That cam does have a fair bit of overlap, but that is not going to prevent you making boost at SOME rpm. A cam swap will not solve the 5.5psi boost problem. It will perform better, sure. But I don't think that is your problem.

 

Stock manifolds on a 408" motor with a cam that big might also be a major factor. Put on some 1 3/4" longtubes so that thing can breathe.

 

I had my hooker LT's on it before the turbo build. I was told to pull them off ang use the stock manifolds for the rear mount.

 

Yeah u dont need long tubes. Is the car lazy ? Hows the bov? Are you sure it dosent bleed pressure? What kind of spring and controller you have? have you tried a different spring one for 10 psi?

 

This new engine only has 450 miles on it now. Before the turbo the 408 was disapointing at the track only runing 12.70 at our elevation. My buddies car wascutting the same time with his 346 MS3, same trans, same convertor, gear. I did blow my stock 10 bolt that day on the 5th launch. I only had about 350 miles on it then and I was there tuning it. I then put the Moser 9" with 3.70 gears and started putting my turbo together. Everything on the car was the same except I removed the LT headers and installed the stock exhaust manifold, did the exhaust new and wrapped it, I did the daul intank pumps, upgraded the fuel lines, added a BRFPR, and went from 60# injectors to the 80# injectors. I finished up and started tuning. The lag is all the way threw first gear. Once it goes into second the boost comes on quick but stops at 5-5.5psi. I did not like the manifold and elbow combination I had on it so I pulled the GMPP carb style intake and Edelbrock Ultra low profile elbow off it and installed the stock LS3 intake hoping it would help the bottom end. It did make the car drive much better, but it did nothing for the lag.

As for the waste gate and blow off valve, they are both NEW Turbosmart valves, the 50mm Racegate and the 50mm Raceport. At first I was only building 5psi and the racegate had a 7psi spring in it. I have a 5psi spring and a 10psi spring. I can put all three in for a max of 22psi, or I can mix them up for different psi. I that at first the 7psi spring was opening too soon so I changed it out with the 10psi spring and I tested the cracking, full open and full close with some specialized valve test equipment from work. It now has the 10 psi spring in it and it did not change the boost pressure. The Raceport would come open if I reved the car up to 2500 rpm in neutral and held it there. I ordered a spring kit with it and changed it out and tested it as well. It was opening at 15" HG vacuum and it now opens at 19"hg. I never see 19+" HG unless I close the throttle at high rpms. I know the Raceport and the Racegate is not leaking or opening when my boost pressure stops building. When I pressure tested the air side I brough the pressure up to 13psi (any more and my plug on the turbo would pop off) and I removed the sensing line to the Raceport, and it did not open and was sealing perfectly.

I am going to represure test the entire system and make sure the are no leaks. If I can not find any leaks, I am going to install two to three pressure transducers, one at the turbo air outlet, one at the turbine inlet, and one in the Y pipe, wire them up to my HP Tuner Scanner, and make a pass to record the pressure at all these ponits. That will give me 4 pressure point readings. If I have a high delta P across the cold side or the exhaust side, then I will change the piping. If the pressure is not showing a high delta P there and the Pressure ratio on the turbo is low, and there are no leaks, then I guess I need to change the cam. I would say go to a smaller AR housing but every one else seams to be doing great with the 1.08 turbine housing on the GT-88 rear mounts.

Other then these test and these possibilities I have no other ideas what to do.

 

Where are the referance points for the gate at? If you referance the gate from the pressure port on the turbo that I know will show more boost then referance from the intake. But for the love of god have the boost controller on its lowest setting, because it will show more boost there than at the intake.

 

pm sent

 

even if your cam has allot of overlap you should still see good pressure at the intake manifold. but boost will be leaking out from the intake valve and straight out of the exhaust manifolds if the cams has huge overlap and that is where power will be lost.

 

I think this cam has about 7 degrees of overlap, wich is not ideal but not so bad that it would cause this. I am going to repressure test everything and make sure I did not miss anything.

 

where are you getting the pressure reading for the boost controller and the wastegate ?

 

There is no boost controller, the sensing line is in the intake manifold behind the TB.

 

Timing could be part of the issue with spoolup. Your timing seem quite low when you are trying to spool the turbo. With 10.25 compression I'd start around 25 degrees until 1-2 psi is made then pull timing from there. As always, be mindful of detonation. I'm at 8.7:1 and run between 35-40 degrees for timing until I make around 3psi and start pulling it from there. My timing is much more aggressive during spoolup than a manual trans car would be. Proper timing can make a big difference on a stalled turbo car. It's worth investigating since it's simple to do. The lowest timing my car sees is 18 degrees at peak torque.

Does your waste gate ever open in 2nd gear when it spools? It should be making a noticeable noise if it's vented to atmosphere. Are you able to spool it on the foot brake? When you tested the gate opening pressure where did it crack at? if it cracked open at 10psi on the bench then it will open at a much lower pressure on the car due to the pressure on the face of the valve.

Once you get everything working that 4l60 is probably gonna eat your lunch lol.

 

I have not tried that much timing even NA, but I will now that I am running VP 113 fuel.

The brakes will not hold the car past 13% tps so it really does noy have a chance to spool.

When I tested the gate with the 7 psi spring it craked at 9 psi and with the 10 lbs spring I want to say it cracked at 12 or so.

Once it gets to 5 psi boost it has a larrge leak sound comong frome the car. It could be the gate but the dyno operator said it was coming from the air cleaner. I was in the car so I do not know for a fact it was not the gate, I will try more spring next I think, maybe 15# and see if it comes up some.

Yeh, I have a 4L80E and the segment swap waiting for me to build up with a transbrake for it. I have yet to decide on a stall for it. I need something to help it boosy faster.

I still want to put in bigger valve springs too.

 

Maybe your cam is a tooth out. The poor performance NA leads me down this path. It would explain poor NA performance and inability to build boost.

On my first stroker motor that I built, I put the cam in a tooth out becuase I had the slack on the wrong side of the chain. The car made 360 rwhp NA. Once I fixed it, it made 490 rwhp. I had tuned so well that besides the power being down, you couldn't tell that there was anything wrong with the car.

 

Interesting possibility with a used chain or aftermarket double roller.
I'm presuming you had it a tooth retarded and didn't have ptv contact?
How did you realize the issue besides low power, backfiring, stumble ???

 

Just low power. Everything else was perfect. By the way, it was a brand new chain. The problem was that I turned it counter clockwise when aligning my dots. This put the slack on the wrong side of the chain.

Obviously, if I had degreed the cam when I was installing it, I would have seen the problem but that entire process is a pain in the *** and I wont run an adjustable timing gear anyway so if the cam is off, then it's off.

 

Is this a street or street/strip car? IMO you need a couple things, one being a new cam. If its more of a street car, you really need to go back to a T4 exhaust housing. Bottom of the wastegate should go to the compressor housing, not the intake manifold. What kind of cranking compression do you have?

 

Well you should be testing in like third gear to see if you do indeed get to spring boost pressure. Testing in first and second is not the best way to test. If you hear something leaking then sounds like for sure you have a boost leak. You seem to have a few problems. One slow spool and another can't get to full boost. Your car should get to full spring boost sooner or later. Spool as said can be helped by playing with tuning, afr and timing and you can play with ar and few other technical things on the turbo itself, change to smaller turbo,etc.

But I still think you have a big boost leak. You shouldn't need 15 psi spring to hit 10psi.
They should be very close. So build a boost leak tester or maybe put the car on automotive smoke machine. You likely have problems at coupler. Use quality silicone couplers with t bolt clamps and beaded pipes if possible.
Many many turbo car boost problems are result of boost leaks. Its number one problem on dsm forums. This will give you not the right boost and also affect spool .

But don't expect super quick spool up with your combo. On the line use two step to build boost and you might want small shot of spray. Go with would think a 3600 or so stall like yank py 3600 or py3400 these are turbo specific application stalls and work great. Gearing well you get different opinions some guys like the 3.08 or so to help things out. I don't have late spool problems and hate low gears off boost so am happy with my 3.54,they are my lowest choice in the dana anyway.

Once you find you got your 10psi or target boost in third then you can work on spool with timing and fuel and launch with two step and stall and then if you still hate your spool up time to change out turbo,go to smaller inlet housing,...etc.
Not actually sure how much cam will make a difference again a lot of theories out there with reverse split ,nomral split, no split and the lsa argument. My cam seems to work good on my car but yours being rearmount should likely copy what the successful rear mount guys are saying. I guess its possible for it to blow out so much boost you can't get up there to where you want but still betting on boost leak and a big one at that and you said you can hear something.

 

I have a rear mount with a TC76 whenever I hear more noise than usual from the rear it's the wastegate opening.
I would cap the wastegate taking it out of the equation all together, find a nice long hill get the car in 3rd or 4th and roll into the throttle SLOWLY!!! and see if that's your problem.

A easy test before doing anything else would to take your finger along the wastegate part that opens to atmosphere and if it's got carbon you know it's opening or leaking.

Most likely if you have a engine problem you should still build boost past 5psi.

 

I personaly witnessed the cam being degreed at the builder, and I personaly checked it at home with my personal degree tools.