[indyav8n]

Can anyone confirm for me that the exhaust size on an original 70 GP with the 455 engine is 2 1/2 inches diameter or is it the 2 1/4 diameter ?

Thanks Wes

[david400]

Well i know its not 2.5 , I thought it was 1.75 " You mite want to consider longbranch exhaust manifolds , like the 69 HO gp cars . They are 2.5".

[indyav8n]

David,
The original owner told me when had the exhaust system replaced they used a 2 1/4 pipe and it was suppose to be 2 1/2. But as I look at the manifolds I am not sure. I need to get it up and measure what is there. Those long branch manifolds sound interesting though.
Thanks

[david400]

I paid one hundred bucks for mine,, but these with the coating

http://www.ebay.com/itm/Pontiac-1965...b9f166&vxp=mtr

if you want to spend that much are probably not to bad of a deal ,,they can be had for less without the coating,, they are repo,, manufactured also.

[indyav8n]

OUCH!
I will do some looking around, I really don't need the coating...

Great job on your GP too!
Wes

[Matt Meaney]

the head pipes for the stock log style manifolds should be 2" and the tails 2 1/4".

[indyav8n]

Matt,
That makes more sense, I did get it up in the air and checked. The headers are 2, I don't know where he got the 2 1/2 from.
Thanks

[rexs73gto]

All the 4 bbl engines used 2" exhaust except the ram air exhaust used 2 1/4". GM didn't start useing 2.5" until they went to the 82 bird when it was just single exhaust. A 70 G/P with the 455HO would have come with the long branch exhaust manifold & the 2 1/4" exhaust pipes. Otherwise if it were just a plain 455 it would have come with just 2" exhaust.

[indyav8n]

Great information and clears up a lot of questions for me, I just need to figure out to upgrade or not!

[Lloyd-TX]

Originally Posted by Matt Meaney

the head pipes for the stock log style manifolds should be 2" and the tails 2 1/4".

Correct!

[Lloyd-TX]

Originally Posted by rexs73gto

A 70 G/P with the 455HO would have come with the long branch exhaust manifold & the 2 1/4" exhaust pipes. Otherwise if it were just a plain 455 it would have come with just 2" exhaust.

Incorrect.

'69 428 HO GPs came with the long branch manifolds and 2 1/4" head pipes. All '70 455 GPs came with log manifolds and 2" head pipes.

[david400]

http://www.ebay.com/itm/65-70-PONTIA...201474&vxp=mtr

Thats not a bad price .

[indyav8n]

Thanks for the heads up on the eBay item David 400
Wes