My '56 GMC truck has a '66 389ci.... GTO? - Hot Rod Forum : Hotrodders Bulletin Board
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Old 02-10-2013, 05:17 AM
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My '56 GMC truck has a '66 389ci.... GTO?

Hi guys,
i have just bought a 56 GMC stepside and it has a '66 389 motor in it - most likely from a GTO??. 335hp.

Now this in standard for should help my truck cruise nice. But i just cant leave stuff alone

Im wanting to change the cam for something a little better but will be adding a brakes booster kit and have read up that some wilder cams can cause the vaccum to be reduced - is this right? If so, can anyone reccomend a mild street cam i should use? Im not intrested in HUGE power just now but want to make it a better driver with some traffic light derby capability

I will be getting headers for it and it already has an edelbrock inlet with holly carb - but not sure hwih ones just yet.

cheers in advance.
Andy
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Old 02-10-2013, 08:16 AM
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Camshafts, intake, Holley Carb and headers are the obvious starting points to make a 66 389 perform...but it would help if you could answer a few questions. Are you sure it's a 66 GTO 335 HP 389? What CFM is your Holley Carb? What are the casting numbers on the heads...(numbers are found on either side of the head on the center exhaust port)? What is the mileage on the engine? Is it an automatic or a standard? What is your rear end gear ratio?

Ray
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Old 02-10-2013, 08:35 AM
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X 2 with Widetrack here.
Some more info is needed.
I would suspect the rear end gear is pretty low , like 3.56 or 3.73.
Kinda too low for a truly streetable ride with a poncho mill as the poncho mill makes more torque and can handle higher ratios much better.
Way kool that you have a GMC with a pontiac engine in it.
I would love to here more about it.
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Old 02-10-2013, 08:48 AM
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hi huys,
many thanks for the replys.
i'll get the info needed asap but might be a week or 2 as i need to get the truck picked up and brought to me so i can go over it properly to make sure she's sound & safe.

It is believed that its a GTO donk, but the numbers will confirm when i get them.
I do have big plans for her but as with most rodders, it wont happen over night

cheers again. Andy
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Old 02-10-2013, 12:37 PM
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1955-1959 GMC Series 100 and 500 with optional Pontiac V8 engines:
1955...287 CI Pontiac V8 engine
1956...317 CI Pontiac V8 engine
1957...347 CI Pontiac V8 engine
1958...336 CI (small bore version of Pontiac 370 CI V8 engine)
1959...336 CI (small bore version of Pontiac 389 CI V8 engine)

Quick ID:
If the V8 engine in your GMC truck has front motor mounts, it is the original 1955-1959 Pontiac V8.
A 1960 - 1979 Pontiac V8 had side motor mounts and no provisions for front motor mounts. .
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Old 02-10-2013, 12:42 PM
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welcome to the forum
looking forward to seeing pics
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Old 02-10-2013, 12:49 PM
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It has side engine mounts, middle of block. from the numbers on the block its a '66 engine.
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Old 02-11-2013, 08:00 AM
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If it is truly a "GTO" engine, the heads will be "93s". 92s were on all other 389s except GTO.

The Pontiac used the timing cover for the "front mount" in those days. The aluminum versions of later years still have the bolt provosinos, but I would hesitate to use one as a "mount' in a high-torque engine.

Pontiacs are very sensitive to compression versus fuel quality. Make certain your static ratio is reduced to under 9.5:1.

Comp offers the XE series for the Pontiac. The XE256H is probably what you're looking for. Decent idle, good low- and mid-range power. The "next one up" (XE262H) is the most popular grind we have for "stockers". Works very well in 389s with TriPower and stock exhaust.

My "dream truck" is a '59 GMC (short bed, step-side) with a 505 Pontiac in it, pulling houses down... (:-

Jim
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Old 02-11-2013, 11:12 AM
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Awesome info guys - many thanks im such a newbie at yank stuff but i learn fast
If it was a VAG or Subaru i'd be well away. lol.

Im happy to have a slighty rougher idle from a cam, it sounds just awesome. I need to find some headers foe it and make up a stainless exhaust as its stock just now and is shot. I also need to look into the rear end and decide what - if needed - axle to go for. i want it to be a good driver for long distance to shows all over the UK and be able to smoke from the lights

Andy
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Old 02-12-2013, 07:55 AM
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Andy,

Let us know what you find for numbers.

BTW, we "yanks" know why you Brits drink warm beer, ya know... You have Lucas refridgerators... (:-

Cheers!

Jim
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Old 02-12-2013, 11:36 AM
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lol Mr P-Body
The block has WW on it but no numbers can be found on the heads at all
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Old 02-12-2013, 01:38 PM
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Quote:
Originally Posted by Mr. P-Body View Post
Andy,

Let us know what you find for numbers.

BTW, we "yanks" know why you Brits drink warm beer, ya know... You have Lucas refridgerators... (:-

Cheers!

Jim
Incredibly funny . . . Lucas, the prince of darkness.

Pat
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Old 02-12-2013, 04:18 PM
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Quote:
Originally Posted by PatM View Post
Incredibly funny . . . Lucas, the prince of darkness.

Pat

That is how I understood it too. LOL
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Old 02-12-2013, 05:00 PM
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chevy cavilers and some colbalts have vac pumps, get one and have at it with a wild hair cam..
many 90's caddy's also have the pumps,,

why not a hydra boost instead of vac booster,,
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Old 02-12-2013, 05:50 PM
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Quote:
Originally Posted by UKstepside View Post
lol Mr P-Body
The block has WW on it but no numbers can be found on the heads at all
The head number that you need to find will be cast on, not stamped.

Bill
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