Hello Everyone,

I picked up a 1973 23' GMC Motorhome this week, it has been sitting for quite some time, and the motor will likely need a rebuild, my initial thoughts while buying this is I would use the motor I currently have in my 72' Cutlass, and build this motor to replace it, (495 kit, aluminum heads), or rebuild the original 350 I still have (434 kit, aluminum heads)
The motor I plan on putting into the Motorhome is a 1968 Toronado 455, it has the N crank, stock rods, l2323 pistons, ported big valve C heads, 8.75-9.0 compression, Harland Sharp adjustable rail type rockers, Holley Sniper system, I plan on using the stock intake and exhaust manifolds, maybe something else if worth it and can hold up in a motorhome application.

My question is how would these ported C heads work in a Motorhome application vs rebuilding the stock J heads. obviously the C's would flow allot better, but my main focus here is low end torque and fuel economy.
also what is a good cam to fit this application? I can use a hyd flat tappet or roller as long as its under 550 lift.

I plan on going thru the trans as well, its a th425, considering a tighter then stock converter, and lowest differential gears available, (3.07 I believe)

this has a 10500 gvwr and using 29" tires.

thank you,

 

1. Verify the front suspension/brakes/wheel bearings before you put in more horsepower. Those things were hard on bearings and brakes and...well...everything down there. Seems to me there are "special" front wheel bearing sets in the aftermarket that withstand the motor-home abuse better than the "ordinary" Toronado bearings.

2. Headers are (or were) available for the FWD motor home chassis. I was told they don't fit the Toronado, which is why I didn't buy a set.

3. I think you have no problems with hood clearance, a high-rise intake manifold could be considered if it suits your intended RPM range.

4. Some folks just slap a Cadillac Eldorado 500 engine in where the Olds used to be. Hard to argue with an extra ~45 cubes. Even in mild tune, 45 cubes is 50--60 ft/lbs.

5. The TH425 can be shift-kitted and modified just like a '64 TH400.

6. There is (or was) a guy selling replacement gear-and-sprocket kits for the TH425, you could alter the gear ratio via the bigass gear and chain set behind the torque converter.

 

I say stroke the 455 for the motorhome and the 350 for your Cutlass. You want your ported C's, they will still have small runners for 500 CI. Keep your compression around 9 to 9.5 to 1, depending on availability of high octane where you are travelling. I say closer to 9 to 1 and run aggressive part throttle timing curves.

 

You may want to upgrade wheel bearings with a modification from Applied GMC MH, (800) 752-7502 in California. Ask for Jim (owner, he isn't always there).
They specialize on GMC Motorhomes. They also have a web site to check out.
Stay with the 455 and go through the TH425. They can probably recommend a torque converter and cam.

 

Thanks,
I plan on going thru everything on this, another thought I had was putting the 2.75 first gear for a th400 into the th425, from what i understand its the same internals as a th400 but running in reverse? Has anyone ever done this? In a toro or motorhome?

 

The CAM you should you is the Melling MTO1. It has the right lift and specs for having the torque you will need at low RPM to move your butt from a stop. It is close to the original stock one, but better than a Street cam that is made for a 3000lbs car

You can use the C heads. I had Ka heads in mine. They are fine. At freeway speeds even with the stock 3.07 ratio final drive you are going to be under 3000 RPM. Even J heads will be fine. The Aluminum heads are fine. Many use them in the motorhome. I am upgrading to those. The aluminum heads will allow you to keep your compression but use 87 octane easier. They run cooler too and the under hood temps on the motorhome are HIGH. anything to keep it cooler under there will make your passenger compartment more comfortable.

Keep your stock manifold that came on the motorhome. You won't get any other manifold to fit under there. The Toronado style low profile convex intake is only one that works. The GMC one is just like the Toronado but NO EGR. It may not be a great manifold for flow and horsepower, but it works, and it you are still in the 215+ horsepower range STOCK motor if I recall? With better heads and cam and rockers and Sniper you may be higher already. It is more about the TORQUE though. MAKE SURE you block off the heat riser passages with block off plates. This will save the manifold long run. They are known to develop cracks under the carb. You CAN use a Donor Toronado manifold but will have to block off the EGR port.

Keep your exhaust manifolds. They are just fine and the headers do not add a whole lot. Save your money there. Plus the stock manifolds are unique to the motorhome on the passenger side.

If you have the Sniper that will work but you will need a Carb spacer, and will need to modify the linkage AND run a super low profile 1991 Jeep Snorkel air cleaner and remote snorkel air filter for it to work. If you have the Quadrajet sniper it might work with few applications though. There are people on Facebook GMC motorhome groups that can help there. I have an MSD atomic on mine

 

Keep the transmission the same. If it works, don't mess with it. They are good long lasting transmissions. If you need a rebuild talk to Manny in Bay area. Or have it rebuilt by someone who is over the age of 50 that has actually worked on TH400 and TH425.

Only thing you really should consider is the final drive ratio. The stock is very low. Some people upgrade to a 3.2 3.5 or 3.7 ratio. This puts your RPM more in the power range of the 455. your RPM will be higher, but the engine won't work as hard, and you may save MPG.

The only trick transmission I know about is the Switch Pitch modification. There are resources on Google to do that. There is a guy in northern CA who has it. IT creates a lockout making it like a mid 60's Tornado. It is not a good idea for Towing though.

As for torque converter, the stock one is very low stall. You will have problems finding one that low unless you have one made custom. The low stall saves fuel supposedly? I changed mine out. The stock one is weak. I put in a 2800 rpm stall, braised fin, torinagton bearing heavy duty one. It instantly made the motorhome have more pick up and go. it is also much better for towing. You might be able to get the best of both worlds having a custom one made. Keep in mind that the heavy duty torque converters may not work with your stock Flex plate. I had to get a new flex plate that did not have the counterweight where stock one is to work with my heavy duty torque converter.

 

and yes you CAN put the 350 in there.....

http://www.gmccoop.com/gmc-motorhome...rown-delivery/

 

How is the mpg with that 2800 stall?
I actually have a good custom 10" 2800 stall I took out of my cutlass, that thing always launched great AND cruised like a stock one.

 

Here is my 2 year average.

The higher averages are highway driving. The ones under 7MPG were around town using it as my 5 mile drive to work car or going to get groceries on the weekend.

The highest was just over 11. With a tail wind WHILE towing a VW Jetta, in Oklahoma. I have a 23' and i was fully loaded and very heavy because i was moving and my whole life was in the motorhome and VW. Those first 11 fill ups were on the 3000 mile trip from Ontario CA to Syracuse NY. You can see it varies with terrain and wind. Mnts kill it. Head wind kill it. tail wins, GOOD! but for towing 3000 mile I would say that was pretty good.

I would say you can use the higher stall torque converter. As long as you keep the engine mild and do not make it a high horsepower and close to stock you will get better milage. going Bananas on the Motorhome is not going to make a huge difference in your 0-60 mph. You are still 10,000 lbs. you are better served making it RELIABLE and bulletproof. Remember there is nowhere to get this serviced in middle of nowhere and is harder to find a tow than a car. if you blow an Engine in middle of Oklahoma who will take it out for you?

If you can get a lower stall converter with the upgrades I would do that if I could do it over again. But I will wait till I need a new Tranny and do it then.

 

Just curious do you know what conditions had you in the 11 mpg rangeand what put you in the 4 mpg range ?

 

the 4.81mpg was a lot of idling at Football games tail gating and charging batteries, driving the hills in the sub Aderondaks where I live, a few traffic lights, and crappy Ethanol gas. The rolling hills here are enough have to shift to S and take them slow up. I do it just to drive it and keep it moving instead of sitting.

The 11 was on FLAT, Texas, Oklahoma, Missouri, Illinois, Indiana going around 55-65 MPH while towing, running the air conditioning, in 110 degree weather and a slight tail wind.

The other good ones later in the graph were through the "mountains" in New York called the Allegany and through Ohio back and forth from Detroit to NY last summer.

Moral of story... Highway driving Good. Around town Bad.

 

How would you say it reacted with that 2800 stall?
When I had the 2800/th400 in my cutlass it drove like a stock car when you were easy on it, but had all the torque when you would step on it, i would imagine a motorhome is a real different world.
I am basically thinking taking that same set up and put it in the motorhome, but change to a more suitable cam (currently has solid flat tappet 520/520 lift, 292/292 adv, 240/240@.050,108 sep)

 

I am VERY heavy on the gas pedal. So that could have a lot to do with it. The GMC moves and has power it needs to get out of the way. It drives like a muscle car even in stock config. You would be surprised how fast it is compared to a modern car. I suppose if I just gave it a little gas and "stayed out of the secondaries" like they would say in the Quadrajet days, it would be better? But then it would not be as much fun to hear that 455 go VROOOOOM.