Engine/Transmission (1994 - 1998) Boost at altitude

[T-Bone]

Is anyone above 6000 feet actually seeing over 26 or 27 pounds of boost with a stock turbo? I’m wondering if it is possible.



Yesterday, Rattlin_Ram, Boondock and I blocked the waste gate line (vice grips on the hose) on two different trucks (Boondocks 98. 4 5-speed and my 97 auto) in an effort to get max boost. Neither truck saw over 27 with the waste gate blocked. Boondock has a #10 and a 3k GSK. I have a #10, 3k GSK and DDIIs.



Thanks,

T-Bone

 

[scaryfast]

I live in Larkspur Colorado and regularly tow through the mountains. I have the wastegate blocked off and can easily get 34+ pounds of boost.

 

[mschoenheider]

I live in Denver, and travel regularly to Laramie (7200 feet). I regularly see 32 psi with my stock turbo, waste gate blocked off. Keep in mind that 32 psi at 6000 feet requires the same turbo pressure ratio as 39. 9 psi at sea level.

 

[Forrest Nearing]

I live in Denver, and travel regularly to Laramie (7200 feet). I regularly see 32 psi with my stock turbo, waste gate blocked off. Keep in mind that 32 psi at 6000 feet requires the same turbo pressure ratio as 39. 9 psi at sea level.

not true... it took me a while to wrap my brain around it, but the 32psi you're seeing on the boost gauge is a measurement of the pressure ABOVE the atmospheric pressue... so the turbo is spinning roughly the same speed and the pressure ratio is the same.



now... the cylinder pressure is lower because the absolute pressure is lower... IE, it would take 39. 9psi at 6000ft to run the same as 32psi at sea level... so you guys in the mountains w/ twins, don't be scared to crank up that boost



Forrest

 

[T-Bone]

So maybe I have some tweaking to do. What do I need to check? Piers set my timing to 15. 5 and I've got an AFE stage 2. The next thing I want to do is get the cat taken care of (still stock exhaust). Could the cat be limiting the boost?



Thanks

T-Bone

 

[scaryfast]

I can tell you I had the cat off my truck for a few months and there was no difference in boost pressure. I can also tell you there is no performance difference either. The diesel cat is very unrestricted.

 

[T-Bone]

Ugh. I was kinda hoping to hear the cat was my problem. I was also hoping for some spool up help without the cat. That doesn't sound like a possibility either. I'm lost on what to check next. Any ideas? Any help would be greatly appreciated. 210hp with a #10, DDIIs, 3k GSK, an AFE stage 2 is a bit of a let down.

Thanks,

T-Bone

 

[mschoenheider]

not true... it took me a while to wrap my brain around it, but the 32psi you're seeing on the boost gauge is a measurement of the pressure ABOVE the atmospheric pressue... so the turbo is spinning roughly the same speed and the pressure ratio is the same.



now... the cylinder pressure is lower because the absolute pressure is lower... IE, it would take 39. 9psi at 6000ft to run the same as 32psi at sea level... so you guys in the mountains w/ twins, don't be scared to crank up that boost



Forrest

I'm not sure what you think in not true in my above statement. The pressure ratio required for your boost gauge to read 32 psi at 6000 feet is 3. 72:1. Pressure ratio is equal to boost pressure plus atmospheric pressure divided by atmospheric pressure ((32 +11. 8)/11. 8). Atmospheric pressure at 6000 feet is approximately 11. 8 psi. If you take the same 3. 72 pressure ratio at sea level (atmospheric pressure at sea level is about 14. 7 psi) you would end up with 39. 9 psi on your gauge.

 

[LHallmark]

I'm not sure what you think in not true in my above statement. The pressure ratio required for your boost gauge to read 32 psi at 6000 feet is 3. 72:1. Pressure ratio is equal to boost pressure plus atmospheric pressure divided by atmospheric pressure ((32 +11. 8)/11. 8). Atmospheric pressure at 6000 feet is approximately 11. 8 psi. If you take the same 3. 72 pressure ratio at sea level (atmospheric pressure at sea level is about 14. 7 psi) you would end up with 39. 9 psi on your gauge.

Forrest and schoenheider are both right... ... They just used different words. When I first read MS's statement i thought he was out of the park. Then read Forrest and thought... right on..... Then reread MS's statement and understood what he was saying... ... . The ratio inside the turbo remains the same regardless of altitude..... The final output pressure changes due to altitude because the input pressure(atmospheric) changes with altitude.

 

[Forrest Nearing]

I really need to think before I post sometimes

 

[mschoenheider]

Forrest,

I hope you aren't taking any of this the wrong way, I am only posting to pass on what I believe to be correct info, not to **** anyone off. That being said, I have to disagree with the second part of your earlier post.



"now... the cylinder pressure is lower because the absolute pressure is lower... IE, it would take 39. 9psi at 6000ft to run the same as 32psi at sea level... so you guys in the mountains w/ twins, don't be scared to crank up that boost"



The cylinder pressure is lower to a certain degree at altitude than at sea level, however I don't think it is to the extent you have said. to have the same amount of total pressure at elevation as 32 psi boost (gauge) pressure at sea level, you would need to have 34. 9 psi boost (gauge) pressure at 6000 feet. Total pressure is boost plus atmospheric, so 32+14. 7 would be the same as 34. 9+11. 8. So in a simplified version of things, you would need 34. 9 psi of gauge pressure to "run the same as 32 psi at sea level". I call that the simplified version of things because I am neglecting a few factors such as air temp, and the extra heat in the compressed 6000 foot air due to the higher amount of work needed to get the air to that "equivalent" pressure.



The reason I bring this up, is because I think your last statement "so you guys in the mountains w/ twins, don't be scared to crank up that boost " might be a little misleading, and could result in a couple blown head gaskets. I do agree that twins are the way to go up here. Once I get me Jeep back together, I am going to start putting the pieces together for my own set.



Mark

 

[Forrest Nearing]

Forrest,

I hope you aren't taking any of this the wrong way, I am only posting to pass on what I believe to be correct info, not to **** anyone off. That being said, I have to disagree with the second part of your earlier post. Mark

no sweat... you're right, I didn't do the math... again, typing without thinking (gimme a break, I just got back from vacation! LOL!)



bottom line is you can run a little more boost at altitude and maintain the same cylinder pressure...



I just got back from spending two weeks above 8,000ft, so my brain's a little foggy I guess.



(the truck coulda used another 10lbs of boost up there too :-laf )



Forrest

 

[JGheen]

MS,

So what your saying is that you should see the same amount of boost(gauge) pressure at sea level and at 6000 ft. , but the turbo is obvioulsy working harder to get to that same amount of boost due to the thinner air? Or would you see a lower boost gauge reading when one drives from LA to Denver, for example?



How 'bout it Forrest, I am I following your boost statements correctly here??

 

[T-Bone]

With all that's been said (it's been very educational!), I now have a much better picture of what's going on (or not going on :{ ) with my turbo. Any ideas where I can get an additional 8 or 10 pounds of boost with my stock turbo? I would like to see 34 or so and am hoping this will help with my smoke and egt issues.



Thanks for all the help so far!

T-Bone

 

[JGheen]

With all that's been said (it's been very educational!), I now have a much better picture of what's going on (or not going on :{ ) with my turbo. Any ideas where I can get an additional 8 or 10 pounds of boost with my stock turbo? I would like to see 34 or so and am hoping this will help with my smoke and egt issues.



Thanks for all the help so far!

T-Bone

What size turbine are you running on your stock HX-35? That is weird how you tried blocking the WG and still only got about 27#. All that fuel your throwing at the engine should produce higher boost. Maybe you have a leak somewhere and that is why you can only get 27#. I would double check all your clamps, boost boots, and any WG or AFC lines that could be leaking. Also, are you sure you governor lever is hitting you #10 plate? I know the #10 is a very versitle plate and that a gov. lever adjustment is probably not needed, but give that a check as well.

 

[mschoenheider]

JGheen,

You would see lower boost (gauge) numbers at 6000 feet than you would at sea level if you drive a truck from one to the other. You are correct that your turbo would have to work harder to get the same total pressure, or even gauge pressure at 6000 feet. The amount of work your turbo can do is basically related to turbine housing size, compressor wheel size an amount of fuel, so if you leave all those the same , you would not get any more work out of the turbo. I have found that smaller exhaust housing sizes, or larger compressor wheel sizes work well to "adjust" turbos for high altitude operation.

 

[boondock]

So what compressor wheel do you think we should try?



Have you tried a 14cm or 16cm housing at altitude? What were the results?



BTW both have stock turbo housings. I would love to get more boost to get my egt's down and then slide the plate further forward!!

 

[T-Bone]

JGheen,



Thanks for the help. I think it is a 12. Not sure though. I had the boys in Castle Rock check it and they said no leak. I'm contemplating buying a boost checking setup to double check. I'm not sure quite what I'm looking for when checking the governor lever. Am I looking down into the top of the pump past the plate? I couldn't seem to see much down in there if that's where I need to look. Any coaching would be appreaciated here.



We slid the AFC some and low end is much better. I get a little bit of smoke off the line, but it clears up when I hit boost. I know I have a bunch of fuel. We slid my plate forward a bit (did not even appear to be half way) and the thing smoked like a freight train. EGTs were at 1300 when I hit 45mph but I didn't "feel" a bunch more power. It just seems strange that boondock and I could only get 27 with blocked waste gates.



I do have a slight injector leak. #3 or #4, but I can't tell if it's the banjo or something else. Any pointers? I'm not sure what I can "tighten" to stop the leak.



T-Bone

 

[Forrest Nearing]

I do have a slight injector leak. #3 or #4, but I can't tell if it's the banjo or something else. Any pointers? I'm not sure what I can "tighten" to stop the leak.



T-Bone

try backing off the nut that attatches the line to the injector and then tighten it back up... I had a couple leakers a while back after I bumped the timing.



Forrest

 

[Rhondasway]

Just checking: you had your whole system tested for leaks? I may have already mentioned having your intercooler pressure tested, and also check the shaft play in the turbo. Are you checking your coolant/oil levels to be sure you're not pushing boost through a head gasket into one or both fluids? This can also happen between cylinders. No exhaust manifold leaks either, right?