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Technical Couple of questions regarding pontiac tri-power carbs

Discussion in 'Traditional Hot Rods' started by semaj4712, Jun 4, 2014.

  1. semaj4712
    Joined: May 28, 2013
    Posts: 98

    semaj4712
    Member

    Hey guys, so forgive me for my lack of knowledge here, but I am having a little bit of a hard time figuring out some things.

    First off I have a 55 GMC with a pontiac 316 V8 in it, Ive been doing some planning and am about to the point where I am going to pull the motor and do a full rebuild on it (has low compression in two of the cylinders) and while I am at it figured I would do some upgrades as well.

    I am really interested in doing a tri-power setup if it makes sense to do so, I have been doing a little research and have found the a few but not many of the 347s in 57 and 58 came with stock option tri-power double barrel carbs.

    I guess my question is what was the cfm on those carbs and how exactly does that work on a tri-power setup? Based on running some figures a stock 316 should have a CFM number right around 400 and a 347 stock should have one right around 425. Which would seem like a rather low CFM per carb, but then I dont really understand how the CFM works with Tri-power setups.

    My second question then is, I am planning on running all new internals in my 316 possibly minus the crank, with a performance cam and a slightly oversized set of pistons. Not sure how much this will really change that much in terms of CFM need, but what would the benifits and negatives to running say a 57 or 58 stock tri power setup.

    Thanks guys
     
  2. .030 over is going to make it like a 321, 322. Watch the cam because changes there mean you need to watch the valve geometry.

    A tri-power is like running a 2-bbl most of the time, when you step on it it opens the end carbs like the secondaries on a 4bbl, easy to have too much carb for the motor with one, often a 4bbl is easier to make perform.

    Really, if you want to hop it up you'd be better off with a '59 389 or even a 60-64 block and machine it to use the iron front cover with a saddle mount. Almost all the internals except pistons are the same as a 400, you gain a bunch of cubes, and externally it will appear identical to the 316.
     
  3. And be careful with cam choices as lumpy low vacuum cams will not perform well with triples and can become a nightmare to tune and keep running well
     
  4. klawockvet
    Joined: May 1, 2012
    Posts: 370

    klawockvet
    ALLIANCE MEMBER

    I bought a new Pontiac Ventura in 1960 with a 389 Tri power and a 4spd. One of the best cars I ever owned. Lots of power, decent mileage and fast. If I was doing anything Pontiac or GMC I would duplicate that set up.
     
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  5. You've posted some good questions. Making Tri-power work well is an on going issue due to bad combinations. Also in reading your post assume your kind of new to motor work. My advice is find a good motor builder/machine shop to work with not just a guy with a boreing bar and a valve grinding machine. We in the Portland aria are blessed with 3 outstanding ones as well as a local Cam grinder that's also a Drag Racer. These are the kind of guys that know how to build the combination for success. The wrong cam will never get you there and correct heads very important. C.F.M. and chamber size are a balancing act for multiple carbs and good performance.
    The Wizzard
     
  6. If you do a search here you should come up with some good info on tri-power setups but here are a couple things to be aware of. Original tri-power center carbs are basically ordinary 2bbl Rochester carbs. Some minor differences but easy to get around, which means finding a good carb to use in the center isn't too hard. The outboard carbs are different . If you look down the bore of a standard carb there you can see space between the throttle plate & bore where air can get past - that's OK, it is what allows the engine to idle. But the outboard carbs have no idle circuts, so you don't want ANY air passing thru them at idle - this would create a vacuum leak and make tuning a pain. The throttle plates are thicker and machined to fit much tighter to the bore when closed. It's something that you can't really see unless you you know what to look for. Finding original tri-power outboard carbs is no easy task. So budget some extra funds for either finding originals or to have a carb shop modifiy standard carbs to tri-power spec.

    There are a few Pontiac forums you should check out, lots of great info on this stuff...
     
  7. 40fordtudor
    Joined: Jan 3, 2010
    Posts: 2,503

    40fordtudor
    Member

    I second that. I had a factory tripower on my 57 and a real lumpy Isky cam---I ran out of vacuum more than once to my brakes. My buddy rigged up a canister for vacuum for me. 51 Norm---cannister cured it.
     
    Last edited: Jun 4, 2014
  8. '51 Norm
    Joined: Dec 6, 2010
    Posts: 684

    '51 Norm
    Member
    from colorado

    I also had power brake problems with a lumpy cam. I don't think that the Pontiac engine or tri power had much to do with it.

    Revving the engine to get vacuum so the power brakes would work properly is somewhat counter intuitive; not the sort of thing that you wanted to be thinking about when trying to stop quickly.

    I have heard many people say that tuning a tri power is difficult. Perhaps I just lucked into a good set up but I never had any tuning issues. I had a very lumpy solid lifter cam and mechanical linkage to the outboard carbs.

    The most annoying thing I found with my tri power was the fuel leaks at the carb inlets and around the bowl gaskets. I eventually got tired of cleaning them up and just left them cruddy.

    The engine is a '63 421. I used it in a '62 Studebaker truck for several years and it is currently sitting under my work bench awaiting installation in the next project.
     
  9. Pist-n-Broke has given you good advice concerning your rebuild. Unless you are building a full on race engine and trying to hit a specific displacement you can't just pic a random bore size and call it good. You bore size is dependent on what it is going to take to clean up the cylinders and make them straight. There is a tech article if it has not been deleted here on the HAMB called "tear down and Inspection" that will be of some help before you start tearing into it.

    If I recall the mid '50s poncho trips are set up with dash pots stock, they are vacuum operated. IE they are dependent on engine vacuum to open the secondaries. Working properly they will not let you have too much carb. The down side is that aside from being very pricey it is difficult to find one complete and in good shape.

    I personally am not a big fan of tri power. I have run them and can set one up but to me for the most part it is just for looks, people think it makes them old school. I actually raced a ford with trips back in the late '60s. A fella named Kilcup helped me set it up and it was right on. he eventually convinced me that I could make more zot with two fours. he was right, I cut a second off my time.

    That said not to discourage you at all, they can be fun to tinker with and it is a blast to spend the time making it run right aside from the feeling of accomplishment when it is right.
     
  10. flypa38
    Joined: May 3, 2005
    Posts: 529

    flypa38
    Member

    Don't have much knowledge of what you're in for, but keep in mind that compression ratio for Pontiac engines in GMC's was lower from the factory than Pontiac car engines. Not sure if that is even a factor here, but might affect your choice of cams.
     
  11. All the more reason to talk to a "real" engine builder not just a guy with a machine shop.
    Porknbeaner-- I'm proud to have been able to call Danny Kilcup a friend and your right, he knew how to make shit run hard.
    The Wizzard
     
  12. semaj4712
    Joined: May 28, 2013
    Posts: 98

    semaj4712
    Member

    This is something I had not considered, but what I am gathering from some of the replies is that I am going to decide between a beefy cam or the tri-power.

    Thanks man, I will probably have to check out a few of the pontiac forums, I do think as someone had pointed out that the compression ratios were slightly different in the trucks for more torque. To get these speicifc numbers maybe the pontiac forums would be more helpful.

    Alot of great info here tho, this is sorta what I had gathered from my research, and I know that original tripower carbs are pricey, I have a guy who has a stock rebuilt set off of a 57 hes willing to part with, thats what got me starting this thread.

    I am not really interested in changing the motor or anything like that, so I guess what I am asking then is, will a stock rebuilt 316 with tripower carbs off of a 57 be a good combo, and if no why not specifically?
     
  13. flypa38
    Joined: May 3, 2005
    Posts: 529

    flypa38
    Member

  14. ebfabman
    Joined: Mar 10, 2009
    Posts: 643

    ebfabman

    There are several aftermarket baseplates available that make converting a stock 2 bbl to a tri-power carb easy. No idle mixture passages/screws, thicker butterflies, sealed fit in the bore, no vacuum passages, and extended shafts. The cool thing about tri-power is you can use a bone stock motor if its in good shape and utilize the tri-power for its advantage. Most linkages are set up to run the motor on a 2bbl til you put your foot down.
     
  15. zeuglodon
    Joined: Nov 17, 2010
    Posts: 88

    zeuglodon
    Member

    First, you want a 57-58 manifold. They are for 3 small-base Rochesters - later intakes use the large base carbs for secondaries. Second, don't worry about finding "real" tri-power carbs. Just find the nicest, most closely matched set of 2G's you can and cough up the $200 for a set of new secondary base plates. Trust me on this - the new ones seal better than the originals ever did. Install the new base plates and remove/block the power valves when you rebuild the secondaries and they're good to go. The simplest way to run them is to connect the secondaries on the right side with a linkage rod you can make from a piece of 3/16 rod and some ends. Then you can use any sort of progressive linkage on the driver side to connect the center carb with either of the secondaries. The beauty of this is that you can use any stock 2G as the center carb and it will require no mods whatsoever. Here are some pictures of the trips on my stroker:
     

    Attached Files:

  16. Your best step forward would be to find a factory setup. Finding matching end carbs may take awhile, and then they have to be converted to fully sealed units. Find someone that can rebuild them AND set them up with aftermarket mechanical adjustable linkage. The end carb then will be able to kick in when you and your engine want them to. Make sure you use a good distributor advance setup and a vacuum advance canister designed for low vacuum. A rump-rump cam will be low on vacuum but the center carb will work if the power valve spring is modified. tuning...tuning...tuning
     
  17. ebfabman
    Joined: Mar 10, 2009
    Posts: 643

    ebfabman

    Zeuglodon I like your set up. My tri-power is completely homemade from three regular 2g carbs. I got lucky and two of them had butterflies that could be repositioned in the bores so that very little to no gap was left. This is somewhat unusual for a normal 2g. Most I've seen over the years would not fit good enough to keep vac leak to a usable minimum. One thing I insist on with my street driven motors is a PCV system, among other reasons is the fumes. The aftermarket baseplate I've seen had no provisions for a manifold vac source. Although I've seen spacers that have vac ports on them. Where there is no power brakes or automatic trans its not a problem, but for the setup on the 51 Merc that has AC, auto trans, and power brakes, I needed the additional manifold vac sources.
    Here is a mock up of the homemade linkage. From here it can be cleaned up and made to look nice. It works well.



     
    Last edited: Jun 5, 2014
  18. If you are wondering about Cfm, 2 bbls are rated differently than 4 bbls. I can't remember the exact reasons why that is or what the difference is all time. Somedays I remember it but today isn't one.

    If you'll be running a factory style set up carbs, your cam selection won't have much effect on tuning them. The end carbs are doing nothing until you put your foot into it. The linkage is progressive and you run mostly on just the center carb.

    Multiple carbs that all have idle circuits are when you run into tuning trouble.

    You will absolutely love a factory style Pontiac tri-power set up.
     
  19. Ye
    Danny was a wizard. He taught me as much about racing (and winning) as anyone ever did. Wouldn't bore anyone on the board with it my interaction with him goes back to the first year I was alive.
     
  20. semaj4712
    Joined: May 28, 2013
    Posts: 98

    semaj4712
    Member

    Some very good info here, I think I have gotten the info I was looking for, which is yes I can do the tri-power on the 316 and I can go a few different ways in building the tri-power, be it factory or not factory.

    This is the info I was looking for, Thanks guys
     
  21. ponchopowered
    Joined: May 27, 2010
    Posts: 438

    ponchopowered
    Member

  22. semaj4712
    Joined: May 28, 2013
    Posts: 98

    semaj4712
    Member

    Thanks, I already have a stock one and an offenhauser and an edelbrock, just need carbs now
     

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