Olds 455 and TH400 is going in place of a olds 307 and 200-R4 in a 1987 Cadillac Brougham.

Swap is going good, everything is fitting great, 455 is stock, no plans on doing any major modding.
Still have to work on getting a drive shaft made up.

Will the factory 2 1/4 inch single exhaust and no catalytic kill the performance of the 455 a lot?
I will use 455 manifolds and a custom crossover pipe for it.

Or should I see if someone can make me a custom 2.5 or 3 inch single exhaust for the car?

 

A 2-1/2 single has 50% more restriction than two 2-1/4s.

A 3 single is about the same restriction as two 2-1/4s.

Just for your information in making your choice.

You don't note your location so make sure you can legally put in an engine without catalytic converter(s).

 

If going single-pipe, try to get at least a 3" pipe.


If you want to keep it somewhat quiet, get the long-case (20" or 22", I believe) AP Xlerator muffler with 3" inlet/outlet. I ran 10's with two of those mufflers on my Falcon, and the sound was nice but mild.

 

YES!


Why didn't you keep the 2004R?

 

My guess would be money. To make it live behind even a stock 455 would take at least $1500 and maybe closer to $2000 due to the hard parts needed to make it live with the torque.
That and he has a good TH400 that cost's nothing but about $350 for a new driveshaft, yoke and u-joints.

 

While I agree with the use of 2.5" duals, I'll also point out that the 425 in my 67 Delta was rated at 365 HP with a single 2.25" exhaust from the factory. The dual exhaust version was rated at 375 HP.

 

My 88 Cutlass had factory single exhaust with no cat that someone necked down to 2" part way down. Even with a glass pack it went flat fast with mild 403. The Hooker Cat Back system with it's 3" branching to 2.5" duals added a lot in the seat of the pants. Go 3 to 3.5" on the single and go at least 2.25" on the crossover pipe.

 

Various sizes and flow chart

http://www.exhaustvideos.com/faq/how...34149169921875

Remember the rpms you plan to use makes a huge difference in what you choose and where you get more power. Lower rpms means lower horsepower seen. Its your average rpms or torque that needs to be looked at and played too.

Dual 2 1/2 can flow 500hp according to Vizard's testing. The chart I linked to has it showing 463 for dual 2 1/2, but its conservative. Most street driven 455s will have less than 400hp. The muffler flow is the real restriction on flow.