I've been battling this issue for a long time, ever since I put the 76R onto the car. I thought the new motor and being auto would allow it to go higher but it doesn't.

I'm pretty sure it's because I don't have ENOUGH back pressure created by the turbo. ITS has told me the R wheel outflows an GTS by quite a bit and then Jose modeled the R wheel in his software and said it should flow about 15% more. This is all great for a front mounted setup, but I'm dealing with much cooler exhaust temps and therefore a denser gas. I have no issues spooling the turbo to 10psi. It spools really good to that point. After that though it kinda goes slower to 12psi and then the last psi creeps to 13 after i've been WOT for a few seconds. The highest my boost controller has recorded is 13.6psi.

The reason I think it's lack of back pressure is because when I first got this turbo it had a .83 divided housing. It would make 6psi really quick and then would take forever to get to 9-10psi by redline even with the wastegate unplugged. After switching to my .70 divided housing it's doing what it is now except the limit seems to be 13psi.

I havent checked my exhaust back pressure yet but i've eliminated every other possibility that I can think of. I can run the car with no wastegate if I want to. I'm having a new y-pipe fabbed up to get rid of the POS SLP pipe that is on the car. The SLP junk is 3" and i'm going to have the new one made from 2.5" to help improve velocity. The other thing I was thinking of was blocking off 1/2 of one of the 2 valutes on the turbo to effectively reduce the a/r of the turbo. I think that should be a good compromise of flow. My goal is to get this setup to run 15psi on 91 with some alky for safety. Right now I'm able to run 13psi with no alky on 91 and the car really moves. I'm thinking of setting the low boost for 14psi on the controller with just a tiny bit of alky spraying and then setting high boost for 17psi with the alky full on. At just 13psi it's enough to spin the nittos at anything below 100 from a higway roll because boost comes in quick.

Guess this is one of the downsides of the rear mount setup LOL. Just gotta keep playing with the setup until it's about perfect.

It does spool pretty quick though, on the transbrake it will make 5psi in about 3 seconds and starts climbing quickly from there.

 

weren't you pushing 15 psi on the 346 Jeremy? It sounds like you are losing pressure, but a leak that big would be detectable. I think when Jon welds you up a pipe, you will see at least 14 on that setup.

 

you have ruled out that it is not a wastegate or boost controller issue?

 

wastegate has been removed and inspected, spring changed to 10psi. I've also bench tested it as well to verify it's operation and opening pressure. Wastegate has been completely disconnected so it says shut. BOV has been eliminated as well. I even swapped a borrowed on one. Pipes have been pressure tested.

This problem has existed for almost a year (about how long i've had the T76 on). The 67 .81 P-trim would make 16psi and I never tried going higher than that due to having a stock motor and 16psi providing no improvement in MPH at the track vs 14psi. My 76 would make 15 if I loaded it hard in 5th gear when I was stick, but it took a long time to get there from around 12.

 

welcome to my world, mine will only go 15psi. Im using a cheesy controller though and it holds 15 rock solid so i cant complain for the money spent. Im using the sts manual controller.

 

wow im glad im not the only one. im seeing 12psi max and i was hopen to see 15-18psi when i get the car home im gonna start going over everything.

 

Is it possible that your exhaust back pressure is high enough that it is pushing the waste gate spring open? This would still happen even if you completely unhooked the pressure reference line from the waste gate. Maybe try putting the pressure reference line on the top of the waste gate to help hold it shut, but BE CARFULL so that if this is the case, you don't over boost the thing.

 

Already tried that, same results. I'm pretty sure the backpressure is to low not too high, especially since changing the a/r from a .70 to a .81 causes me to lose 5psi of boost.

I guess i'm going to have to block off one of the valutes in the housing partially and see if that works. Only problem with that is the t4 flange is one of the orignal pieces left over from STS and it's a really shitty design that will not let me do that. Gonna have to have my friend fab up a new pipe with a new t4 flange so I can block some exhaust flow off.

I'm going to put a gauge on the exhaust to mesure my backpressure. My guess is that at 12psi i'm < 1:1

 

Don't know if it would make any difference, but didnt you go with a higher gear ratio, like 2.73s? I would think that would lower the RPMs of the motor, causing less backpressure. For instance, motor would not be spinning as high, causing less exhaust and less pressure. Probably a long shot, but just wondered if it would make that much of a difference.

 

Don't bet on it.

You might try installing a block-off plate on the wastegate. I tend to believe that it's opening and this test would tell for sure.

Also, replacing a 3" y-pipe with a 2.5" will not help spool-up measurably. Velocity in the y-pipe does not equate to velocity in the turbine volute.

 

Zombie... what are you doing to keep up the heat in the pipe? Coating? Wrap?

 

Why would my ability to make boost drop to 6 psi on the .84 housing vs 12psi on the .70 with that being the only change in the system? Wastegate has been swapped previously, different springs have been put in. My .67 p-trim .81 turbo was able to make 16psi and i never ventured higher. Process of elimination points to too much flow.

I also know that the y-pipe will not do much but my existing y-pipe is a POS donkey ball sucking piece of junk that was made by SLP and hangs 1.5" off the ground on the drivers side at stock ride height.

 

A couple suggestions that "might" help (basic physics here.) 1. Try wrapping the exhaust pipes to keep the velocity of the gasses higher....hot=faster.

2. Use a stepped down pipe to the turbo. My STS gained significant low RPM spool when I went from a straight up 3" I pipe to a stepped 3" to 2.75 to 2.5" pipe. The gas has to speed up as the pipe gets smaller....giving more velocity to spool the turbo.

Anyhow, just my $0.02.