Alum heads vs iron heads on street car

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.

As for the original post I think the hemi may lose a bit more thermally due to the massive chamber.

I'm having trouble finding it now but thought I read about a test on an ss/ah Hemi cast iron vs aluminum heads with the same port program and aluminum was down 40hp.

Could be wrong, that is just from memory, I'll keep looking for the info.

EDIT-

This is where I read that-

https://board.moparts.org/ubbthread...55708/re-aluminum-heads.html#Post1355708

How does built up heat in the head get to the crankshaft? If both motors are maximized for their head material aluminum wins every time.
NOBODY fast runs iron heads unless rules of their class dictate they must.

I find this very interesting... I wanted to start with my cast iron heads... then later switch to aluminum in a about 2-years... but I was told that my pistons (-20 dish) dictated cast iron heads only because of the lower CR (~9.75:1). I've since installed the cast iron heads, and also cc'd chambers and cylinder... calculated the CR at 9.9:1

I'd still like to eventually swap to aluminum heads... per allowable racing class rules. Cast iron allows me to run in more classes and be more overall competitive, but giving up a little to the classes which allow aluminum heads.... weight is factor for me... more than engine power.

I don't know that anyone said aluminum heads " need " more compression.
They ARE more tolerant of higher compression , and less likely to experience detonation.
As far as over thinking , I guess most of my thoughts are in taking weight off the front of the car.
Better handling is one claim , though few of us drive where that difference is really appreciable.
But less weight is still less weight.
I don't know that they are ever a bad idea , but that being said , I am looking at a set of J heads on my current project.
There is a certain pleasure to underthinking.

The internet wisdom premise is...... if you changed from iron to aluminum heads, all else being equal, you’d lose power...... because the aluminum absorbed some power away.
And...... you’d have to up the CR for the aluminum headed combo to be equal to the iron head combo.

The test in the link I posted did what I feel is a pretty thorough test of that in a very “man on the street” type of build.
The difference in power was with the margin of repeatability for the dyno.

That’s using heads that are as close as possible to being the same, other than the material they’re made from.

In the case of what most people here are doing......if they’re going from iron to aluminum....... you’re almost never comparing equal heads.
In most cases, you’re replacing iron heads with better flowing aluminum heads(otherwise.....why are you changing the heads?).

It’s going to pretty hard to come up with some sort of “high performance” build combo where the better flowing aluminum heads get out powered by old iron heads.

I don't know of any mopar heads that are available in both materials that are identical architecture wise.

DART sells sbc heads in both iron and aluminum, but not for Mopars. Going back to back with those heads on a dyno on the same shortblock would tell you.

Were the old 440-C Indy heads the same as the 440-1?

Maybe the MP hemi heads?

Pretty sure those are Edelbrock castings with MP cast into them instead of Edelbrock.

Kevin

Yes I thought Edelbrock made them but thought they were closer to the cast iron heads than their Victor heads, certainly could be mistaken on that but thought Victors had larger valves and ports.

Edelbrock says that Mopar would not allow any improvement.

Thanks for all the input guys. I’ve got two builds on the plans right now. The one with a 440 is a easy deal. He has more money to spend so custom pistons, 440 source alum heads and Cometic head gaskets are not a problem. CR will end up at 9.8 to 10 to one. It’s the budget 383 I’m building with off the shelf pistons, limited cam choices low CR right now. By the time I fix his 452 heads, I’m 70% there to 440 source alum heads. He is really tight on money. That’s the big deal. I may end up with seal power forged and cut some valve notches.

My alum heads vs iron head power question with all things being equal was from and SS/BA friend from years ago. In a 65 acid dipped car, he had problems keeping the front wheels on the ground, so weight transfer benefit was no help to him. Maybe the big chamber of the hemi was different.

70% of the way to the selling price? Or the price including shipping? Or the price, plus shipping, plus having them checked out?

Good point on adding everything up when making a comparison.

When I compared the 440Source Stealth heads to the Edelbrock E Streets, the E Streets came out cheaper due to the fact that you can get them from Summit or Jegs with free shipping and they usually have some kind of spend over $1000 and get $100 off sale. So for the same price or cheaper, you can get the E Streets which will flow a little better than the Stealths. Seems like the only reason to go with the Stealths these days would be if you want something you can paint and look stock. Or if your headers can deal with angled spark plugs, you can go with the ProMaxx heads which would be even cheaper than the E Streets.

The 84cc E streets have straight plug holes.

If the OPs budget 383 uses angle plug friendly headers, the 75cc E streets would certainly go a long way to upping the compression.

Well when you figure EVERYTHING, more like 50%. New bolts, better head gasket. He has cast iron manifolds and intake. I imagine I will end up with fixing the old 452. I can get him up to about 8.8 or 8.9 CR and use a smaller cam with about 218@.050 and survive. I may have to fly cut the pistons a little for clearance. Comp says their cam with a 110 centerline is ground with 4 degrees advance already. That’s as big as I dare get.

I highly doubt there would be any clearance issues with a Comp 268H cam.

I got a similar dilemma with iron vs Alum heads.

I got a well built running mild built street 360 motor 9:1 mild cam Hyd cam with Hughes Stage 2 ported heads with SS valves. Its in a street cruiser.

I also got a spare set of built 305cfm Alum Hughes ported/coated heads sitting on the shelf.

Trying to figure out if its worth installing on a street motor, I like the weight weight loss, and not sure what I will ever do with them.

What to do?

Course its not my statement but I think he means the iron heads wont disapate the heat as much as aluminum so the heat exspansion in the cyl will be more with iron heads since they hold more heat and can add to more power pushing on the piston which will go to the crank of course.

Myself I agree with most that the difference is so minimal between iron and aluminum heads that many overthink it to much. As said most aluminum heads are newer and more modern with better combustion chambers and better flow then 50 year old iron heads. I have all the equipment to do the valve work on iron heads and set them up but to me its a no brainer in my street/strip car to go with the aluminum heads. And for the price of aluminum heads today most will spend as much paying someone to work iron heads for them anyway. And they do shave a little weight off the front end. Ron