Decrease In Compression w Alum. Heads

[Rickd]

This question is directed to you technically advanced guys with engine building expertise. I've been advised that by switching from iron heads to aluminum heads I will automatically lose 1 point in compression due to the cooler combustion chamber in the aluminum head motor (fater heat dissipation). My motor has a 9.0:1 so it would drop to 8.0:1. This comment was made by a cam shop and reiterated by a cylinder head shop. Does that mean that if / when I buy a new set of AFR / Trick Flow / or Edelbrock aluminum heads, I need to have them milled to compensate and maintain the compression ratio at 9.0:1?? I appreciate your comments. Rick

289 FIA, #9299, Viking Blue, 302 HO, T-5

[Brent]

The first question is, what is the combustion chamber size of the heads you have now compared to the new cylinder heads? Some of the aftermarket aluminum cylinder heads have smaller chambers and the effective compression ratio actually would stay the same.

The aluminum head will perform better with more compression, the aluminum head dissapates heat and reduces combustion temp. I would not drop below 9-1 no matter what.

have fun!


Brent
427 SC Chassis #4279401
Candy Apple Red/White
402 FE/Toploader/3.54

[TurnpikeBoy]

Compression RATIO is completely, utterly unrelated to the material the head is made of. Period.

Ratio is just that: a mathematical comparison of the volume of the entire cylinder, inclusive of chamber, from when the piston is at Bottom Dead Center, to the volume of the entire cylinder, inclusive of chamber, when the piston is at Top Dead Center. Volume BDC divided by volume TDC equals compression ratio; example, you have a TOTAL volume of 38 cubic inches in one of your 302 cylinders at BDC - but you have a TOTAL volume of 4 cubic inches at TDC. 38 / 4 = 9.5, or more properly expressed, 9.5:1.

Nothing more, nothing less. Aluminum has vastly superior heat-transferral qualities compared to cast iron, and will therefore remove heat from the combustion chamber faster than an iron head will - this can result in a lower BMEP (Brake Mean Effective Pressure - an average of cylinder combustion pressure) in concept, but I know of absolutely NO evidence or data to support the concept.

By the way, if you overbore your engine you increase compression ratio as you have now increased the TOTAL cylinder volume. Just an FYI.

Yeehah

[Naumoff]

Yea, What he said.[8D]

Mathematical, Yea.

Tony:D

Unique 427 #4279480 427s/o Nascar toploader 3.31 Jag IRS

[ralphscott]

[Aluminum has vastly superior heat-transferral qualities compared to cast iron, and will therefore remove heat from the combustion chamber faster than an iron head will - this can result in a lower BMEP (Brake Mean Effective Pressure - an average of cylinder combustion pressure) in concept, but I know of absolutely NO evidence or data to support the concept.]

I agree as well. In a static environment I can see heat transfer characteristics having such an effect, but not in a dynamic environment where heat dissipation rates will normalize quickly under engine operation.

Ralph

[Naumoff]

It is true that temp. increase is directly related to pressure increase and vise-versa, but that is not compression ratio.
I cannot see a draw back from that, only a benefit of lowering the chances of pre-ingnition of the air/fuel mix in the cylinders.
And we are all trying to keep are engines running cooler anyway.
The only thing that will lower your compression ratio is if the new heads(be they aluminum or cast iron) have a larger combustion chamber.
Tony



Unique 427 #4279480 427s/o Nascar toploader 3.31 Jag IRS

[Brent]

I am SURE he did not mean that the type of metal reduced the actual static compression ratio. I think he was suggesting a point more compression for the characteristics of the aluminum head. All things equal (ports,chamber,valves,etc.) a iron head will actually out perform an aluminum head most of the time. You do not want to spend 1,500.00 and gain nothing.
Something else to consider is you are going to a larger camshaft with more duration (I think). To put more of a charge in the cylinder.



Brent
427 SC Chassis #4279401
Candy Apple Red/White
402 FE/Toploader/3.54

[nolastyankee]

Rick,
While it is true that the mathematical compression ratio is independent of the material, I have also been told the same thing with regard to aluminum needing a point higher compression ratio to compensate for the reduction in head temperature associated with aluminum. Speaking with both World Castings and Comp Cams during the building of my engine, they recommended a different compression ratio for their iron vs. aluminum heads to deliver our target performance.

Perhaps a call to engine master Bill Parham will settle the debate?

Brian Carlson

Unique 9122 - 289 FIA Cobra - The Legendary "Spence" Car

[Justin Upchurch]

Brent is correct about the aluminum heads not making as much horsepower as the iron due to the heat loss. More heat in a motor makes more power. The Bantam Altered that I drove made less power after switching to a KB aluminum block from the iron KB.
Because of the characteristics of the aluminum you can run a point more compression. You may want to consider running 10 to 10.5:1.

[Rickd]

The "cam guy" made the following example. "If you have two identical motors in every respect, but one has aluminum cylinder heads and the other has iron heads, the motor with aluminum cylinder heads will run with effectively 1 point lower compression ratio due to the heat dissipation characteristics of the aluminum heads".

Now every article I've read about people switching "stock" heads for AFR 165's and Trick Flow, the result has been significant HP increase. No loss of power. A lot of the stock 302 cylinder heads have 58 cc combustion chambers. The AFR's also come with a 58 cc while the Trick Flows have a 60 cc. RD

289 FIA, #9299, Viking Blue, 302 HO, T-5

[TurnpikeBoy]

The biggest sources of heat loss/transfer in an engine are the pistons and the valves. Yes, there is some loss thru the cylinder head itself, but the walls of the cylinder represent about a 500% greater surface area (read: transfer area) for heat movement. It is absolutely true that the hotter you can run an engine, the more efficient it becomes. Limits here are lubrication and component strength - this is why you MUST use oil to cool the bottoms of pistons. If iron, by itself, truly made more power you would think pistons would be made of iron. They were, once, but were so heavy that they caused numerous other troubles - and aluminum pistons became typical once lube and hot-strength issues were resolved.

If you are able to run more compression with an aluminum head than an iron head - all other things being equal - and not have problems with detonation, it is probable that the reason is chamber shape and flame-front propagation efficiency. I cannot state categorically that aluminum's heat removal ability has no effect on this, just that I have seen no evidence in support of the position. But it is no secret that (1) aftermarket heads are designed and manufactured to be better (more-power-producing) pieces than stock stuff, (2) aluminum is much easier to fabricate heads from than iron, (3) aluminum is 1/3 the weight of iron and thereby desireable for our applications, and (4) it's just trick to say they're aluminum.

Rick, the reason you see power gains with better heads is a function of their ability to simply move more air in a given time period.

Yeehah

[ralphscott]

BMEP http://www.epi-eng.com/ET-BMEP.htm

Also look at http://www.jaytorborg.com/engine_design.htm Go half way down the attachment to the title Cylinder Heads

Ralph

[TurnpikeBoy]

All good stuff. There's lots other on the benefits of aluminum heads out there, too, but on the issue of going to higher compression with them, nothing concrete. I suppose that lots of dyno experience has given rise to the position that aluminum heads can run higher compression; and I also suppose that GM or Ford could demonstrate design-wise this to be true.

Still. All I've seen is simple statements (as in Torborg's) that since aluminum transfers better, that you can run higher compression ratios - that ain't proof. I am aware that NASCAR and Indy builders like ceramic coatings in the chambers to keep as much heat in as possible, but use aluminum heads too - for weight? for efficiency? I can't tell you because they certainly won't tell you either, and I don't run a dyno for a living and - thereby - I'm not qualified to state definitively which is better. NASCAR has minimum weights for their cars; if iron heads were SO much better for power production I would think they would be willing to give up whatever miniscule Center-Of-Gravity changes they would generate in exchange for the power they would ostensibly make. For that matter, do you see ANY racing cars that chucked aluminum heads for iron?

? - just thoughts. Not reaching for huge efficiency in my engine so I'll go with aluminum heads for the weight advantage. Not to mention the much better flow figures available in them. Anybody seen ported stock iron heads (or anybody's iron heads) that flow what AFR 185's or TFS heads can? I sure haven't.

Yeehah

[Rickd]

Looks like a plan is slowly coming together.

AFR 165's
Speed Demon Carb (to be sized by Don Gould)
Racer Brown Cam .220 .220 / .500 .500 lift

Should do the trick, alon with my existing Edelbrock RPM intake.

Rick


289 FIA, #9299, Viking Blue, 302 HO, T-5

[TurnpikeBoy]

Sounds like an excellent combo. Will make for a very streetable, decent package.

Get it together and drive the n*ts off it! [:P]

Yeehah

[TurnpikeBoy]

http://popularhotrodding.com/enginemasters/articles/hardcore/0305em_bigbang/

Another opinion. These guys report on eveybody's favorite engine competitions. Their take on iron goes to its' inherent strength over aluminum.

Yeehah

[Rickd]

Ralph, thanks for the link to the engine primer! Interesting read for a rookie. Turnpike Boy & Brent, I appreciate your input and comments. Looks like my "cam guy" had a point about aluminum heads and compression, but basically had it kind of backwards. Aluminum heads, because of their more efficient heat dissipation characteristic, have the ability to handle in general a 1 point higher compression ratio than iron heads. Thanks again. Rick

289 FIA, #9299, Viking Blue, 302 HO, T-5