68 Bluebird Conversion

jepopp · 01-14-2014, 08:28 PM

Hey everyone! Glad to be here---not much luck in the powertrain forum, so I'll try here. I plan to start a thread and/or blog on my bus project soon, I'll post when it is up. Any help you can give me is much appreciated!

I own a 1968 bluebird school/transit bus, about half done. It is actually the same bus that was being given away on here 4 years ago. I am the 3rd owner since then, and it is now running again.

Anyway, 3 major drivetrain questions:

1. This baby has a 478 big block V6 in it. Basically a 305 sbc with 6 big pistons, I guess. I have done some research but not a ton. Are these things worth anything to anyone? It runs alright--stubborn to start, smokes a little oil to begin with, good power and idle after a few minutes--but it does alright going down the road. Am I better off sticking money into rebuilding this? Are parts hard to come by? Or can I get any money for it? Would it be feasible to put in a 350 instead? Or a 6.2 diesel (I have a cheap line on one of those)?

2. It has (I believe) an Allison MT-40 auto transmission. Would it be easy to swap a 350 (or whatever) to this transmission? What does anyone know about these transmissions? Good/bad/whatever?

3. The rear diff plate reads this: "Rockwell Model L148PX17 Shell Serial 500375." I am winding up over 2500 at 55 MPH, this thing must have been a city transit bus? Anybody know about this rear end? How can I re-gear to something that will get my up to 70mph? What am I looking for? How much does that cost?

Anyway, that's where I'm at. I love the carpentry and electrical in the bus, but as you can see, I'm no mechanic. Any help at all would be huge! I'm planning on starting a thread of my bus and its progression this spring, once it isn't -35 degrees ouside. Thanks everyone!

Jeremy

Tango · 01-15-2014, 05:41 PM

Howdy Jepopp...and welcome to the asylum! --- WOW...that is one strange motor alright. Must be one of the biggest production v-6's ever. Wish I could tell you more without lying but that's about it. My guess is it costs a fortune both to repair and to run. Personally, I'd transplant a diesel and something like an Allison. There are quite a lot of them hanging off Cummins in Dodge trucks these days.

As for the rear gears, stat out by determining the ratio you have. There are very likely a number of ring & pinion options for the Rockwell. The do some calculations to work out the right ratio/tire size and rpm for whatever motor you wind up with . Below is a link to a very handy online calculator...

http://www.csgnetwork.com/multirpmcalc.html

Did look around a bit and found this...might be some decent engine info there. Best o'luck with the build and do post pix.

http://6066gmcguy.com/V6powered.html

JDecker · 01-20-2014, 08:16 AM

I'd agree. If you'll have the bus for very long and/or plan on driving it very far, you're better off chucking the gas motor in favor of a solid diesel engine that will give much more reliable service and get much better fuel economy. A Cummins 5.9L/Allison MT643 would be a good find.

You say you have a line on a cheap 6.2L... I'm getting out of my personal experience, but I've known guys that had them and hated them, and read nothing but bad things. You'd likely get much better fuel economy out of it than the monster that's in there, but it's not an engine I would take cross country.

JDecker

ol trunt · 01-20-2014, 11:07 AM

Just my opinion but I recall that the 6.2 was first designed to be a boat anchor and then they turned it into a diesel engine. I have owned at least a half a dozen Chevys with that engine and all were poor examples of dieseldom.

opus · 01-20-2014, 11:17 AM

All good thoughts but do you have an bottomless wallet? You'll have to do some math for your gear ratio, It is going to be real low [high numerically] just because it was a gas engine.

sojourner · 01-20-2014, 05:19 PM

I agree a 6.2 is a bad idea. Great motor for light trucks but medium duty, not so much. I can tell you with my DTA408 I have 4.78 gears and turn about 2200 or 2300 rpms if memory serve me right (yea, I know, I need to take it around the block). I have a MT643. The Cummins will probably be your easiest to find and better price than the international but if you can get an international get one. I love mine. Bullet proof engine with a wet sleeve design. The Cummins is a parent bore block (no replaceable sleeves) which is it only downside, if there is one. I think the Cummins is lighter if the suspension is an issue compared to the gasser.

jepopp · 08-01-2014, 09:18 AM

Bumping this post----couple new questions now that I have had the bus out on the road this summer.

I can cruise this gas big block v6 at 55-60mph, but the RPM is 3000+. The bus seems to be screaming at that RPM to me, but maybe Im just not used to being in a loud vehicle...?

Is that too high of an RPM to cruise at? It isn't floored, she still has more power to go, but I don't really want to push it. I'm having the engine tuned, new belts and plugs, etc this week. I'll see if that changes anything---but is this motor made to cruise at over 3000 all day?

My mechanic, who mainly works on grain trucks and semis, said there is no other rear end for this. He said I could add an aftermarket OD to the tune of 3 grand----which is ridiculous. I can't believe there aren't any other gears for this? He said this engine wouldn't be able to power a high gear ratio, but like I said, I have power to spare right now...? ANy ideas?

Tango · 08-02-2014, 11:14 PM

Hard to believe there are no other ratio options for any Rockwell. Do some homework and I'll bet there are plenty. If it is screaming rpm's at highway speed, that is not good for motor, trans mpg or anything else. I set my Cummins up to run at 1700-1800 at 65 -70. Ya...it is a slow turning diesel but so long as you nail the sweet spot (peak torque), life will be good.

My dos centavos anyway.

SmokinJ · 08-05-2014, 08:14 AM

My first bus was a 40' 1953 GMC Superior. It was already converted when I purchased it from a Christian rock band called the evangeliers. lol it had the 478 in it as well. We drove it about 10000 miles before we swapped in a diesel. It got the worst fuel mileage ever! About 7 mpg. Was the most underpowered vehicle on the road including VW busses! I disected the motor when it died. The pistons in it were easily as big as my head.
Get a diesel power plant. You won't regret it.

Stu & Filo. T · 08-09-2014, 05:31 PM

Quote:

Originally Posted by Tango

Hard to believe there are no other ratio options for any Rockwell. Do some homework and I'll bet there are plenty. If it is screaming rpm's at highway speed, that is not good for motor, trans mpg or anything else. I set my Cummins up to run at 1700-1800 at 65 -70. Ya...it is a slow turning diesel but so long as you nail the sweet spot (peak torque), life will be good.

My dos centavos anyway.

Which Cummings do you have & what gear ratio?, Mine is 8.3 250hp with if I remember correctly 4.33, max speed is 67mph but screaming rpm's

Tango · 08-09-2014, 09:30 PM

I'm putting a little 4BT in my shorty. 130 hp and 327 pounds of torque. The new rear axle is a Dana 80HD with 4.30 gearing and the trans is a new Allison 6 speed with double OD. Final drive is .65 and the new tires will be 33.6 inches. That all works out to 1817 rpm @65. I was hoping to get closer to 1700 rpm (peak torque) but they don't make tires these days quite as tall for 6" rims. I'm shooting for mpg near the 30 mark but will be happy at 25-28.

The original 6 cylinder gas engine was 90 hp and about 190 pounds of torque hooked to a 1 to 1, 4 speed trans and a 6.17 rear end with 36 inch tires. It was only good for about 45mph at about 2800 rpm. Course you have to remember...this bus was built before there was even such a thing as the National Highway System.

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