Choking Off the Power

I've got a few interesting dyno pulls that make me think that I'm choking the engine off with the exhaust somehow. It may be the cam, 224/218 on a 114, or it may be the turbine housing, ptrim 1.0 a/r, but it seems to be choking off if I look at these dyno curves. what do you guys think.

383ci LT1
250cfm heads
stock manifolds to 2.5" cross over to a 3.5" downpipe, no exhaust
T76 turbo


The first run is a full pull pre spooled and everything, the second is a partial pull spooling as we go. Just form looking at the spots where the boost is the same I see a huge difference. 5-6K is where I see the boost the same The thrid pull is a low boost pull just for reference of a hopefully not choking run.



EngSpd 17 ManPrs 15 ManPrs 13 ManPrs
RPM
3600 15.2___ 0.0___ 7.6
3700 15.2___ 0.0___ 7.6
3800 15.3___ 0.0___ 7.6
3900 15.6___ 0.0___ 7.6
4000 15.8___ 0.0___ 7.6
4100 15.8___ 11.5___ 7.7
4200 15.7___ 11.7___ 7.8
4300 15.8___ 12.3___ 7.7
4400 15.6___ 12.9___ 7.8
4500 15.6___ 13.4___ 7.8
4600 15.4___ 14.1___ 7.9
4700 15.3___ 14.4___ 8.0
4800 15.1___ 14.7___ 7.9
4900 14.9___ 14.6___ 7.9
5000 14.7___ 14.6___ 8.0
5100 14.5___ 14.4___ 7.9
5200 14.2___ 14.4___ 8.0
5300 14.0___ 14.2___ 8.0
5400 13.9___ 14.1___ 8.0
5500 14.0___ 14.0___ 7.9
5600 13.5___ 13.7___ 7.9
5700 13.3___ 13.5___ 7.8
5800 13.3___ 13.4___ 7.9
5900 13.4___ 13.5___ 8.0
6000 13.1___ 13.5___ 8.0
6100 13.0___ 13.6___ 8.0
6200 13.1___ 13.2___ 8.2



Horsepower

EngSpd 17 STPPwr 15 STPPwr 13 STPPwr
RPM CHp CHp CHp
3600 441.4___ 0.0___ 339.9
3700 454.4___ 0.0___ 343.7
3800 465.3___ 0.0___ 356.7
3900 482.4___0.0___ 368.4
4000 505.7___ 0.0___ 380.3
4100 514.0___ 452.6___ 396.6
4200 525.0___ 466.6___ 406.7
4300 536.7___ 491.0___ 420.2
4400 553.5___ 508.0 ___429.1
4500 559.5___ 533.1___ 446.8
4600 563.2___ 550.8___ 455.4
4700 566.8___ 563.7___ 464.4
4800 564.6___ 575.3___ 466.9
4900 566.9___ 584.0___ 474.9
5000 569.8*___585.8___482.1
5100 568.4___ 588.7___ 489.4
5200 562.5___ 590.8*___495.6
5300 558.5___ 586.8___ 500.5
5400 557.6___ 588.0___ 500.1
5500 559.0___ 579.9___ 501.9
5600 549.7___ 575.3___ 503.0*
5700 542.0___ 567.5___ 500.6
5800 539.7___ 570.8___ 492.0
5900 524.6___ 557.3___ 489.2
6000 506.3___ 548.8___ 489.4
6100 494.8___ 539.2___ 478.8
6200 495.8___ 525.9___ 478.4


Torque
EngSpd 17 STPTrq 15 STPTrq 13 STPTrq
RPM Clb-ft Clb-ft Clb-ft
3600 643.9___ 0.0___ 496.0
3700 645.0___ 0.0___ 487.9
3800 643.1___ 0.0___ 493.1
3900 649.7___ 0.0___ 496.1
4000 664.0___0.0___ 499.3
4100 658.4___ 579.8___ 508.0
4200 656.5___ 583.5___ 508.6
4300 655.5___ 599.8___ 513.2
4400 660.7*___606.4___512.2
4500 653.0___ 622.2___ 521.4*
4600 643.0___ 628.9___ 519.9
4700 633.4___ 630.0___ 519.0
4800 617.7___ 629.5___ 510.9
4900 607.6___ 626.0___ 509.0
5000 598.5___ 615.4___ 506.4
5100 585.3___ 606.3___ 504.0
5200 568.1___ 596.7___ 500.5
5300 553.4___ 581.5___ 496.0
5400 542.4___ 571.9___ 486.4
5500 533.8___ 553.7___ 479.3
5600 515.5___ 539.6___ 471.7
5700 499.4___ 522.9___ 461.2
5800 488.7___ 516.8___ 445.5
5900 467.0___ 496.1___ 435.5
6000 443.2___ 480.4___ 428.4
6100 426.0___ 464.3___ 412.3
6200 420.0___ 445.5___ 405.3


Any kind of comments that you guys might have would be helpful. Thanks.

Looks like it would be the turbo to me...

The first run is where you run ~14psi right? Peak power @14psi is at 5000rpm and at 8psi you peaked at 5600rpm. I would think that its the turbo/p-trim setup. You be pushing very inefficently (hot air)...seems like the turbo likes low boost.

The problem with that is that you need to watch the boost as well, it's on a downword slope at that point. By the way the PSI numbers are 1 or 2 too high compared to my MAP sensor readings. I'm not sure what's causing the drop but it's probably related. The IAT's were more than at the lower boost levels but still around 120* or so compared to the 100* at lower boost levels.

the only port I have in the pre turbo header system is the stock EGR ports on the top of the manifolds, can I use those to get a back pressure reading or are they too close to the exhaust ports?

A 383 with a T76 p-trim is not going to work very well at all. Then add the stock manifolds to that and its a blown headgasket waiting to happen. Here is my suggestion. Go with a much larger turbine wheel, like the S-trim, that will free alot of backpressure up. The S-trim wheel only allows for a .96A/R from PTE since they don't cast anything else for it. Also get those manifolds either ported or swap to headers. I bet your backpressure #'s are close to 3-4:1. The cam maybe somewhat hurting your output , but it should not hurt the power #'s that bad...that is a steep decline in power indeed.

A 383 with a T76 p-trim is not going to work very well at all. Then add the stock manifolds to that and its a blown headgasket waiting to happen. Here is my suggestion. Go with a much larger turbine wheel, like the S-trim, that will free alot of backpressure up. The S-trim wheel only allows for a .96A/R from PTE since they don't cast anything else for it. Also get those manifolds either ported or swap to headers. I bet your backpressure #'s are close to 3-4:1. The cam maybe somewhat hurting your output , but it should not hurt the power #'s that bad...that is a steep decline in power indeed.


Blown head gasket waiting to happen? We blew it two days ago, we're working on getting that back together. I talked to somebody at your shop yesterday, but I don't remember who it was. They told me to go up to an s-trim, but a quick easy,but not the best, fix would be to switch to a non-divided big a/r housing. I'm just trying to grab some more opinions. I'll be running low boost, ~7psi, until everything is broken back in properly anyways, so I've got a few weeks to get it all figured out.

Did anybody have an comments on using the EGR port to measure back pressure?

Did anybody have an comments on using the EGR port to measure back pressure?

Tapping off the egr port on the exh. manifold should be fine.