P90 with SU intake?

[steve91tt]

I am in the market for a new head for my 3.0 liter bottom end. I am currently using the stock 1973 E88 head and cam with about 10:1 compression. The car runs great if I keep the timing less than 30° total advance on 93 octane. If I run more timing or less octane then I get detonation. From what I have read much of the issue is the shape of the E88 combustion chamber.

I like what I have read about the performance of the P79 or P90 head. I may loose a little compression compared to the E88 but it should flow much better in my setup and allow me to run more timing.

Question: If I run a FI head with my stock SU intake will the intake runners line up or will FI notches in the head cause issues? Do I need to have the FI notches welded up?

[Arne]

Lots of people running SUs on P79/90 heads. I had a P79 in my yellow car for a while, and beandip has been running SUs on his P79 for years. The notches in the head are covered by the manifold fine. You'll probably want to find a P90 so that you don't have to swap to a new round-port header or exhaust manifold.

If the stock P79/90 compression isn't enough, I've read of several people who have shaved their P79/90 .070", shimmed the cam towers back up by about the same amount, and use the earlier L28 valves that have .070" longer stems. That should get you close to your current 10:1 ratio but with the better chamber shape.

Edited April 5, 2011 by Arne

[steve91tt]

Outstanding. Thanks Arne.

[LeonV]

If the stock P79/90 compression isn't enough, I've read of several people who have shaved their P79/90 .070", shimmed the cam towers back up by about the same amount, and use the earlier L28 valves that have .070" longer stems. That should get you close to your current 10:1 ratio but with the better chamber shape.

Change that to .080" shave and .080" longer stems and that would be correct.

Speaking of combustion chambers, a more efficient one will require less MBT timing so if you plan to go that route don't just advance as much as possible as you may pass MBT. Just FYI.

Edited April 5, 2011 by LeonV
Typo

[Arne]

Thanks for correcting that, Leon. I was shooting from memory, couldn't remember the exact difference.

[steve91tt]

Sorry, what does mbt stand for?

[LeonV]

Sorry, what does mbt stand for?

"Maximum Brake Torque"

It is a term for the highest torque value seen while varying spark timing, holding load and rpm steady.

[steve91tt]

Thank you!

[Diseazd]

Steve....I'm running the P79 shaved .080 and towers shimmed .080 on 2 of my Z's with the F54 block. I'm also running SU's with the early intakes. No problem on the fuel injection notches. I'm also running the early stock exhaust manifold and gasket.......again....no problem. We took a household dremel and opened up the exhaust manifold a little to match the ports. We run the Isky Stage III cams in both engines. They run awesome with no detonation problem what-so-ever! Here's a thread that may help. http://www.classiczcars.com/forums/showthread.php?t=34448

[steve91tt]

Great build thread. Thanks for the link.