Where did the boost go.

SmokinCAT

SmokinCAT

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Recently I tried a set of twins on my 855 cummins, this was more to see what would happen instead of out of need. In doing so I didn't gain hardly any overall boost maybe 4-5psi.

The manifold turbo is a HT4B holset with a 84mm inducer and the low pressure turbo is a HX82 with a 108mm inducer.

As a single the manifold turbo would make 38-40psi, so I was expecting more from out of the combo than 45psi at the most, the LP turbos is making between 20-25psi.

I looked fairly closely for boost leaks after the manifold turbo and there aren't very many places to check. 4 gaskets and one boot. Never did check my boost gauge though.

Could there be something in the size of the turbos that isn't allowing them to work properly together.
 
SmokinCAT said:
Recently I tried a set of twins on my 855 cummins, this was more to see what would happen instead of out of need. In doing so I didn't gain hardly any overall boost maybe 4-5psi.

The manifold turbo is a HT4B holset with a 84mm inducer and the low pressure turbo is a HX82 with a 108mm inducer.

As a single the manifold turbo would make 38-40psi, so I was expecting more from out of the combo than 45psi at the most, the LP turbos is making between 20-25psi.

I looked fairly closely for boost leaks after the manifold turbo and there aren't very many places to check. 4 gaskets and one boot. Never did check my boost gauge though.

Could there be something in the size of the turbos that isn't allowing them to work properly together.
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I think you don't have enough fuel to spin those both.
 
4-5 extra pounds boost isn't exactly little on 855 cubes. Not sure what your doing for fuel but most likely needs more.

Really interested on your outcome as we've got a couple tractor's I've been itching to put compounds on.
 
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Joesixpack said:
4-5 extra pounds boost isn't exactly little on 855 cubes. Not sure what your doing for fuel but most likely needs more.

Really interested on your outcome as we've got a couple tractor's I've been itching to put compounds on.
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It is definitely in the works to add more fuel.

I guess I am just not able to figure out why the turbo that went from making 40psi dropped to 20psi in this setup, wouldn't the extra air into the engine drive the manifold turbo even harder than it was as a single?

Most likely I will be going back to the single I was running, when I pulled with these twins I seemed to have lost quite a bit of upper rpm, 3-400 rpm down the track.

What size tractor are you thinking about doing this too?
 
SmokinCAT said:
It is definitely in the works to add more fuel.

I guess I am just not able to figure out why the turbo that went from making 40psi dropped to 20psi in this setup, wouldn't the extra air into the engine drive the manifold turbo even harder than it was as a single?

Most likely I will be going back to the single I was running, when I pulled with these twins I seemed to have lost quite a bit of upper rpm, 3-400 rpm down the track.

What size tractor are you thinking about doing this too?
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How did the egt's compare? Twins to single...
 
I wouldn't give up yet, remember the LP turbo is working with much denser air than 1 atmosphere and 20 psi isn't accurate for figuring out its workload.

Frankly I can't imagine how you lost top end. 45 pounds out of a compound arrangement HAS to be stronger that 38-40 psi on the single.

JD 8960's
 
maybe fighting the stroke of the engine.. and no to mention elevated exh pressure with overlap acts as EGR. i would try tighting the valves .014"-.027" Maybe .012"- .024.
 
whats max exh restriction 40" h20? may be your exh system cant handle the flow with the additional air.. what muffler do you have?
 
I thought about the exhaust side of the manifold charger not being large enough to handle the flow and this could be what contributed to the lower max rpm, I have another HT4 in the shop that has a larger exhaust housing, but the two aren't interchangeable, it also has a larger compressor wheel so that could cause issue.

The exhaust is only about 5 feet of 5" pipe into a 7" stack.
 
I have no doubts that it ran stronger, but the loss in rpm has me scratching my head, the previous pull it revved well over 3000 and at the next pull with the twin it almost seemed to be governed to 2800.

The pump next year will be set to 3500 under power so with the bigger injectors more issues could come up.

Do those 8960s have the 531 in them?
 
855's in them. JD didn't have an engine for them after the V8 mess and went to Cummins for the 89xx series.

How much pump pressure are you running now?
 
I have another option that may work, maybe a HC5 with a HT60 or HT3. I would think that the HT4B would work well in your situation, what rpm do you run them at when you work them?
 
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