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Engine/Transmission (1998.5 - 2002) Turbo Huffing

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Engine/Transmission (1998.5 - 2002) EDGE EZ & 47RE Auto

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I'm interested in bombing my '01 6spd, and since I live in the Portland, OR area, I talked to Rip Rook. I'm looking at an Edge Comp, DDIIs, 4" exhaust, HX40 turbo, AFE intake, guages, pump, clutch, etc... .



Rip was kind enough to take me for a ride in his similarly equipped truck, although his has an automatic. After proper warm up, he had a couple of occasions to "punch it" for about 100yds. This thing was FAST... and we approached slower traffic in a hurry, so Rip had to completely back-off the throttle, causing a "huffing" in the turbo. As I understand it, this huffing is caused by the excess pressure that has built up in the intake side of the turbo trying to drive the impellers backwards due to the sudden lack of exhaust pressure to keep them spinning. I've heard that this "huffing" can cause turbo shafts to break in lesser quality units. I also understand that this is why there is a wastegate.



Anyway, with a 6 spd, I'm really concerned that if I'm "driving aggressively", or drag racing, that I'll get this huffing with each shift. Not good, in my opinion. Would any of you folks with HX40s and manual transmissions please post your experiences/comments?
 
The turbo backsping or huffing is caused by excessive boost pressure at the intake to the cylinders being force back through the system to the turbo during a sudden lift off the throttle. The boost pressure works to stop or backspin the turbo and can break the turbo shaft.



It is that sudden lift that does it. It can happen when you change gears but it usually has to be after a fairly high boost level is obtained and the lift has to be fairly quick so that the pressure is not being taken into the cylinders.



I worked on a system to stop this from happening by measuring differential pressure between the output of the turbo and the intake horn on the other side of the intercooler. When a higher pressure was detected on the back side of the intercooler, a solenoid driven valve would bleed the pressure off before it could backspin the turbo. The cost of the valves was pretty high so I put the project away for a while until off the shelf technology is available.



Want to avoid backspin? Don't do sudden lifts at high boost pressures.
 
I don't know if i would use the term "huffing" since that usually means you blew up the turbo. The more common term is "turbo bark. " Do a search for turbo bark and you'll find all kinds of info.



One way to prevent or lessen the effect of turbo bark is to stay in the throttle when you shift. Also, if you're considering a HX40, I get the ones from Piers.



Charles



disclaimer: not affliated with any vendor
 
On large engines

We "commonly" call it turbo surge . If you look at most turbocharger compressor performance maps (compressor wheel pressure ratio vs. air flow), they will show a "surge line". Any condition above that line (low airflow with high differential) will cause the manifold pressure to backflow through the compressor.



Like Alan said, it is the sudden pedal lift under high intake manifold pressure conditions that causes your surging. What happens is, you cut the fuel to the engine; this reduces the energy available to drive the turbine. However, your manifold pressure is high from the previous load condition. The compressor is a non-positive displacement centrifugal element that can only overcome manifold pressure when it is driven fast enough to dynamically compress the intake air to the manifold. Anyway, you lose your power to drive the compressor and you can't flow enough air to overcome the manifold pressure. The air in the manifold then backflows through the compressor wheel. We get it bad enough on some of our engines to blast the air filters flat occasionally.



I would check with Holset as to the destructive possibilities of surging. Our large engines use Elliotts, Air Research T-18s and the big ABB turbochargers. They list surging as detrimental to performance, because you slow the compressor down too much and get longer lag when the load comes back. However they don't list it as being destructive to the turbocharger. Thrust bearing design is a big factor in that.



Shaft reversal only occurs when you have an exhaust by-pass that hangs open.
 
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Here is a good article about turbos

This is a paper from a research group at K-State that our engine manufacturers use for data and turbo comparisons.



NGML Paper
 
OK... I understand the physics on why turbo-bark happens... but...



"Want to avoid backspin? Don't do sudden lifts at high boost pressures. "



So. . if I'm drag racing, how do you avoid sudden lifts at high boost pressures? Remember, I'm driving a manual transmission truck. Are you saying I should shift when the boost level is at or below a certain psi? If so, what's that max psi that owners are typically finding to be safe?



If I don't lift suddenly when shifting, aren't we talking about "power-shifting"? At the hp and torque levels we're talking about, couldn't that damage the powertrain?



As always, thanks to those that have responded to my questions so far, and thanks in advance to those about to respond!
 
So. . if I'm drag racing, how do you avoid sudden lifts at high boost pressures? Remember, I'm driving a manual transmission truck. Are you saying I should shift when the boost level is at or below a certain psi? If so, what's that max psi that owners are typically finding to be safe?



If I don't lift suddenly when shifting, aren't we talking about "power-shifting"? At the hp and torque levels we're talking about, couldn't that damage the powertrain?

A fellow NWBomber told me that when 1/4 mile racing, his PDR-40 turbo "barks" on nearly every shift. I've only experienced one "bark" with my turbo, when my foot slipped off the (wet)pedal while at nearly full throttle. My "normal" aggressive driving hasn't made the turbo bark.



-jon-
 
Me4osu, Good explanation on what's happening.



As far as how do you shift, you could consider bumping the throttle quickly as you shift or power shift. Not knowing how fast you are shifting, I can't give you advice on the how. I know the Holset is a strong reliable turbo, but the shaft will break after excessive "barking". Maybe I'll go back to looking for reasonably priced valves and build a system for you that solves this problem. :p
 
Originally posted by rrivelli





If I don't lift suddenly when shifting, aren't we talking about "power-shifting"? At the hp and torque levels we're talking about, couldn't that damage the powertrain?




Sounds to me like you are unintentionally doing a borderline "power shift" anyway. You might be so close with disengaging the clutch and lifting the throttle, that you are causing the high MAP and surge. Maybe lift the throttle just a little sooner (if it doesn't hurt your ET's).



I'm not a big drag racer, but I do take off from some stop lights quite quickly, and only hear a little wheeze between shifts. Of course it may be my measley little HX-35, and I get to a maximum of only 27 psig MAP before shifting.
 
Barking Tubo

I have been over 400 hp for a few years now, and always drive my 5 spd hard and never bark the turbo between shifts while racing. I have barked the turbo a few times in the past while towing. At the time I still had the 12cm housing and was pushing 44lbs of boost. While I was pulling 15k pounds up hills losing power, and doing quick downshifts, the turbo would bark. This resulted in turbo failure. A few times the compressor wheel hit the housing and I wore the edges off three blades. It was just a matter of time(1 month) until the compressor wheel became enough out of balance the I blew up (Huffed!!) the turbo. I now have a PDR HX35 with a 16cm housing (37psi max boost) and don't ever bark the turbo. I drive it hard empty and speed shift it. But while towing heavy loads I always feather off the throttle while downshifting under high heat and high boost .

Garry.
 
Wastegate is to control peak boost pressure, nothing to do with barking.



The only time I've barked mine was lifting at high rpm without using the clutch--like you do with a gasser to get nasty exhaust tones. I've learned not to do it with a turbo engine.
 
The only ways to eliminate "turbo bark" is like others have said... don't suddenly lift off the throttle at high boost.



I am looking into building a "blow off valve" control system for my truck... should help out a lot!
 
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