wants it and runs best . I HAVE to mix it with 100-110 octane. Getting AV gas is out of the question for me
My base timing is currently set at 14-15 , but it likes it better at about 19-20 base ( dont remember total timing. It was dialed back almost 4 years ago now when I bought the car )

Im in southern Az and drive the car year round. And on real hot days if I get stuck in heavy or stop and go traffic it warms up to about 200-210 temp range. When that happens it will rattle on accel . So I just drive it a little easier until it cools back down.

But for the most part it stays right around thermostat temps in the 180-190 range

View attachment 1699942

What is your cranking cylinder pressure?

 

[Cranky]

Don't remember my cranking pressure (but I know it wasn't 190_ but the static compression came out to 10.3 with a fairly mild cam in a 340 in a 71 Cuda with stock but mildly worked heads. It would ping on hot summer days on 93 unless I had at least a gallon of av gas in it. In the winter time, no problem with 93 but once the temp went above 185 or so, it would ping but not bad but any pinging is bad. I've driven cars that would ping on mild acceleration and that's not even good. I also ran my timing a bit up there with both initial and advanced numbers. It just ran so much better with the initial and total being higher.

 

[33 IMP]

Is that 10.3 with the IRON heads? What pistons does it have in it? With a mild cam and gears, you will need to run better than 91 for sure, and probably better than 93. Do you have access to av-gas?
I'm running a 10.5 to 1 440, and it will run on pump 91...... but its got a 250+° pupleshaft, 3.91 gear, and a 4000 converter, and I still add av-gas cause I can.