1967 L-71 vs. 1968 L-71/L-89

[Jean C.]

I assume that the '67 and '68 L-71 engines have the same internal config, i.e., cam, compression, etc. Correct?

Do the '68 L-89 AL heads have the same performance related specs as '67 & '68 L-71 cast iron heads?

Thanks and best regards,
Charlie

[Duke W.]

Re: 1967 L-71 vs. 1968 L-71/L-89

Same internal configuration - only differnce might be carb and ignition map calibration (emissions).

My understanding is that the L-89 heads have different port geometry and might flow better than the L-71 CI heads, but this could be offset by the greater heat tranfer. Maybe Clem, Joe, or other BB experts will have something more specific.

Duke

[Jean C.]

Thanks Duke *NM*

[Mark #28455]

Re: 1967 L-71 vs. 1968 L-71/L-89

The aluminum heads have the 1.84 exhaust instead of the 1.72 in the cast iron heads. The closed chamber aluminum 842 head ports actually leave a lot to be desired compared to the newer heads, but the 840 iron heads aren't great either!
Mark

[Michael H.]

Re: 1967 L-71 vs. 1968 L-71/L-89

Charlie,

There are several +/- factors for both the AL and the cast iron heads for 67-69 big block but these factors seemed to cancel one another most of the time. If there was any increase in HP because of the configuration of the chamber, valve sizes and ports in the AL castings, it was eaten up by the fact that the aluminum itself ate HP because it more quickly absorbed heat (energy).

The basic advantage with AL heads would have been the reduction in overall vehicle weight, especially in the nose. The cars did handle a bit better with AL heads but all else seemed to come out about even.

Most engine builders will agre that the AL heads are easier to modify, save weight and are also much easier to repair but there is a slight HP loss if all else is equal.

[Warren F.]

Re: 1967 L-71 vs. 1968 L-71/L-89

Michael,

There is significant differences in aluminum heads. Besides some minor external differences for '67, '68 & early '69 heads, in temp sending locations, pipe plugs, plus a slight advantage with the larger exhaust valve diameter. The L-88/ZL-1/LS-6 head differs in many ways. Larger exhaust valve diameter again, intake port floor contoured differently, exhaust port shaped differently, combustion chamber shaped differently. Tests shown in various magazines of the day(era)showed showed significant improvement in horsepower gains over cast iron equivolents. The amounts shown varied from each magazine articles from 20 to 30hp to 15 to 20%.

[Michael H.]

Re: 1967 L-71 vs. 1968 L-71/L-89

Warren,

Don't believe all that you read in the hot rod magazines. Yes, there were several casting changes over the years but most had to do with open exhaust, high RPM operation. Many of the port modifications that occured in 1968-69 actually reduced air velocity (not volume) which made the "on the street" and "through mufflers" HP numbers decrease instead of increase.

For typical on street operation, the HP numbers for cast iron/aluminum heads were about the same.

If we're to believe the magazines claim of 15-20% increase in HP, that would change an L71 from 435 HP to somewhere around 500+ HP and mysterious magical things like that just didn't happen. I think GM was pretty close when they rated both engines at the same HP number.