I had a Super Stock hemi racer friend of mine that could run alum or iron heads on his car and always preferred to run the iron heads because he said they made more power than alum. He was limited to the same compression ratio with either head. On a street motor I believe the rule of thumb is that you can close to a full point higher than iron heads and not have detonation problems because aluminum dissipates heat faster. So my question is this. What if you run an aluminum headed motor with 9.0 CR. What would be the consequences?

Aluminum heads are more tolerant of compression than iron is what I understand.
If I had a 9:1 street motor and was going to run aluminum heads , I'd strive to get the compression up over 10 :1
10.5 :1 would still be pump gas friendly.
Unless Hemi's are radically different , I think the same would be true.

I have read LOTS of theory on this subject, most all of the 10-1 iron head engine street strip I have built even with the proper quench had detonation issues on 93 pump , never have I done a back to back comparison of Alum vs Iron heads, my last build that is in my 4100lb 68 Charger has Alum heads, .035 quench 9.7-1 com, VERY mild build, & it makes more power than I thought it would, 11.04 @123, 3.54 gear real street car drive it across the country with the ac on, I can run 87 pump & have never heard or seen any sign of detonation,my opinion is alum & steel heads make the same power.

I think you have something backwards. iron doesn't tolerate the higher compression ratios because it holds heat. alum sheds heat and you can get by with a little more on the street but I never use the "1.5 points higher" the internet/magazines tell you. an iron head, with everything equal will make more power. "everything equal" is the issue. I've always thought the greatest advantage to alum is it's much easier to repair. I think the whole thing is more complicated than people want to get into. chamber design, piston configuration, plug location, etc. can you control the heat the engine makes or is it a problem? I have a set of iron heads I firmly believe make more power than the edelbrocks i'm using, but the edelbrocks burn cleaner and they're easier for plug changes. I don't think the whole issue is cut and dried and I definitely don't prescribe to the culture that alum is always the way to go. keep in mind that the top of the piston is the bottom of the combustion chamber and the bottom needs to match the top for best efficiency. if this is about the 383 your working on, then things do get narrowed down a bit. a 440 is more flexible with parts selection.

sounds like I may be driving something similar to yours. 9.8:1 alum head, mild mech cam, very pump gas friendly, more power than I need, excellent driver cruiser; but mine would never run the times yours runs. i'm with you on the iron heads with 10:1, or more; won't work very well on pump gas.

Any power loss doesn't compare to the weight loss of aluminum heads. There is no downside to aluminum heads.

One reason why they are more knock tolerant is... less power. The heat loss is power not reaching the crankshaft.
This is why you do not remove carbon from the chamber and exhaust ports: it's a thermal barrier.
Back to back tests done on BBC 50 years ago: iron wins.

Most all class racers will coat the chambers of Aluminum heads, and in class racing usually there is a horsepower factor penalty on Aluminum heads.... sometimes its worth it sometimes it's not.

For a street car there is no down side to an Aluminum head.

I built my 493 at 9:1 with Edelbrock heads.

500hp/600 ft/lbs and it runs on 87 regular without complaint on all but the hottest days and that could be addressed with a change in the timing curve.

Kevin

That and the fact that the cast iron stuff is well over 40 years old and quite simply used up!

I run 10.6:1 CR with aluminum in my 505 and it will run on the cheapest pump swill out there. Don't typically do it, but it can if I need to.

The best part was sending them off for cnc porting for well under $1k and getting back heads that flow better than the OE Max Wedge head.

As for timing, I have a little knob inside the cockpit that I can adjust it on the fly, so I have some flexibility.

I have over 7,000 miles on a set of aluminum heads and there have been zero downsides.

When the Eddys first came out, I bought a set for my 500" stroker project. At the time, I had a very worn 440 in the Cuda that only had a cam, intake/carb swap and headers. At the end of the driving season, I decided to throw the Eddys on the engine just to see what they would do. Some said that the car would actually slow down due to the low compression. To be honest, I don't know what the ratio was.

Anyway, with no other changes, from one weekend to the next, the car picked up nearly a full 2 tenths with just the head swap. That was obviously due to the better flow of the Eddys over the stock iron heads, but if the aluminum hurt anything in this case, it was not much nor noticeable.

Just my experience.

The notion that you “need” more compression with aluminum heads has been tested at least once with a SBC, where you can buy heads that are the same in iron or aluminum........ and the results are that the power was the same.

Imo...... it is totally a non-issue, and falls into the “over thinking the combo” category.

Run the aluminum heads and enjoy.

The factory engineered 6.0 in my 2004 Chevy truck seems to run just fine with aluminum heads and “only” 9.4:1CR.

Oh...... and as for the NHRA stock/ss thing...... the aluminum heads actually come with a HP penalty(car has to be heavier or run a higher class)...... and a lot of the time guys run them anyway.

https://www.hotrod.com/articles/ccrp-0602-iron-versus-aluminum-cylinder-heads-test/

My experience has pretty much been the same. I went from stock iron 452s to Edelbrock E Streets and picked up nearly two full tenths in the 1/8 mile. I think there's more in it too because on my one run since swapping, the car was sputtering and then lost all power in the top end due to a loose alternator stud shorting out.

It also seems to be idling and running smoother and getting better gas mileage since swapping to the aluminum heads. I wish I would've just built my 440 with the aluminum heads to begin with.

Depends on the engine and style of heads. In the ‘90’s, the LT1 chevy, iron headed, had 10.1 compression and ran on 87 octane. The aluminum head version had 10.5 compression.

I went from stock 906's (215cfm?) to ported BV 906's (260cfm?) with no other changes and went from 11.2@118+ to 10.7@125 at 9.8:1 comp on the race car....only 2/10ths from iron to Ally?..there not race cars? but I'm sure you hooked them ok.

That engine is also reverse flow cooling,, cooling the cyl heads 1st.

I swap from a set of mildly ported big valve(2.14,1.81) 906 to a set of MCH CNC ported Eddy 84 CC RPM heads(same CC as the 906) and my old pump gas Duster went from 10.69 at 124 MPH to 10.49 at 127 MPH with no other changes There was around 50 CFM flow differences between those heads also, 260 to 310 CFM
EDITED: Both heads had 84.Cc so the compression ratio stayed the same at exactly 9.25 to 1.
I change the crankshaft stroke the next winter to increase the compression ratio, went from 4.25 with the piston deck height at -.025 to a 4.300 stroke crank that made the pistons measure zero on the decks and that raise the compression ratio up to 10.29 to 1 which help the car run low 10.30s at around 127MPH also due to not having enough fuel volume in high gear back then that was with alow deck eddy six pack intake with 1970 440 six pack carbs set of by C&J engineering in Whitier, CA
I ended up swapping the heads again later to a set of Indy SR M.W. ports with 75.0 C.C. combustion chambers and a Indy 400-3 intake with a Holley #9375 non HP 1050 CFM Dominator and fixed the fuel delivery problem which help make the car run 10.00 at 134.+ MPH, one 9.993 at 134.89 run through the full exhaust with the air cleaner on running Oregon 91 octane non ethanol pump swill

mmmm, so that tells people something perhaps and does suggest that iron heads can make more hp......so I went from a stock head that was dead @around .450>.500"....to one that was good to .600+" and I ran a .650" sft....and got .5 in ET and 7mph...which=+71fwhp

the new parts are only as good as the old parts are bad?

Perhaps in my case, but the numbers on heads are the same, but not in Cab's case, he already had decent heads and still only went 2/10ths faster with much better ally heads, perhaps if he had upped the CR by 2pts it would've gone quicker?...........Also didn't the Indy 440-C iron head make as much if not more power than the -1 if I remember well with equal porting.