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Discussion Starter #1
Hi guys
I have a problem with a turbo 402 I'm tuning for a mate. My thoughts are cam timing is out so il need to check that


Combo is 402 Gen iv block
Ls3 heads and manifold
Bullseye billet 80mm
Comp turbo cam mid 230s and 624th lift
1 3/4" headers with 2.5" crossover.
60mm gate
3.5"down pipe.

T400 trans with 9.5 5000rpm converter.
With 9psi and 16deg on e85 it made 520whp
But power peaked at just over 5000 rpm and just fell off.
Ive since added 4 more degrees and 3psi and only gained 25 hp. And the power cure is still shit falling over at about 5500.

What's everyone's thoughts
Should be making like 700 I would think
 

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Discussion Starter #3
Yeah it's a off the shelf cam. Owner is bringing the cam card up to me tomorrow. And it is a t6 1.32. Would have run larger dump but being in a vy clubsport we wanted to squeeze it between the engine and chassis rather than cutting the inner guard
 

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Verify timing with a light, what's compression?

I had a similar engine, but more efficient drivetrain, and a PT88-

2 psi made 300
4psi made 400
6 psi made 500
7 psi made 540.

It took 5 psi just to get back to NA power.
 

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I would also measure backpressure PRE-turbine. Power falling off is a sign of improper turbo. I would also measure
Boost Pre intercooler, and see what pressure
Drop is.
 

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Discussion Starter #6
Yeh good suggestions. I did verify timing with a light. Well at idle only. But thought if that's ok the rest hopefully is. I didn't think it would have high back pressure yet at such low power. The converter has about 12% slip but put that due to low power not locking the converter
 

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Gigity
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cam is fairly large, it will want lots of rpm to work right, but the 3.5" down pipe is probably a big restriction once rpm comes up. the intake from what Ive read from efi guys, is not happy above 6-6500, maybe less on your 402CI motor, so its working against the cam.
Last your converter being such a high stall, is wasting all the power you are making at lower rpm, and not getting efficient untill much higher rpm - which works with the cam, but not the intake and not the restrictive downpipe.
If this was my setup I would get a bigger downpipe on it first, and then decide if you want a 7500rpm combo , or something lower? - then you can get the cam/intake/converter sorted out.
The thing should flat out rip once the combo of parts works together
 

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Chupa Cabra
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I would also measure backpressure PRE-turbine. Power falling off is a sign of improper turbo. I would also measure
Boost Pre intercooler, and see what pressure
Drop is.
Listen to Louis here.

He's on to something.

I would also suspect an inefficient intercooler core, or high pre turbine pressure.

What exhaust wheel is this using? 96/88?

3 pedals is also dead on. Great information in here.
 

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Discussion Starter #11
Thanks again guys your advise is appreciated. We knew the converter would be a bit large as it wasn't built for this combo, he had a 6.0 with a larger cam previous that was going to get this turbo added but decided to build this bigger engine. The intake was going to be a holley high ram but we used the ls3 cause we had it and thought we will change later. As for the timing I expected it would need more than it has as my own turbo 355 has about 26deg at 12psi to be happy but this thing wasn't responding to the 4 degrees I added. I'm about to head to the shop and try a few things today we'll see what the dyno says.
 

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Discussion Starter #13
Yeah we are trying to get it ready for the track this Sunday. I think you guys are onto something with the dump pipe. The way he has it is the gate blows back into the dump about 8 inches back from the turbo. But the angle the wastegate pipe is it almost blowing back against the turbo putting back pressure into the dump. I just took it off and run a mock up 5" over the mudguard and it picked up 5 psi just from that. But no power Increase. I think il make a screamer pipe for the gate and see how that goes with the 3.5 dump.
 

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Gigity
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Would you put a single 3.5" exhaust on a 700+hp n/a motor? No, because it is a restriction, same way it is a restriction post turbo on you engine. My point is do whatever you have to, to get a bigger down pipe on it
 

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Discussion Starter #15
Yeah I understand that. My own engine has 5" and I was expecting the 3.5 to be a restriction just not this much. Another engine I've had a bit to do with made 811whp with a 4" so I thought this would go 650ish
 

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He said he was running 9 psi. At 9 psi and 9:1 compression, you can run NA timing easily. My data
Comes from countless cars I've tuned the last 15 years. 9:1 ZR1s on 9
Psi on pump fuel run 27*!

Your mileage may vary, but certainly, good fuel, cool IAts, baby boost, you need spark to make power.
 

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Id say more timing, mines kinda pushing but we have 23* at peak torque, and if those heads are unported thats a big issue as the flow of an ls3 head actually drastically decreases over .600 lift
 

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and im running that timing at 15psi and occasionally 18psi but I see a slight but of coolant in the overflow at 18. Im on c16 with ngk 9s btw
 

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My lil 5.3L in my work truck made 500/535 on 8.5lbs on 22's and 33's, but it made peak power at 5200.. I've got something up my sleeve and am going to REALLY hurt some feelings with it..
 
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