I probably should not bring up DCR after last time but here goes. Looking for opionions on the following from experienced owner/builders:

With completely identical 496ci Big Blocks, which would make a more powerful street motor. An engine with aftermarket aluminum head (110cc) with a static comp ratio of 9.3 (DCR 7.3) or a engine with reworked Iron Heads (97cc) with a static of 10.3 (DCR 8.1)?

Click here for specs on my motor (http://www.72chevelle.com/engspecs.htm)

I'm concerned with the low static/DCR number and considering switching to a Cast head with smaller chambers.

Thanks

Iron heads, no doubt. It's widely accepted that with aluminum's superior heat dissipation properites, it needs 1/2 to 3/4 point higher compression to hold it's own against cast. In a pump gas situation, the 2 main reasons to think about aluminum are repairability and weight savings. If it's power you're after and weight isn't a big concern, cast is the best pick. How about milling the aluminums down to raise compression a bit and get the best of both worlds?

You have it just the opposite,aluminum heads allow you to run 1point compression more.And on big block you can save about 100 lbs.

Imho, you two just said the same thing using different wording is all....

pdq67

The alum head is likely much better flowing, however, imho, its all about cylinder pressures, so as long as the iron head can fill the cylinder, you'll make more power with more compression. The various debates on iron v alum are spurious as hell, no one has any scientific evidence to back anything up.

Your basing this on comp. alone?....MyBoTy has it right on.....cut the aluminum heads graemlins/thumbsup.gif ...that is assuming the heads are decent heads to begin with......

I agree, dont just compare compression numbers. Aluminum soaks up heat and dissipates it faster, so it doesnt hold as much in the chamber. Horesepower is heat engergy and when you dont keep it where it belongs(the combustion chamber) you dont make as much power. Assuming you had an iron set of heads and an aluminum set of heads, both flowing identical, if compression was identical iron heads would make more power with that amount of compression. Basically, aluminum heads dont have the same thermal efficiency as an iron head. You have to compensate for the aluminum and add more compression, cause the additional cylinder pressure adds to the amount of heat present, and you need that extra heat to compensate for what will be lost through the aluminum. With that extra compression, the aluminum cylinder head can then perform as well and possibly better than the iron head with a little less compression. Way back in the day, the big guys used to use aluminum heads for the weight advantage as well as the ease in working on and reparing aluminum, but the guys really trying every trick to go fast would often coat the chamber of their aluminum head with cast iron, in hopes to retain more heat in the chamber area. Nowadays there are so many different types of thermal activated composite type stuff, I cant tell you what anybody is doing, and what they may or may not use. My guess though is that experimenting with cast coatings is probably not very common with all the options availabe and changes in technology. Either way, you have to compare flow numbers first cause thats where its at. If the aluminum head are far superior to your iron heads flow numbers, the little bit of thermal efficiency you gain with a cast head wont make up for the large amount of potential airflow you lost by ditching the aluminum heads. Also, even if they flow close to the same, its still not a no-brainer cause once again, the possible power increase due to additional thermal efficiency may not be enough to make up for the weight difference. Check all the numbers, not just compression cause it may not be worth it. Your compression would have to be really in a bad place right now in order for some extra compression to atually offset a head that probably doesnt move as much air.

Originally posted by JeffK:
I probably should not bring up DCR after last time but here goes. Looking for opionions on the following from experienced owner/builders:

With completely identical 496ci Big Blocks, which would make a more powerful street motor. An engine with aftermarket aluminum head (110cc) with a static comp ratio of 9.3 (DCR 7.3) or a engine with reworked Iron Heads (97cc) with a static of 10.3 (DCR 8.1)?

Click here for specs on my motor (http://www.72chevelle.com/engspecs.htm)

I'm concerned with the low static/DCR number and considering switching to a Cast head with smaller chambers.

Thanks Jeff,
This is basically an impossible question to answer without knowing what the two heads in question flow & the port sizes.
I don't give a damn what the compression is when you are talking less than a point. the better flowing head will almost always make more power period.
An engine is an air pump & as such the more you pack into the cylinder in a given time the more power you are going to make.

You will make more power with more compression, that is a given but going from 10.0 to 9.0 will lose you worse case about 3%, probably more like 2% & a set of heads that flow even 20cfm more in the range you are opening the valve too is capable of close to 40HP & a really good set like the Brodix that is in the 340-350cfm area with a 270cc port is obviously capable of even more.
If you are talking about a 500HP engine, as I have said before,,, I will let you do the math ;)

I agree with Joe & MyBoTie completely, assuming the aluminum head is the better of the two, raising the compression & using it is a win-win deal.
Plus the already stated fact that you just took about 100lbs of the front of your ride :D

G.M. Iron heads generally weigh about 68 lbs. each with valves, springs, and other small parts oppossed to aluminum heads that run all over the place. Weight savings will be about 70 lbs. per pair but the question is how modern a design are the aluminum heads? A little more information would be helpful in accessing your choices.

r3ecently switched from an iron head with 9.0 comp to and alum upping the comp to 10.0 from 72 chambers to 64 and the iron head felt like it made gobs more torque than the alum head.

zl1 Madness, youve gotta look at all the numbers like valve sizes, port size and quality, combustion chamber quality, the tuneup after the swap and flow numbers from low lift to peak. More than likely your iron heads had smaller ports and valves. This means that although they may not be capable of moving as much air total, the smaller port/valve combo is capable of better mixture velocity at low revs. And that equates better low end grunt. Of course, there is no guarantee that rhe reason why you feel a difference in the torque can be totally attributed to a difference in port velocity. The tuneup is a consideration too. When the airflow charactoristics of an engine change dramatically, you need to make an effort to get the tuneup right for whats basically a new engine combination. So, getting your hands into carb and ignition timing settings might get things back in check. Depends on the whole combo. The heads you have now could be easily capable of more torque and horsepower than you had before, but you never know. Thats why you need to work with it. If you went from a small oval cast head though, to a big ol aftermarket 345cc rectangle port head with 2.30 valves, there is a surefire bet that your motor just cant come close to moving enough air at low revs to fill the big ports and produce the kinda torque and response you did with the old heads at low rpm. The benefit is that a cylinder head like this will allow for huge horsepower at higher revs. Thats why its all in how well the total combo is matched, and of course the tuneup.

I've run both iron and aluminum with good luck from both. Like the others said you just want to optimize the compression ratio and cam for the combo and take the cylinder head material into consideration.

70 lbs is worth roughly half a tenth and its off the nose of the car which could help with weight distribution.

A bit more compression in an aluminum head engine makes up for the extra thermal loss (about 3/4 of a point IMO). So if you just swapped identical flowing iron heads for aluminum without changing anything else the iron would probably win -- but if you bump the CR with the aluminum head engine, power will probably be a wash between the two.

The only actual comparison I've seen was in one of the mags a while ago. They used World heads -- identical castings except one was iron and one was a CNC chamber aluminum. The aluminum head engine made like 2 HP at peak more than the iron head engine and that was without optimizing the compression for the aluminum head engine. I'm sure the aluminum head's CNC chambers helped a little with flow but prob only a couple CFM -- I sure wish they had just run cast chambers on both.

I'm not suggesting that the magazine article was a definitive test, just that it was the closest I've personally seen to a head to head aluminum vs iron test. The test left me with the impression that the iron vs aluminum thing just isn't that big of a deal.

We'll I don't want to tear into the bottom end to change pistons, so I'm staying with the flat tops. These are the choices I have for heads:

Aluminum: Straight from the box GM part #12363390
with 2.25/1.88 valves and 110cc chambers.

Iron: GM cast #3872702 machined for 2.19/1.88 valves, manganese guides, competition valve job, mild port job and unshrouded valves.

So basically is comes down to Iron heads with 10.3 CR or Alum heads with 9.3 CR.

JeffK

Do you know what the 702's flow with the work done on them? Are the aluminums the GMPP roval head? To me it's a no brainer to go with the better flowing head but if they are close I would go with the 702's. They can flow 320cfm at .650 lift with the right work and with your cubes you need good head flow. Why are you confined to those two choices anyway? Just curious

No idea of flow on the 702's. Yes the aluminums are GMPP Ovals. I believe the same ones that come on crate 502's.

Not confined to those choices, just own them already. What other heads would you suggest, keeping in mind the flattop pistons.

If you own the 702's I would definately bolt them on and try them out. The stock GMPP heads don't really flow that well anyway and if you have quality work on your iron heads the bonus now becomes the increase in compression with the smaller chambers.

Those 702's sound like the ticket to make very nice power in your motor. Your quench will be good and those 98cc chambers will absolutely love the flat tops pistons and flame travel will be better. If the 702's are untouched, either take them to a reputable machine/race shop or do a basic port job on them yourself. Don't worry about gasket matching them to your intake, just knock down the casting flash(do NOT polish!) in the intake runners, smooth and blend the bowls(under the valves) and work the exhaust side really good(polish them if you have the patience!) and you will have a very nice setup.
I did all this with a set of #290's and it REALLY woke them up!

Whoa, which GMPP heads are you talkin about. Any flow numbers Ive seen for those heads are real darn impressive as is. For example, 300CFM of flow at .600 inches of valve lift, and if youve got the camshaft to take care of it, they flow 318CFM at .700 and 327CFM at .800. High lift numbers arent the only story though. They do well enough at lower lifts, and have a strong .233 at .400 and .272cfm at .500. Not knowing how well your 702 heads flow, its hard to say what would truly be best, but I can say for sure that the 702 in stock form doesnt flow air like the 290cc GMPP aluminum head. If you could, take both heads out and have em flow bench tested on the same bench. That way, even if the flow numbers on the particular bench you get em flowed on vary to any degree from what I just provided, if they are both done on the same bench, you will know which one flows better. Look at all the data too, not just peak lift numbers, look at the numbers over the whole lift curve. That would be the best way to verify which head fits the bill.

My 215 casting closed chamber 103cc with 2.25/1.88 valves flow better than that on the flow bench and I don't have to run much dome for 10.7:1 with a zero deck and .039 quench. If those 702's are done right they are the way to go on his combination without changing the pistons.
But I agree JeffK should either flow both his heads or bolt them on and try them for comparison sake.

This is for a 99% street driven car, so I would have thought a jump in compression would be better than maximum flow numbers.

I'm leaning towards the iron heads now. Would be nice to try each, but would rather put together the best running street combo up front.

Thanks for the input so far.

As a follow up, I was wondering what the best use for these 110cc alum heads would be. They cant be used with domed pistion, so the most comp that a person could get is around 9.5:1 which seems to be a waste for the heat dissipation properties of aluminum. Maybe used for weight savings.

In hindsight, I probably should have went with a domed pistion and 118cc chamber alum heads.

Jeff,
SRP has some newer pistons that are 14cc domes that will work pretty well with those heads with the right deck etc.
And you are completely correct, the max or peak flow numbers mean absolutely nothing in an application where you are not running a cam that opens the valve that far.
The important numbers are the low & mid numbers where you are most of the time.

Chances are the two heads are going to have reasonably close numbers to .400 or so & possibly higher,, depends on the job done on your iron heads.
Also, the iron head will have a smaller port then the alum one's you have which are like 290 I believe.
In your case it's kinda a tossup not knowing anything about the flow so I would probably go with the head with more compression & smaller port for a mild street deal.

Or.... you could sell them both & I will be happy to set you up with some real good flowin Brodix ovals :D

well, i have a pretty good comparason. my iron extensively ported 215's fitted with 2.25/1.88 valves had 10.44 static compression and cranked out 195 cranking compression. i ran 11.21 at 121.2mph with those heads. bolted a set of brodix race rites alum heads with the cnc chambers. went down to 10.01 compression and 180 cranking compression,same exact combo except for the heads
and now i'm running 11.10 at 123.2mph, even though i lost compression across the board i'm clearly making more power. the race rites are 271 cc the 215's measured 261 cc on the intake ports.

Hi Ray graemlins/waving.gif
Not a bad head for a little ole oval huh :D

Keep me posted next year on the progress graemlins/beers.gif

Originally posted by JeffK:

As a follow up, I was wondering what the best use for these 110cc alum heads would be. They cant be used with domed pistion, so the most comp that a person could get is around 9.5:1 which seems to be a waste for the heat dissipation properties of aluminum. Maybe used for weight savings.

I probably should have went with a domed pistion and 118cc chamber alum heads. I'm currently running the same GMPP heads you have with TRW-2465 domed pistons. The pistons were cut for valve relief for an earlier combination, so I don't know how much that effected the piston to head clearance. The block was decked to give .005 in the hole and I'm using a .039 gasket for a calculated compression 10.44 .

Tom.

mike, i'm looking to get 125 mph from those heads next season. the cnc chambers really wake up the exaust flow numbers according to brodix, and i guess that would explain why uncorking the exaust and just running 18" extensions i went from 118mph to 123mph, a 5 mph difference!

you guys lost mestarting with Greg's first post. could someone simplify this please? graemlins/clonk.gif

mr68, What size exhaust did you uncap that hurt you 5 MPH? My son had 3" Borlas and the car actually ran a little faster with the exhausts capped. 10.50s @ 126-127 MPH and only lost about five hundredths in E.T. It seems the trick is to put the mufflers as far back as the body will let you and run short turn downs off them.

Originally posted by young gun '71:
you guys lost mestarting with Greg's first post. could someone simplify this please? graemlins/clonk.gif Originally posted by young gun '71:
you guys lost mestarting with Greg's first post. could someone simplify this please? graemlins/clonk.gif =
For this discussion,,,

Better flowing heads that don't have a port that is to big will make more usable power :D

This is assuming the head flows better in the "window" the cam is giving it.

If the "window" isn't open far enough the air can't get in (or out)

The "window" in this case is the valve

How's that graemlins/clonk.gif

That's perfect!! LOL

harold, i run 3" flowmaster 2 chamber(loud ones) turned down at the rear end(pumpkin),muffelers are pretty far back. very simple no kinks very little bends or restrictions