[Jay S]

I did 91 octane pump gas 64 389 engine with flat tops, stock 716 heads, and log manifolds earlier this year. It would be great to know what the Erson cam is in the last 389 example.

My highest compression we have done is 12.2 on 91 octane with iron heads. Pontiac’s the highest is just over 11:1 in 91 with iron heads. Highest on 87 octane with iron heads is 10.5 SCR with 500+ cid’s. Most instances the those high compression pump gas car’s would be quicker with race gas and a different cam. If your content with how the engine runs, and don’t mind adding race gas like the Jeff “geeteeeohguy” , then leave it alone, no reason to change things.

The term bigger cam is usually associated with more overlap, more overlap with a bigger cam will just “move” were the engine has spark knock. That last post was exactly right on that.

With only 150 to 160 crank pressure, the engine may have a tuning issue. But it does not appear at 150-160 that the cylinder pressure needs to be spread out more and lower by using a different cam. As is, with the cam in it, lowering the compression, you would have to drop the cranking pressure down another 10 points to get down to 93 octane. Lowering the compression with thicker head gaskets is probably the worst option. The thicker head gasket fairs a little better than putting the piston down in the deck .100” like what Skip mentioned. But not a lot better, I am not totally sure what that is, my guess is the improvement is mostly because of a difference in turbulence.

Steve25 really nailed some of the issues with this engine in post #9. Read #9 again, start by reading the plugs. The pumping numbers are really pretty low for as much compression the engine has. Dropping the compression will lower the pumping number more yet. That isn’t necessarily good with the cam in it. I attached events for the Lunati cam in it. It has a lot of seat timing, and as much overlap as a Melling SPC-8 (041) cam. It would pump similar compression also. But the SPC-8 opens the exhaust valve up 10* earlier. Those early 389s need help in that area to run well on pump gas. That is worth about 2 point in octane on this engine with the same compression. That would get it down to around 93 octane with no other changes and you should not have add race gas anymore. I am not suggesting that is what it needs, it is just a basic example. There are MUCH better cam solutions that that. The Lunati cam was a pretty poor pump gas cam pick with the 77 heads and 10+ compression.