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Performance Tuning for the Tahoe

6.2 turbo said:
Got this thing running again. This time a stock compression 6.2. Turned the fuel back a lot. Having trouble getting any power past 3500 rpm. Boost peaks at 25,and drive psi is never more than 25 psi. At low boost, drive psi is lower than boost psi. More timing helps only slightly . Midrange power is fine. Low end power is totally gone. At least it starts easily. I said it before, I think it matters if it's too lean.
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I think 18:1 for air:fuel ratio is about the best for power at high load. So not very lean at all.

Could you swap the gauge inputs to see if the gauges are effecting the boost/drive pressure readings? Is this with the 16cm^2 HX50 turbo housing?

Glad you got it running.
 
I could swap gauge lines,the gauges are identical 100 psi. Could be off a little at low readings. Still the same HX 52,just wanted to check if the drive psi was too high. I have the same pump on,but with a leaf spring and stock cam ring. The combination of shoes and spring will allow it too be turned up farther than stock.
 
I backed the timing off. Now the turbo lights,30 psi boost and 40 psi drive. It also will get 20 psi drive with free rev at around 4000 rpm's. At 20 psi boost they are even,but after that ,drive psi climbs with rpms. Boost climbs the fastest,and drive psi catches up,and on past.
 
6.2 turbo said:
I backed the timing off. Now the turbo lights,30 psi boost and 40 psi drive. It also will get 20 psi drive with free rev at around 4000 rpm's. At 20 psi boost they are even,but after that ,drive psi climbs with rpms. Boost climbs the fastest,and drive psi catches up,and on past.
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Sounds pretty good, you must have had a good amount of advance.

I wonder if the pulsing exhaust messes with gauge readings down low. Still shows the turbo is pretty efficient turbo if its about 1.3:1 at 30psi and lower below.
 
I installed an external wastegate,tonight. It opened at 8-10 psi boost. Clamped the hose and it opens at 27 psi boost just from backpressure. Couldn't see the drive psi,light's not hooked up. Seemed to be pulling pretty good at 4000 rpm's. Not sure were my timing is,but smokes white if revved while cold. I might shorten the wastgate spring so it opens sooner,and advance the timing some. Pyro is not connected,but I'm sure the egt''s are high,which makes boost.
 
Got the wastegate set at 20psi boost. Finally might have an answer to the lacking top end power. After a heat soak it wouldn't start,dumped a gallon of water on the pump,then it started. When the pump was maxed out it hid this problem. Also 3200 psi pop injectors make it worse. I had pumps before that acted really weird at high rpms if they were worn.
 
I installed another head and rotor,it starts hot now. I tried a T-70 turbo,a S200,and a hybrid hx 52/55. The T-70 ,and HX 52 would make power at 4500 rpm's ,only if wastegated. None waste gated they all stop the power at 4100 rpm.
 
I adjusted charge psi lower, 60 psi idle,and 100 at higher rpms. Trying to create a perfect timing curve is pretty much impossible. Although I did manage to get power past 4500 rpms . No matter what turbo,timing,or injectors I try, this motor has no power under the turbo. My 6.5 would stall the convertor to 1500 rpms .the 6.2 can barley get it to 1200 rpms. I put the T precups in,but I think they are killing the low end power .
 
Have you checked compression on the engine? Is it possible that it does not like the 60psi charge pressure since transfer pump output is probably usually only about 30psi at idle normally.
 
The compression is stock,didn't check the psi. When this pump was stock it had 50 psi charge pressure at idle. One issue is that the charge psi drops now at full throttle,and retards the timing. I been down this road before,and ended up locking it solid.
 
6.2 turbo said:
I installed an external wastegate,tonight. It opened at 8-10 psi boost. Clamped the hose and it opens at 27 psi boost just from backpressure. Couldn't see the drive psi,light's not hooked up. Seemed to be pulling pretty good at 4000 rpm's. Not sure were my timing is,but smokes white if revved while cold. I might shorten the wastgate spring so it opens sooner,and advance the timing some. Pyro is not connected,but I'm sure the egt''s are high,which makes boost.
Click to expand...

EGT's do not necessarily make boost. I get the same boost 17 PSI at 1100 EGT as a do at 1500 degrees.
 
It might have somewhat to do with what the egt's are caused by. Less timing always makes more boost and egt's for me. Same thing with more fuel. They are directly related . Timing can make around 10 psi boost either way,and fuel will max the turbo to whatever boost it can make.
 
I restricted the advance piston travel,and cranked the charge screw 2-1/2 turns. The turbo lights now. The timing on this motor is so touchy,1/16 inch can make a huge difference. If I ever pull the heads,the T cups are going to fly. I think they are screwing with how it runs under the turbo,and maybe more so since its only a 6.2. It makes absolutely no power until it has boost,and compression is stock.
 
It may have poor throttle from too low of advance at idle. The 6.5 with DS4s are pretty gutless off the line at 9 degrees crank advance, and some programmers and even GM put them up to 22 degrees crank advance at idle. That might only be the beginning of injection when the metering valve closes, but you can see there was a huge difference in timing of the DS4s and it really does make a difference in power pre-boost. Unfortunately it may also cause slower and less spooling on larger turbos, but with the DS4 you can drop timing to whatever you want after idle rpms. Just varying my advance from 9 to 16 made a big difference, and even some later stock went 6 more degrees to 22. Those are crank values so what a scanner would show is cam timing I programmed from 4.5 to 8 degrees, up to 11.

What precups did you have before, the smaller 6.2 dotted cups?

Is this IP running the transfer pump with a normal lift pump pressure input or some other setup for lift pump pressure? Do you have a reference for what idle charge pressure is normally?

And is it possible your torque converter has failed somehow, because it should stall at 1200rpm, it should be at least 1500rpm like you said. That could be whats messing with your power.
 
My 6.5 had the medium size,square stamp I think ? My converter stall is lowered from stock,and possibly bad. But low end power changed dramatically,when I swapped in the 6.2,even before touching the ip.
 
I doubt the precups are hurting you. 1200rpm stall converter will hurt low end for sure, make any truck feel a lot less responsive. Is your idle RPM the same as well, or have you lowered it?

You had a modded IP on the 6.5, which showed signs of failing not long after the engine swap, and now a fixed IP. My first IP was putting out tons of fuel making my truck pretty quick and belching smoke even with boost. I thought it was normal, until it failed a couple months after I bought the truck. New IP had no smoke, less get up and go power, but much smoother. There might be something significant with the transfer pressures and advance piston in the IP making it run differently.
 
Idle is about 1000 rpms. The converter is so tight it will go up a very steep hill at idle. With the 6.5 it was just as tight but stalled more from torque. Another thing I changed with the engine was the Monarch nozzles. I had stock 304 nozzles in with the 6.5 ,but didn't notice less low end power then. It just seems to depend totally on boost,like it has low compression. I can get power from 2000 rpms up to 4700 rpms,depending were I have the timing. Some time this week I'm going to try and check full load timing. I can do this now that my timing is basically locked,and varys only with fuel. I don't think my motor would idle very well at 22 degrees,10-12 degrees it will .
 
Well you can see the dyno of a regular 6.5 with a large turbo on it. The turbo lites around 2000rpm and builds a lot of power out past 4000rpm, but it does have low torque at the bottom. Maybe you are just more normal now, and have to find out what made the last setup work so well.

And the compressor side of WW's turbo never ran out. I have the same turbo and it will easily go past 20psi at low EGTs. Something else was holding him back, wouldnt burn any more fuel to generate higher cylinder pressures that will end up more drive pressure.
 
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