ELECMAN

The previous owner of the car had his mech put a 455 motor in the car.it originally had a 350.he used the exhaust manifold's from the 350 motor.he blocked off the down pipe hole when using single exhaust.is this a bad thing and can this cause the exhaust manifold gaskets leak.

mugzilla

Not bad but not good ...

The back of the manifold usually cracks and leaks, inspect, replace ...

You probably have a lot of back pressure with that set up ...

ELECMAN

What would back pressure cause

mugzilla

exhaust leaks, slow accelleration, poor mileage ...

But better low end torque ...

gearheads78

An engine is a air pump. You have to get air in and out efficiently. When you block the exit you can't get air in because the exhaust is still in there with nowhere to go. A very restictive exhaust can easily cut 100HP or more out of the power you are making.

Eric Anderson

If you are talking about the blocking off where the crossover pipe connects when running a single exh. setup, thats normal when running dual exhaust on a 350 they came that way from the factory. As for 350 exh. manifolds, IMHO, they are not very good, find a pair of W Z manifolds (would be my choice) or headers if you want to go that route.

ELECMAN

THE LEFT MANIFOLD HAS A bIG #1 380001 GM ON IT AND THE PASS HAS A BIG # 4 ON IT IS THIS A 350 EXHAUST MANIFOLD.IF I WOULD PUT THE CORRECT EXHAUST MANIFOLD,S ON WILL THE EXHAUST SYSTEM HAVE TO BE ADJUSTED AND IS THERE MAYBE A KIT

joe_padavano

As noted, the factory used a cap on the crossover port for dual exhaust small blocks. The problem is that the SBO manifolds are crappy for a 350 and are a significant restriction for the BBO. Why bother swapping the motor, only to give up 20-30 HP? Change them.

ELECMAN

What is a good set of headers for the car

silverriff

I am about to get a new exhaust system on my 455 and after doing a ton a research on headers and decided not to get them and just stick with my stock w/z manifolds. I read that unless you spend a pretty penny on the headers, any cheaper headers are going to require some modification of either the frame or headers themselves, just because the fit isn't so good. Take a look at some of the other forums on here and read some of peoples personal expeiren es.

ELECMAN

What size is the opening coming out of the 350 exhaust manifold,is it the same size as the w & z heads or do i need adapters

agtw31

get rid of that whole exhaust system and go 2 1/2" all the way back.

i bought some dynomax ceramic coated for my 72 U code 455 ragtop years ago,i didnt have any problems installing them.
my brother has Hookers on his W-31,they fit perfect.

only problem with headers is blowing an exhaust gasket out now and then,if the flange isn't perfectly straight.