VNT tweaking

[glhs875]

I have a 90 Shadow VNT that I'd like to tweak a little. And a VNT car is kinda new to me. First of all, what is the boost cut-out set to on these cars? Mine is going to a max of what looks like 15 to 16 psi according to the guage in the higher gears already. If I were to put on a ported exhaust manifold, remove the cat, and 3" exhaust, would the boost raise alot and be out of control so to speak? How much boost can these turbos make and still be efficient and reliable? I'd kinda like 18 to 20psi, but in 3rd, 4th, and 5th gears only. And how exactly is the boost contolled on these turbos, and can the boost level be raised or lowered with some type of boost controller? On the intercooler, is it higher flowing or lower flowing when compared to a 87 GLHS intercooler? I'd like to get 30 HP to 50HP more out of it while still remaining pretty much stock. I just want a good running daily driver with AIR COND. It's freaking HOT down here!!! One more question, has anyone tried a 2 piece intake on one of these cars with good results?

[thx138]

From my experience, opening up the exhaust will result in spiking in the lower gears and you will run into OB in the upper gears. In cold weather it can get uncontrollable. The OB on these is a delayed reaction thing. You will get a second or two of OB condition before it kicks in and then the engine will stutter. The OB is not the big 'slam' like the smec OB. The IC is basically the same as the T2 coolers except it is one cooling row narrower. Not sure if the tube design varies fromt the T2 coolers. You can bleed the top port on the actuator but I only ever got a spike out of it. I switched over to a normal T2 turbo and would not go back unless it was to restore the car. The vnt is great for daily driving with quick boost, but really lacks for tuneability and the strip where you want top end power and you are never below 3000 rpm. (for my two cents worth I only ever got mid 14's on street tires in a g-body - 2.25 60 foot times).

[nBALLS]

On my old Daytona C/S VNT, I bled the top part of the actuator, using a simple brass bleed valve I bought from Lowes. I had it set to 11-12psi, as it would spike to 14-15 under certain conditions, and it didn't pull very hard past 4500rpm. It went 15.2@87 2.2 60ft, the MPH there is a good indication of the lack of pull my car had on the top end. :)

I had this boost controller on for 6-8 months, and shortly thereafter, the turbo started making the squeak of death(sounds like a monkey screaming), and I kept driving it, as it was my only running vehicle at the time. Once I got got my Voyager back on the road, I looked through the air box to the inlet of the turbo, and in there was a lake of oil in the inlet, EEK! When I bought the car, the Turbo maybe had 2-3K on it, as it was a brand new, off the shelf unit that was put on when the engine was rebuilt.

:)

[Directconnection]

You have a PM.

[GLHNSLHT2]

Oh come on steve, I wanna see your recipie for souping up the VNT setup.

If I had a VNT and wanted some more performance without killing the turbo I'd put a 3" intake on it, a high flow big intercooler out front of the rad, and a 2 piece intake. Keep the boost as low as possible and get more performance from the higher flow in the intake.

These things are spinning about 50k more than a T2 turbo at the same boost levels. So cranking the boost means catastrophic rpms.

[nBALLS]

Or just chuck the VNT, like Steve did. http://smiliesftw.com/x/hsugh.gif

[Directconnection]

Oh come on steve, I wanna see your recipie for souping up the VNT setup.

If I had a VNT and wanted some more performance without killing the turbo I'd put a 3" intake on it, a high flow big intercooler out front of the rad, and a 2 piece intake. Keep the boost as low as possible and get more performance from the higher flow in the intake.

These things are spinning about 50k more than a T2 turbo at the same boost levels. So cranking the boost means catastrophic rpms.


hehe.

If you had some $... a Spearco like TU or FWD sells would be a GREAT investment. I wouldn't be too overly concerned about freeing up cfm on the intake tract vs the exhaust side. 5digits had a great writeup on the sdml years ago on minimum obtainable boost (MOB) and the VNT. I never did this mod and wish I did as it would have yeilded even more performance. In a nutshell, you want to basically make your MOB as little as possible via creative grinding on the vane lever outside of the turbo. This gives the vanes a more WOT position as all vnts differ a bit. Wide open helps give what he called "brakes" so the vnt doesn't overshoot it's boost goal, and also allows more exhaust gas to bypas the turbine wheel to make the boost goal instead too much of the exhaust gas cramming through, which makes the turbo overspool, and then the computer tries as hard as it can to minimize via timing retard which raises egts, which makes more exhaust gas energy, which spooles the turbo harder...round and round. There's more to it than i am explaining. But, you want to adjust it (the lever and also other tuning variables) so if your boost target is 15 psi, your MOB isn't making 14 or 13...or especially 15. It needs the brakes to control and also, like i said, allows more exhaust to bypass the turbine when it hit's it target and has headroom so not all the exhaust is forced to pass through the turbine wheel.

My setup was nothing special, but it performed rather well. Mild ported head, FM ported exhaust manifold, no cat, super turbo muffler, port matched the stock TB as all 1-piece intakes and 46mm tbs are mismatched from the factory, stock airbox and filter, and a custom 14.3 psi cal from a DC individual (best mod). Car ran 76 mph in the 1/8th on 14-15psi. I later removed the balance shafts and some weight, but never re-ran the car. Would have been nice as I used to over rev the poor thing those years back shifting at 6,00-6,200k argghh.

The other VNT car... just had a ported head similar to the other car, the same custom 2-bar cal and I tossed on the 3" system with ultraflow and no cat. Man, did that car wake up. Pulled alot harder in 4th the the other car. Should have installed the 3" on the other car and seen what it would do then. Reason it pulled great in 4th was due to it overboosting some with the safe tune saving my arse.

If i were to do it again, spearco and a 3 bar cal for maybe no more than 16psi. 3" exhaust but maybe something less freer flowing to tame the boost unless i did the MOB mod. Ported stock head and manifolds, retard the cam 2-3 degrees, 52mm TB and balance shaft removal.

The key to all this is the spearco that I wish I had back then (have now) Hell, even the stock T-II would be a gain as we saw the '90 VNT shadow I converted for a friend, then sold to another friend that sports a S60 turbo, mild portwork, 3" exhaust and S5 cal made 245whp at 20psi on the stock T-IV cooler. I suggested to switch it out as they didn't want to fabricate the large NPR i picked up for the car right away, and install a stock T-II cooler. whp went up to 271 the following weekend on the same tune on the same dyno.

[glhs875]

I've got a 87 GLHS intercooler. I may try that out. I will probably leave the exhaust alone for now as the boost goes as high as 16 psi in the higher gears now. If I do modify the exhaust I may look into adjusting the vane arm so it doesn't shut all the way to help with boost spikes. The car actually runs pretty good. Feels like a low 9 second car in the 1/8th. It could use a little better top end pull. I've already made it pull some harder up top by retarding the cam and fine tuning the ign. timing and fuel. But I'd like a little more. If I can make it pull hard to 6000rpm, while keeping it a VNT, I'll be happy with it. The stock VNT turbo may not allow it though I don't know.