I was hoping to buy a set of them but they are back ordered. Thornton says at least six months before they will make them. I am thinking about using their X W manifolds because I was told the other W Z repos are no good. Has anyone had good or bad results with the other manifolds? Mild 455 going into a 1977 442. TIA

 

The 69-72 W/Z manifolds (W #402295 LH and Z #402294 RH) have individual runners inside and flow quite well. The 65-67 X/W manifolds (X #389269 LH and W #389268 RH) are open log manifolds that do not have this feature. I would not use them for any application other than a correct restoration. It's a waste of money. The other good choice would be the 73-74 X/Z manifolds (X #412287 LH and Z #402294 RH) if you can find good used ones. The X/Z pair are also internally divided and flow about the same as the W/Z pair.

 

Are the Thorton manifolds made with a 2 1/2" outlet instead of the stock 2 1/4"? GTO and Firebird owners can get repro 'ram air' exhaust manifolds that look stock but have the bigger than original outlet. They dyno close to what headers put out.

 

Do you have proof of that from someone other than the manifold manufacturer?
In my experience that’s not remotely true, Pontiac or Oldsmobile. Did a 375 Stroker SBO. It made over 30 hp more virtually everywhere on headers vs manifolds. Plus the headers cured a less than perfect fuel curve.

 

Some cars really suffered from exhaust manifold design. My engine builder saw a 100hp swing on a stroked BB Mopar on a dyno run, you can't leave that much on the table.

 

I am not building a race car. I just want to know if anyone has used manifolds other than Thornton and what their experience was.

 

I hear ya. But whats forgotten is a tuned, free flowing exhaust is one of the few things you can do that will make more power and most often, make it more efficient overall.
Just sayin.

 

On the Performance Years Pontiac forum numerous people have posted dyno sheets showing headers vs the ram air manifolds. Pontiacs have the common 'log' manifolds which most cars received. They had the ram air manifolds starting in '67 on the hotter optional engines and there was the 'long branch' manifolds on hotter engines installed in big cars in the early 60s and '67-'69 Firebirds. Ram Air Enterprises makes a reproduction ram air manifold with larger outlets which performs very well; even on 455+ cid motors with big roller cams.

 

Ram Air Restorations shows an 'oversize outlet' W&Z set of manifolds but it doesn't look available right now. Photo but no price.

 

I dont know if there is more than manufacturer making those w z repros? I would expect theyre a relatively low volume product made by i manufacturer and sold by more than retailer.

 

I talked to someone who had a bad experience and recommend only Thornton. I talked to Thornton and they said at least six months before they make more. Some suppliers are out of stock and sell Thornton. Others are out of stock and say more will be available later this month. Others have them in stock and when I ask who makes them they can not give me an answer.

 

I found some Thorntons. Nice manifolds.

 

Came across this post, i am considering the Thorntons in my 64 its been converted to a 455 and the manifolds on there now are crap. How have the Thornton's been for you? Do you have any feed back or pointers on how they would go into my 64?
Thanks
Rob

 

Hear this...

 

Has anybody used these? They are expensive but are impressive looking. That is some of the best welding I have ever seen.


Sorry I could not get the image any bigger. Here is the link.

https://www.ebay.com/itm/38449649061...53.m2763.l2648

 

Here you go Glenn

 

i have them and im very happy with them .The price has gone up considerably since i bought mine a coupe years ago

 

Thanks guys. I am wondering how much HP I would pickup on my stock 75 Hurst Olds. It currently has true duals on it now with stock manifolds. The right manifold has a slight leak now and is missing the middle bolt in the manifold. I doubt I will spend this much but these headers are really attractive.

 

Not to hijack this thread “but” any opinions on the HP gain from 1969/72 W/Z VS 1965/67 X/W manifolds? Asking for a Friend.

 

The manifolds are well made, easy to install ,and look good but I don’t think you will feel much of a difference in hp.

 

Thanks Jeff. For the money and work required I think I will stick with what I have.