These are the rotors upfront after replacing the master, calipers and pads. It was barely groaning. The new rotors were percesion cut and just beautiful in their appearence.
With the heavy return spring on the WG. the PCM limits boost by cutting fuel for a fraction of a sec. in pulses. It only happens at WOt and I'm thinking of increasing the pot. beyond 10k ohms to the three wire full pot.

Dude wheres my rotor facing: I upgraded to power stop cross drilled rotors & carbon metallic pads ala JC Whitney, some folks don't like them for me they work not much difference in price of cross drilled vs stock.

QM stop diddling with boost sensor output; increasing resistance may be part of the problem, in OBD II boost code is a problem if boost is too low just as boost too high, 5 K setting on the pot is all you should need to keep from setting a code or giving a range the PCM can only deal with by going limp. I'm not sure if same holds true with OBD I but suspect it is.

What is IAT could be a bad sensor clipping fuel IIRC you are doing some post turbo cooling, make sure it's still cooling & IAT sensor is giving proper value.

One more thing that did not show on mine until swapping to reflash was a bad MAP sensor, that went hi, my boost fooler was hiding condition, but when I hogged down on my TM as you are more or less doing with your spring mod, I was mechanically making more boost by gauge, but electronically to the PCM was making mega boost 35psi and I was clipping that way, swapped the MAP and problem fixed. Do you have access to a scanner that can view boost signal seen by PCM. I'll be in Houston next week will have mine with me if you want to try and get together.

With the heavy return spring on the WG. the PCM limits boost by cutting fuel for a fraction of a sec. in pulses. It only happens at WOt and I'm thinking of increasing the pot
I've seen this quite often with trucks that had a wastegate solenoid stuck to WOT, overboost and PCM cuts fuel and truck Pulses like you say under full load. If your cheating the MAP and BARO and the PCM still sees enough too much boost to trim fuel like that, your in serious danger of turning that turbo into a fireball... or a grenade! 10lbs of boost is MORE than plenty with these engines, in stock configuration. Just be careful ;)

Hmm..
I suspected the PCM detected overboost. Iat's should be close to the '94's it has the same 3" pipe and spring on the wg
PCM can't cut boost so it cuts fuel. It's never happened in the '94.

OBDI PCM should kick you into limp mode upon tripping Boost out of range code, OBDII will temporarily cut fuel back causing the pulse.

I"m serious though, that kind of over spooling of any turbo is not good for them.

No problems with 18 psi boost to the turbo, not effective though as you start getting backpressure pretty high and you start putting big squeeze on engine with 21:1, many run sustained boost in the teens, mine will run there when hauling heavy, running empty as you say 10 psi is plenty of boost. Good call on fuel clip I meant to add that as well but forgot to add it to the post. PCM tries to dump boost as 1st corrective action, then if it can't do that goes for fuel, I did not know OBDI went into limp mode.

On OBD I, does it stay there or reset itself after X amount of time or do you need X number of normal start stop cycles to reset it.

HI IAT causes PCM to do same thing as overboost actually I think anything that PCM feels it is losing authority over will cause PCM to defuel in effort to save the engine .

QM you still need to have someone look at what the PCM is seeing for boost and IAT, I suspect tho since you aren't reflashed to allow higher boost operation TDGs idea of overboost is more probable culprit, as without reflash you are bumping GMs upper limit even boost fooled, only way to know for sure is to check on a scanner what is making PCM want to defuel, or go back to stock and see if it clears problem. If it does incrimentally add resistance on WG spring until you get to a point it starts acting up again, you might have too must resistance there, where did you source the spring from.

On ObdI it stops the boost for a sec if you have vac system only. It will set the ses light with a spring, nothing else I noticed.
I could reduce the spring but increasing resistance should cover it.

What is the code you are getting if it is Boost out of range that can be out of range high or low the PCM at least in OBD 2 is smart enough to know if the engine is running at x rpm it is supposed to be making some boost at a given range, keep on adding resistance for minimal output and you will set a code that way also. You really need to scan while driving to see exactly what is going on rite now you are throwing darts at a moving target.

If you can't do that go to 5K on your pot and take off the WG spring see if the code clears, then dial in on pot more resistance to see when the code sets for too little boost leaving your spring off. once you find that boundary then you can add your spring back.

I suspect though it won't come back unledd you have something else going on 8K is where I had my fooler set IIRC for 17 boost, when I was running a fooler before the reflash.

Just out of curiosity, you still have a boost gauge right QM? What kind of boost are you making and what are you trying to get to? If you really want to make more boost, you should consider atleast a stage 1 rebuild on your turbo, machine the compressor cover a little to fit a larger compressor wheel. Our turbo guy here is working on swapping a Duramax wheel into one for me, as soon as he gets a chance.

Has he got a solution for the real problem with the GM-X series?

The turbine is sized for a 3-litre engine - gets Boost up, starting at ~700-1000rpm, where 6.5L produces exhaust flow rate similar to a 3L at ~2000rpm.

We get full boost at ~1700rpm, where the 3L would need ~4000rpm.

We get max volumetric flow rate at ~3500rpm, where the 3L would need 8-9000rpm.

Diesel volumetric efficiency is ~1.0, gassers are ~0.78 due to venturii and throttle plates, and such. Modern FI gassers are a little better - no venturii.

I'm just doing rough approximations in my numbers-addled head - here's the calcs for your convenience -

(cid/1728)/rev x rpm/2

gives cu ft per revolution, at any rpm divided by 2 for four-stroke operation, for flow rate in cfm.

So....doesn't take much for the 6.5L exhaust volume to exceed the flow limitations of the 3.0 litre exhaust turbine.

That is the problem - not the compressor wheel.

PCM limits Boost - not the compressor.
(I run vacuum-controlled 20psi regularly with GM-8 and charge-air cooler - just not as effective at 3-3500rpm as it is at 1700 rpm)
(And - I'm dyin' if I'm lyin' - man, is it effective at 1700-2500rpm! ! ! )

PCM doesn't know, or care, what size wheel is making Boost - just that Boost is above a spec'ed level.

3.0L GM-X exhaust turbine limits over-all efficiency of Boosted 6.5L system.

At low rpm, it's just what we need for instantaneous torque - maximum combustion pressure over the longest moment of the crank arm.
At the high end, it limits horsepower, which is derived thru many combustion events per unit of time, producing the high flow rate

Fact

Word up

You can take that to the Bank

Etc........

where are you getting 3.0L exhaust wheel from?
I"m talking about the 6.6L duramax wheel, bigger, more air with less RPMs, thats my only goal. Haven't found a good replacement for the shaft/exhaust wheel, but upgrading both ends would be my first preference.

So, unless I miss my mark here . . . GM has on-purpose sized this turbo small. (?) Here's my question, what was their rationale in doin' this - was it because about 97% of engine's usage down the road was going to be in the 1800 rpm area, moreso than the dragstrip 3500 rpm that mo-power people are desiring?

I don't see why I couldn't get a relatively larger exhaust and intake wheel and hone the housings on my drillpress to make them fit. Even the gm-8 wheels would be an upgrade on the 3&4. Wouldn't there be an advantage to a larger wheel in the smaller gm-4 housing? Wouldn't this increase your turbo's efficiency by reducing the top end overspooling? Wouldn't it move a greater volume of air after spool up but still be restictive enough to spool quickly?

Correct, kn.
GM calls that 'driveability' - power available when\where it is needed.
Instant spool-up - Boost - from idle, settle down to good fuel mileage with adequate power to turn those 3.42's, and tote the load as required.
Which also meant slowing down and \or pulling over to allow the 6.5L engine to cool down.

The 6.5L offset - to the tune of 28mpg - the 4-8mpg of the bad-boy 454 lite truck engine, adding a turbo gave it similar or better towing power.

It was not meant to hot-rod.
The indirect-injection system gave better starting\drive-away\economy, but the heads created over-heating problems.
The truck body-design did not allow for a charge-air cooler, the cast crank did not allow for any significant power increase, as did not the low nickel-content block.

The over-all 6.5L design was for good fuel mileage at 135-150hp, with good low-end torque, to meet enforced CAFE requirements.
The EFI PCM allowed no power improvement, even by a super-wrench (you know - such as.....myself! ;)).
(Good thing for hackers!)

Not many people wanted a Diesel street-racer, nor has one ever been available - till now.
Check the Cummins\Dodge and Isuzu\GM drag and speed records for pickups, probably also in the tractor\truck pull events.

Got some really BAD Diesel pick'em-ups, available to the general public, right off the showroom floor.
With rpm red-lines around 5500-6000rpm.

A 360cuin six-cylinder Diesel holds the world land speed record for a pick-up truck - 222mph.
Supposedly, it's a daily-driver, was driven to the event, and prepped on-site for the record run.
Can read about it on the Banks Diesel website.

Work'em er race'em - they ain't yer grampaw's Diesel, anymore. ;)

he tows a trailer with his extra parts and tools to the events also. he claims that the engine internals are bone stock with 30k miles at the time of the record run in the salt flats.

QM - the compressor wheel, shaft and turbine wheel are identical in the GM5 thru GM-8 IHI turbos.
The wheels are identical size and shape in the GM-4 thru GM-8.
The GM-4 compressor housing, cartridge and turbine housing - snail - were redesigned for increased efficiency in the GM-5.
The GM-8 wastegate shaft and bushing were re-worked due to early failures in the GM-5.

Some turbo shops shave the turbine wheel, and rework the snail to increase the A/R, which allows some increase in flow, but I have not read any verifiable data which would indicate a significant improvement.

Boost is PCM limited - has nothing to do with the compressor wheel size - my observed 20psig Boost does not fall off at 3500rpm.
But, my engine is seeing only brief excursions to 3500rpm - sustained 3500, as in sporting events, may be another story.

Exhaust flow is limited by the small turbine - wheel and snail - so high EGT's increase quickly at 20psi and 3500rpm.
Rapidly to a point, then much more slowly at 2500rpm.

Altering the wheels is VERY dangerous - ANY imbalance at 100,000rpm can result in rapid expansion and dispersement of rotating wheels and housings.
'Nother words, they can and will 'grenade' on the unsuspecting shade-tree experimenter.

Pick up a used 7.3 Powerstroke turbo, and rework the inlet\outlet flanges to fit - that would be a better experiment, imho.

And, safer.

GM1 thru GM8 are all the same compressor wheel. Only difference in 5 and 8 is that 5 doesnt have the extra flange on the side for the support bracket to the intake manifold.

Of course, when/if we do this experiment, the wheel and shaft will be rebalanced.

TDG brings up a good point on balance.

The welded turbine wheel and shaft are balanced as a unit, the compressor wheel is balanced separately.
They are then assembled, and the rotating assembly is again balanced with the nut, such that the nut and compressor wheel must be at that precise relational position to the shaft and turbine wheel.

Not a relationship to be taken lightly.

Correct - the support boss was moved from the -4 cartridge, longer because it also contained the wastegate mech, to the longer exhaust elbow in the -5 thru -8 series, when the wastegate mech was also moved to the elbow.

I did some work on the 96. I sodered in two fixed resistors in place of the 10k pot. moved the pump driverside 2.5mm and did TDCOL manually.
I had to move the a/c compressor to get at the pump bolts.

What was your final TDCO reading?

Engine fully up to 190deg operating temp should have shown ~+8.5deg Desired and Measured timing at idle.

I took a chance that it was stock, marked it and moved it 2.5mm driverside. I've been considering the OBDII since I got the OBDI scantool. It was due for it he has every mod I do 'cept no optical bump, yet. It was obvious once you drove it, fine in the lower rpm but weak over 2500 rpm where pcm had previously clipped fuel before with only 11.11K ohm resistance. Seat of pants says it was a move in the right direction.

QM What did you move "toward" the driver side I'm guessing, The complete IP pump?
Who's truck was this ?
I guess I missed what this post was replying to
Sorry just Lost in space here this morning

Bow,

I moved the pump housing on the timing cover 2.5mm to achive 5 deg pump advance on my dad's '96. I have yet to bump the optical otherwise I've had my way with this one. Darth vader's milage is 240,000 mi. now. I'm glad this one doesn't have the oil channels in the block.

These forums are like a "living" book, constantly updating.

They can be used like a book, by going back to any page (post) in any chapter (thread) and re-read the info posted at any previous time.

Anyone can be an 'expert', merely by correlating any or all of the previously posted data in any or all threads.

Quite handy, for a dyslexic high school drop-out - or anyone else with access to the data stored here.

Try it - you'll like it.........

These forums have been great for me. I find every shared experience enlightning and I look forward to reading/answering the new posts every chance I get. We're just one big diesel community.

YEapper while thats true It is easy to get off subject and start going down rabbit trails and infomation gets lost in the translations. We also must always remember where we came from on these trucks and that others are starting at that point to in their quest to enjoy their trucks too.
Thanks to everyone here I have a better understanding of whats happening in my truck and look to continue to learn from all whom share knowledge here.

Guys,

I'm not so sure 2.2K ohms + the 1K back to ground is optimal for a '96+ OBDII. I was thinking with the way the PCM reads baro through the MAP at start-up it's likely to be added to the resistance felt by the MAP after startup. I might have to back the resistance down and see how she does. Perhaps 1K ohms on the MAP wire and .5K ohms back to ground. This would be cutting it in half and if it is added then this might be what I'm looking for.

I was lookig for some feedback if anyone has tried this.

Sorry I have no feedback to help you QM, BUT I am watching to see what I can learn from you on this subject.

The Fixed resistor set-up that worked was .262 ohms off the B wire and 1.390 K ohms back to ground.

A shot of the space available for IC