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View Full Version : What engine do I put in a 76 GMC Motorhome


Billb66
01-05-2007, 10:13 PM
I have a 76 GMC Motorhome, 455 Olds Gas V8, TH400 auto tranny, front wheel drive, ( Toronado drive train). In the interest of power/fuel economy I need to replace this 212 HP @ 3400, 344 f/# torque @ 2400. What diesel most closely will match, physical dimensions (to fit), power, and ability to adapt or bolt up to tranny?

Bill

76 Palm Beach
Orlando

a5150nut
01-06-2007, 09:10 PM
Since no one has hazzared a guess, I will. If it were me, I would look at the 6.5, newest model AMG block set up with DB2 injection pump. It has the mecanical linkage and you wouldn't be installing ECM and related wiring. Use the KISS therory. I believe it will bolt up to your trans but I would rebuild trans and beef it up, probably a converter change to get the right stall speed.

Go through your cooling system and maybe have to upgrade there to a larger radiator. You will need to check capasity. If you don't have, you will need an oil cooler also. Goes right infront of radiator.

This should get the thread started. JUst sit back and read and ask more questions.

a5150nut
01-06-2007, 09:49 PM
I was browzing arround and found this in another thread and though it might be usefull..............This was a reply to another member asking about using a t400 trans.

Ok first off
HI there
You need the 6.2 TH 400 flex plate
The cast aluminum lower cover
The stabilizer bars that go from the motor mounts to the lower cover
(this stops the tranny case from breaking if you get wild)
There is also a large steel bar that goes from the right side rear of the t case to the right side middle bolt on the tranny bell, this stops linear flexing of the tranny and t case.
Now the controls, you will need the column shifter or some other means to control the tranny.
The single electrical plug on the tranny goes to a switch that is activated by flooring the throttle. The switch is connected to 12V and is normally open,
You can get one at a bone yard or the parts store, (Kick down switch) Its used on the 400 only.
The 400 is a vacuum modulated tranny and you need to get the vacuum switch to control it. This switch has two ports on it one goes to vacuum and the other goes to the modulator on the right front of the tranny.
The switch bolts to the IP where your little electrical switch is now bolted.
Make sure your engines vacuum pump is working good (26 inches at idle and steady) Install a vacuum storage can as it helps make things work better.
The switch will need to be adjusted so the tranny shifts correctly. Too much vacuum too long and it will shift late and hard. Too little and it will shift soft and quick. If you dont have access to the factory manual for the specs on setting the switch you can start with the switch installed with the bolts in the middle of the slots and go from there.
Looking at the switch from the passenger side, turning the switch clockwise will make the shifts later and firmer.
Be sure you fuse the kickdown supply wire.
You may run into drive line issues but factory shafts can be found easily as this was a common setup. Oh the wires on the switch you spoke of are of no concern, take the switch off and toss it on the shelf. The connector on the tranny is only the kickdown. Neutral safety is on the top of the column and usually in the same switch as the backup lights. Kickdown mounts on the throttle pedal braket and the switch usually has a peg that goes through the long slot in the bracket.
Fairly straight forward. You may have to improvise to make it work right.
If your truck has/had the clutch switch that required the clutch be depressed to start use these wires and run them through the neutral switch to get the safety factor needed.
The kickdown is a no brainer, grab a 12v source that is switched (IGN) and be sure its fused and run it through the kickdown switch and on to the tranny single spade.
Lets see now HMMMMMMMMMMM Motor mounts are the same, tranny rear mount and cross membershould be ok too as the t case sits in the same spot.
Good luck and keep us posted

Robyn
__________________

You might check with Robyn, she know her trucks and would be a lot more helpful than me. One way or another we will get you some help!

JeepSJ
01-07-2007, 11:02 AM
Isn't there going to be an issue with the BOP vs GM bellhousing pattern? That would require an adapter plate. As for the TH400 and Toronado drive train...I thought the Toronado used a TH425 transaxle, not a TH400 transmission?

JeepSJ
01-07-2007, 11:15 AM
Found some things to throw into the mix...


The Toronado engine package was absolutely north-south. The FWD trans was essentially a TH-400 mounted along the driver's side of the engine. The TH-400 (redesignated the TH-425) was split aft of the torque converter, the transmission itself was reversed and mounted along side the block with the output shaft facing forward. The chain drive couples to two halves together. The torque converter is mounted on the flywheel in its normal location at the rear of the engine and a massive chain drive couples the engine to the trans. The differential is mounted at the end of the trans (about under the #3 spark plug), with the right side half shaft running under the oil pan.

rustyk
01-09-2007, 10:54 PM
I have about 2,500 miles on a new AMG/Peninsular Diesel 6.5L TD, rated @230HP (~395 lb.-ft torque @2,700RPM) in my 28' Barth diesel pusher.

Its performance blows the original (very early) 190HP 6.5L TD away. For over $10K, it should :o . Anyway, with a GVW of 16K lbs. (I usually run around 15.5K) I got around 9.2 mpg with the original engine. With the AMG (custom built, with Phazer gear drive, new-design DB2 IP, non-wastegated turbo, and hi-pop injectors), it's now very close to 11 mpg turning the 4L80E tranny.

The engine itself is computerless. As mentioned, you would need a vacuum pump (my M/H uses an electric, which runs only the HVAC flappers and the cruise control); you probably already have the kickdown switch. The stock cruise control and throttle would have to be redone, as the AMG/GM are vertically-mounted in a bracket above the IP throttle lever.

I'd wonder if the chain drive could handle the additional torque - the tranny surely could be beefed up to do so.

More Power
01-10-2007, 03:39 PM
In general, there are two GM V-8 trans bolt patterns - BOP (Buick, Olds, Pontiac) and Chev/GMC V-8. The 6.2/6.5 use the Chev/GMC pattern.

Jim

butch7
01-10-2007, 07:30 PM
I'd wonder if the chain drive could handle the additional torque - the tranny surely could be beefed up to do so.

This transmission was used in it's day with Cadillac and Oldsmobile engines wity torque figures in the 400 - 500 ft lb range. I would think it could handle it and built for more. 500 cu inch Cadillac motor used to be a common swap in these motorhomes using this trans. I've also wondered how a 6.2 or 6.5 might work in a GMC seems like it would be a natrural at first glance.

sidehackbob
01-11-2007, 07:00 AM
I know this is the diesel page but.... Your GMC is a really neat ride. Why not repower with a solid rocket 455? The next option is to find a 5.7 diesel and bolt it up to the BOP bellhousing.
If you are totally bent on a 6.5 then bellhousing adapters are available the oil filter may have to be relocated. Engine mounts will need to be fabbed. a vac source will be needed. The AC will need new lines built for the new style AC pump. you should already be converted to 134 ac right?
Take a careful look at where the turbo will sit in the engine bay, will there be plenty of clearance?
The drive train should hold up fine. One of the Olds perfomance guys used to have one of these with a really snorty 500" engine that would do awsome burnouts.
BTW the 500" olds is pretty simple, 425 Olds pistons, 455 Buick rods and a competent machinist to offset grind a 455 crank. big torque, low bucks.
Good luck with a cool ride..

Colorado Kid
01-16-2007, 12:50 PM
The impossible part of this proposed conversion is making the exhaust fit on the transmission side. That's why only Cadillac and Oldmobile engines are found in these things . . .you have to have an Eldorado or Toronado manifold to make it work. My uncle had one of these FWD RVs and we tried like heck to make anything else work . . . ended up rebuilding the 455.

Craig M
01-16-2007, 01:46 PM
Well Cadillac, Oldsmobile (and Buick) had front wheel drive 5.7 diesels engines (Eldorado et al). They could be mounted to the typical GM motor mounts. But power is something else. A Naturally aspirataed 6.2 is not really sufficient for a big motorhome, a 5.7 would be severly defficient, call that slow!!

butch7
01-18-2007, 04:27 PM
The impossible part of this proposed conversion is making the exhaust fit on the transmission side. That's why only Cadillac and Oldmobile engines are found in these things . . .you have to have an Eldorado or Toronado manifold to make it work. My uncle had one of these FWD RVs and we tried like heck to make anything else work . . . ended up rebuilding the 455.
I guess that alone explains why we don't see any of these conversions...good information.