Mystery of the ZL/X

[markjohnson]

I was going through some old magazines the other night and ran across some interesting advertisements from all the big Supercar dealers. Motion and Berger quite a bit. Both of these dealers offered something called a ZL/X. This is not something that I see talked about very often. I was always kinda under the assumption that a ZL/X was a an exact copy of an open-chambered ZL-1, but only in affordable cast-iron. I even owned what believed to have been one back in the Eighties. A 1971 over-the-counter 427 with a 512 block with open-chambered 12.5 GM pistons and cast iron #026 heads. If you look these heads up, they are shown as 1971 LS-6 heads. Well, we all know that that didn't happen. As far the LS-6 Corvettes in 1971, they came with aluminum 074's. That leaves the orphan'ed #026's as the prime candidate for being ZL-X heads with their cast-iron construction, rectangular ports and open chambers. It's probably even a superior head, although heavier, than the aluminum #074's with their terrible giant exhaust port.

[ANDY M]

Mark, you're forgetting about the -990 heads. Cast iron, square port open chambered head. Sold over the counter, with an L-72 long block, made for a mean motor.

[ORIGLS6]

Didn't someone post a scan of an old B/M ad here a while back that listed all the GM P/Ns for the ZL-X? I know I've seen it somewhere.

[markjohnson]

Yeah, I know about those #990 heads. There's also the #188 heads. I've seen several of those heads on LS-6 and LS-7 crate motors all the way up into the '90's. I think the 026's, 990's, and 188's are all VERY similar. I'll tell 'ya, I've had some friends go VERY fast on those 990 heads. How about 8.60's in a 10.5 Shootout-style 72 Nova!? That was around 540 inches with a pretty heavy shot of the juice, but still that's really impressive for an old out-dated cast-iron head when everybody says you have to have the latest, greatest most modern aluminum heads to go fast.

[LS6 RAT]

Actually, the 1971 LS6 Corvette engine option is a direct result of Duntov wanting to keep the Corvette as the flagship performance vehicle in the Chevrolet lineup. This option was derived from Duntov still persisting on bringing the LS-7 option back in the '71 model year. Several features of the originally planned LS-7 option for 1970 were incorporated into the '71 Corvette LS6 option. Duntov was able to put this engine option together and get GM's approval of it, because he could assemble the pieces without much new tooling costs. GM knew this engine option would not be available the following year and did not see any point to spending a considerable amount of expense to an option that would only be available for a part of one year. Duntov's insistance was instrumental in this option seeing the light of day during the 1971 model year. In fact it was not approved yet when the '71 brochures were printed in August 1970. Several original owners of LS6 Corvettes placed their orders for this option back in October, November, December of 1970, only to be told in written letter form from Joe Pike [editor and public relations for GM and Corvette News magazine]that the LS6 option was not available at this time. The option was approved for production by upper GM management in January 1971 and several revision sheets with specs for the option were printed and sent to Dealers to replace previous pages in their Dealer Sales Albums for the 1971 model year options and specifications. Even the earlier 1971 Corvette purchase order forms were revised twice to change the information from LS7 option of 450hp to LS7 425hp to LS6 425hp.

[markjohnson]

I've always been a huge fan of the 1971 LS-6 Corvettes. Factory aluminum heads, aluminum intake and Holley, and the fact of being they were the last solid lifter'ed big blocks after a complete absence of such in 1970. Duntov's last stand......I'd have to agree.

[LS6 RAT]

They really were a complete packaged option. Most LS6 Corvettes were M22 ordered [122 out of 130], also came with MA-6 [heavy duty dual plate clutch assembly] and magnetic pulse transistorized ignition system [TI].