I've pretty much completed my backpressure testing on the T-76 GTS. Here's what I got:

Recent customer build using a 3.5" downpipe and 377 cid:
10 psi boost - 18 psi backpressure
18 psi boost - 26 psi backpressure


My personal car using a 3" downpipe and 363 cid:

With single 3.5" cat-back and a 3.5" Hooker Aerochamber:
11 psi boost - 35 psi backpressure
21 psi boost - 48 psi backpressure

With no cat-back, 3" downpipe only:
19 psi boost - 34 psi backpressure

The next step is to purchase a Magnaflow 3.5" straight-through muffler to replace the Aerochamber and hopefully keep the backpressure down, but keep it quiet.

The interesting part is that I used the exact same boost controller setting when I removed the exhaust and saw a boost increase from 11 to 19 psi. It seems as though the boost controller and wastegate work more off of backpressure than boost since both had the same ~35 psi backpressure.

Another interesting point is that removing the exhaust caused a change from 21 psi/48 psi to 19 psi/34 psi but, even with a huge reduction in backpressure (-14 psi), the quarter mile mph didn't seem to improve. Hmmm. . . 140 mph before and after. But, switching from a bias ply to a radial tire netted a 3 mph increase to 143.

Mike

 

very good information . Now I just have to get to work on making a 3.5 DP

 

Interesting numbers there Mike. I'm still trying to make sense of them. I'll be logging backpressure before the turbine whenever I can get to the track or dyno next.

One thing to note about removing your exhaust at the track and the lack of gains in MPH. I see the definite potential for gain since you went the same MPH with 2 psi less boost. Crank it back up to 21 psi and see what happens.

 

True. 2 psi = 50 hp. Something that also occured to me later is that those runs were on 2 different nights. When I ran 21 psi/48 psi it was cool (70 deg) and dry, while a couple weeks later at 19 psi/35 psi it was warm (90 deg) and humid.

 

Good info Mike...

I finished my plumbing last night, and will be logging my bp today. Its a 306/MPT70/.68 car....I will post the results later...

 

also mike (oh and thanks for the data),could a full system be exiting in a area of lower pressure ,eg at the back of the car ,makinmg the pressure in the exhaust reduce the faster you go ?
regards robert.

 

3.5" Dp and no muff's

Shahram

 

I am in the same boat man. I hate my 3" DP, this is some awesome data though.

 

Thanks!!!

I'm glad I stuck it out with the 3.5.

 

anybody have any pics of their 3.5 DP? prefer a PTK kit w/ it.

 

On my webpage shows putting the 3 1/2" together about 1/2 way down the page. Link in sig.

 

Mick got a couple of questions that I'll pm you about to not hi jack the thread

 

You kit didn't come with 3.5 dp? My Gen II kit with T 76GTS came with a 3.5 dp.

Shahram

 

Great information Mike......maybe one of the moderators can make a sticky for this kind of information since its quite helpful and few people have posted their BP results recently.

 

Very good info...Thanks..

 

they come w/ 3in unless you specify 3.5 you have any pics?

 

What muffler?

How does that sound? I always liked the sound of aerochambers (have one on my truck). What about running a 3.5' DP and a 3.5 or 4' aerochamber?

 

Sorry. The 377 with the 3.5" downpipe had no muffler. Actually, to clarify, this was done on an engine dyno, so it used a large industrial 6" dyno muffler.

The 3.5" Aerochamber sounds great and I've previously had very good luck with them. However, the addition of 14 psi backpressure is unacceptable. My guess is that running a 3.5" downpipe and using a 3.5" Aerochamber would yield somewhere around 40 psi backpressure at 20 psi boost, but that's just an extrapolation (educated guess).

Mike

 

Well, my point was that I would assume that the 3.5 or 4' aerochamber flows much more then a 3', so it shouldn't be as bad a restriction, but I've never actually seen flow numbers for either one. I know that the 3' fit just below the straight through mufflers and well above most of the other performance mufflers flow wise, part of your issue is that a 3' exhaust is really just too small for that kind of hp, not really muffler design.

I'd love to hear your car both ways, with the hooker and without it'¦

 

FYI, last night, I made 2 runs back to back and recorded the boost pressure. With a 3.5" Aerochamber muffler, it made 12 psi boost. I pulled the muffler (and only the muffler) and with not other changes, it made 19 psi boost.

My new 3.5" Magnaflow is on order.

Mike