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So who has bumped the power on a 6CTA cummins

Hoefler

Active member
1,096
20
38
Location
White Bear Lake,MN
I have looked over the TM's and on-line 8.3 stuff to learn how to adjust the idle on my 8.3. Have found a simple sketch from a motorhome site that deals with 8.3 Cummins engines-looks like the low speed adjust is near the throttle shaft-a flat blade screw with lock nut on bottom?
Is this correct? Does anybody have the TM for this or image.
My idle is 500 RPM but needs to be bumped up to spec.
Thanks,
Pete
 

hklvette

New member
373
7
0
Location
Christiansburg, VA
I have looked over the TM's and on-line 8.3 stuff to learn how to adjust the idle on my 8.3. Have found a simple sketch from a motorhome site that deals with 8.3 Cummins engines-looks like the low speed adjust is near the throttle shaft-a flat blade screw with lock nut on bottom?
Is this correct? Does anybody have the TM for this or image.
My idle is 500 RPM but needs to be bumped up to spec.
Thanks,
Pete
You are correct. Break the jam-nut loose first, then loosen the flathead screw to increase idle speed. Tighten the nut back down when done.
 

hklvette

New member
373
7
0
Location
Christiansburg, VA
No. You adjust it while it's off, then test fire it.
I'm curious why you would adjust the idle screw with the engine off? We may be talking about two different screws. The one I'm talking about limits how far back the throttle lever can travel through mechanical interference, and I see nor found any particular reason it can't be done with the engine running. Not looking to be a contrarian, just trying to learn.
 

Jason O

Member
107
2
18
Location
Lebanon PA
I turned up the full fuel / rack travel with good results. I noticed the sound of air rushing through the mushroom cap intake screen on my m923A2 when the motor was wound up under load. There is not a lot of surface area for the air to enter. My 931A2 tractor has a different shape inlet cap and likely flows better.

Ill take a few measurements with a manometer and report the results. The restriction may be minimal, or may be good for a pound or 2 of boost if eliminated. Higher inlet pressure = higher discharge pressure.
 

HETvet

Member
395
7
18
Location
Bedford, texas
As far as air restriction, I personally would make the intake (turbo inlet foward) the same diameter at the turbo inlet and make any bends smooth, along with trying to eliminate bends if at all possible. Also, increase the flow rating of the filter, and increasing the size of the tubing foward of the filter. I say all of this thinking out loud as I have never popped the hood on a 6CTA powered truck before.
 

Jason O

Member
107
2
18
Location
Lebanon PA
The intake piping and air filter canister look like they should flow pretty well. What would an acceptable pressure (vac) reading be post filter, before compressor inlet?
 

hklvette

New member
373
7
0
Location
Christiansburg, VA
It's not the idle screw you are adjusting. It's the fuel screw under the hex cap on top of the ip
We are definitely talking about two different things. The idle-speed screw is not under the hex cap. It is just aft of where the throttle shaft enters the injection pump body, and is adjusted without removing any caps from the IP.
 

Carlo

New member
1,364
20
0
Location
palazzago italia
I turned up the full fuel / rack travel with good results. I noticed the sound of air rushing through the mushroom cap intake screen on my m923A2 when the motor was wound up under load. There is not a lot of surface area for the air to enter. My 931A2 tractor has a different shape inlet cap and likely flows better.

Ill take a few measurements with a manometer and report the results. The restriction may be minimal, or may be good for a pound or 2 of boost if eliminated. Higher inlet pressure = higher discharge pressure.

When does get start getting dangerous as in to high temps and possible pistons melting? Are you controlling the Exhaust temperature?
 

HETvet

Member
395
7
18
Location
Bedford, texas
The intake piping and air filter canister look like they should flow pretty well. What would an acceptable pressure (vac) reading be post filter, before compressor inlet?
this I don't know.

I'm drawing from my time with the LLY Duramax engine. the compressor map in stock configuration has the turbo running at the surge limit. this is because of a poorly designed air inlet pre-turbo to post air filter box. By changing out the "turbo mouth piece" or the turbo inlet pipe to the stock LBZ piece, you drop IATs by 75 degrees, go from 22 PSI MAP to 19-20 PSI MAP, increase MPG by 2 MPG, and make more power. With the addition of larger and smoother intake pipes to the air box and cutting holes in to the air box, these advantages become even more pronounced. So If your truck is making these noises, I would assume that there is a restriction there and by doing something like what I described might net you gains like what is experienced with the other engine.

This works because the computer is commanding XXPSI from the turbo. The MAP sensor is telling the computer that its seeing YYPSI. So the computer begins to command more from the turbo until it sees XXPSI from the MAP. In essence, the computer is telling the turbo to put out 20PSI. The MAP is telling the computer its seeing 18PSI at the intake manifold. So the computer begins to force the turbo to put out more boost until the MAP is seeing 20PSI. But, at 20PSI MAP, the turbo is working as hard as it would to make 23-24PSI. The reason for this starts at the turbo mouth piece. Its got a 90 degree bend in it and at that bend and through the bend it pancakes down from a 3 inch round inlet to a 1inch by 2.5 inch rectangle shape back out to a 3 inch round discharge in to the turbo inlet. Moving forward, the intake tube goes from 2 inches to 2.5 inches and the air box has one hole in it that is sealed to the fender and is 3inches by 4.75 inches. By replacing all this with free flowing equipment, you lower the PSI, but allow the turbo to flow more CFM per PSI.
 

Ford Mechanic

Active member
1,805
6
38
Location
Edenton, NC
When does get start getting dangerous as in to high temps and possible pistons melting? Are you controlling the Exhaust temperature?

I would also like to know what the safe EGT limit is on these engines. I know the LDT 465 in the M35A2's EGT magic # was 1200*F for a lasting engine.
 

Csm Davis

Well-known member
4,160
393
83
Location
Hattiesburg, Mississippi
I turned up the full fuel / rack travel with good results. I noticed the sound of air rushing through the mushroom cap intake screen on my m923A2 when the motor was wound up under load. There is not a lot of surface area for the air to enter. My 931A2 tractor has a different shape inlet cap and likely flows better.

Ill take a few measurements with a manometer and report the results. The restriction may be minimal, or may be good for a pound or 2 of boost if eliminated. Higher inlet pressure = higher discharge pressure.
I have never seen a different mushroom on any 939s but some have a additional screen under the mushroom that is cone shaped.
 

Ford Mechanic

Active member
1,805
6
38
Location
Edenton, NC
Do ya'll know the max boost out of the 8.3L with just turning up the pump? I'm buying a pyro and boost guage and trying to decide 35psi or 60psi guage.
Thanks!
 

Jason O

Member
107
2
18
Location
Lebanon PA
CSM Davis - You are correct about the intake caps. I removed what I thought was a different design to find it was only an additional screen as you described. Thanks for setting me straight.

Took the truck for a quick ride with the cap removed. No more sucking sound through the screen. That motor really pulls in the air when wound up (put your hand near the intake). Ill get some pressure measurements later.
 

Jason O

Member
107
2
18
Location
Lebanon PA
For those of you who have installed new GSK's, have you considered using a better harmonic balancer? Only reason is that I remember seeing notes in the parts catalog that the OEM balancer was for 2100 RPM or lower.
 
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