My dad's '79 GMC K15 towing woes (long)

[77 K20]

So in 1988 or so my dad bought his GMC truck from the original owner who was a mechanic- it was in great shape and had a 400 small block in it. For the next 17 years or so it towed his small 18' 5th wheel with the greatest of ease. Then the motor got tired. I told him to rebuild it. He heard of the famous 400 overheating issue and I guess that scared him off from doing it.

So he decided on getting the Chevrolet Perfomance 350- the 290hp version:

Specifications:
Horsepower: 290 @ 5100 RPM
Torque: 326 ft/lbs @ 3750 RPM
Max Rec. RPM: 5100 RPM
Compression Ratio: 8.0:1
Heads: Cast Iron 76 cc Chamber - 1.94 Intake valve/1.5 Exhaust valve
Camshaft Lift: .450 Intake /.460 Exhaust
Flat Tappet Duration @ .050" (222°Int/222° Exh
Lifters: Flat Tappet
Connecting Rods: Powdered Metal
Block: 4-bolt main
Pistons & Pins: Aluminum
Crankshaft: Cast Iron Nodular, 3.480" Stroke
Rocker Arm: 1.5
Ignition Timing: 34° BTDC total advance
Fuel: 87 octane


It doesn't tow his trailer worth anything. So he then has the dealer install a Edelbrock performer 2102 camshaft to go with his performer intake manifold.

Helps out some- but still not acceptable. Then someone notices that his RPM is pretty low at 65 mph. Find out his has 3.08 gears. I tell him to get 4.10s since he only tows. He has 3.73s installed in both axles. It is a bit better- but still not good for hills. (tires are only 235/75R15s)

He now has a lot of extra money dumped into this and still wants it fixed. The next thought he has is this winter putting some different heads on it (possibly vortec heads) and then ditching the factory exhaust manifolds and going with headers. This will help also- but don't think it will be a "fix".

Is the edelbrock cam to mild to use for towing?

duration @ .050: intake 204* exhaust 214*
lift @ valve: intake .420 exhaust .442
lobe seperation 112*

 

[bucket]

Well that sucks. What's the timing set at? Was anything else changed along with the motor? Was the truck out of service for a long time for the swap? Maybe he just remembers the 400 being more powerful than it actually was?

 

[77 K20]

Well that sucks. What's the timing set at? Was anything else changed along with the motor? Was the truck out of service for a long time for the swap? Maybe he just remembers the 400 being more powerful than it actually was?

Both my dad and a shop he uses have timed it and re-adjusted the carb and it didn't help. Don't remember what they set it to- but it sounded right. Only the motor was changed. They did the swap in a single day.

Since he bought the truck from a mechanic there must have been modified a bit. It did have a holley carb and an edelbrock manifold but there must have been more. I remember in high school I had a load of wet apple tree logs in the back (a lot of weight) and raced one of my friends in a '67 Dodge Dart GT. I still won. And that was with the 3.08 gears. It got pretty good gas mileage too...

Basically don't know if the 350 will ever be able to compare against what he had. Heads and headers will help out some- but would hate for him to still be disappointed.

I did just recently read the 350 he bought has dished pistons... so his compression is crap. It was advertised as 8.5:1 but most people say it was under 8:1.

 

[GreaseDog]

3.08 gears are probably your biggest enemy right now.

 

[77 K20]

3.08 gears are probably your biggest enemy right now.

He did regear it and runs 3.73s now.

 

[HotRodPC]

What intake and exhaust is he running? To get that 290hp 326ft lb, it's required to have an Eddy Performer Intake and Headers. Also, That Eddy cam you're asking about is smaller than the cam that is in the motor now. But NO, I would NOT go with the Eddy Cam. That's more of a performance type cam.. (Guess, haven't looked at the specs in awhile). Bottom line here, forget about horsepower. He doesn't care about horsepower. What he's looking for is Torque. Get the right cam grind, headers, intake and carb and he should do just fine.
Yes, 4.10's would have been better, but an 18 foot camp trailer is not all that much really, so the 3.73's should be able to work just fine. I'm just wondering if his expectations aren't set a little high. This isn't going to tow like a 5.3 LS powered truck in Tow/Haul mode and if he's expecting that, then he probably won't get there without a piston and head swap to get the compression ratio up.

 

[77 K20]

Found more info on his 290HP 350. With the dished pistons in it is has 8:1 compression only.

I'm thinking of telling him to get a set of aluminum heads with 64cc chambers. Using some online calculators it says it will raise the compression +1.41 (if he stays with a stock thickness head gasket) to give him around 9.41:1 compression.

Will that and a set of headers or ram horn manifolds be enough? Or since the engine will be torn down get some flavor of Lunati Voodoo cam also. They have 6 different ones.

As far as his expectations he has never towed with another truck other than his '79. So he isn't comparing it against anything newer. He just cannot keep the truck in 3rd gear for hills. Montana has a ton of hills. Some big, some small. With almost any hill it means dropping down to 2nd gear and then doing 45 mph or so. Then he gets cars stacked up behind him, or cars blasting by throwing gravel/sand all over the truck.

 

[HotRodPC]

You're saying he originally had a SBC 400 in the truck and had no problems with that? Granted the cubes make a bit of a difference, but the biggest difference is that longer stroke on the SBC 400 over the 350. That is nothing more than pure torque gain. As I said before, he's not missing the horsepower, he's missing the torque. Kind of a bummer he has this motor now, but a 383 stroked 350 is what he needed. That's all the stroker 383 is. It's a 350 with added torque due to the longer stroke.

I really wonder if that 8:1 is accurate. I know you got the info from the Jegs site. But even the smaller motor, which is 260hp is 8.5:1 with the same heads and it also had dish pistons. To the best of my knowledge, the only difference between those 2 motors is the camshaft. I got this info from a magazine article that was all about the Chevrolet Performance Small Blocks and they had both motors for their testing. Their goal was to get 450-500 hp out of one of those crate motors. They did cam, piston and head swaps to get them there. It seems they said, save your money and get the 260 hp unit since it's the same motor as the 290 hp unit other than the camshaft. Maybe it was pistons too. I don't recall now. I'll check my favorites and see if I bookmarked the article.

Maybe you need to go with some Vortec heads. They're not real expensive and will give a significant rise in hp and compession which should add some badly needed torque.

Again, what intake and exhaust does he have? That's important too. 290hp is no slouch of a motor and even then, there is torque to go along with that 290hp. I think it's 326 advertised IIRC and that should tow a little 18ft 5th wheel trailer. You sure it's only 18ft? That seems small for a 5th wheel.

 

[HotRodPC]

OK, I see about the exhaust now. Yeah, get some exhaust work done. Headers or the Ram horns should be good too. Those breath almost just as good as headers. Not quite, but better than the later OEM manifolds for sure. You can also do a little port work on those ram horns too.

Those aluminum 64 cc heads aren't cheap, as suggested, look into some vortec OEM cast iron heads. They are off of 96 and later 350's. Nornally 96-98 trucks, but the Vortec 350 stayed in Vans a little longer so it's possible to find some I think up to 2002 at least.

On the head gaskets, since this is a new motor and should be a trued deck, and if you got new or new resurfaced heads, you might go with metal shim head gaskets too. That can be worth .25 more compression. They not very forgiving on head bolt torque though, so they'd require retorquing at least once, maybe even twice.

 

[77 K20]

Thanks for the input Hotod. I'm just trying to make sure he doesn't make another expensive mistake on his truck. I too wish he had went with a 383. I showed him a few different ones- but he said they were too expensive....

The 8:1 compression (nominal) I even found at GM's own website:
http://www.chevroletperformance.com/EngineShowcase/tech_specs.jsp


The GM iron heads I've found for $708 for a pair. But then he would need new valve covers and intake manifold. The aluminum heads are $920/pair and he can reuse his intake and valve covers. But the choice will be his... he had no interest in going to the junkyards to look for used ones to rebuild.

And I did find this dyno chart (for the edelbrock aluminum heads) that shows pretty much every part he has. The only difference is the EPS intake manifold vs just the performer one. According to Edelbrock the only difference is 5 more ft/lbs from 3000-4500. And with 64cc heads he would be in the 9.5:1 compression ratio like the dyno engine. I wish it showed from 2000 RPM and up, but with those numbers it should make for a good engine. Only other difference is the carb. Instead of that edelbrock carb he has a holley (do not know the part number) but remember it seemed properly sized around 600-650 CFM.

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[HotRodPC]

Then the difference must be a bigger dish in the pistons. Cuz the heads are 76cc on both motors. I guess that wouldn't make a difference anyway as far as that article went since they changed pistons, cam and heads. IIRC, they went to flat tops and 64cc heads that had a mild port job and a big cam. So in that respect they'd be right, just buy the cheaper 260hp motor cuz everything else is the same that they are NOT swapping. I have that EPS intake on my CP crate 350 motor. It runs real well.

 

[77 K20]

For the sake of people searching in the future I'll throw in an update on this.

Late last fall my dad had the Edelbrock E street heads installed along with the 2.5" ram horn exhaust manifolds (like mine). Just last week he got the 5th wheel all ready and went camping with it.

He is very happy:

Truck had enough power for all the hills and passes.

Truck didn't "run away" when going DOWN the passes. And engine with more compression will hold your speed back better.

Gas mileage is MUCH improved. Used to get 7 mpg towing. This first trip out was only 400+ miles, but he averaged 10.9 mpg. And when driving the truck empty he said it went from 10 mpg to 13-14 mpg.

 

[HotRodPC]

Nice. So the raise in compression and bigger valves helped out alot then. I'm sure it's more the raise in compression that did it.

You did say he had already swapped to the Eddy 2102 Cam right? He should have a small powerhouse in that truck now.

 

[77 K20]

Nice. So the raise in compression and bigger valves helped out alot then. I'm sure it's more the raise in compression that did it.

You did say he had already swapped to the Eddy 2102 Cam right? He should have a small powerhouse in that truck now.

Correct- the Eddy 2102 cam was the first thing he had done.

 

[hirschdalechevy]

Crate motor blue's , I hear alot of these stories , I have seen gm's 260 hp motor's with the piston's so far down in the hole that they would be lucky to be 8 to 1 with 76cc heads , ( more like 7.8), no quality control. Glad you got it worked out , now you know next time to build your own. It's all about cylinder pressure that comes from compression and cam choice , and then fuel and air flow and when you build your own you know exactly what you have and or want , (as long as you use a good machine shop) hard to find but worth it. Glad you got it worked out.