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onetrick56's 1964 Grand Prix

2020 September
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Thanks for the add into the group first. So I have a 1956 gmc truck with a370 c.i. Pontiac engine. I’m interested in putting a 1965 389 ci 4 barrel manifold and carb on it. From my research it seems the 370 and 389 are the same block. My worry is the 9 years separating them and if I’ll run into any problems involving the cooling system. Thanks in advance for any help

2B789D9A-9551-414D-8F46-1CBA5E1E4832.jpeg

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Mike - welcome to FP and the madness.

I also happen to be president of the GMC Truck Specialty chapter to the Pontiac Oakland Club so I know a thing or two about these trucks.

The Pontiac engines in GMC trucks was a four-year deal in the US. In '55 was the Pontiac 287, 316 in '56, and the 347 in '57. In '58, a 336 was used.

So if you are saying you have an original 370 motor, I am suspicious. GMC also used Oldsmobile motors in larger meduim-duty GMC trucks. In '55 and '56 the Rocket 324 was used. For '57-'59 the Olds Rocket 370 was used. Given the photo of your truck and it's dually axle, I suspect you could have an Olds motor.

Can you take some pictures of your engine and post them here please?

Now assuming for the moment your engine is indeed a Pontiac motor from the period, replacing it with a 389 will require you to replace your radiator. That is because the early 50s Pontiac V8s had a reverse flow cooling system, the 389 is not. So coolant ran into the heads and then the block in the early Pontiac V8s. So you will need a radiator to match your 389's water directional flow. Fortunately radiators for the 326-455 family of Pontiac V8s are plentiful.

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It has an emblem right in the front 370 and I have a title for it it’s a 1956 GMC one and a half ton truck I know Oldsmobile never made a 370 from my understanding the 370 block was the precursor to the 389 they just bored and stroked it correct me if I’m wrong I don’t know that’s why am on her asking questions thank you for your reply

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Posted (edited)

I'm curious about the 370 engine too..

I agree with Frosty...The truck in the pic looks like it's a 1 ton...(dually)..

The Olds engine used in the GMC heavy truck is a 371 CID engine..4.00 bore × 3.687 stroke...and had some differences from the car engine...Such as hardened valve seats, heavy duty valves..(larger diameter valve stems)...beefier rods...different camshaft.. ETC..To make it more durable for heavy duty truck use...

This same Oldsmobile 371 engine Was branded ( for whatever reason) as a  "370" in larger 1 ton GMC trucks....

The 370 Pontiac engine...4.0625 bore × 3.5625 stroke....I'm pretty sure was  used...Or was it...??? in regular pickup trucks..???? and not in 1 ton trucks....But I'm not 100% sure about that...I would love to see engine pics too.....

Welcome to the site..

Edited by TWO LANE BLACK TOP
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Regardless of power train, That is one Killer Looking GMC 👍 

Looking forward to seeing come along.

Cheers

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On 5/1/2020 at 9:37 PM, Mike T said:

It has an emblem right in the front 370 and I have a title for it it’s a 1956 GMC one and a half ton truck I know Oldsmobile never made a 370 from my understanding the 370 block was the precursor to the 389 they just bored and stroked it correct me if I’m wrong I don’t know that’s why am on her asking questions thank you for your reply

I totally agree with Sprint6, it is a killer truck. Mike if you would not mind, can you give me the first six characters of your VIN. That will give me the model and build location of your truck. That should tell me what your truck was built with. The VIN will be on the drivers side door jam.

Two Lane, I don’t know if there ever was a 370 cubic inch Pontiac engine for cars or trucks. So I believe Mike may have an Olds, especially since he says it is a 1 1/2 ton. Pontiac engines I know of were used in light duty pickups.

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Posted (edited)
19 hours ago, Frosty said:

I totally agree with Sprint6, it is a killer truck. Mike if you would not mind, can you give me the first six characters of your VIN. That will give me the model and build location of your truck. That should tell me what your truck was built with. The VIN will be on the drivers side door jam.

Two Lane, I don’t know if there ever was a 370 cubic inch Pontiac engine for cars or trucks. So I believe Mike may have an Olds, especially since he says it is a 1 1/2 ton. Pontiac engines I know of were used in light duty pickups.

Just in case anyone is interested.....Here a little background info on the Pontiac 370 CID engine ( 4.062 bore × 3.562 stroke )... Was introduced as an improved version of the 1957...347 CID engine (4.00 bore × 3.562 stroke) with different cylinder heads with larger exhaust ports and different better breathing exhaust manifolds...The 370 Was only available in 1958 and was  used as the optional engine for the 1958 Bonneville with the Rochester mechanical fuel injection set up...Rated at 310 horsepower and 400 ft-lbs of torque....But could be had with tri-power or single 4 barrel carb...They were also used for one year only (1958) in the factory backed Pontiacs for NASCAR racing although without The Rochester fuel injection set up..The 370 was replaced with the 389 starting in 1959...There were only about 500 or less total 370 engines produced and only in 1958....

 

Edited by TWO LANE BLACK TOP
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14 hours ago, TWO LANE BLACK TOP said:

Just in case anyone is interested.....Here a little background info on the Pontiac 370 CID engine ( 4.062 bore × 3.562 stroke )... Was introduced as an improved version of the 1957...347 CID engine (4.00 bore × 3.562 stroke) with different cylinder heads with larger exhaust ports and different better breathing exhaust manifolds...The 370 Was only available in 1958 and was  used as the optional engine for the 1958 Bonneville with the Rochester mechanical fuel injection set up...Rated at 310 horsepower and 400 ft-lbs of torque....But could be had with tri-power or single 4 barrel carb...They were also used for one year only (1958) in the factory backed Pontiacs for NASCAR racing although without The Rochester fuel injection set up..The 370 was replaced with the 389 starting in 1959...There were only about 500 or less total 370 engines produced and only in 1958....

 

Thanks for the history lesson Two Lane. Much appreciated. Using it on the fuelie Bonneville makes sense now. It explains why I am not very familiar with it.

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  • 4 weeks later...

Something that has not been mentioned is the change in heads/ intake manifold. Earlier Pontiac engines had 6 bolts on each side of the intake manifold later ones had 5. I spent a long time looking for a 6 bolt 4 BBL manifold for my '63 and eventually gave up and used a Holley Sniper. Never did get it running right and when it launched a rod bearing I gave up and put in an LT and 6 speed auto out of a 15 silverado but that is another story. 

Point being is that if it is a Pontiac engine 6 bolt intake manifold a head change might be necessary to get a 4 BBl manifold. Don't know if there were block changes that would affect a head change someone that knows more about Pontiac engines than I do would need to address that.

Also I went to High School with someone that drove a GMC pickup with a nail head Buick in it backed up with the early 4 speed cast iron hydro from the factory. So maybe another engine possibility?

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The earlier Pontiac engines are reverse flow cooling...I'm pretty sure (not 100%) the changeover to the conventional water flow was some time in real late 1964 or The beginning of the 1965 model year...Because of that the earlier cylinder heads won't interchange or Visa versa with the later model engines...

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Two lane,

Last post I read of yours, as memory serves, you had backed the race car into the wall on the first? race of the season. Did the season improve?

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Posted (edited)
20 hours ago, TWO LANE BLACK TOP said:

The earlier Pontiac engines are reverse flow cooling...I'm pretty sure (not 100%) the changeover to the conventional water flow was some time in real late 1964 or The beginning of the 1965 model year...Because of that the earlier cylinder heads won't interchange or Visa versa with the later model engines...

From Wikipedia.....

"All Pontiac V8s from 1955 to 1959 were reverse cooled, known as the "gusher" cooling system. It was removed from the design for the 1960 model year because designers moved the generator and the power steering pump from atop the front of the engine down to the front of the heads to accommodate a lower hoodline. However, the 1959 389 engines had the generator in front of the heads with reverse flow cooling still in use. This suggests that the cost of the reverse cooling was the reason for the change to "equa-flow" cooling."

So essential a 1960 389 and up have "normal" cooling. All the 50's era Pontiac V8s are reverse cooling. I was not 100% certain of exactly when until I started looking but I generally knew that '50s Pontiacs were not interchangeable with '60s-and-up Pontiacs. Now I know more clearly why and when.

Edited by Frosty
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I wasn't 100% sure....Thanks for clarifying.. I learned something.... I don't want to post inaccurate info if at all possible...

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On 5/30/2020 at 10:28 AM, TWO LANE BLACK TOP said:

I wasn't 100% sure....Thanks for clarifying.. I learned something.... I don't want to post inaccurate info if at all possible...

I understand completely. I try not to either.

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