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Gmc 305 v6?!?!?

Discussion in 'The Hokey Ass Message Board' started by blown240, Dec 26, 2010.

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  1. If you search there is a tech thread and I remember a guy saying his would get 10 MPG be it empty, or loaded beyond what it was rated for.

    That motor started out as a diesel in the 1950s and was converted to gas for some reason around 1960, giving GMC it's own gas engine (replacing the inline six?) rather than use a Pontiac (some bigger trucks had Buick or Olds motors too).
  2. squirrel
    Joined: Sep 23, 2004
    Posts: 46,902


    A half ton pickup should get 10-12. The GMC V6 isn't any more of a gas hog than an FE (Ford 352, 360, 390 etc), which is a gas hog too.
  3. joe_padavano
    Joined: Jan 18, 2010
    Posts: 263


    Sorry, wrong on both counts. The GMC 306 is a 60 degree V6. Kinda hard to see how ANYTHING on that motor is shared with the 90 degree V8 BBC. And what exactly is a "modern 305"??? If you mean the 4.3 liter Chevy V6, well better check your math. 4.3 liters = 262 cu in. Yes, THAT motor is 3/4 of an SBC.
  4. 12 isn't much worse than any of my late model 350-powered Chevy beater trucks have been, either.
  5. squirrel
    Joined: Sep 23, 2004
    Posts: 46,902


    "modern" meaning 80s-90s

    The rod bearings on a gas GMC V6 are about the same size as the main bearings on a BBC.
  6. blown240
    Joined: Aug 2, 2005
    Posts: 1,656

    from So-cal

    I talked to the guy this morning. The truck gets about 8mpg. It runs and drives like it should, but it is a total work horse. Pretty much what I thought.

    It has a bit of rust in the rain gutters, and some bondo in the door, the floors are solid. But it is REALLY inexpensive. I am going to go drive it tomorrow. I do really appreciate the fact that the guy was seemingly honest with me about the bad stuff.
  7. little skeet
    Joined: Jan 27, 2008
    Posts: 302

    little skeet
    from huston

    Put higher rear end gears and bigger rear tires on it. Then update with H.E.I. distributor with proper advance. Get rid of the old carb and put on a new one and jet according to the altitiude you are at. That engine will last a long time and does have lots of torque like everyone says. But you should be able to help the gas mileage issue with proper iginition, timing, and jetting.

    Good luck
  8. GMC alwayshas its own six cyl. gas engines 228-248-270-302 inline six complete different design than a chev.The V6 GMC design didnt start out as a diesel. The early GMC diesels where 6-71 two cycle detroit diesels no intake valves just exhaust valves . ports in the sleeves and supercharged by a blower. That where the Top fuel racers got their blowers. The 478 V6 was avaiable as both a gas burner or a Toro Flow Diesel. I had a 62 Pk with a 305 V6 about 4 mpg. Also had a 478 V6 diesel in a single axel truck tractor about 6 MPG loaded or empty. GMC V6,s where all gas hogs. OldWolf
  9. 1952henry
    Joined: Jan 8, 2006
    Posts: 735


    Just to add, the GM diesels were available from 2 cylinders to 16 (two eights put together). The blowers were used to "push" the exhaust from the cylinders to make room for more fresh air. Later, turbochargers were added to boost the intake charge.
  10. Vimtage Iron
    Joined: Feb 28, 2010
    Posts: 538

    Vimtage Iron

    OW is correct the V-6 was its own engine, in about 63 or 64 GM used that same design and block for the Toro Flo diesel, now that was a poor set up.
    I've got a 61, 2 ton flat bed, when I was hauling with it a few years ago it got around 12 mpg, same weight same round trip milage each day, I got to topping off the seat tank every other day just to keep checking, I still don't believe what it was getting for mileage, I think my pencil or the station pump was screwed up.
    But if your going to use that pickup for a run around town getter It will do just fine, those big V's have a sound totally their own, there not a race horse but will run all day, I think the truck is 3800 rpm on the door tag.
  11. No way are a GMC 305 and a BBC of the same engine architecture.....they are the same in manufacturer alone.
  12. Four Banger
    Joined: Jan 6, 2009
    Posts: 214

    Four Banger

    Good grief! I have never seen as much negativity in one place for one engine, than this thread. Let's take a deep breath and think this through. These engines had a great many strong points, as well as a few shortcomings. However, if they were as bad as everyone on here is suggesting, how in HELL could GM keep selling them year after year, in the numbers they did? If you were a GM fan, you could buy Chevy trucks identical to the GMC's only with a wide variety of different engines, ALL of which surely must be superior to the big V-6??? I'll tell you my lone experience with the V-6 Jimmy, and it mirrors both sides of this arguement. I bought a 62 GMC half ton when I was in high school. It had the 305 with a one barrel carb, and the 4 speed hydromatic. Guy said it needed a tune up badly....boy howdy, it used a quarter tank of gas in the 17 mile trip home, and was gutless beyong anything I ever drove. It ran really smooth, though, which puzzled me to no end. So, I got points, condenser, cap, roter, wires, and plugs, and set to work. Installed plugs with correct gap, set points to correct gap, fired it up, and turned the distributer till it sounded best (my timing light was broken). It still started hard, and had NO power. A compression test revealed normal compression. A carb kit was purchased and the carb rebuilt. Same results. Now, I could have parted ways with that piece of shit, and I would have been one of the guys here saying those things are total turds. However, I took off down the road in the thing on an errand, and it just quit running. Timing chain jumped. I went and got a new chain and gears and put the thing back together, and HOLY HELL......It had plenty of power, even with the hydro, and consistently gave me 12 miles per gallon (not great, but not the 4-8 I see in this discussion). So, my greater point is this: As I look back, I've had a great many old engines that people praise daily, that ran like shit and sucked fuel. And every one of them was either worn completely out, or was terribly out of tune. If I judged every engine strictly on one lone encounter, I'd be bad mouthing half the modern engines made, and ALL of the old ones! So, yes, these ARE big heavy engines. Yes, they DO rev very slowly, and yes, they DO use a bit more fuel than some other six cylinder engines (but if you compare apples to apples, they aren't far off). As a trade off, they are durable, dependable, and long lasting if maintained correctly. I wouldn't hesitate owning one today.
  13. RDR
    Joined: May 30, 2009
    Posts: 1,352


    have always thought a 4 bbl carb would cure the 305V6...I remember my dad had a 1948 KB8 IHC I-6(around400CI) engine in his log truck in 1950-60 and it had a HUGE bendix 2bbl carb on it. It would start hard after warmed up and suck gas like 3-4 MPG...he milled the manifold off redrilled it for a early Carter 4bbl off of a 51 Cadillac and the mileage jumped up to 6-8 MPG and it always started easy,hot or cold....
  14. Four Banger
    Joined: Jan 6, 2009
    Posts: 214

    Four Banger

    Yep, I'd bet you're right. Something with smallish primary bores, like a Carter AFB, or the ubiquitous Rochester Quadrajet. Also, if a person didn't need to tow anything heavy, these engines could pull really tall gears. With just a tweak or two, these engines would work nicely in a half ton/ daily driver/ shop truck application.
  15. blown240
    Joined: Aug 2, 2005
    Posts: 1,656

    from So-cal

    I have an extra Q jet laying around. Perhaps I will look into doing a swap if I buy this thing.
  16. nocoastsaint
    Joined: Jan 5, 2006
    Posts: 413


    I think that a quadrajet would not be the way you'd want to go. The Holley 500 would be a better place to start, I think. has a Demon carb set up to run in pairs for the GMC V12. It might be something to look into.

    We have a 61' GMC with the 351 V6 and I am going to try an HEI six cylinder distributor. the 6066 guys have an information about this and it seems to be a pretty good way to go. It sure helps with the 548 IHC engines.
  17. larry607
    Joined: Feb 18, 2012
    Posts: 2


    I VERY much want a well running 305 v6. Anyone interested in selling please contact me.
  18. Truckedup
    Joined: Jul 25, 2006
    Posts: 3,864


    Four Banger says,
    Do you remember the Vega, Olds Diesel, Caddy 4.1 aluminum V-8? GM continued to sell plenty of those clams :D And the 305 was discontinued after about 9 years.It's a fine engine in a truck but was it really better than the other light truck engines of that time?
  19. Wild Turkey
    Joined: Oct 17, 2005
    Posts: 902

    Wild Turkey

    Just to add a bit to the conversation -- the GMC 305-478 family of V-6's are true 60 degree engines. Thye are also all built using the same exterior dimensions, so swapping a 478 into a 305 truck would be doable, but check the weights!

    Take a look at the 6066 page and you'll also notice the spark plugs are on TOP of the engine.

    They are heavy engines made for heavy work -- if you want revs you'll need to lighten everything, which will really lighten your bank account, but if you want TORQUE at low speeds they may be what you want.

    I've got a 401 (out of a New Idea Harvester) that I'm planning on putting in something soon.

    But please understand I started driving Poppin' Johnnies when I was 10 and don't like engines that rev over 3k.

    Also went to sleep for years listening to 354 Chryslers running at full load on our irrigation wells.

    Ah, to be young . . .
  20. barry gmc
    Joined: Dec 14, 2011
    Posts: 2

    barry gmc
    from spokane wa

    Ive got two of the 305s a 60 3/4 ton with hydromatic, spicer 2021 auxillary trans set up as a overdrive with 4.10 gears. In direct drive around town it gets 10 + or - abit depends on how much your foot weighs. iN overdive on the road it gets about 12. The other is a 66 3/4 ton with 420 trans 3.54 gears and 52000 miles. Averages so far 8-9 mpg havent run it on the freeway since the 3.54 gear swap. before it had 4.56 and got 5-6 but engine was turning 3000rpm @ 60+ to fast for an engine with peak power between 1600-1800 rpm. I have also seen a lot with leaky exhaust valves. To little lash causes the valves seats to erode. they must be 18 thousands hot. once the heads are fixed and properly tuned they really wake up. Way better power than the chev sixes. I also have a 72 chev 3/4 ton 4x4 with a 350 auto 4.10 gears, yes it has more power but it gets no better mileage. I also think GMC made a mistake with the 305 351 v sixes in the light trucks by not bumping the compression a bit adding a 4 barrel carb and better breathing exhaust and heads. had they done this I think peoples views would be different . Its like comparing a 350 v8 out of a 2 ton truck to a 350 in a 70 camaro. Long live the odd ones I also have a 348 turboglide 58 impala. but thats a diff story
  21. geary harlan
    Joined: Mar 20, 2016
    Posts: 1

    geary harlan

    Not true ... I've had mine over 20 years and do a lot of hard work with it Heavy Hauling Roofing everytime I go through the dump truck and trailer is 13 - 15000 pounds...I get 10-12 mpg when not Towing but always have ladders tools heavy equipment etc. My same year Chevy with a 327 gets the same ...for awhile there it had an overdrive and mpg was 13-14 taking it ez on the hwy.
  22. Ive owned several V6 GMC trucks of different sizes. And they all where great at pulling. and they all used way too much fuel. And the timing chains where not worn out. My current daily is a 1/2 ton GMC. 250 six factory SM420 granny four speed. and It can pull my fully loaded trailers 60 MPH no trouble and dont get hot ever. Easy to work on. And gets 12 MPG or better. Back in the early 70,s the GM pickups all got bad fuel economy. The service bullitin reccomended fix was to change out the speedometer gear for one that had fewer teeth. sped up speedometer by about 5 MPH. the folks drove a bit slower and the odometer showed more miles. Put a accurate hub meter on that old V6 and check the fuel milage.
  23. I drove the 4 valve 238 hp 671,s .And you could hammer on them all day. hold it flat on the floor. Never could hurt them. They got one with the Turbo ahead of the blower. 250 Hp and I promptly blew it up. Burnt the pistons and man did it really blow oil out the air box vents.
  24. I have a 1965 305 gmc V6 with a thrown rod and ventilated block. see my ad in misc. for sale. cheap.
  25. Pete1
    Joined: Aug 23, 2004
    Posts: 1,805

    from Wa.

  26. 6narow
    Joined: Jun 1, 2008
    Posts: 517


    I had a '65 big window long bed with the granny 4-speed and the "E" motor for a short time in the late 80's.
    Good ol' rig. Used to belong to an indian who used it for clam digging.
    Didn't like to drive over 45 mph, but I could cruise 60 on the freeway if I had to.
    Started off at 8mpg, but tuned it up, fiddled with the carb a little and learned the art of driving one and I ended getting around 13-14 mpg with it (it helped that my dad used to work at a GMC truck dealership back in the 60's when we lived in Salina. He had a lot of helpful hints that I used to make that truck run better).
    6 lug wheels gave away its 1/2 ton capability, but if you're going to run a truck over its rated capacity, that was the one to do it with! (not a safe thing to do on a regular basis, though. NOT recommended).
    I remember a buddy and I had a repair biz once and we borrowed an air compressor from another friend.
    I wanted to see how well my truck did with a decent load, so we used it to take over to my buddy's place.
    Thank God I had an 8 ft. bed, because the tank on that compressor was an exact fit. Pushed up against the front of the bed and we had just enough room to lock the tailgate shut (we were younger and dumber back then. Just strong enough to hop that thing in, and then out, of my truck).
    Truck held that load just fine.
    According to a youtube video I recently found, it appears those engines really like 4-bbl. carbs and dual exhaust.
    I sold it because Washington State instituted the mandatory car insurance law that year and I couldn't justify insuring two cars. My other ride was a diesel Rabbit and it was in much better shape than the truck and got 19 times better fuel mileage....wouldn't mind having that ol' truck back now, though. =)

    Last edited: Oct 21, 2018
    travis.vermeulen likes this.
  27. I am of the opinion that internal friction was a big part of the poor fuel economy. The pistons where big and heavy and had 5 rings per piston. The rod and main bearings had a lot of surface area. The valve train was heavy also. Back in the day. Ive seen lots of the 351 & 401 engines that blew up because the heads of the exhaust valves popped off. they had sodium filled valve stems. And like the Y block ford GM only made them about 10 years. and the Small block chevy is still being manufactured.
    travis.vermeulen and Hnstray like this.
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