Engine and suspension swap

[Staci Newsom]

I have a 1978 GMC c15 heavy half ton that is mostly stock. It currently has a 350ci engine (which has recently been rebuilt and has an edelbrock intake and a holly 4 bbl carb.) It does go fast. It also has weight distribution shocks in the rear. I am not sure if the leaf springs are very good (they seemed a little soft to me) but they are supporting the truck. I am not sure if its rated 6050 gvwr or the larger number that escapes me at the moment.

I want to beef up the rear end and power train to handle towing a travel trailer that is fully loaded at gross 7000 lbs and dry weight of around 5200 lbs.

Is it possible to push it to a much higher gw with stiffer leaf springs bigger engine, different gear ratio in the rear end. Given the frame limitations how far could I push it and does this sound crazy (if I could find a 3/4 ton I would). I used to own a 73 Chevy c20 camper special and I miss that truck)

 

[Bextreme04]

3/4 ton an 1/2 ton pickups have the exact same frame. What rear end and springs does the truck have now? Pics? What makes you say it is a "heavy half".. is it a "camper special"? Big ten? Do you still have the SPID label in the glovebox with the RPO numbers on it? That would tell us the factory spring/axle/gearing package that it left the factory with.

 

[Bextreme04]

I have a 1978 GMC c15 heavy half ton that is mostly stock. It currently has a 350ci engine (which has recently been rebuilt and has an edelbrock intake and a holly 4 bbl carb.) It does go fast. It also has weight distribution shocks in the rear. I am not sure if the leaf springs are very good (they seemed a little soft to me) but they are supporting the truck. I am not sure if its rated 6050 gvwr or the larger number that escapes me at the moment.

I want to beef up the rear end and power train to handle towing a travel trailer that is fully loaded at gross 7000 lbs and dry weight of around 5200 lbs.

Is it possible to push it to a much higher gw with stiffer leaf springs bigger engine, different gear ratio in the rear end. Given the frame limitations how far could I push it and does this sound crazy (if I could find a 3/4 ton I would). I used to own a 73 Chevy c20 camper special and I miss that truck)

Also, what trans? Pics of the springs, trans pan, and rear end(both the cover and axle ends) will give us an idea visually if you don't know what they are already.

 

[Staci Newsom]

3/4 ton an 1/2 ton pickups have the exact same frame. What rear end and springs does the truck have now? Pics? What makes you say it is a "heavy half".. is it a "camper special"? Big ten? Do you still have the SPID label in the glovebox with the RPO numbers on it? That would tell us the factory spring/axle/gearing package that it left the factory with.

 

[Staci Newsom]

It's heavy half ton based on vin I can get whatever I got and can add pics if that will help. There's no rush right now but I think you guys will tell me what I can do.

 

[Staci Newsom]

It's heavy half ton based on vin I can get whatever I got and can add pics if that will help. There's no rush right now but I think you guys will tell me what I can do. The label in the glove box is missing I will check the door frame closer.

 

[yevgenievich]

Gm documentation does actually specify thicker frame for a 3/4. But it should be ok with standard 3/4 ton hardware for the stated weight

 

[Staci Newsom]

Here are the pics asked for and I will have more shortly

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The tag in the glove box reads

Model TC13903
C60 All weather AC
C63 Fleetside
HE4 Rear axle - 3.40 RA
L/S 9 V8 ENG - 350 CID 4 B
NL2 Auxiliary fuel tank
L78 L78-15/8 TBLS HWY BL
L78 L78-15/8 TBLS HWY BL
Gages - volteter
XGG1 Green Custom VI

A01 Soft-Ray Tinted Glass
F44 Heavy duty Chasis
K30 Speed and Cruise control
NA2 Standard Emission
N41 Power steering
V43 Painter Rear step BU
YLB L78 = 15/B TBLS HWY BL
Conventional 2 ton
Scottsdale Equipped
46 Holly Green 12 FR

I think I got it right

 

[Bextreme04]

Pics definitely help. I don't think the VIN actually distinguishes... but if you have RPO F44 on your SPID label then you have a "BIG-10" which is what some people call a "heavy half". You'll have a little bit heavier duty front and rear spring and a little bit taller rear gearing than the standard C-10 as well as a little bit bigger rear brakes. Front brakes stay the same.

Gm documentation does actually specify thicker frame for a 3/4. But it should be ok with standard 3/4 ton hardware for the stated weight

You're correct... it isn't much thicker, but it is a little bit. Frame modulus on the C10 is 3.14 and the frame modulus on the C20 is 3.92. It gets a little more apparent when you go up to a 1-ton and the frame modulus shoots up to 6.2.

None of that is going to help you pulling a heavy trailer though. You need some more power/torque and to beef up the trans while adding a good cooler. It would also be beneficial to beef up the rear end, you probably have a 10 bolt in there. You can find open carrier 14bff axles with 4.10's in them for $150-300 all day around where I'm at. I'd recommend dropping a 454 and built 700R4 in there with a true-cool 40k cooler and a 14Bff with 4.10's. You'll be able to pull anything and still cruise on the highway when unloaded.

 

[Bextreme04]

Here are the pics asked for and I will have more shortly

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That GVWR means it is the lower weight rating of the F44/Big-10 package and had a 350 from the factory.

Here's the vehicle information kit for 1978 from GM. https://www.gmheritagecenter.com/do...its/Chevrolet-Trucks/1978-Chevrolet-Truck.pdf

A picture of the SPID sheet on the fender well or inside of glovebox will also help identify what options were fitted from the factory.

 

[Staci Newsom]

Here are some more pics

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[Staci Newsom]

and more

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[Curt]

12 bolt rear.With that numerical low (high gearing),not so great for towing unless you have a lot of torque.Trailer brakes will help tremendously but 7000 could still push you with 1/2 ton brakes.Probably be ok if no hills involved.

Tranny cooler or TH-400.14 bolt rear axle would be ideal

 

[Raider L]

@Staci Newsom,

I agree with @Bextreme04 in that you will be better off going to a lower gear in the rear end for starters, a good trans. freshen up, and please do something with that drive line. New universal joints, heavy duty ones Dana Spicer and take that swing bearing apart and the slip joint end of the long shaft. You'd be surprised that most people don't ever clean that part of a split shaft out and find out there's no grease in it. Put some fresh grease in it. Make sure that little seal where the swing bearing end of the shaft slides back into the long shaft joint is okay, that's what keeps all the grease in the slip shaft.

You say the engine is newly rebuilt? What was done to it other than an Edelbrock intake and a four bbl. Pistons? Crank? Stroked, what? That will tell you whether you really need anymore torque. You may be fine in that area. Just your trans and rear end.

And something some people over look when they start towing and that is brakes! Make sure your brakes are in tip top shape. If you have shoes in the rear make sure obviously new shoes, you put in new spring sets on both sides. Turn your rotors! If you are hauling 7,000 lbs, you want to make dang sure you can stop the truck and the trailer.

For years I used to haul my mother-in-law's pickup and 22ft. travel trailer along with a camper on her pickup up to Northeastern Texas lake to a park on the lake. The highway was nothing but East Texas windy roads and lots of hills. And I'm telling you she had the camper and the travel trailer loaded down with everything including the kitchen sink! But she took care of her truck, trans, brakes, all that stuff and it never missed a lick going and coming.

 

[Raider L]

@Bextreme04,

What does that part of the glove compartment build codes that says, "conventional 2 ton"? Does that mean it's a two ton hauling truck, not 1/2 ton, not 3/4 ton, not 1 ton, but 2 ton? Can't be.
@Staci Newmon, also, don't forget to check out your trailer brakes to. Are they electric over mechanical, or just electric. I built house trailer frames at a company that just did house trailer frames, many were double wides, when I first became a welder and I didn't know how they stopped until this guy explained it to me. Interesting, electric brakes.