Engine/Transmission (1994 - 1998) 5th gear part throttle turbo barking....

[JGheen]

I can't figure this one out, guys. Whenever I hit the steep hills of the I-5 Grapevine out here in Cali, I can't keep the turbo from barking when I lightly back off the throttle in 5th gear. It only does it in 5th gear and I am not just suddenly letting off the go-pedal to make it bark like during a shift.

I can be into the throttle, humming up the hill @ 30psi boost, 1900 rpm, 70-75 mph and then say I want to step the boost up to 40psi and head up the hill a little faster, as soon as I back off the throttle nice and steady it seems to bark as I lower the boost back to say 25-30psi. Engine speed doesn't matter, it can happen at any rpm but only in 5th gear and above 25psi of boost.

Is the air moving through the turbo going so fast at that gear and engine speed that when I back it off throttle just a bit, it just what happens? It's like the turbo is also spinning a lot faster in that gear @ 3/4 to full throttle than the lower gears and when I back off throttle the thing just makes a cha cha cha cha cha sound as well as it seems like it is kind of choking when I really try to hammer it with high boost going up a very steep grade. Does anyone elses turbo do this? Would installing a muffler to put some back pressure on the turbo reduce this barking??



Thanks, Joe

 

[crawler]

Its just that the drive pressure drops suddenly and that combined with the air pressure (boost) pushing on the compressor wheel you are going to experience some compressor surge.





Later,

Wes

 

[JGheen]

Is there any way to reduce that surge? It can't be good for the turbo over time.....



Joe

 

[Matthug]

Buy a turbo-guard, but that only helps if you completely let off the throttle.

 

[JGheen]

So this is just something I have to live with and try to keep my foot as steady as possible in 5th gear with high boost?? Why does it only do it in 5th gear, I never get any surge in 4th or below except during a high boost shift but never when I let off the go-pedal slightly during cruising in any other gear??



Joe

 

[Matthug]

5th puts the most load on the engine therefore the boost will be higher, higher boost=more likely to surge.

 

[JGheen]

Thats what I was thinking of originally, so again, I guess there is no way around it and this is just a character of running a large single?



Joe

 

[B.G. Smith]

So this is just something I have to live with and try to keep my foot as steady as possible in 5th gear with high boost?? Why does it only do it in 5th gear, I never get any surge in 4th or below except during a high boost shift but never when I let off the go-pedal slightly during cruising in any other gear??



Joe

Another reason it doesen't do it as much in lower gears is also because the engine is turning higher rpm's and uses up more of the boosted air before it can blow back through the turbo. It is true that the engine is working harder in 5th gear but not necessarily true that the boost will be highest in 5th gear in all situations. bg

 

[Matthug]

Thats very strange to have the HTB2 do that though, especially with the 14 housing. If you had a SP66 that would be a different story.

One thing you could try is to slide the plate back some and give it a little less fuel, you'll have less power obviously but it might solve the turbo bark problem.

 

[RustyJC]

Please don't take offense for my asking the obvious question, but how's your air filter and the rest of the air inlet system? The compressor impeller will be more prone to surge as head (i. e. , compressor discharge pressure minus compressor suction pressure) increases. Restriction on the suction (inlet) side of the impeller can increase head and walk the impeller over the surge line when drive energy is cut as you ease off the throttle - to the compressor, inlet restriction is no different than higher boost pressure.



Rusty

 

[JGheen]

Thats very strange to have the HTB2 do that though, especially with the 14 housing. If you had a SP66 that would be a different story.

One thing you could try is to slide the plate back some and give it a little less fuel, you'll have less power obviously but it might solve the turbo bark problem.

Well, the 'ol #100 plate is full forward right now, but wouldn't my smoke level and EGTs tell me it could be moved back some? Right now EGTs don't get crazy in fifth if the RPMs are up and my smoke level is probably low to medium on a scale of it just pouring out like it does before boost comes on in the low revs.



Joe

 

[JGheen]

Please don't take offense for my asking the obvious question, but how's your air filter and the rest of the air inlet system? The compressor impeller will be more prone to surge as head (i. e. , compressor discharge pressure minus compressor suction pressure) increases. Restriction on the suction (inlet) side of the impeller can increase head and walk the impeller over the surge line when drive energy is cut as you ease off the throttle - to the compressor, inlet restriction is no different than higher boost pressure.



Rusty

No offense at all Rusty , to tell you the truth it has been quite a while since I cleaned my AFE, it is new since middle of last year and I have not cleaned it since but it's not coated in dirt to where the media isn't even visible. It is definately due for a cleaning which I plan to do this weekend.

So basically, it's just blowing back the boost pressure it is discharging since that pressure is higher than the pressure coming in on the suction side of the compressor... ... ... I think I read that right?



Joe

 

[Matthug]

Well, the 'ol #100 plate is full forward right now, but wouldn't my smoke level and EGTs tell me it could be moved back some? Right now EGTs don't get crazy in fifth if the RPMs are up and my smoke level is probably low to medium on a scale of it just pouring out like it does before boost comes on in the low revs.



Joe

I'm not saying you're running too much fuel for the turbo to handle, I'm just saying your turbo spools too quick. Some of your mods IE the ported ATS and twin ram intake that most people don't run help spool the turbo faster so the turbo it actually undersized on the turbine side for your BOMB's. It spools faster than it should with the amount of fuel you are running so it increases the chance of surge. Try centering the plate once and see if it will still bark.

The PDR HX50 would probably be a good choice for your setup unless you tow, possibly with the 17cm housing but that might be a little laggy for a street truck probably go with the 15.

Do you have an aftermarket cam too?

 

[RustyJC]

So basically, it's just blowing back the boost pressure it is discharging since that pressure is higher than the pressure coming in on the suction side of the compressor... ... ... I think I read that right?

Joe,



All the compressor impeller recognizes is the pressure ratio across it. A higher ratio can be caused by a higher discharge (boost) pressure - or a lower suction (inlet) pressure that could be due to inlet losses. The compressor impeller doesn't know what the source of the higher pressure ratio is, and the higher the pressure ratio, the more prone to surge the turbo will be under changing conditions.



Rusty

 

[JGheen]

I'm not saying you're running too much fuel for the turbo to handle, I'm just saying your turbo spools too quick. Some of your mods IE the ported ATS and twin ram intake that most people don't run help spool the turbo faster so the turbo it actually undersized on the turbine side for your BOMB's. It spools faster than it should with the amount of fuel you are running so it increases the chance of surge. Try centering the plate once and see if it will still bark.

The PDR HX50 would probably be a good choice for your setup unless you tow, possibly with the 17cm housing but that might be a little laggy for a street truck probably go with the 15.

Do you have an aftermarket cam too?

No aftermarket cam, just the stocker. I won't be going to a new or different turbo anytime soon, as this one cost me an arm and a leg as it is. Not sure if HTT offers a larger turbine housing but lots of guys out there are runnint the same housing I am(14cm) with an SP66 turbo and mine is only a 62. I don't believe it spools faster than you think it does, cuz it does take a decent amount of time to get boost unless I am already up in pressure. Thing is the problem I am having doesn't tell me it's spooling problem, it just surges real bad when I ease off the go pedal climbing a steep grade @ 30-40 psi boost. I think Rusty is onto something and I just need to clean my air fliter and open up the inlet side more.



Joe