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Old 12-11-2022, 12:01 PM   #1
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boost pressure

so....

how much turbo boost pressure does the factory run, how much can i run?

mechanical '93 cummins 4bta

i've screwed it up and trying to reset.....

i finally bought a boost gauge and im only getting 5 psi boost at full throttle.
i think i have the afc set up for 15-20 psi.

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Old 12-11-2022, 01:47 PM   #2
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on my 24v, it runs 25psi with a factory tune. I have an HX35W. I thought the turbo changed around your year of cummins. I assume you have a 6bt or do you really mean 4bt in something other than your bus?
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Old 12-11-2022, 04:55 PM   #3
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Yeah I think he has to have the 6BT as his profile says it's a 5.9L and a 4BT is only like 3.9L that they used in delivery trucks.
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Old 12-11-2022, 05:45 PM   #4
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yes, 6bt

i had a brain fart earlier.

i got the boost gauge hooked up and all assembled and took the bus out for a test drive.

in the driveway pushing the throttle, i could only get the boost up to 5psi.
on the test drive, under real load, i got the boost up to 12psi.

the floor by the turbo charger is always hot. i think the waste gate opens too early and just pushes the boost out the door.

tomorrow, i'll try and tighten up the waste gate spring.
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Old 12-12-2022, 08:28 AM   #5
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20 or so on a stock 6bt.

Boost is heavily dependent on load(fueling and rpm) and altitude. Sitting stationary free reving you might only get 5, a lot of engines don't even get that.

Being a mile high, 12 might be all that you get for that set of circumstances. Is there smoke at all when you're doing this? Do you have a pyrometer installed? If there's smoke, tighten the wastegate up a little bit and try it again. If there isn't any, then what you got might be all that you're gonna get. You don't want a super high psi number for the sake of having it. Ideally you want enough boost to keep your exhaust clear and EGT's in check. Any more then that is a waste IMO.
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Old 12-12-2022, 08:51 AM   #6
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20 or so on a stock 6bt.

Boost is heavily dependent on load(fueling and rpm) and altitude. Sitting stationary free reving you might only get 5, a lot of engines don't even get that.

Being a mile high, 12 might be all that you get for that set of circumstances. Is there smoke at all when you're doing this? Do you have a pyrometer installed? If there's smoke, tighten the wastegate up a little bit and try it again. If there isn't any, then what you got might be all that you're gonna get. You don't want a super high psi number for the sake of having it. Ideally you want enough boost to keep your exhaust clear and EGT's in check. Any more then that is a waste IMO.

its also not a bad idea if you are going to tune in this engine to put a boost gauge on the exhaust manifold side of the turbo and monitor drive pressure.ideally you want to keep your drive pressure to be about 10 percent lower than your boost pressure.. but they are often even or a little "upside down" which is OK.. but you dont want a Huge amount of backpressure compared to boost or you will almost certainly run the EGTs off the scale..



unless you are really going to take this thing to the moon you probably dont need to worry alot about drive pressures or valve springs.. however its another tool if you are going to tune. the HX35 isnt going to like much more than 35 to at max 40 PSI..
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Old 12-12-2022, 09:40 AM   #7
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its cold today, so i may not get to try any changes......

but,
my egts are higher than i think they should be. 10k-ish at wot highway speed.

i want to go to arizona in january, towing, and i'd like to set it up so i don't abuse the motor more than i need to.

the low wastegate pressure is from my exhaust manifold replacement a few years ago, so its been low for a while. i tightened the wastegate up on another thread a few years ago, but i need the boost gauge to dial it in. yesterday, i got the boost gauge.

how can i lower egt's on hill climbs under load? thats my goal!

my afc is almost al the way turned in. (maybr out), it smokes pretty good.
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Old 12-12-2022, 10:22 AM   #8
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If you're seeing EGT's of 1K. Then your boost should be higher. Plug off the waste gate and see what it does. The stock WG actuators were not noted for being that good. There was also what was called a boost fooler made to delay WG opening.

I would do a pressure test of the IC. With age and wear and tear it could be more of a sieve than a IC.

BTW the 5.9 is rated for 1250 EGT's in a continuous run mode. Myself I wouldn't do above 1150. Number 6 cylinder doesn't get the same amount of air as the rest. That cylinder runs hotter. If you search online forums for #6-cylinder wall problems. It shows how expensive it can get turning up a 5.9 to much.
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Old 12-12-2022, 10:51 AM   #9
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1000 degree pre turbo isn't bad. I wouldn't go above 1200. 12psi seems low, especially if it's at 1000 degrees, but then again you're in colorado which is much higher then where I am.

To a point you can lower egts by increasing boost, until the charger becomes inefficient. Last hx35 I worked on the spec for wastegate to open on the bench was 20 psi. On vehicle in operation that # will lower because of exhaust pressure acting against the waste gate. Shorten the rod to increase boost, and try your test again. Always do 1 turn at a time and test. Run the same route so that you can eliminate variables.

Once again, you don't want too much boost as that will create too much back pressure. Too much back pressure blows head gaskets.

FWIW, very few times I've seen back pressure lower then boost pressure, more often then not it's the other way around. If you can get ebp to boost at a 1:1 ratio, you're doing pretty good, very rarely does it get better then that, especially if you're intercooled.
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Old 12-12-2022, 11:25 AM   #10
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1000 degree pre turbo isn't bad. I wouldn't go above 1200. 12psi seems low, especially if it's at 1000 degrees, but then again you're in colorado which is much higher then where I am.

To a point you can lower egts by increasing boost, until the charger becomes inefficient. Last hx35 I worked on the spec for wastegate to open on the bench was 20 psi. On vehicle in operation that # will lower because of exhaust pressure acting against the waste gate. Shorten the rod to increase boost, and try your test again. Always do 1 turn at a time and test. Run the same route so that you can eliminate variables.

Once again, you don't want too much boost as that will create too much back pressure. Too much back pressure blows head gaskets.

FWIW, very few times I've seen back pressure lower then boost pressure, more often then not it's the other way around. If you can get ebp to boost at a 1:1 ratio, you're doing pretty good, very rarely does it get better then that, especially if you're intercooled.

but that 10 percent rule i mentioned is EBP 10% Above boost.. but when you dont have things set up right you can easily end up with a 20% or more EBP over boost which will raise EGTs despite pushing out more boost..
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Old 12-12-2022, 11:55 AM   #11
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re- reading your posts and figuring out what all the initials stand for.

i think i get it.
i think i am under boosted, possibly making that ebp higher than it should be.
the floor by the turbo really cooks. the passenger has to watch where they put their foot. imo, the wastegate is open at wot. going to have to test and see.

im not really trying to tune it.... just want to get it it back to factory or a bit better. is 30psi too much? if its warm enough to work on it, i don't mind turning it up.
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Old 12-12-2022, 12:05 PM   #12
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boost only helps if you are fueling to burn it.. boost is just more air in the cylinder.. if you are blowing black smoke or running really high EGTs then you can boost to lower them.. I dont consider 1000 EGT (measured manifold before turbo) too high under a good load.. boost is obviously made by more exhaust.. burn fuel make exhaust and make boost..



trying to achieve high boost numbers arent in and of itself beneficial.. if your turbo is waste-gating then its trying to make more boost then you allowed it to.. it cant hurt to make a a small adjustment to the weastegate, drive it and see how EGTs and boost do. the HX35 should stay pretty efficient up into the 20s for sure.. 30 is still safe for it.. if setitng the wastegate higher doesnt make any change to your EGT or clear up any black smoke then id put it back...



theres a lot of factors in play... on my redbyrd I notice colder weather is a big factor in EGTs and how fast they run up.. cold weather means more dense air and likely better intercooler efficiency... ive thiought about going to a water / air intercooler which would make the intercooling more consistent as road heat wouldnt be near a factor
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Old 12-18-2022, 12:58 PM   #13
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Second gen 12 valve can do 32 psi stock but I don't think that the 93' with the rotary pump will get much more than 24-25 psi.
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Old 12-18-2022, 01:23 PM   #14
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one of the oddities between dodge and cummins.......

the '93 dodge 5.9 uses the VE rotary pump.
the '93 school bus version is the bosch p pump.

maybe its a truck thing? idk, but dodge and cummins are not the same thing.
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Old 12-18-2022, 02:05 PM   #15
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one of the oddities between dodge and cummins.......

the '93 dodge 5.9 uses the VE rotary pump.
the '93 school bus version is the bosch p pump.

maybe its a truck thing? idk, but dodge and cummins are not the same thing.
I believe Dodge made the change in 1998. Consumer vehicles often don't have to make dramatic model changes until a redesign cycle whereas commercial vehicles don't live and die by consumer populism.
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Old 12-19-2022, 07:30 AM   #16
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Commercial always used the inline pump as far as I know. Dodge used the ve rotaty pump, upgrading to the inline p7100 in 94.

They all upgraded to the vp electric rotary in 98/98.5, when they went to ISB, depending on the vehicle.
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Old 12-19-2022, 11:22 PM   #17
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Turf you have a p-pump or VE pump? Either way it seems like your boost numbers are really low. I have a 93 VE pump cummins in my 63 crewcab, stock it was mid 20’s for boost. Now with the pump tuned, HE351 turbo, and head studs it makes around 40# of boost. I’ve owned a few cummins trucks and they all made over 20# of boost.

If you do have a p-pump I feel like it should be making mid 20’s for boost ?

Also, where do you have your boost guage installed?
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