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GMC 305 V6, The baby of the series!!!

Discussion in 'The Hokey Ass Message Board' started by revkev6, Jan 5, 2007.

  1. revkev6
    Joined: Jun 13, 2006
    Posts: 3,351

    revkev6
    Member
    from ma

    I just stumbled upon a site dedicated to the big block series gmc v6 engines and was extremely suprised to hear the 305 cube engine was the SMALLEST of the v6's that gmc made. between 1960 and 1974 gmc made the following:

    sizes for v6 went as follows:
    305
    351
    401
    478:eek:

    and get this a 637cube V8
    and lastly
    701 cubic inch V12 :eek::eek::eek: at a hefty 1480lbs
    [​IMG]
    [​IMG]
     
    Jacqueline likes this.
  2. revkev6
    Joined: Jun 13, 2006
    Posts: 3,351

    revkev6
    Member
    from ma

    MO54Frank likes this.
  3. Wesley
    Joined: Aug 12, 2006
    Posts: 1,670

    Wesley
    Member

    I bet that beast was easy on fuel.:eek:
     
  4. Frank
    Joined: Jul 30, 2004
    Posts: 2,321

    Frank
    Member

    Yeah, its been discussed on here before. I believe its covered some in the thread "Oddball engines" or something like that.
    They would look different, but not much from a performance standpoint. Check out www.blastolene.com. I think they are building a rod to use one.

    An odd thing about them is that they had plaid valve covers. Looked just like a Scotch kilt.
     

  5. squirrel
    Joined: Sep 23, 2004
    Posts: 47,086

    squirrel
    Member

    I'm pretty sure that only a few of the GMC V6 engines had plaid valve covers, most were just red like the rest of the engine.
     
  6. revkev6
    Joined: Jun 13, 2006
    Posts: 3,351

    revkev6
    Member
    from ma

    the plaid valvecovers are discussed on the site listed. performance to weight these motors are not very amazing but the wow factor and torque would more than make up for it IMHO
     
  7. scottybaccus
    Joined: Mar 13, 2006
    Posts: 4,109

    scottybaccus
    Member

    Now I will have to rethink that jeep swap I was planning! :) I wonder if that V-12 has a 1 pc crank with 2 blocks lined up that way. Cool to see propane fuelers on it.

    Oh, wait. That is a one pc block. Dang. Who wants to blueprint that engine?
     
  8. That engine series has been discussed on here many times. The general consensus is that they are heavy, low-speed, low hp truck engines that have no hi-po parts available.

    Given that, knock yourself out using one of you want.
     
  9. xlr8
    Joined: Jun 26, 2006
    Posts: 650

    xlr8
    Member
    from Idaho

    The 637 V8 was interesting in that it was also a 60 degree V, basically the 478 with two more cylinders added. I overhauled one once, it was in a tandem axle truck, approximately 50,000 lbs. gross. The truck got 1.5 MPG, loaded or empty.
     
  10. BigBlockThug
    Joined: Apr 8, 2008
    Posts: 26

    BigBlockThug
    Member

    Isky Racing has hop-up cams etc, for the bigblock six...
     
  11. squirrel
    Joined: Sep 23, 2004
    Posts: 47,086

    squirrel
    Member

    I get the impression that the gas and diesel versions of the engine were designed at the same time.
     
  12. A 305 V6 would get about 4 mpg in a 1/2 ton PK.:eek:
     
  13. they also made a diesel conversion of the 478. It would make 3200 RPM very high for a diesel. Most only turn 2100 to 2450 RPM,s
     
  14. I heart big engines. :)
    We had a dump truck on the farm with one of these... what a beast!

    JK
     
  15. Hnstray
    Joined: Aug 23, 2009
    Posts: 11,637

    Hnstray
    ALLIANCE MEMBER
    from Quincy, IL

    Back in the 60's I worked in a shop and was assigned to do a valve job on GMC pickup......it had the 305 V6......The engine was very bulky appearing, a good candidate for a boat anchor I thought, especially after pulling the heads....those puppies were HEAVY!.......sluggish, poor fuel economy, excessive weight for the pickup application.........GM wised up and started using Chevy based engines (there were minor differences) in GMC pickups and sales improved dramatically.
     
  16. if I remember right the diesels were called Toroflows, we ran them with twin turbos and got diecent service out of them in busses.
     
  17. Thanks for posting, I love all of the old-school style of illustration art contained on that link.
     
    Jacqueline likes this.
  18. Driven several V6 305's in mid 60's pickups, if you drove them hard the 3spd's came apart (hypothetically of course)

    I saw this gem at the Mitchell show

    [​IMG]
     
  19. Hnstray
    Joined: Aug 23, 2009
    Posts: 11,637

    Hnstray
    ALLIANCE MEMBER
    from Quincy, IL

    That V12 is from Bendtsen's Adapters in Ham Lake, Mn, I believe.........impressive looking, that's for sure.
     
  20. Bib Overalls
    Joined: Aug 3, 2001
    Posts: 4,071

    Bib Overalls
    ALLIANCE MEMBER

    Starter looks wimpy to me.
     
  21. Locomotive Breath
    Joined: Feb 1, 2007
    Posts: 711

    Locomotive Breath
    Member
    from Texas

    Here are a couple of pictures of my '50 GMC COE powered by a 702 V12 and 4L80E transmission.
     

    Attached Files:

    j-jock and Atwater Mike like this.
  22. Heres a little about these motors I leared back in the mid 80's. My grand father gave me his 1965 GMC 1t 4x4 that came with the gas 351c v6. I rembered from a kid that this old truck would out pull anthing and never broke down. The old 351 was getting tired with about 200k on it so i decided to rebuild it and of coarse hop it up. So I started looking into performance imfo on these motors. That put me in contact with Babe Erson. He told me about the gas 478M motor and how with some work the big 478m v6 will easily make more tork and hp than a mildly built 454bbc. So I bougth a 478m motor from the junk yard and went to work per Babe's rec's. The first thing was to throw away the stock 4 ring pistons and masive rods that weight about the same as a complete hemi motor. I had a crank grinder turn the rod journals down to a much smaller size to fit a 426 hemi rod. I had BRC make some new lighter 3 ring pistons that also raised the cr up from 6.5 to 9.0. Engine dynamitics lightened the crankshaft alot and rebalanced every thing for me. Babe made me a special cam and all the vale train parts. I ported the heads and polished the combustion chambers myself. I also made a sheetmetal cross ram intake with 1 850cfm Q Jet carb and a set of home made 1 3/4" headers. Once I got all the tuning done on this motor it was one tork animal that would rev easily to 5.5k. It broke the transfer case right away so I put in a beffier transfer case out of a newer ford truck I think then I blow the old gmc rear end so I upgraded it to a dana 70 with posi. I also upgraded the old truck 4 speed to a newer 5 speed that could be shifted much faster. I had put big 38" tires on this truck and could smoke them in 2nd gear. It did not do to bad on fuel also at about 12 mpg fully loaded with a 10ft camper and pulling a 30ft inclosed trailer running about 65mph. I wish I know where this truck (James) is today as I would try to get him back
     
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  23. Rocketile
    Joined: Mar 31, 2010
    Posts: 12

    Rocketile
    Member
    from USA

    The main issue is that they never were designed to turn many RPM. Not that the crank and valve train isn't capable... the fuel and oiling systems aren't. I suspect this gave some bad experiences in the past but as gearheads, these are solvable. The issue on weight will not be overcome BUT... the trends in rat rodding trucks and uniqueness brings these behemoths into the "interesting" category. www.thunderv12.com has started to resurrect these to make thing easy. Check out the 5000 rpm run and the fact that you are pushing 630+ ft lbs of torque across 3000 rpm ranges. Myself... I got bit and now have three 702 V-12 motors with the first project being a "sport-truck". BTW... 1960 vintage medium to heavy trucks are CHEAP and easy to come by.

    Onwards!
     
    j-jock likes this.
  24. I seem to remember reading somewhere that the basic design began as a diesel in the late 50s. That would explain the low-rev design, the later GM 6.2L diesel is the same thing - they really stop making power after 3000 RPM and are governed to 3600 or so so you can't make them blow their guts out.
     
  25. The diesel was a 478 cu in toro flow. I owned a 69 truck tractor equipped with one. It came after the 305 and 351 gas engines. The deisel was governered at 3200 RPM very high for a diesel most diesel max is 2100RPM. A gas burner would only get about 7 MPH max.
     
    j-jock likes this.
  26. rjaustin421
    Joined: May 1, 2009
    Posts: 337

    rjaustin421
    Member

    We rebuilt quite a few of them over the years...it was a hoot seeing one in a 3/4 GMC ton pickup.

    There may be better V/6 big 3 engines....but there are none heavier!!!
     
  27. Rocketile
    Joined: Mar 31, 2010
    Posts: 12

    Rocketile
    Member
    from USA

    Locomotive breath, What are you running for carbs on the V-12 COE?

    The engines have 4 bolt mains and Nitride hardened crank journals... if you want to turn past 3500 rpm (yes there are cams even for the V12!) then you need to address the oiling system. These engines were designed to go to 200,000 miles with no overhaul... side clearances on the rods are too tight and the single pick-up on the oil pump cavitates as the pump tries to suck nearly 25 gallons per minute at that engine RPM through a 3/4 inch hole. At 5000 rpm it is trying to pump over 34 gpm which is more than a nitro funny car does at the same.

    We are hot-rodders.... off the shelf high performance parts are not how hot-rodding started is it? The V12 with cam and NOS can produce over 800 HP at 3000 rpm with the crappy heads. Better, continue that avg HP to beyond 5000 hp. OMG! Who cares about weight, I just hope the asphalt holds up. Overdrive this and try a rolling start from a "high" number and see what happens huh?

    Ever talk to someone who raced Daytona when it was a banked sand track on the beach? They will tell you about hot rods. Of course, sometimes the thrill is finding the treasure... photo of 2 V12s I found less than 15 miles from my house.

    Onwards!
     

    Attached Files:

    j-jock likes this.

  28. they get 10 mpg i had a 64 gmc with a 305e and it had a 4 speed
    3/4 ton with 411s it got 10mpg all day long, loaded or empty, up
    hill down hill with the wind blowing with you
     
  29. Locomotive Breath
    Joined: Feb 1, 2007
    Posts: 711

    Locomotive Breath
    Member
    from Texas

    Hello, and welcome to the HAMB.
    I am running 500 cfm Rochester 2bbl's that have been modified by Jet Carburetors for high performance applications. They feature replaceable Holley main jets for easy tuning. I personally prefer the Rochesters over the 500 cfm Holley 2bbl's. I made the base plate adapters to allow the large base carbs. to fit the V12. Here it is running.
    http://www.youtube.com/watch?v=wG9VnCOoM9Q
     

    Attached Files:

    j-jock likes this.
  30. petes65usa
    Joined: Mar 11, 2010
    Posts: 254

    petes65usa
    Member

    This is an awesome truck 702 v-12 wow..............:cool:
     

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