1970 GMC questions
 

Hi, I have been lurking here for a while, but I recently found a 1970 GMC LWB truck I could not live with out. I have owned between 10 and 15 of the 68-72 era trucks and forgot how much I liked them. In fact, I bought this truck to try and resell it. But I liked it so much, I sold my late model Dodge Ram 4x4 instead. The GMC is a Sierra Grande and is fairly loaded (buckets, tach, cruise, et al). It has already been converted to HEI, which I like, but they ran the plug wires over the top of the exhaust manifolds. This of course causes problems with plug wires. Does anyone know what HEI plug wires I can buy that are long enough to route correctly? Also the truck has the side tool box option. It is pretty solid, but does have some rust on the bottom of the door and the mounting part of the bed. Does anyone make patch panels for these? Sorry for the long first post. I have already learned a lot just by reading the back threads. Elrod

First of all, welcome to the board from central ohio.
Your truck sounds like a pretty sweet rig, we all know GMC's are better looking, and a loaded version makes it all that much better.
75 corvette plug wires will fit the bill. They still had the ram horn manifolds, and HEI too.

PS...we need pics. ;)

Welcome to the board, it sounds like you have a nice truck. You ought to post a pic.

Jeg's has HEI under header wires for $29.99. They also carry under header wires from Accel and MSD etc. http://www.jegs.com/cgi-bin/ncommerc...86&prmenbr=361

I'm sure Summit carries under header wires and places like Autozone should be able to get them too.

Welcome to the board. We need pics and the 75 'vette wires work good I have them on mine. Did I say we need pics?:D Especially of a 70 GMC.:D

Pics
 

OK, I will try to post pics. Thanks for the info about Corvette plug wires.

Yep, I'd have a hard time selling that one too! Nice truck.

Rg

Welcome to the board!!!!! the only truck coolr than a GMC, is a GMC with a V-6, ;) :) Nice truck!!! I would've kept it too!!! good call!

that truck is pretty sweet lookin.... is it all original, or is that an older resto?

Very nice truck!

You write a good thread man, you sound like a lonesome man who's best friend just came back into his life! Almost like a commercial!

That is a nice truck. Good choice.

WOW, what a sweet truck. Keeping that one was definitely the right call.

Thanks all. The truck is almost all original. The PO had converted to HEI as I mentioned. Also changed turbo hydro-matic to 700 R4 tranny. It has the stock 411 gears so the 700 R4 helps. I think I am still going to go to a much taller tire and swap the rear gears too. Should I dare go clear to 308 gears?

nice truck, welcome to the board

What size tires are on it and what size would you be changing to?

3.07 gears would be too much, drivability and MPG would suffer.

:agree: 3.07 gears work good for a non od tranny, but the low end sucks.

Longhorn Man, why would MPG suffer? I can see why the low end would be bad, as Krue suggests, but the reason I thought about going to a higher gear was to increase MPG. It sounds like 3.07 is too far.

sharp truck!

I agree that 3.07 gears are too high speed. I think the reason you run into problems is the trany has a hard time maintaining high enough RPM's to stay in OD. So it either lugs the motor down in OD or shifts back and forth to 3rd all the time killing longevity.

3.42 gears might be allright as long as you stay with something like a 255/70R15 or 235/75R15 tire. Even with 3.42 gears if you're heavy footed at all, it will shift out of OD more often than you might like.

I'd recommend 3.73 gears to maintain good take off speed. You'll still have better fuel economy and lower RPM's, at cruising speed, relative to a TH350 or TH400 with 3.08 gears. 3.73 gears with a 700R4 in OD would be the equivalent of 2.61:1 gears in nonOD trany.

HEI and ramshorns. 75/76 vet plug wires are the way to go.

Nice truck!

Imagine you are pushing a huge shopping cart, loaded with heavy bricks or something to that effect. Now, if you are on flat smooth pavement, you can push it fairly easily at X mph... we'll say 5 MPH...just a round # I pulled out of my rear.
Now, same cart, same flat smooth pavement...try to push it at a steady 2 MPH.... it'll wear you out.
A 3.07 and an overdrive will put your cruising RPM's below the power curve. Your engine will struggle to maintian the steady speed. In all reality, you'd probably use more throttle just to maintain that speed than if you were to go down to 3rd... 1:1 gear.

Low engine speed does not always mean high MPG. My last small block was happy at 3000 RPM. When it was in my first truck...a 70 3/4 ton Longhorn, it got 14 MPG, with a record 16.5 MPG. (highway figures, cruising at 70 - 75 MPH most of the time) This truck had a 4.10 gear...and your typical 9.50 16.5 inch tires.
Same engine, with NO chainges what so ever...plucked out of the (now totaled) Longhorn, and put it in my 2nd truck, a SWB '69 GMC 1/2 ton. The same tranny made it's way over too. I was expecting awsome MPG with the much lighter truck and the 3.07 gears out back.
Nope, Dropped to 10 MPG with a best of 12 (at normal speeds) 65 MPH came in at 2300 or so RPM's....the engine was working harder than before. When i bumped the speed up to 80 MPH, I got consistant 12 MPG...with a best of 14. This was in a smaller lighter lower truck.

I recently found a 1970 GMC LWB truck I could not live with out.

Wow I can see why. Nice find. John

Nice GMC! Welcome from Alabama!

Old Yeller 1970, I have to admit I have always been perplexed by tire sizes. I currently have P235 70 R 15 and these look tiny in the wheel wells. The truck feel like it sits too low also. I would like to get a taller tire that would help offset the steep gear (411) in the back. I have a set of later model chevy six lug wheels from a suburban. They have 31 inch tires on them. I think I am going to throw them on and see how they look and how it drives. They might be the answer.

They won't fit without modifications...pluss they'll be tucked in a bit too far IMO.