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Old 04-27-2011, 09:48 PM   #1
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Default PT88 turbo LS6 wont pull past 5k, w/dyno graph

Sorry for the long post... wanted to provide as much info as possible.

Taken on a dynojet
Click the image to open in full size.

Here's the combo:
  • Precision PT88 turbo, new 3/2011
  • Tial 44 WG, Tial 50 BOV
  • stock block and crank with forged rods/pistons
  • upgraded PCV system, roughly based of mighty mouse's setup
  • 80# injectors
  • TH400 w/PTC turbo converter
  • ford 9" w/3.25 gears
  • stock cam, intake, tb, 317s
  • Comp 26926-16TI-KIT springs
  • Pushrods are comp hardened 7.400, preload is ~.05
  • nasty performance stage 3 dual intank pump -8 feed, -6 return
  • base fuel pressure was set at both 50 and 60psi - didnt change the dropoff
  • fuel pressure according to the autometer gauge does not fall off or flutcuate
  • pumps were swapped, running either pump as primary did not impact the prob
  • coils were all replaced with another set of GM coils
  • injectors were sent out for testing, all tested within spec
  • 3bar OS, w/GM 3bar MAP
  • PCM was replaced in Nov 2010 to troubleshoot the same problem (we suspected injector drivers to be the issue)
  • AEM truboost controller

This problem is eating up my brain and I can't stop thinking about it.

As you can see in the graph, the car will not pull past 5k. You can also see that boost creeps up after the HP/TQ curves flatline at 5k, which I would guess to indicate load of some sort? Changing boost level does not change the 5k drop off point.

At one point in time, when the PCM was replaced, pulsewidth was 16.1ms every time it would begin to fall over. Now it seems that the 16.1ms isn't consistent after scaling to get resolution in the timing tables, we're seeing pulsewidth in the 12-13ms regions when the curve flatlines.

The harness was ohmed out to the injectors and coil packs, all checks passed.

MSD wires were all ohmed out at 11-12ohms. New ones were installed JUST IN CASE. No impact.

The plugs were replaced as the car was put on the dyno. NGK BR7EF (PN 3346) were installed, first at .22 gap. We later suspected blow out, and gapped them down to .17 to ensure that wasn't happening. Changing the gap had no impact on the 5k flatline.

We suspected valve float. I had 918s in the car when it was strapped down. Everyone on hand agreed it could be float, so we installed 928s in the car as it sat on the dyno. No impact on the 5k stop.

I'm strongly considering chopping the ground wires that run to the back of the heads, and splicing in new ground wires right at the PCM and running them back to the battery to ensure the PCM is being properly grounded. I guess the PCM has 3 grounds, all run to the back of the cylinder heads. The grounds are responsible for the injector and coil circuits as well.

I'm at a loss for what to try next. I'm tired of throwing money at it. I really need good ideas, I'd love to hear fresh ideas.

Thanks for reading everyone.
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Last edited by Eagle02; 04-28-2011 at 11:14 AM..
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Old 04-27-2011, 10:52 PM   #2
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My opinion would be if it were a grounding issue it would be more sparatic and not happen at the same exact rpm point every time, thats just my guess though. With all those mods i didnt see any mention of the tune, since youve ruled out what would be some of the issues i would guess, its possibly something in the tune. From here its hard to tell but it looks like it goes lean when it starts to break up. Im assuming who ever has this dyno is tuning it? I would also think that if it were a PCM problem the feedback from the PCM would be all garbled and not give good feedback.
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Old 04-27-2011, 10:59 PM   #3
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Yep, there is a professional tuner on the tune.

The AFR on the dyno graph is not accurate as we did not log AFR to the dyno, rather just to HPTuners while the car was on the rollers. We tried several different points ranging from 10.8 to 11.6, no fueling changes impacted the 5k fall off.
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Old 04-28-2011, 10:05 AM   #4
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Bump for the morning crowd. The reason I suspect a ground problem is because its at the point where boost comes in fully and is first fully loading the injector and coil circuits. It's a stretch, granted. But at this point, I'm really running out of ideas.
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Old 04-28-2011, 10:25 AM   #5
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You have too much airflow and boost trying to keep the valves open at higher rpm. Your 928 springs are way overmatched for a PT88, even with a stock cam. Get some quality dual valve springs and your problem should go away.
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Old 04-28-2011, 10:38 AM   #6
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Perhaps I listed the wrong PN... when we put the springs in it on the dyno it was 3am. Here's what I installed:

Comp 26926-16TI-KIT

Comp Cams site link: HERE

The springs provide up to .675 lift and 129# seat load, 470# open load. Should be more than sufficient.
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Old 04-28-2011, 10:52 AM   #7
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I'm wondering if the cam timing is off, not sure how that would look on a dyno.
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Old 04-28-2011, 10:55 AM   #8
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It wouldn't hurt to go a step colder on the plugs (TR8). One thing you need to check is the connection tightness on the plug boots. If there is play in the boot to plug connection they will drop out at a certain load. This is what happened to me on the dyno and at the same rpm as well.
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Old 04-28-2011, 11:03 AM   #9
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Great point, because we too questioned the wires. MSDs are notoriously loose, but all my wires clicked on both ends and have only been removed and replaced 1 time since new.

What wires are you running now, 93formto98T/A? Do you have a dyno sheet from your car doing this by any chance?

Right now, IMO, it feels like the car kinda hits a wall. As if it's somehow fighting itself. Seems like its still working... but the dyno doesn't reflect it.
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Old 04-28-2011, 11:47 AM   #10
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Sorry if I missed it but how much boost? If you turn it down, will it pull thru? When it "hits the wall", is it cutting in and out or it just hangs there? Does it do the same thing on the street?
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Old 04-28-2011, 11:49 AM   #11
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Is it possible the dyno isn't putting enough load on it?
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Old 04-28-2011, 11:51 AM   #12
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Quote:
Originally Posted by 98Z28CobraKiller View Post
Sorry if I missed it but how much boost? If you turn it down, will it pull thru? When it "hits the wall", is it cutting in and out or it just hangs there? Does it do the same thing on the street?
Any boost, but the graph attached is with the controller set @ ~12.5psi, it crept up to 19.5 by the time we shut it down.

@ 7psi (straight on the spring) it does the same thing, falls over right at 5k.

When it hits 5k, it doesn't "cut out" it just more hangs there. Seems like its fighting itself internally somehow???

I haven't honestly pulled on this thing on the street much. I've never noticed it on the street.

We went to the dyno this time with the intention of putting c16 in it and running low to mid 20psi but never got there due to this issue. This was all on pump gas.
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Old 04-28-2011, 11:53 AM   #13
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Quote:
Originally Posted by 01Z28Camaro View Post
Is it possible the dyno isn't putting enough load on it?
The day before my car was on the dyno, there was a turbo mustang that put down this graph:

Click the image to open in full size.

Several cars have made over 1000WHP on this very dyno.

Not sure if its dyno load related or not.
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Old 04-28-2011, 11:53 AM   #14
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Is the converter spec'd out for this build or one that you were using before? Does it pull past 5K in 2nd gear @ any boost level?
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Old 04-28-2011, 12:17 PM   #15
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The converter is a PTC 9.5" 15 blade unit. Just spoke to Tim @ PTC (great guy BTW) and he is damn near sure its not blowing through the converter. He says to have the trans drums checked.

When the TH400 first went in, it pulled fine, no issue. Over time last season this issue started creeping in.

I think I'm going to pull the trans and have it checked out tomorrow by my local trans guy.
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Old 04-28-2011, 12:23 PM   #16
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You can look at the last page of my build thread. There's a video on there with a pull then the dyno graph. Looks similar but worse than yours. Same symptoms as you listed above. Put your car in neutral then rev to redline. If it does this fine then put it in drive and try first gear slowly with partial throttle to redline. If no issues here than it's most likely an electrical related issue when under full load.
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Old 04-28-2011, 12:25 PM   #17
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like pat says it sounds like the springs. maybe float
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Old 04-28-2011, 12:33 PM   #18
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Quote:
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like pat says it sounds like the springs. maybe float
Springs are top of the line $600 kit from Comp. I do not believe its the springs.
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Old 04-28-2011, 12:35 PM   #19
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Quote:
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Any boost, but the graph attached is with the controller set @ ~12.5psi, it crept up to 19.5 by the time we shut it down.

@ 7psi (straight on the spring) it does the same thing, falls over right at 5k.

When it hits 5k, it doesn't "cut out" it just more hangs there. Seems like its fighting itself internally somehow???

I haven't honestly pulled on this thing on the street much. I've never noticed it on the street.

We went to the dyno this time with the intention of putting c16 in it and running low to mid 20psi but never got there due to this issue. This was all on pump gas.

Pump gas? with stock pistons and 317"s? e85 is pump gas.......is that what you meant? Or are you running an iced down liquid intercooler?

Somethings strange......19+psi on a 88 mm turbo and 570 ish hp?
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Old 04-28-2011, 12:42 PM   #20
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93 octane, BP flavor IIRC. lol

air to air IC.

Of course something is strange, that's why I'm here. I'm expecting more 850-950WHP.
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