Square peg round hole: or using a round header on a square head

[Kurbycar32]

I have a L28 motor with N42 head in my 260z, it was like that when I bought the car. A friend of mine had a header literally lying around his shop and gave it to me. Its got round ports and appears to be ceramic coated so score. The main goal is to be able to reduce under hood temps which I figure I can do by wrapping these headers, I have done this on dirt bikes and v8's with excellent results. My N42 head has square exhaust ports, this is a round header..

1. Will this mount properly with standard gaskets?

2. Obviously there will be some restriction going from the square hole to the round one, but will it still perform better than the stock manifold?

3. I plan on buying a head that's already been rebuilt in a few months, should i move to an N47 to get the better valve seats and round ports?

[ksbeta]

I have the opposite problem. Want to send me that N42?

[ozconnection]

I have the opposite problem. Want to send me that N42?

I've just finished putting a square header onto a round port exhaust head. I swear it makes absolutely no difference. No leaks. The liner in the round port makes it much smaller than the square port header opening.

As for a square port and round header...dunno but I have the feeling it may not work quite as well as the other combination. I could be wrong.

[Kurbycar32]

I have the opposite problem. Want to send me that N42?

Actually your thread made me start thinking about the whole thing.

[John Coffey]

1. Will this mount properly with standard gaskets?

2. Obviously there will be some restriction going from the square hole to the round one, but will it still perform better than the stock manifold?

3. I plan on buying a head that's already been rebuilt in a few months, should i move to an N47 to get the better valve seats and round ports?

1. No.

2. Hell no.

3. Don't make you head purchase choice dependent on a free header.

Sell that round port header on eBay and save your money for the correct header for your cylinder head or install the stock exhaust manifold.

Edited September 26, 2012 by John Coffey

[Kurbycar32]

1. No.

2. Hell no.

3. Don't make you head purchase choice dependent on a free header.

Sell that round port header on eBay and save your money for the correct header for your cylinder head or install the stock exhaust manifold.

Thanks John. I wouldn't buy a head to match my free header but maybe you can chime in on the differences between an N42 and N47 head. After reading some threads on this site and others my understanding is the N42 was built for leaded gas, has brass valve seats and square exhaust outlets. The N47 head has similar flow but steel valve seats and round exhaust outlets. Since I plan on buying a rebuilt head anyway I figured I could hold on to this header and buy an N47. The only good machine shop within about 50 miles of me closed (probably because they were assholes) or I would just have my head rebuilt locally. My motor is in an running fine with the stock manifold BTW.

[John Coffey]

If you're going to have the head rebuilt, replacing the valve seats on a 40 year old head is a good idea regardless of the head casting. Given that, the N42 is probably the preferred choice just because of the performance potential down the road. But for anything under 225hp either head it fine.

[Kurbycar32]

Thanks again John. Ill see about getting my N42 rebuilt or exchanged for the same model and sell/trade this header. Problem solved!