Did a quick search on vortec heads and couldn't find the answer to my question. I've always read that you couldn't put Mark IV heads on a Gen V big block because the coolant holes are shaped different. Also read that GM corrected this problem with the Gen VI big block. I'm wondering if you can put a vortec head on a Mark IV block. Anyone ever tried this?

I dunno if they fit or not, but I wouldn't bother. I put a set on my Gen VI, and didn't really pick up a whole lot, maybe 2 mph max. The real issue came with the compression. It detonates on anything less then 91 octane, and that's with the timing set at 30 degrees total advance..

pay a few hundred more bucks and pick up a set of merlin ovals....

just my 2 cents worth.!

I figured the static compression ratio would go up to 9.2 or so with the 100cc chambers. Thought this might be a good thing but sounds like you had a problem. Do you think you were maybe a little lean with the vortec heads?

You're supposed to run no more than 28 degrees total advance with the Vortec heads!

That explains the detonation. 28 degrees because of the static compression ratio?

The Vortec heads in both small and big blocks have fast burn combustion chambers. They don't require ignition as early as conventional heads. 9:1 is the stock compression and should work with 87 octane. You can raise the compression and run higher octane but the timing requirements don't change. In fact, while doing some testing on a MPI vortec engine, the stock Merc setup was only running about 24 degrees total timing. We modified the engine and reprogrammed the ECU to end up at 28 degrees maximum.

The vortech heads need porting to make them work. Chamber work, full exhaust port, bowl port on the intake would be the minimum.