Pontiac Owners Club UK

Hi guys does anyone know what is the best diameter exhaust to use with a Pontiac 455 in a second gen trans am is 2.5 inch or 3 inch best I am thinking of getting a magnaflow exhaust but not sure what is the best diameter to use . thanks

Don't really know much about this but i would say 2.5,bigger is not always better although it might be louder,that's why i started putting 3" on mine,thinking it might make it louder,lol.Not bothered about performance gains,probably gonna go with 2.5" when i re do the exhaust.
I was told years ago by Peter Cooper (parts shop in Pickering) that 2.5" was biggest i needed on a 400.
I know there is single pipe systems that can add more performance than duals.

thanks mate mine is a 455 so would 3 inch be better my engine isn't standard it has forged pistons etc etc

I have ordered this exhaust now http://www.ebay.co.uk/itm/380676803034? ... EBIDX%3AIT

I have seen 455 cars with 500 bhp that use 2.5".

I struggled to find the magnaflow 2.5 system it was easier to find the 3 inch system so I just ordered 3 inch

cool I went with the 3 inch simply because it was easier to find so got the 3 inch magnaflow 15899 its gonna need a little alteration at the back to fit the tail pipes but I have bought them as awell

i think with 3 inch you may suffer with loss of torque. its like having a 6 inch diameter single exhaust. For 7.5 litres and roughly 350hp thats way too large and scavenging will be affected.

The Autopontiac race car which is 600hp+ and has 540 ft/lbs torque is on 2.5 inch pipes.

bhm1712 wrote:i think with 3 inch you may suffer with loss of torque. its like having a 6 inch diameter single exhaust. .

dose not work out like that
3 inch works out about 7 sq. in
6 inch works out about 28 sq. in

yes true, so really its like having a 4.5 inch single exhaust diameter.

but either way, in my opinion 3 inch duals is still too large for a 455 motor for the best scavenging and performance.

would be nice to be in a position to do some dyno testing with several different exhaust sizes on the same engine to show the difference in HP and torque.

I would say 2.5 is plenty on duals

ahh ok I went with the 3 inch as it was easier to find