How high a compression on pump gas

572 Hemi, aluminum heads, what’s the highest compression I can run safely on 93 pump gas? I know timing and cam is a factor but just a general rule of thumb idea. Pistons are 10.5 now but I can deck to get more or add thicker gaskets to get less, street and occasional blast down the track by adding race fuel.

Talk to your cam supplier. If you give him your engine particulars and what octane you want to run, if he's any good he can tell you what compression ratio to run. On an AMC 401 I went this route and with iron heads, stock intake and exhaust, running 87 octane pump swill, the recommendation was absolutely no more than 9.2. How did it run? Still putting it together so we shall see.

10.5 might be okay in a wedge with perfect quench. I bet it is too much in Hemi for pump gas.

We have a member here than has ran his 12:1 Hemi on pump gas, but i'm not sure he raced it on pump. From what i have seen, your about max to race on pump gas also, seems Hemi's are not as sensitive to compression increases as wedges are. Best told me you need 13:1 or higher to really make a difference in power with compression increase from 10.5:1. Between 11:1-13:1 seems to be a bit of a no mans land with Hemi's. That being said, I will be running 12:1 with my 572 because i ordered the pistons before i was told better. Best Machine has made 850hp on pump gas with 10.5:1 compression with a 572 Hemi, so seems there really is no need for higher compression for a street Hemi.

We have run 12:1 with our Hemi...I would even go more with an aluminum block.....

The Hemi chamber is actually pretty detonation-resistant; one of the reasons it was employed in the Gen1 engines.
I had an iron-head 426 Hemi @ 10.8 measured, never an issue @ 36 degrees total, + vac advance, even on CA 91 octane.
It did have a fairly big-for-the-time old Crower FT cam, IIRC 247 @ .050, lift in the .540s, which bled off some dynamic CR (and idle vacuum).
With aluminum heads, that engine could have gone more like 11.3-11.5 CR, I'd bet.
4-speed, 4.10s, 3700-lb RR.
I wouldn't push a wedge that far, from personal experience.

Sounds like I’ll leave it, it’ll be down in the hole about .065 anyway..? (haven’t checked the deck) (rods are 7.045) with a 1.36 ch. 56.6 dome 170 chamber, right at 600 cu in. I’ve heard so many ‘what’s’ I wanted some real examples to do once and not go back into and change, thanks all

I'm at 10.4-1 with my 605. It will run on 89.5 but I prefer to run 91.

Elevation is a factor too, the sea levelers will experience problems at lower compression. On the other hand they make more power with the denser air.

a lot of factors go into to building detonation resistance. a hemispherical combustion chamber is more detonation resistant than a wedge chamber. allow for some carbon build-up in the build to be safe.

I wouldn't worry about 10.5 to 1 compression ratio on any aluminum headed Mopar, especially on any hemi motors that have not been CC on the actual dome volume in the cylinders
Hemiroid motors are a completely different animal when it comes to true compression ratio
I've seen hemi pistons with advertised dome volume have 15 CC less when actually measured
Depending on the stroke and bore size that can mean the difference between 12.2 and 10.8, the bigger the motor the harder it is to get the compression up on Hemiroid whiney

I'm 10.0:1 on the dot with iron heads and a pretty small SFT cam on my Hemi. No problems with 93. I wouldn't be afraid to try 11:1.

Static compression has very little to do with what octane fuel your engine needs, it is dynamic compression or cylinder pressure that determines much more the octane needs of an engine. Performance trends engine program will make a prediction on spark knock and will generate a spark curve to use if knock is predicted, very cheap insurance when designing an engine build. I have built pulling truck engines that had to run on pump gas with 13 to 1 static compression, and it did not detonate.

Depending on the overall setup of the build, where the compression/quench zone ends up at on the engine, and to remove some worry to make things a little more forgiving, you probably should look into having the chambers softened on the heads.

The Hemi I just sold was 12.3 and had zero detonation issues on pump swill..Steel block aluminum head.

So you really know where you're stating from, I'd advise to measure the actual 1" down volume subtracted from theoretical flat (to manually calculate your dome volume) along with chamber CCs, and use these to manually calculate your CR....before you order gaskets.

Closing the intake valve later to reduce CCP works at cranking and low RPM, but its effect on anti-knock is gone well before your torque peak.

^^^This is what we did with my dads 528 pump gas HEMI. 12.5 on 93 and he beats the hell out of it and still going after 8 yrs.

Controlling cranking compression can help with pre-ignition. Compressing a gas increases its temperature. Starting with a colder intake charge also helps.
Detonation is an uncontrolled combustion event which occurs after the spark event. Chamber / piston shape, spark plug location, and quench distance can help or hurt.
Either way, you want to avoid hot spots in the chamber and on the piston.
There are a bunch of dynamics that I don't understand. As RPM goes up in may reduce the chance of detonation, but not sure how the relationships of Time (decreasing with RPM) vs Air/exhaust flow (increasing with RPM) vs Engine load (how fast the RPM is changing.) I think the slower reving engine (think heavy truck) us going to build more chamber heat than a light vehicle with the same power.

So it makes it hard to compare just compression ratio vs. octane as there are a bunch of unknown variables.

“chambers softened” Not sure what this means

Probably not applicable to Hemi heads

Definitely out of the norm for HEMI heads, but was applicable, although an experimental gamble with someone who knew what they were doing. My dad had a well known big name shop in N. Carolina do them. It worked out.
DJ

Kevin at Performance Trends has greatly impressed me.

After buying two Engine Analyzer versions and one Fuel Economy Calculator program
his on the phone support was top notch,
and twice when I mentioned “It would be great if”
he did a software mod at no charge.

I have seen the new motorcycle engine's have 11.0 on the the compression, it is all about the volume you have in the engine .

My new 6.4 Challenger is 10.9

Some are 14.0-1


Lots happening for that to work

Anyone?

I'm going to go with this, softened means they have been deburred and any sharp edges that could be a hot spot removed.

I had one of those same 6.4 SRT8s in my 2011 ram 1500 4 door 4wd but I had more compression, I had eagle heads swapped onto it that are about 10CC smaller, ran great on 91 octane (3000 foot elevation) but would knock if it even sniffed anything lower. I don't remember the exact number but I think it was around 12to1 actual.

Softening the chambers is something that is done usually to nitrous engines to keep from melting pistons. I have never seen it done on a Mopar engine, I have seen the mod on a few Chevy engines. The last one was a big block chevy with raptor heads on a mud racer. Basically they CNC the chamber to give it a taper from the quench to the valves. Reher Morrision did the work.

John,

Keep in mind this procedure is mostly done in high performing race engines. In the OP's case it could be purely unnecessary, but has been utilized on HEMI heads. Chamber softening is where the chamber is laid back and cuts out sharp areas that create hot spots and slows the flame travel down. We all know each combustion chamber design will vary with its efficiency, but from talking with Monte he said its great if you can start out with a head that already has efficient chamber design. Removing area's in the chamber will allow heat to soak in and give more area for expansion. Of course quench & chamber shape are important when doing this. Most importantly its finding someone who has a good program and knows what they're doing. This is typically done with nitrous or other power-adder applications, and "some" N/A setups. My current heads chambers are softened by Slawko. There are several dudes who know what they're doing that have programs for Mopar heads. It all comes down to heat. As we know the whole deal in an engine is the overall combination and managing heat. In my conversations with Monte, he stressed emphasis on being all about timing, valve events, combustion efficiency, ability to fill the cylinder, etc. Man that dude is missed.

Wes

for sure, Monte knew his stuff and is missed by many here !
if a guy were to knock off the sharp edges by hand with sandpaper only, such as around the plug hole, and any other machined defined edges, would that be a waste of time, or just something that couldn't hurt ?
kind of like dressing the sharp edges of the valve reliefs on pistons.

Totally agree and great explanation, but hemi heads dont work like wedge heads..... There is no quench or 360* of quench, depending how you want to look at it. I believe chamber softing isn't supper applicable to a hemi's chamber. Works great on a lot of wedge heads, and have also seen it done on pistons.


Fwiw, ive been 11.3:1 on pump gas and made 986hp with 572 cubes and a indy 1RA head.

As for Best machines theory on compression....... I took a motor out of my racecar and changed pistons and cam. Went from 15:1 to 11.3:1 and made more power. Compression doesn't work the same in hemis.

Like to here more details about that 572-1RA deal.

Thats because high compression requires a very tall dome which inhibits flame travel. J.Rob

So after all this tech talk, what was the answer?

I have run many engines on the street and at the track too.
Iron/iron, iron/ali, ali/iron there's lots of variables here...

But when folks ask me these days, I just say 10 to 1...

Its safe and its old school, flat top piston closed chamber head.

But when it comes to hemi's its a whole new ballgame!

I got a street 'all iron' hemi from a guy a few years back with 13 to 1 pistons, driving it on pump gas.
Yes the pistons were worn, but it seemed to run ok?.
0.008 thousanths clearance in the bores and smoked when hot etc...

He said it ran good, cos its got a RACE camshaft...
Yep it was huge around 700 lift...

''Bleeds OFF all the compression at low RPM's''!

I heard that a lot over the years and maybe its TRUE???

FWIW...my 575" hemi is right around 11:1 and I run it NA on pump gas. On spray I do run race fuel.