[Formulajones]

Dennis is right on the money. I also run the Pypes transverse setup on my bird. When I switched from the stock Walker system that was 2 1/4" and upgraded to the mandrel 2 1/2" system my car picked up a solid 3 tenths and a little more than ~3 mph with no other changes at all.

['ol Pinion head]

Originally Posted by Formulajones

Dennis is right on the money. I also run the Pypes transverse setup on my bird. When I switched from the stock Walker system that was 2 1/4" and upgraded to the mandrel 2 1/2" system my car picked up a solid 3 tenths and a little more than ~3 mph with no other changes at all.

And Jones, if you would have had the day one original corked up '70-72 stock muffler system, what do you think the car would have picked up? .5, .6?

Have been buying Walker Dynomax mufflers for 20 years, all of them have been noted for higher flow than stock, that's why have bought them.

[Formulajones]

Originally Posted by 'ol Pinion head

And Jones, if you would have had the day one original corked up '70-72 stock muffler system, what do you think the car would have picked up? .5, .6?

Have been buying Walker Dynomax mufflers for 20 years, all of them have been noted for higher flow than stock, that's why have bought them.

My appologies, I said Walker. I had that on the brain when Dennis mentioned their mufflers as I use them on another car here. I meant to say I had a Gardner system on the firebird, which is about as close to the original as it gets.
Back then, I switched to a Flowmaster transverse Force II kit as that was (if I remember correctly) all that was commercially available at the time which still used a factory transverse style setup. It was 2 1/2" mandrel bent and used an "H" pipe. This system is where I found another 3 tenths in the car.
A few years ago Pypes came out with their own 2 1/2" mandrel system, made of stainless, and uses an "X" pipe. I also liked the design of this muffler a little better with inlet/outlets in more of a stock location.
So I bought this system and installed it in place of the old flowmaster kit, curious to see if it had any further gain over what I had. If so I think it would be marginal at best, but I agree with Dennis, it's a very nice system, fits excellent and sounds fantastic. Very mellow and nice for cruising, yet will make a little noise when on the go pedal.
I haven't raced the car since having this system so can't report any feedback on possible gains.

Edit: Ol' Pinion, just to add, knowing these old Flowmaster kits have had a reputation of being restrictive (so people say nowadays, I don't know for sure) it's possible that the .5 or .6 gain you asked might have been possible if I had a better system available to me at the time. I'd love to run the car again with this Pypes system and see if that would be true. I have all the runs with the car over the years well documented with DA numbers to keep accurate records of any tweaks I found usefull, so even today I could run the car and look back to see if it was worth anything.