What is normal boost?

[kjenckes]

I have a 95 ext cab 6.5. I bought it this spring. The previous owner did the exhaust and put on a mechanical wastegate. I have put in a new IP and remote PMD. I put in a Sunpro Boost Gauge today hooked up through one of the boost bolts from Leroy Diesels. Haven't driven it yet. My knowledge of boost and numbers is fairly weak. I have searched for this and learned some but need to know what to expect for boost under different circumstances, IE: idling, accelerating easy, foot to floor, towing etc. I have no idea what the mechanical wg is set at or how he made the settings. I wouldn't know what is too high or low to adjust it. Thank you.

Kevin

[Robyn]

Hi

The boost on these engines varies widely.

There will be zero boost at idle and minimal at real low throttle applications.

Boost will come up with medium throttle on up through hard throttle applications.

Normal moderate to hard throttle boost should not exceed 10 psi

When the rig was stock and the system was working as designed, boost would top out at around 8 psi, maybe a tad more.

Highway cruising at medium throttle should see about 2-4 psi max


The original wastete control was via a vacuum diaphram that was controlled with a pulse width modulation of the vacuum source. This allowed the waste gate to keep the boost pressure within the desired parameters.

The mechanical unit does the same thing, only it uses a spring to do it.

As the drive side pressure reaches the desired maximum level, the gate blows open against the spring and the boost is held at that level.

Depending on the waste gate controler thats installed, it may be adjustable with a nut that is tightened or loostened.
The more tension on the spring, the more boost.

Overall, a diesel engine drives the turbo via the volume of hot gases that are passing though the turbo as they leave the engine.

The turbo is actually two parts, the TURBINE, that is driven by the hot expanding exhaust, and the COMPRESSOR, that is connected to the turbine with a shaft. The compressor pulls air in, compresses it (boost) and delivers it to the intake manifold under pressure.

Hope this helps

Missy

[kjenckes]

Thank you very much, that was a great explanation. I am turning 2-4 on the highway depending on hills, mild acceleration, etc and 7 wide open throttle.

Kevin

[Robyn]

What you are seeing is right in the area that the GM engineers designed things to run.

The 6.5 can run happily in the 10 psi range when at full throttle.

Once you leave the range you are running and go higher, you really need a pyrometer to see whats happening with the exhaust temps.

Missy

[enormiss]

Is it normal for boost on a stock rig to drop off a lot as RPM's increase?

[DennisG01]

Is it normal for boost on a stock rig to drop off a lot as RPM's increase?

No. Boost increases with RPM. There's more a technical explanation to this and I'll let that to someone else, but... Boost is made by exhaust gasses. The more exhaust you're making (higher and higher RPM), the more boost you make.

Inside the turbo there are two "boat propellers". They have more blades than a prop has, but this is an easy way to imagine it. Each prop is mounted at opposite ends of a common shaft. The exhaust rushes past and spins up one prop (known as the turbine). Because they share a common shaft, the faster the turbine spins the faster the other prop (known as the compressor) spins. The compressor "compresses" the incoming air as it goes into the engine.

The ECM (through moderation of the wastegate) will control how much exhaust gas is allowed to spin up the the turbine. The wastegate is a "dump" or an outlet (think of a door opening) for excess exhaust gas (pressure). As the pressure builds to what the ECM thinks is allowable for the conditions, it opens the wastegate.

[enormiss]

Thanks for the reply
I understand the basic operation, just wasnt sure if by design the boost was lowered/controlled at higher RPM or what PSI it should be.
I lose boost (and power) at high revs, motor prefers to lug along in low rpms & high psi.
Think accleration actually slows when the trans downshifts :eek:
Would this most likely be an issue with the vacuum pump (not that old), the modulator, wastegate, or turbo itself.
Just not sure where/how I should start troubleshooting

edit, I recently removed the soot trap thinking maybe it was plugged and causing the problem.
That didn't help anything but I can hear turbo whistle now

[DennisG01]

Well, let's get some actual numbers and scenarios. Can you quantify what you're talking about?

[Robyn]

High RPM without a moderate to heavy load will not make much boost.

The boost is going to be at its highest in the 2500 RPM range with a good load on the engine.

About 7 PSI is what you should see, maybe a tad more.

If the stock programing is in the ECM, and the boost exceeds 8 PSI for about 5 seconds or so the ECM will defuel the thing and full power will not be reachable again until the key is cycled (engine off and restarted)


OVER REVING (running it up against the governor) will see the boost fall off.

During a normal med to hard throttle acceleration run, the boost will climb within a second or two of mashing the throttle and then as the load lessens due to the vehicle reaching speed, the boost will fall off some.

Normal highway cruising speed should see about 2-3 PSI
There will be Zero at idle and or just reving the engine.


Let us know what you are seeing


Missy

[enormiss]

I don't have a tach in the dash so no exact RPM's
Have mechanical boost gauge that reads from 0 to 15psi.
From a stop to full throttle boost will hit 10lb for a moment and then fall quickly to about 2lb as rpm's climb.
Everytime the trans upshifts boost will increase maybe to 7lb for a moment and rapidly fall with rpm's.
The same thing to a lesser degree with mid throttle.
I don't think I ever recall holding 8psi.
It hates highway accelerating with a load, I soft pedal it to overdrive then get back on throttle but not too much to cause downshift.

[DennisG01]

Try this... With the engine running, see if you can manually move the wastegate arm (try to pull it down). It should be awful hard to move - don't worry, you won't break anything - pull hard.

If you can move it at least somewhat easily, you might have a cracked vac line (or a failing vac pump). Follow the little plastic line that leaves the wastegate to the wastegate solenoid. Then continue to follow the other line to the vac pump. Look for cracks.

[enormiss]

I'll pull the doghouse when I get time and check.
I know there's atleast some vaccuum because I still have control of the heat & AC vents.

[DmaxMaverick]

I'll pull the doghouse when I get time and check.
I know there's atleast some vaccuum because I still have control of the heat & AC vents.

HVAC controls are electric, not vacuum. Vacuum HVAC controls went away during the GM Truck 88-91 tooling transition.

[enormiss]

This is a van, trucks may be different?
I replaced the vac pump awhile back due to no boost and vent control came back too.

[phantom309]

cheat a little,. put a spring on the wastegate to hold it closed,. drive it see how it works,.

Nick

[More Power]

Is it normal for boost on a stock rig to drop off a lot as RPM's increase?

Yes.

Maximum boost in a stock application will peak in the 1800-2200 rpm range +/- at full load. Boost at 3000 rpm will be approximately half that seen at 2000. All in a stock/unmodified installation.

The DS fuel injection pump delivers max fuel at 2000 rpm +/-.

The combination of the above demonstrates why we need to run the engine at 1800-2200 rpm to get the most out of a 6.5 while towing or for fuel economy.

Jim