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Discussion Starter · #1 ·
Ok.. Sorry if this is long..

Picked up an 82 15' P-30 on Friday that I got through DoveBid.

What I have figured out about it so far:

Engine is a Cummins recon, dated 9/98. CPL 857, BTA, 120HP. JWAC equipped.

Transmission is a TH400.

I can give you the back story if you want, but it turns out the engine was rebuilt about 10K ago, and the trans is solid. Also, the pump was "turned up", but I don't have any details of what was changed.

I can say this:

Tire size on the truck is 8R19.5. TH400 is 1:1 in top gear. And, on flat ground, this thing moves along at 75MPH with ease and no noticeable smoke. On hills (at least the ones between Chattanooga and Knoxville on I75) it drops back to 70MPH, but still don't see noticeable smoke. Held it pretty much wide open from Chattanooga until I hit the traffic snarl that is West Knoxville.

I'm not relying on the speedo in the truck that was not showing the correct speed after 60MPH for this measure, but rather the one in the chase vehicle that my friend was driving behind me, and I KNOW it's right..

Truck originally came with a gasser straight 6.

Now, on to the questions (and sorry, but they're more of a P-30 nature than 4BT):

I suspect that the pump has a higher RPM governor spring installed (thus the 75MPH where most only do 55-57MPH). To check this, I need to figure out what final drive ratio this thing has. I found one reference online that indicated you can use casting numbers on the pumpkin (on driver's side, front), but according to that it's 2.60:1. Sounds a bit fast to me, especially considering the original powerplant, but if my math is holding up, it would put 75MPH right in the torque sweet spot. Before I go out, jack it up, chalk mark the driveshaft, then start spinning and counting, anyone got a better way / numbers / ??

Also, I got the P-30 with the intention of repowering my 90 C2500, but after driving the P-30 home, I'm thinking it'd be a better truck in the end if I put the cab / bed off the C2500 on it. Tack on the fender flares for the duals and go. Both trucks are same overall length, and have the same wheelbase as far as I can measure. Anyone know if the body mount locations for the P-30 chassis are arranged such that trying to swap a pickup cab and bed onto the thing would be a fabrication nightmare?
 

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I think the rearend are 4.10, but sounds crazy if you were able to do 75 mph.

Only problem I see in swapping your cab and bed over might be the lack of drop in the frame for the cab. look at your frame and see if it has a drop in it. i'm planning on swapping a 57 chevy cab onto one, but those had a straight frame to begin with. you probably won't need fender flares with that rearend, its a cab and chassis setup, and isnt any wider than a single wheel pickup, so the wheels will fit under the bed.
 

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Discussion Starter · #3 ·
Sounds about right...

I think the rearend are 4.10, but sounds crazy if you were able to do 75 mph.
Plugging that into a calculator makes sense, and confirms what I thought: 3200 RPM spring. (Calculator gives 3121RPM at 75MPH, then add about 100RPM for torque converter slip..)

Guess once the yard dries up I'm going to be doing a fair amount of crawling under stuff (since the P-30 won't fit in the garage I don't have.. :eek: )

Thanks!
 

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A real common rear axle ratio was 3.42 Allowing for the usual speedometer error & 32 inch tires that would give you a top speed of about 67 mph against the governor at 2500 rpm allowing for transmission slippage.
 

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Discussion Starter · #5 ·
Looks like I'm jacking up a wheel soon.. :)

I do know that the speedo in the chase vehicle is within 1MPH at 75MPH.. Let's just say it's been confirmed a couple of different ways recently. (Well, radar and laser are different, right??)
 

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Looks like I'm jacking up a wheel soon.. :)

I do know that the speedo in the chase vehicle is within 1MPH at 75MPH.. Let's just say it's been confirmed a couple of different ways recently. (Well, radar and laser are different, right??)
Most speedometers read higher from the manufacturer. Common is around 3 mph at 55 and gets proportionally greater at higher speeds. When you get a traffic citation the cop will usually knock of a few mph in your favor unless you act like a total a-hole. I consider my speedometer as relatively accurate since the previous owner was the man who was on the end of issuing the traffic tickets. So get your oil change pan out and crawl under and pull off the axle cover and start counting teeth. It's the only way to really find out. ;)
 

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actually the numbers are stamped into the ring gear, so all you gotta do is rotate it until you can read them, and divide one into the other. should be like 41 10 or something in that neighborhood.
 

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Discussion Starter · #8 ·
Most speedometers read higher from the manufacturer.
That's sorta how the testing session got started.. A discussion about speedo accuracy overall and at different speeds. Add a bored off-duty officer of the law and bingo! Next thing you know you're looking for a safe place to check it out..
 

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Your van might be different I guess, but my donor P30 had a huge monstrosity of a frame under it! I'd say half again as heavy as my stock '67 Dodge frame and that's pretty stout by most standards. What I'm saying is you might be hauling a whole bunch more weight with the P30 frame as with the stock Chevy frame and that costs fuel mileage. If that's not a problem then it seems like an easier way to do a swap, and we like easy. :)
 

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I just checked my 18ft p30 and it has 4.56 gears in it. only way to know for sure is to check I guess.
I think most of those P30's had a corporate 14 bolt in them. The lowest gearset for those would be 3.42's up to 4:56 from the factory.
 

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Discussion Starter · #12 ·
That answers that question

And the verdict is: Cab / bed swap isn't likely. P-30 definitely has straight frame, and the frame on my 90 C2500 drops under the cab. Ah, well..

Also, the rear wheels are standard dually width, so the fender flares would have been needed. But that is one big honkin' frame under that thing.

Hmm... I could just raise the bed to match up the body lines.. :idea:

Does appear to be a corporate 14 bolt, but still a wee bit too wet here to go crawling around under it.
 

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If you want reasonably accurate mph, us a gps.
Even the old ones usually have a mph function.

When you check the ring and pinion ratio, jack up both sides and turn the drive shaft. This way you don't put the spider gears in to the mix, which will change what you think is the final ratio.
 

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Discussion Starter · #15 ·
The frames are cab and chassis ,so hump for the bed ,flat bed !

CrewCab59
Don't tempt me... ;) Or, was that you volunteering to help? :eek:

Ok, so it was dry enough today to play. After I mowed, of course..

Put it on the jack (just one side), marked the tire and the shaft, started spinning. Turned the tire 4 revs, counted 8 and just shy of 1/4 turns of the shaft. With only the one side turning, that should be pretty close to 4.10 final ratio, right?

So, if that's plugged into the calculator at http://www.4lo.com/calc/gearratio.htm with 33.1" (or 33.4, found a couple of different figures for 8R19.5 size, and they are fresh recaps on the rear) tires, 75MPH and 1:1 top gear, comes out right at 3100, add 100 for TC slip, 3200RPM. Cool..

Now to scrounge up a 4L80E...
 
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