difference between heads

[whitedevilstealth]

What is the difference between the DOHC NA heads and the Twin Turbo heads?

[stealthee]

TT heads flow better.

[Amphiron]

I believe the TT heads are ported a bit more, or the valves are a bit bigger. The exhaust cam has more duration as well.

[stealthee]

Valves are the same size.

[tein]

Can you bolt TT heads on to an NA has an upgrade?

[Tsi_shinobi]

yep the ports are different between turbo and NA heads

[J. Fast]

The parts and internals are identical from the 1st gen N/A and TT heads, valves, lifters, springs, etc.... The exhaust ports are slightly larger on the TT heads but can be matched with about and hour or so of port work. I would say they're identical as according to Jeff Lucius' findings from bench testing their was a flow variation of 3-5 cfm.

On to the 2nd gen heads. The heads are completely different between the 2nd gen's. There was a revision on the mid 1994 models as the intake and exhaust ports are dramatically different. There is no way to port match 2nd gen N/A heads to the same spec as the 2nd gen TT heads as the water jacket on the N/A is to close to the wall to port to the same spec. The internals are nearly identical on all models with the exception of the cam profiles.

Awhile ago at co3s we had a dicussion involving N/A heads and TT heads being identical. I stated they were. I based that off of comparing a set of 1st gen N/A and TT heads. After comparing a set of 2nd gen TT heads with a set of N/A heads, Gary Melcher had at MHM, I realized there must have been a revision, hence the dramatic change in the exhaust and intake port shape and size.

I would suspect the reason why first gen cams were found to make more power with 2nd gen heads would be the result of the INCREASED PORT SIZE ON THE INTAKE AND EXHAUST combined with the difference in lift duration/overlap between the 1st and 2nd gen cams. Ofcourse it's a big assumption however I'll throw a guess out there and say 1st gen heads have a smaller port so the cam profile was more agressive to make more power. When mitsu increased the ports on the 2nd gen they changed the profile to a more economical profile, hence swapping a set of 1st gen cams on 2nd gen heads resulted in IPO's discovery of 25 more hp.

2nd, there's a few threads in some closed forums with cutaways of our engines I can't point you to them but if you use your google finger you may be able to dig them up 8)

I can't say anything about 3rd gen heads as I've never seen a set of NA or TT heads from that generation so I have no idea if they modified the design again for those years.

Jeremy

[GTOJOE]

If you take the time to read the post that Steve linked it is a question I asked about JDM NA and TT heads and the differences. The pics he posted show the differences. Obviously the USDM heads are different to the JDM heads. But worth a look.

[J_Parker]

I think I'll have to take a wild stab at a downgrade man. We all know that N/A's like their backpressure.

[stealthee]

Backpressure is a myth. You want free flowing heads with the highest velocity.