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Well the Z24 isn't a barn burner to begin with. The stock exhaust manifold is not going to restrict flow enough to worry about. It's an expensive way to find out that there's really no improvement in this direction. Save your money and spend it on the following...


What will improve the flow is removing the catalytic converter (if you do not require it for smog checks*) and replacing the stock muffler with something larger or a 'turbo' style muffler. Not a straight through style. Have a shop bend you a 2.25" exhaust system from the manifold back or anything 1/4" larger** than the pipe you have and match the 'muffler' to it. I think the 720 uses 2" pipe but it might be smaller.


If you just 'have to have it' be sure to not throw the stock manifold away because when you get tired of the header you will want to put it back on. Get a ceramic coated one or have it coated (not cheap) to make it last a little longer***. Expensive headers usually fit. Almost all tend to come loose, leak, rust, increase under hood temperatures, and are noisy. Add to that you have to butcher your down pipe and the labor time to fit it. They never just 'bolt on'.   


* If you do need it for a visual smog check, remove and empty it out by removing the insides so it is just a hollow container.


** Never go too large with exhaust pipe. Going from 2 to 2 1/4" is an increase if flow of 26%!!!! More than you will ever need.


*** Headers do not last forever. They are just muffler pipe after all.

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* If you do need it (catalytic converter) for a visual smog check, remove and empty it out by removing the insides so it is just a hollow container.



When I gut mine, I find a piece of exhaust tube that fits inside it, so it's a nice smooth strait through passage.

Sometimes I use a piece of perforated tube, and turn the old cat into a pre-muffler.

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L block headers are over rated, I have only one made of thick metal(never used yet), all the others last around 5 years before they need replaced(they leak sooner sometimes and I fix them), I am at about 5 years now and I plan to use an L16 exhaust manifold when I take it apart in the near future, I have a few new L block headers in my attic but I am tired of fixing them when they leak, I likely have went thru 4/5 headers in the last 20 years, it is time to get out of the header loop.

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Maybe the climate where you live, and/or the rest of the system is inducing stress into the header.


The one on my 620 was old, and abused when I bought the truck 11 or so years ago, and it's still in one piece.

But I live in Texas, so very few cold mornings, and little rain, so very little condensation in my exhaust.


My Pacesetter on the Cressida was repainted several times over a 2-3 year period, until I got fed up with 'high temp' paints, that weren't.

So I had it ceramic coated.

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I only have them on my work truck which is a daily driver, it don't sit and rust because of condensation like Datsuns do in fields, but it is not covered and never has been.

I may use that thick/heavy header I bought next, but I also have an L16 exhaust manifold and head pipe/exhaust pipe all the way back to where the muffler was.

I don't know what I am going to do yet, in the next couple months I am going to transfer the work truck cab and drive train to a 1986 Nissan 720 frame I have, as the modified 521 frame either needs new 521 frame welded in, or something custom needs to be made that will work with the power steering gear and the idler arm, if I just use the 720 frame I don't have to re-invent the wheel again, as it is made for power steering which is causing my issues with the 521, it was never made for power steering and the side pressures I have the way I did it.

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I would have used a stock manifold, if I could have found one of the ones without the huge road blocks at the mouth of the inlet side.

Seen pictures of them, but haven't ever been able to find one for sale.

So I have headers now, kind of like wayno, all sorts of headers........

Besides, they weigh less.

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The very earliest would be the best. Early with air injection holes but discrete 4 into 2 piping next best and last the non-separated 4 into 2 manifold.


The air injection holes may only be on the ribbed ones. I don't know. The ribbing was for the ATC and that is benign. On my L20B (U67 head) I have injection holes that someone had previously put pipe plugs into, I ground the bumps down, they aren't much. In any case The L16/18 manifolds are better than the later 4 into 1 L20B exhausts.

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