Which head to use?

[240ZMan]

http://www.classiczcars.com/forums/showthread.php?s=&threadid=11830

I asked a similar question earlier, but didn't get an feedback and since then have learned more ... and still have questions.

I am getting an 80 280z engine complete with the FI. My current plan is to put it into my 73 240 and run with my round top carbs. I have 2 main questions:

1) am I giving up any HP by using carbs vs. FI?

2) my existing engine (200+k on the bottom end and bad compression) has an E31 head with unknown mileage. The new engine's N47 head (and block) have about 80k miles. Which head should I use

I've read that the N47's exhaust ports aren't the best shape and that the exhaust manifold won't bolt up to my existing down pipes. I've also read that the E31's valves are smaller. It would be easier and cheaper I think to put the E31 head on with my current manifolds and exhaust system. Cheap is important right now as I need to wait until later to make further improvements for breathing.

Any advice would be much appreciated.

[zeiss150]

hmmm there isnt a big huge differance in CR between heads the E-31 is 42.4 CC and the N-47 is 44.6 CC. The L28 block your using has dished pistons and that is your main problem and it really lowers your CR. even with the E-31 head your CR is only 8.56.1. TheN-47 head has the steel liners in the exaust to burn off any unburned fule. From what I have read It doesn't interfear with exaust flow. The N-47 does have bigger valves but the low compression(8.29:1) is no fun. The best of both worlds would be to have your E-31 head rebuilt with new steel seats and larger valves. But if money is a problem that would set you back a good $400.00. If your E-31 head has never been rebuilt it really should because it has brass valve seats and they sink over time, but so does the N-47, so either way you probably have bad vavle seats. As far as bolting up your exaust manifold I belive that all the heads are exactly the same, I could be wrong though. Using the engine calculator the best engine set up would be L-28 block/ dished pistons, E-31 with new valve seats and larger (N-47) vavles, Use an HKS 1mm head gasket that would give you a CR of 8.75:1. the other option is to get a flat top piston L28 block with your E-31 head with a stock head gasket gives you a 10.2:1 CR:D . Then all you will need is a Header and exaust packedg a good set of SU's electronic ignition a mild cam and you should have yourself a bad arse rocket . I hope this info was helpful.

Matt-

[mdbrandy]

Originally posted by zeiss150 As far as bolting up your exaust manifold I belive that all the heads are exactly the same, I could be wrong though.

E31 has square exhaust ports, and the N47 has round ones. The square ones are supposedly better. Couldn't tell you why, though. Probably flow.

[240ZMan]

Thanks Zeiss150, that's helpful. Sounds like it's a wash between the two if I don't rebuild. Of course, I do plan to do a rebuild later. I think then I'll go with the E31 head, larger valves, more agressive cam, thinner gasket, keep my SU carbs, and a better exhaust and life will be good. :classic:

In the mean time, it seems like whichever bolts up to my existing exhaust will be the better choice. I read somewhere that the flange on the N47 exhaust manifold is different than for the E31. Anyone know about this?

Also, I have a chance to get the exhaust from the donor car. He says it includes a Dynomax super turbo muffler and 2-3/4" mandral bent aluminized

pipe 1 year old. Anyone know if I can use this with my 73? The donor is an 80 280z. I'm sure it won't just bolt up, but if it basically fits and I just need to attach new hangers that would be sweet.

[san_maru_zed]

The N47 does NOT flow as well as the E31. I have seen flow ratings for both heads(although the E31 did have oversized valves) and the N47 is definately lacking.

[240ZMan]

So it boils down to 2 choices for me:

1) use the N47 head with its stock exhaust manifold and find a way to get the aftermarket exhaust (2.75") to work on my 73.

2) use my E31 head with the 73 stock exhaust manifold and the existing exhaust (Monza 1.75" more than 20 years old).

Both will use my rebuilt roundtops. Which would you choose?

[zeiss150]

Hey Daniel,

How about option #3...

Use the E31 head (don't for get to put in new steel seats) buy a motor sport header 6 into 2, 2 into 1 (which will bolt right up to that head) then get a 2 3/4 inch Y tube that comes with the header and run your exaust. I have the 2 1/2 inch exaust with a dynomax muffler and it fits perfect on my 72 240z(E-88 head). I dont know if the 80 zx pipes have the same bends in them, if anything you can use the muffler the pipes aren't that expensive. But what ever you do dump that stock exaust manifold in the trash and go with a header. My header from motorsport was $154.00 with the Y pipe and the gasket. Just so you know... that E-31 head is a very popular head, they are hard to find. Most people wanted them to make a high compression racer. but it could make a bitch'n street car.

Matt-