Iron head 435 v. Aluminum head 435

[Geoff C.]

Dear Big BlockHeads:

An Iron head high horse Tripower is rated at 435 HP, the same as the Aluminum head version. Is there really a difference? Are they both under or over rated with regard to horsepower? Is there an old car magazine article that dyno tested them when new that has figures ? (Not a 10-15 year old article in Corvette Fever or other latter day pulp source that tests a restored or old car.)

Geoffrey Coenen

[Duke W.]

Re: Iron head 435 v. Aluminum head 435

Tough call. With the same CR iron heads might make a little more power because they don't transfer heat as rapidly, but this can be gained back with higher CR. An aluminum head will take a quarter to a half point more CR on the same octane fuel due to the increased heat transfer, which keeps surface temps a little lower.

The other variable is port geometry and valve size. I know the L-88 heads have slightly larger exhaust valves than iron, and I also believe the ports flowed a little better in OE machined form. I'm not sure if this same head configuration was used for L-89.

In any event, some judicious pocket porting/port matching with a multiangle valve job will improve top end power in the 5-10 percent range without any impact on low end torque.

Duke

[Jerry Clark]

Re: Iron head 435 v. Aluminum head 435

In STOCK configuration, (heads) the aluminum L89 head will make more horsepower than the iron. I have run both on the same engine.
Jerry

[Warren F.]

Re: Iron head 435 v. Aluminum head 435

1967, 1968 & partial 1969 L-88 & L-89 head same configuration. Several improvments made with the 1969 ZL-1 head, used on partial year 1969 L-88's and all 1971 LS-6's. 1969 L-89's did not use this head.

[Joe C.]

Re: Iron head 435 v. Aluminum head 435

Geoffrey:

The difference, as you know, is in the weight savings (50 pounds, approx.) Also, aluminum is much more "sexy" than iron. With these facts in mind, we all know that the L89 was considered "head and shoulders" above the 1967 L71, and the price premium proved it. On the downside, of course, is the differential expansion rates. This will cause larger variations in valve lash, and more head gasket sealing problems over Fe-Fe, or Al-Al interfaces. The ZL-1, although stillborn, was absolutely, positively the ultimate MKIV engine. The spectre of the Federal Emisions standards killed it, though.
I do not know whether the L89 heads were configured similarly to the L71 heads, but Joe #930 clearly shows, in a thread below, that the L88 iron head had a smaller chamber volume than that of the L71.
Bottom line:
Unless someone can show me an actual "apples-to-apples" dyno test result,comparing the L71 to the L89, then I would put my money on the Fe head engine.

Joe

[Joe C.]

CHANGE: Line 5

"L88 iron head" to "L88 head"