Finally got the car dynoed after 3 years of having it together. NA 405hp and 539tq (converter flash) with the exhaust uncapped. And this on a Mustang dyno with a th400! It made 370hp through the flowmaster. A big thanks to Charlie and Cindy at Auto Motion in Louisville, KY

 

Very nice. Do you have any details on your build?

 

that's nice through the TH400.

 

Wow, 35 RWHP for uncapping the flowmaster!!! I was hoping it would be more like 5-10, seeing as how I have a FM 80 series and 3" pipe. Glad I added a cutout. Might need an electric actuator, too, for that kind of potential increase on tap.

Is yours 3" exhaust, with the cutout in the usual spot under the rear pass seat?

 

I know another person that gained 30+rwhp with a cutout on a flowmaster catback, its not unheard of.

 

Does Flowmaster suck? Would Magnaflow, or other catbacks, exhibit similar restrictions?

 

The magnaflow catback would be less restrictive, but it starts becoming a restriction at higher power levels. Thats why I'm running true duals, I have 100% of my power all the time.

 

I'm sure at some point any exhaust becomes a restriction, but a 9.5% increase just seems like a lot to me. I'm certainly no expert on these cars. I wish I could find a flow comparison that included all the major brands of catback. The only one I've ever seen didn't include Magnaflow, which is one exhaust I'm considering for my own car. It would be cool to make 405 hp at the wheels.

 

Scat forged crank, Scat 6" h-beams, JE pistons, LE heads, nitrous cam, Crane gold rockers (1.6), Comp R lifters, ported stock intake, 52mm TB, 37lb injectors, Walbro 255 intank pump, CSI ewp, AS&M 1 3/4" headers, Flowmaster catback with cutout where the cat used to be. PTC 3800 converter.

I would also like to point out that the base tune from Bryan @ PCMforless was pretty much spot on. Charlie at Auto Motion didn't really have to change much.

 

The Flowmaster is a piece of crap....Even the 4".

I gained 2 mph by taking it off of my old 396 LT1 and replacing it with a 4" Dynomax Ultraflow round.

 

I love my magnaflow. And like someone else said, there is a point where ANY exhaust will become a restriction.
For flowmaster that point is... any point really. Its probably the most restrictive "performance" exhaust out there IMO. Its built purely for sound really, not so much flow. Which is why its so popular with the rice crowd, they just want to be loud.
I made more power switching the flowmaster w/ gutted cat that was on my car when I bought it, to a magnaflow WITH a cat(hi-flow).
To me its pretty clear which is the better exhaust... Magnaflow.
But to each his own, and every car is different.

 

405rwhp at the tires is strong on a Mustang Dyno for sure. It's a easy 20rwhp at least lower than a Dynojet.

Bret

 

I personally gained 33rwhp and almost 40rwtq when I opened my cutouts right off the header bypassing a flowmaster. It was a rediculous difference.

As for 405rwhp on a mustang dyno, that sounds a bit high to me. Are you sure the dyno wasn't set to a specified correction factor. I know the dyno at my buddies shop can print out in numbers almost identical to a dynojet.

But I guess the proof will be in the track times, 405rwhp through a th400 should be equivilent to 450ish rwhp in a 6spd. So maybe mid to low 120mph? Good luck with the car sounds like a good setup.

 

I don't know. I will ask when I go back to get a new fuel pressure safety switch for the juice.

 

I know of a 550 rwhp LS1 that made more hp with a 1 chamber flowmaster than it did with SEVERAL other mufflers!

Maybe because it was a 1 chamber?