I need to read more about Hemi builds, but now I'm wondering,
since there is no squish area, do you have to run less
compression ratio than a 572 wedge ? For pump gas that is.

Or if you have the compression on the Hemi, do you also need
more cam duration than the wedge, to bleed off some pressure ?

As an aside, I think that in general, bigger bores are more
detonation sensitive than smaller bores, and a 572 Hemi has a
big bore as well as no squish.

I don't overthink that stuff. Normal line-of-thought is 9.5 for iron heads and 10.5 for aluminum heads. And all that quench crap? Well, Hemis and Mountain Motors don't have that and they run pretty good.

Just was wondering what exactly is your application for this motor. This 572 Hemi you using Iron or Aluminum block and what heads.

Hemicar, still collecting parts. Have an old KB alum block,
will eventually get Stage V heads. 1970 Chall 4-speed / Dana
3.54 is the goal. Drive it everywhere hopefully.

With a 4.5" bore there is a "quench ring" around the outside edge of the Piston/head that hangs over the bore.

We run as high as 12:1 on pump gas in a Hemi...NO issues, and I am not alone in this. many others have as well.

A friend has a 572 that was built by FHO. It made just under 700 HP on the dyno. It is a pump gas 9.5 to one. I looked at the build and there is plenty left to make a lot more HP from this motor. He built it to put the motor in a 1968 Charger and drive it any place like hours of driving. It is a World Aluminum Block and Stage V Heads. The cam lift is very small and I talked to Tim of FHO about the lift and it was the right choice for this motor and what my friend wanted to do with the motor. I would have to look at the specs of the cam on the card to give you any information on the cam, do not remember what exactly they are since I have not look at the specs of the build for years. Since you live were you have to drive to get any place this might be the kind of build that might suit you. 700 HP seems plenty for a driver.

One of the reasons Chrysler went to the Hemi in the 50's was because it could run a higher compression ratio than other engines with the same octane fuel. A lot of motorcycle engines run the Hemi chamber so they can run higher compression ratios as well.

W/r/t "it could run a higher compression ratio than other engines with the same octane fuel"
Source of this is?

Late IVC to "bleed off some compression" is very effective... until about 2,000 RPM. The engine has its full static CR (and more if VE > 100%) during the entire torque range.

I built a Mopar Mega iron block 572 C.I. pump gas hemi motor several years ago, it has EFI with dual 4150 1150 CFM dry throttle bodies, it had right at 10.8 to 1 true compression ratio and made 799 HP on Oregon 92 octane non ethanol pump swill on a DTS engine dyno in Chico, CA with a set of CNC ported Stage V heads form Tim at FHO
It's going into a 1971 Challenger with a 727 with 3,23 gear with 29 inch tall rear tires, no O.D. or lock up torque converter
I'm going to help my customer get it running, it is a complete ground up build from front to rear. The car was strip completely by a thief several years ago while stored at a friends of his house
Hemiroid motors are a pain in the butt to get the exact true compression ratio, I use a 100 CC burrette to measure the volume in the cylinders with the piston dome down enough to have from .010 to .050 clearance above the dome to the block deck and then calculate the true down volume off of the specs for that motor
Good luck on your build

11:1 on my 604 Pump Gas Hemi.

Seems like the squish in a Hemi is in the piston dome on either side of the valve notches. How close can you make them fit the chambers? .040"-.060" clearance, smaller radius than the chamber.

Mine is at 10.4-1, I feed it 91 octane as that's the best stuff you can buy around here. I have in a pinch had to put some 87 in the tank as the closest 91 pump is 21 miles away

Thanks All.

I don't think I'm being nit-picky on the compression issue,
just cautious.

Here's a lttle more info. I don't want to take the motor
over about 6000. This should assure some good low RPM street
manners. In view of this, I won't need huge intake ports,
manifold, or carb. So all that limits cam duration to something
maybe around 250 degrees ? Less ?

That is not a big cam, so what can I get away with for
compression with aluminum heads, polished chambers and
piston tops, and zero quench ?

Well I know it takes real gas to run a 12/1/2 high dome hemi piston with iron block and heads and huge duration cam, old school build I did years and years ago from old parts I had laying around and that motor is still in my Challenger. I do not know were the Feds and each State is going with their corn syrup gas. I read about they are now doing tests on this 10% Eth. to see if there is any bad side effects and all of us know it eats gaskets and hoses. I have seen were it has a shelf life of about 4 week before is starts to separate and becomes crap and evaporates and gums up a carb. Never could figure out why corm is used for gas and not to feed Humans and Animals but Government seems to want to appease the tree hugger. Also read that there is going to be 100% plant life gas in the near future.

I will not argue the shelf life or the damage it does to hoses in time. I ve always said if the government is going to make us use it, it should at least have alot more octane like 100.
But....when corn is used to make ethanol the chief byproduct is Millersgrain. It's a very hi energy livestock feed. Cattle love the stuff, it's like feeding a kid candy, they can't get enough of it.

With no lead and other things taken out of gas and improvements in refining why is a food source being used to run a Vehicle. In the Dakotas alone there is something like 250 years of Oil still in the ground. North America has plenty of oil to be processed, the Alberta Tar Sand has plenty. Texas still has plenty, Sask. in Canada is becoming a big producer but we have to have this 10% Corn Syrup added. Just wait till we are made to have an Electric Vehicle in the next ten years and you will have charging stops everyplace. Who will be making the money then it is not GREEN looking at everything it take to get it done.


Getting back to the topic, 11.1 maybe the high and 9.5 the very low 10.5 in the middle, so 10.5 to 11.1 is were I think you should try for. Who are you going to get to build your motor. is that Washington on the West or East Coast you are from. Cab Burg states that he is in Oregon so if you are on the west coast you could talk to him. East Coast has plenty builders like Susquatch on Moparts also.

There is no special trickery to run 12:1 in a Hemi....on pump gas...Keep timing in check and make sure its not lean, and keep engine temp reasonable.

It's not the same corn.
All you need to know: corn = Archer-Daniels-Midland = Iowa primary = ethanol subsidy.

W/r/t "Seems like the squish in a Hemi is in the piston dome on either side of the valve notches. How close can you make them fit the chambers? .040"-.060" clearance, smaller radius than the chamber"
In some British bikes it is, but the dome shape has to be really accurate. Making it out of an existing piston would need more positive deck or a head mill (!!), and removing some of the dome volume at 12 o'clock to keep the CR.
Even if you don't get actual quench (something no wider than about .060"), you can get squish (higher turbulence) by simply making the piston dome radius curve away from the chamber wall as it approaches the plug - the charge will flow to the larger volume.
You might be able to build some swirl into that as well, but:
1. would you want to?
2. which direction?

Gold Star, Manx, G80 500 singles are much easier to work on!

Hemicar, I'm west coast. I am definitely going to hire
someone else to build this motor for me, but I will have input
on the design parameters. I would like to get a little better
handle on the cam. I "feel" 10:1 is probably safe ! If you
found the specs to that 9.5:1 572 that would be great but
I don't want to be a pest.

Cab, do you recall the cam in the 10.8 572 you built ?

Dragula, what is a typical cam you use in a 12:1 pump gas Hemi ?

Gsmopar, same question if don't mind, cam in your 11:1 Hemi ?

Quickd100, I really like that truck of yours, always like
reading about it. What cam in your 10.4 motor ?

**********

As long as I'm here typing, I'll throw this out. Because
of my design limit of 6000 RPM or so, and the fact that
it will be an aluminum block, I'm considering a hydraulic
roller cam and using titanium valves. The hydraulic cam
would take up lash from block growth, and the Ti valves
would allow using fairly light valvesprings to help out
the hyd lifters.

I don't really care about overall horsepower with this
motor, no specific goal, it will be healthy enough.
I look at it as being a torque generator.

Again, I want to say thanks for the guidance, there is
an amazing amount of collective knowledge on Moparts
compared to many other websites !