Best engine/trans choice for my Silversides

[keithshotrodshop]

I am in the process of converting a 1947 GMC PD3751 Silversides. My current engine/trans combo is the old 6-71 and four speed. Although I love manuals, this one has gotta go so my wife can drive it. What would be the best choice for my application. Also, my motor is tired, and although I may rebuild it, I want more power. Should I stick with the 6-71 and maybe convert it to a turbo, or swap in something like a 6v92t (which one is availble locally for a fair price along with the auto trans and everything else nessesary to to the swap). I would appreciate any and all opinions. I am new to the bus game, so any info would help.

[RJ]

Keith -

The least expensive option would be to teach mama how to drive the OEM powertrain.  Not that difficult, she might even do it better than you!   

Not sure if a 6V92TA can be stuffed in a Silverside's engine compartment.  A 6V92TA / V-730 powertrain is the hot ticket for GMCs, but there are a lot of changes that must be made.  For example:

1. Driveline alignment:  On your coach, the bevel gears that change direction are located after the gearbox.  Starting with the later model PD4106, the bevel gears were located between the clutch and transmission.  This relocation also changed the offset of the rear axle pumpkin, from curbside to driver's side, IIRC.  If you try to use the 92/730 with your stock rear axle, you'll have terrible vibration problems as well as short-lived U-joints.  Throwing a 4106 or later axle under there may or may not work - those models all have air ride suspension, yours is steel spring.

2. Cooling:  A 6V92TA puts out a LOT of heat.  Moreso hooked to an automatic.  The small radiator opening currently on your Silversides probably wouldn't allow enough radiator to keep the beast cool.  Unless you want to carve up the sides to enlarge the opening (and destroy the classic lines of the coach in the process).

3. Weight:  A 6V92TA / V-730 is approximately 500 - 1000 lbs heavier than the OEM powertrain you currently have.  May or may not affect the handling of your coach.  But it will affect fuel mileage - it will be worse.

4. Fabrication:  May or may not be an issue for you, personally, but structurally modifying the engine compartment to stuff the bigger powertrain inside is not for the weak of heart.  The engine bulkhead is a major component of the chassis - any modifications require reinforcement to maintain structural integrity.


You mention a 6V92 available locally - be careful.  Is it a RH or LH engine?  If it's a RH engine, you could end up with only one forward gear and three/four in reverse.  There is insufficient room to stick a T-drive powertrain in a Silversides unless you lengthen the rear of the coach behind the axle.

Putting a turbocharger on the virtually unbreakable 6-71 would bump the HP up to 275, or about 40 more than a four-valve head stock unit.  That's about the maximum the bevel gears can take w/o breakage.

Finally, a Silversides is NOT a BMW!!  If you drive it like it's supposed to be driven, then you'll spend many an hour enjoying the journey.  If you try to drive it like a beemer, you'll spend many an hour frustrated.  It's a tortoise, not a hare (remember the kid's story?) 

FWIW & HTH. . .

[gus]

Keith,

I faced the same thing when I bought my 4104. I wanted my wife to be able to drive but it turned out that she loves the ride and is not unhappy not to be able to drive it, so all is well.

Being able to drive with an AT is good but it is very difficult for the average driver to get used to stopping a vehicle with this much weight.

The 6-71 may be one of the best engines ever made.

I wanted a 3751 or 4151 because they are so neat looking and lightweight, and that column shift is so great. Keep it the way it is.

[DrivingMissLazy]

I really can not understand why anyone would want their wife to drive their coach. To me that is the fun of it, even if I have to drive 12 hours a day. No way would I want to turn that fun over to her.
Richard

[keithshotrodshop]

I drove the coach from San Diego to Detroit and the only time I wanted to dump the manual trans was in heavy traffic. My favorite Idea so far has been to turbo the motor, but I'm not sure where to start. Seems I am getting conflicting information about the turbo motors. I've heard some say that I can simply retrofit a turbo on my existing 4 valve motor, others say I cannot and need to start with a turbo motor in the first place.

The turbo motors I  have been able to view pics of all have the supercharger on the pass side of the motor, wheras I need the supercharger and turbo on the drivers side of the motor. Anybody know if this combo has ever been built, or do I have to piece it together.

Also, is there an auto I can use with my existing 6-71? I am not afraid to swap out the rear axle, I've done this on many cars and trucks, and have the knowhow to do this.

Lastly, part of my concern about horsepower is the fact that any slight uphill climb means downshifting to first or second in my bus, and doing less than 30 miles an hour. In all honesty, this can be a danger in any area, and illlegal in michigan (45mph minimum speed limit on freeways). I am not looking for a hotrod, but only to be able to keep a safe 40-60mph speed in most conditions. Considering my coach is gutted now, another several thousasnd pounds of conversion weight will not help the situation.

[TomC]

I see two easy choices.  One is that you could turbo your engine.  If you install an air to air intercooler, upgrade your blower seals, and stay with no larger than the 7G75, you can turbo your engine.  Course, personally, I'd have it completely overhauled since you said it is tired.  By going to the air to air intercooler (mounted on the right side door) with the turbo above the trans, 7G80 injectors, bypass blower, you could easily have an engine that puts out 300hp with 900lb/ft of torque, compared to your 210hp and 600lb/ft.  50% more hill climbing power that would maintain the power up to 10,000ft with the turbo (I know the 4104 trans was rated at 1200lb/ft-don't know if it is the same).
But- I think the 6V-92TA and V730 would be the trick.  Little more power and good reliability.  With this setup, I'd use an air to air intercooler, or install an extra radiator in the right engine door.  Good Luck, TomC

[4104bigred]

hey keith this is chris in cadillac::::

how was the trip???  maybe you can share the ups and downs with all of us

on the low power problem my advice is to rebuild the 6-71 as you say it is tired.my 4104 has a new rebuilt 6-71 and has plenty of power for our michigan hills. go see david bryson at the harsens island ferry, ask him if he will help you rebuild the DD for more power I know he did the engine in johns silverside and he is really good with DD 6-71 BECAUSE THAT IS WHAT HE RUNS IN ALL THE FERRIES

chris

[keithshotrodshop]

When I talked to John, he said Bryson wrapped up more than $10,000 in his rebuild, and I cannot afford that any time soon. I have the knowledge and means to rebuild it myself (if it even really needs rebuilding). You are probably right about the michigan hills, but my wife and I fully plan to travel much of north america in this bus, so I want it to be very versatile.

As for the 6v92t swap, I'm not ruling it out yet. When I have time, I am going to take alot of measurements and see if I belive it to be feasable.

Lastly, on my trip from california, it was actually fairly uneventfull with exception to a few things. First was the fact that I had to add more water every 100 miles because compression is leaking into my cooling system. It heated up and the mototell shut the engine off right in the middle of a border patrol check station. Since it seemed to run better at night, I drove through the night, and even most of the day, so I made the 2500 mile trip in 3 days. Another thing is that at some time, someone wired the turn signals wrong, which I only figured out when I got home. Whenever I turned them on the front was flashing a different direction from the rear. Also, I had never driven anything this big, or with air brakes before, so my drive home was also my learning experiance. I managed only to clip one streetlight on the sidewalk in front of a cracker barrel on my way home. I figured out real quick I needed to make wide sweeping turns with the bus!

[4104bigred]

YOUR PROPOSED 6V92 SWAP IS GOING TO BE VERY COSTLY sorry

the guys i have talked with that have done that swap have between 20 and 40 thousand dollars into it when done
also you can have dave and john help with advice and such and rebuild what you have for around 5 grand total

chris

[4104bigred]

FWIW   I feel that our rebuilt 6-71 has a enough power ,not a hot rod by any means and when we go through tn and ky we never go below 3rd gear

chris

[lostagain]

My '57 Courier 96 with 4-71 DD is slow up the hills like yours. I have researched and humd and hawd the last several month about an engine swap, and have settled on installing a turbo on my 4-71. I am going to leave the engine as is: N 65s, 4 valve head, trunk type pistons. The only thing is I am taking the blower to a DD dealer to install the better oil seals to withstand the added boost. The rad will stay the same. I have a Airesearch turbo on order that will give me 12 to 14 psi of boost max. I will mount it on the drivers side. That's the side the blower and exhaust manifold is. I'm mounting it onto the existing manifold. I'll have to build an adaptor, and plumbing for intake and exhaust. I'll keep the whole assembly below the level of the valve cover because I don't want to raise the dog house. I'll use the existing oil bath air filter I hope. That's my winter project. Fun, fun. I'll let you know how it goes.

[TomC]

Lostagain- I highly recommend you change the pistons from trunk (one piece aluminum?) to the two piece with iron tops.  Your aluminum pistons won't withstand the pressure, and more the heat of turbo.  You should also install an air to air intercooler for better results.  I am doing the same with a 8V-71N, but it already has the two piece pistons that are high compression, so we're keeping the boost around 10-12psi.  Good Luck, TomC

[lostagain]

Tom, I have talked to 3 different DD dealer mechanics who tell me that my pistons shoud be fine. I will ask again though, since you have me wondering now. The only difference between a regular trunk type piston and a turbo-charged one is the one peace oil ring on the latter. The crown is the same.This is also partly the reason we are staying with the N65 injectors, so we dont create much more heat. (For the rad too). They're saying N70 might be OK too. I'll look into the intercooler idea as well. Is that for cooling the air between the turbo compressor and the blower? How does it work? Is it like a little rad? I see the one discribed in the DD service manual is a water cooled unit.

[keithshotrodshop]

I'm not sure where the numbers came in for the engine swap, but I can have anything I want from the drivetrain of the bus with the 6v92t for $1,500. Being that welding and fabrication doesn't scare me, I'm not sure how I could possibly reach those number. All in all, I'm willing to bet that if I do the whole swap myself, I could keep it under four grand.

[tekebird]

I think you will find yourself SOL.

You won't find anything that will fit trans wise without alot of cutting same goes for any V engine

Turbo is an option but there is not enough radiator room for the biggrer rad your gonna need.

Rebuild what you have, it will push it nicely when fresh