I've been running a T-76GTS on 363 - 383 cid for a year now. I'm currently using a Tial 40mm wastegate and TurboXS boost controller. I measured backpressure and found 35 psi backpressure at 11 psi boost, and 48 psi backpressure at 21 psi boost. It ran over 140 mph in the quarter at 3700 lb like this.

In dyno'ing the airboat motor (377, T-76GTS), it became apparent that much lower backpressure was possible (only 26 psi at 18 psi boost), so I unhooked my cat-back exhaust (3.5" piping to a 3.5" Hooker Aerochamber). Now, keep in mind that I've had very good luck with Aerochambers in the past. My 550 rwhp supercharged combo only picked up 5 rwhp when I ditched the 3" Aerochamber.

Anyway. . . I unhooked the cat-back and went to the track. On the first pass, it blew both head gaskets. I have no idea what the boost went to, though. I brought it home and fixed the head gaskets - lucky no pistons were harmed (thank you, Methanol).

After new head gaskets, I took it out on the street and hit the gas on "low" boost. "Low" boost only applies pressure to the opening side of the wastegate, so it should go to the base spring setting in the wastegate. At "low" boost, what used to be 11 - 13 psi, is now 19 - 20 psi with the cat-back unhooked.

Has anyone else experienced a drastic increase in boost from removing the muffler?

I didn't get backpressure data today, but can I assume that I have about 35 psi backpressure at 20 psi boost now since the wastegate setting is hasn't changed from the previous 35 psi backpressure / 11 psi boost setting?

Mike

 

you was fighting the exhaust exit out of the turbo. kind of like putting your finger over a straw and trying to blow through it. remove the staw more air moves faster. get more boost that way ive heard of it before on here. definately a huge restriction you had on there. i would think you could re adjust the wastgate for this. you could have hurt the diaphram on it to because of how much it took to make your desired boost? or your wastgate is now to small for your setup?

dont quote me i have no experience with this im just suggesting what i think could have/be happening. hope this helps a little

 

Mike
I've seen the inside of one of those Hooker mufflers and it doesn't belong on your car. I'm not surprised that happened.
Dale

 

It's called boost creep. The more freely your post-turbo exhaust flows, the greater the pressure drop is across your turbine housing, which will cause the turbo to spin faster. You can compensate for this with a larger wastegate, so if your 40mm WG isn't quite enough for the amount of air you're running through the engine, then you will definitely run into this problem.

Look at it this way...if the exhaust pressures before and after the turbo were equal, then there wouldn't be much turbo impeller spinning going on (low flow). Increasing the flow of the post-turbo exhaust means that the exhaust will move through the exhaust turbine quicker, and more of it as well.

Mark

 

Mike, as the exhaust becomes less restrictive their is less pressure being applied to the turbine wheel in a counter rotational direction. So controlling turbine wheel speed becomes harder, and thus a need for a larger way to vent more exhaust gas. This is quite normal for big turbo/small gate setups with no exhaust except downpipe.

Jose

 

Sounds like what happened to my car. I had a GT42 on my 406 and it had tons of backpressure but would boost to whatever spring I had in my Tial 44mm. When I put the Majestic 88mm turbo on with the 10psi spring in instead of the 14psi spring it made 14psi, on the street with no traction. First time at the track in the water box I put it in high gear and whammo!, 30psi creeped fast and my rods exited the block.

 

Now I'm really confused. . . At 20 psi boost and open exhaust, I've had to cut my Methanol waaaay back to get it to quit sputtering when it comes on. It suddenly got richer. Hmmm. . .