Nice project you've got there. The school bus motor will be truck spec, not stationary industrial. @char1355 will be able to help you out with any CPL numbers you can find on the tags.

 

Consider installing this 6bt engine without any performance modifications - I am having trouble following where you are headed on this - At one point you say "I want to DETUNE it" - and then you say "12v pushing a modest 375hp"...

Just assemble it with the stock school bus 6bt. If you don't drive it hard (i.e. Clutch dumps, burnouts, etc) it should get you to school and back. The gearing concerns me, you might find that the combination of RPM limiter, rear axle ratio and tire diameter limits you to a slower than 65 MPH. Use the following calculator to get an estimate of top speed - AND - you really do not want to have to drive with the engine on the rev limiter.

https://www.grimmjeeper.com/gears.html

Russ

 

The same forged con rods are used in all the 6bt-4bt motors, pickups, vocational or stationary motors, nothing special about a 190hp bus motor as far as the rotating mass, piston bowl design and injector size, will be the main differences... it's been said the p-pumps have a couple of different governor weights to control the fueling rate between different motors, my 230hp pusher motor seems to fuel differently (slower) than my 95 160hp 12v motor, something I will look into since I'm planning on pulling to pull the motor and place into my 91.5 w250 build... imo your 190 should be left alone until you beef up the frame, it will be the weight, not the hp that needs to be addressed, your 58 1/2 ton has a very light frame compared to first gen dodge frames that the 6bt first appeared in back in 89, and should have a fully boxed or boxed front half frame plus a reinforced crossmember in place before placing the motor in service, just to put this in perspective, guys will box-reinforce these early gm trucks just using a 105hp 4bt... your build, your decision, just my 0.02, looking forward to seeing more of your build...

 

Ok thank you for this piece. I think ill just go ahead and do that up front instead of waiting for problems.

 

Cool project! I’ve done a 6BT swap too detuning’s tricky but doable. Try backing off the fuel screw and AFC spring. Smaller injectors help, and sounds like you’ve got that covered.

 

Initially my rig was a 4BT at the 200hp-400ft/lbs power range the truck was 8,000# unloaded at that time I was running the ford M5odr2 transmission which is a 1/2 ton truck trans
that mane consider a weak unit. It served well for me until I needed to move 5 trips towing trailers finally did it in with a 500 mile interstate run at 16,000# CGVW being the nail in it's coffin, with that being said IMHO it held up well pulling grades in OD for the most part and it did not fail I KILLED it asking too much from it. Your trans being that it was also used in medium duty trucks should hold up if you treat it right unless you really overload/over work it or get froggy and introduce shock loads, if you do that with a b series Cummins
there are very few drive trains that wont fail. The b series engines like to be driven smoothly with easy throttle and clutch work the immense torque that exists starting at idle
is truly a irresistible force just let it do what it does, I tend to "short shift" it until I'm in 3rd by then there is enough load on it to where it can start to really work and it pulls like a locomotive. Getting your gearing right is very important for the drivability in a b series as the useable power band is pretty narrow compared to what most engines have,
they are happiest when living between 1,500 and 2,000 your rig is light so you will have a couple hundred rpm below 1,500 and while they will run above 2,000 OK they really start to use fuel. I turn 2,000@70mph and on the highway every 150rpm beyond that costs me approx. 1.5-2 MPG...$.02

 

Initially my rig was a 4BT at the 200hp-400ft/lbs power range the truck was 8,000# unloaded at that time I was running the ford M5odr2 transmission which is a 1/2 ton truck trans
that mane consider a weak unit. It served well for me until I needed to move 5 trips towing trailers finally did it in with a 500 mile interstate run at 16,000# CGVW being the nail in it's coffin, with that being said IMHO it held up well pulling grades in OD for the most part and it did not fail I KILLED it asking too much from it. Your trans being that it was also used in medium duty trucks should hold up if you treat it right unless you really overload/over work it or get froggy and introduce shock loads, if you do that with a b series Cummins
there are very few drive trains that wont fail. The b series engines like to be driven smoothly with easy throttle and clutch work the immense torque that exists starting at idle
is truly a irresistible force just let it do what it does, I tend to "short shift" it until I'm in 3rd by then there is enough load on it to where it can start to really work and it pulls like a locomotive. Getting your gearing right is very important for the drivability in a b series as the useable power band is pretty narrow compared to what most engines have,
they are happiest when living between 1,500 and 2,000 your rig is light so you will have a couple hundred rpm below 1,500 and while they will run above 2,000 OK they really start to use fuel. I turn 2,000@70mph and on the highway every 150rpm beyond that costs me approx. 1.5-2 MPG...$.02

Very well said. I agree there is no need to de-tune, sensible operation is all that is necessary.

 

You need to consider a few things. First off, the 190 HP engine is mid line for the 6bt. The Dodge versions were mostly 160 HP and the truck models like yours went up to 230 HP. That was the most powerful road 6bt offered. Your transmission is OK but lacks an OD gear. Depending on your trucks rear axle gear, you may only reach 50-55 MPH at full throttle. You need an OD geared transmission like the NV4500 or NV5600. The stock GM bellhousing was only on the 4bt which came in Chevy vans. It isn't common but is available from Cummins. It is very expensive. The rare part is the flywheel. Most GM vans were automatics. Your engine probably came with an SAE flywheel housing. You could use that depending on which one you have. You gave an engine serial number so I could look that up. You'd just need a flywheel and clutch to go with it. There is a company that has a setup to mate a NV4500 to a SAE3 set up. Now, nothing is going to be cheap. Forget that idea unless you just luck onto something. Need to know what rear gear ratio you have. If your engine has 400 lb ft of torque then it is exactly the same as the 160 HP used in the Dodge. You're not going to turn it down. Forget that idea. Up yes but down no. Your big problem is going to be engine speed. Now you could add a gear splitter behind your transmission and have an OD unit. Those are a bit scarce too.

 

When Dodge 1st put the Cummings in their Pick-ups... we would see them in the shop... Bad Differentials, twisted off drive shafts, a few cracked frames.. It ain't the HP.. its the torque. Put a load on it and and give it the throttle... and if what you are pulling resists... the early Trucks would break shit.. If you are looking for show... Might be wonderful.. you start playing tough and it will break shit. If you change your mind... a 4bt is much more reasonable in terms of weight ( yep you'll have to reinforce the front suspension for a 6BT.. Then you better plan on good power steering up-date.. I have driven many front heavy trucks without... it ain't no fun.. ) If You are in WA. state area... I know where there is a 4bt total rebuild, my 1 cent worth

 

You bring up a very good point the weight of the 6BT it is double the weight of the existing engine and is gonna make for a real ugly front to rear weight distribution,
when building my '68 F-250 the 6BT's weight was for me a deal breaker as it is with the 4BT I'm still over 1,000# heavier than with the V8.
I'm very much OCD about weight distribution due to IMHO handling, drivability and safety issues that it brings to the table, many moons ago I owned a 1st generation AMC AMX
it was a fun fast and cool little mussel car but its short wheel base and nose heavy setup brought some truly UGLY traits the worst was it doing a 180* spin under heavy breaking.
IMHO a 6BT in a 1/2 ton truck is like a 4BT in a Wrangler or mini truck, it will NOT be your friend when something goes wrong or someone does something stupid in front of you,
it will be just the opposite and it will stab you in the back. Now plenty of people have swapped 4BT's into vehicles of that size and wheelbase but you could not pay me to do so...$.02

 

1986 Ford F150. The 4-cylinder 4bt replaced an inline 300 CID 6-cyliner engine. The shorter engine moved the engine center of gravity back, so I did not notice any sagging in the front end.

Disclaimer: I never drove the 300 CID engine on the road - it was so worn out - that it barely had the power to drive into the shop...

Russ

 

Russ I need to clarify the 1,000# increase on my front end is due to the crew cab and winch/bumper not the 4BT but it was the reason for not doing a 6BT which my truck would have been happiest with. Still a nose heavy vehicle is not your friend.

 

My 1986 F150 has a reproduction front bumper (my guess is it is lighter than the stock bumper). The rear bumper is the stock bumper for the short step side bed. This bed is a 1952 Bell Telephone Systems service box. The braking is well balanced.

Picture taken about 4 years ago - The Lowell district of Bisbee, AZ.
Russ

 

One item you must consider is the 190 HP 6bt is not exactly the same as the one in the Dodge pickups. You note that even though the HP is up by 30 HP, the torque level is unchanged. There are some internal changes in the injection pumps. Just because both have a P7100 doesn't make them the same. Be interesting to see a power curve for both engines. And there was one model in the last year of the Dodge that got the really good P7100 with all kinds of potential. Those are kind of scarce. 400 lb ft of torque is not excessive but can break things if abused. Have to drive sensibly.