How high compression ratio are you getting away with on pump gas? Ive heard as high as 12:1 with aluminum heads. I know the chamber allows higher than some wedge stuff.

What pump gas? Non ethanol? 91 or 93? 11.1 with aluminum heads is about the max in the REAL world IMO.

whats your combination? Are you seeing detonation at 11?

No sir but mine is one of those 16 spark plug, flat top piston NOT A REAL Hemi.....Hemi's. It runs like a missle with 23* of advance. I shouldnt even be wasting your time by commenting lol! Are you building a new combo or wanting to make an existing combo work on pump gas? Is E85 an option?

New combination/new parts with old design gen 2 stuff. E85 is out for me, no place close by that sells it. 😟

I ran 11.6:1 in my 484 with no issues for years. I have a 605 hemi now, and I am right around 11.0-11.1:1 with no issues. The Hemi's are very resistant to having issues with detonation. I would be very comfortable running 12:1 on 93 and watching my timing and tune up. What we use around here is E10, so until they get rid of it, that's what I am running mine on.

I would not run any lower than 11:1 on pump gas with a hemi. If your building it, build it for 12:1 with .027 head gaskets, and switch to thicker ones if you have to.

Just finished and dyno'd my 528 with a true 10.5:1 and it almost made 840 hp and 671 TQ.

I'm guessing on pump gas. What timing and cylinder pressure?

Sorry didn't catch the Hemroid part.

I've built and dyno tested several different stock Mopar iron headed pump gas 426 type motors, 434 C.I., 3.91 stroke and 4.150 stroke motors for the same customer. All of them where right at 10.5 to 10.8 to 1 compression ratio and ran fine with CA pump swill back in the early 2000s I built the same customer a D5 alumimun headed 526 C.I. motor with a stock OEM Mopar cross ram with stock OEM Holley SS carbs., it struggle to make 50 HP more on the DTS engine dyno than the 4.150 stroke motor did with the new heavy Mopar iron replacement heads and a stock 1966 inline intake manifolds and stock 1966 AFB Carbs did That motor had a little over 12.5 to 1 compression, I probally should have treid it also on CA 91 octane pump swill

My 605 is 10.4-1, the reason I didn't go any higher is the best fuel locally is 87 octane. I have to drive 20-40 miles to buy 91. If you build a big bore hemi be very careful about using a head gasket .042 or thinner. My 528 had 4 pistons kiss the heads at zero deck and .042. I over revved it twice.Dave

I wouldn't go over 11.1 with the fuel in my area. Rated at the pump at 93 the rating is from when it left the refinery. A lot of the octane is achieved from the ethanol so as it ages I'm sure the octane drops. I run 10.4 with aluminum heads. I still mix in some race fuel when I race it. Unless you are trying to make max horsepower per cubic inch there are other ways to make horsepower than raising the compression.

Mine is 11:1.

I bought my 540 reciprocating assembly and cam from Ray Barton. He spec'd the CR @ 10.5 for this aluminum headed street/strip motor. I asked him about more comp since I have ran 11:1 in my aluminum headed 499 wedge for years with no problems. He said there wasn't enough additional power for a primarily street driven engine to warrant the extra hassles.

Care to share the combo?

Yes, I am very interested in your combo as well. I love to learn new stuff that works.

our pump swill here is 85 octane, so I kept my 572 at 9.5 to one, that way I know that I can run any gas anywhere. Tim

10.8:1 Iron block and heads 477 CID. No problems on pump gas. The cam plays a big part in how much compression you can get away.

any trouble with 12:1 on a hot day? That would be my concern.

10.4:1 on 87 is doing pretty good.

Dave, what would be the extra hassle he is reffering to? If you have to get pistons anyway, would cost much more to bump it some i wouldnt think

Never had an issue at 11.6 and I drove it a bunch in 90 temps.....You have to remember anything with aluminum heads that heats up loses compression as it gets to temp...The Hemi has a huge combustion chamber....And drops compression a little at temp. Its not really 12:1 at 185 degress anymore.

I think he was referring to issues with gasoline quality and detonation.

Mine is nothing special. Mopar Mega block, 4.500 bore x 4.150 stroke, Mopar Performance heads, with Barton valvetrain, .800 solid roller cam with 4/7 swap. Indy intake.

ive been at 12.2 to one with my 606 cu hemi runs on 93 octane no problems , but the big motors with good heads don't like much timming either that helps out

Yup 29-30 degrees is what have seen.

My 10.5 540 liked 30* on the dyno this week.

Mine liked 33 Degrees best. Lost some power at 32 or less & also lost some at 34+.

My 572 likes 34°. Atleast it did...I have not changed my base timing in 10 years, lol.

Thanks for all the info here guys.

We put nearly 15 dyno pulls on my buddy's all aluminum 572 Hemi and it liked 29 the best. Lost 20hp & 20 torque at 30 degrees....My 605 is 32-33 and I have not played with it. So far best 1/8th mile was 6.32 spinning....Lots more in it, but I am only legal to 6.50, so we left the timing where it was, and the mufflers on it.

Vid of one of the 572 dyno pulls
https://www.youtube.com/watch?v=5QjkTW-o1QY

My 6.36 pass against y buddies slower full race car....
https://www.youtube.com/watch?v=NVTE82X6ZQ8