321 GMC inline 6 header

[Ron Golden]

New guy here with a problem. I'm building a 321 inch Jimmy for 1950's type drag racing (HA/GR dragster) and the rules require a stock style cylinder head. Since the exhaust has simease ports on cylinders 2&3 and 3&4 with cylinders 1 & 6 being single ports, what is the best configuration for a header?

I'll be dynoing the engine and tuning the header but I'm at a loss as to what is the best header to start with.

Any suggestions will be appreciated.

Ron

[JackD]

Inliners International would be the best place to start.

[panic]

Never mind.

[power58]

motor books has a  chevy six banger book out that covers a lot of head mods that should work for the GMC also. I don't have it in front of me right now, but I remember they added a divider wall to each exhaust port to make 6 exhaust ports. The flat head ford guys do a similar thing on the center exhaust port.
  If you cant make those mods to the head, a divider in the header extending into the port with two small pipes would help also. Please send phots when you can sounds like a torque monster.

[power58]

found the book listing.  Chevrolet Inline six-cylinder power manual.
ISBN  1931128154  List $19.95 most places on web  have it for $15.96
saw it on the motor bookstore page. List table of contents and other info. Bought mine at Borders. I think you would get some good info from it. Good luck with you're project keep us posted.

[mtkawboy]

Most of it is on late model Chevy 6's, not much on early Chevy or GMC engines. Page thru it first if possable & see what you can use. I have one but cant find it. I know it deals with siamese ports but dont recall any header stuff in it. Its a good read anyway

[interested bystander]

Looked in Smith's Scientific Design of Intake and Exhaust Systems and not a lot of clues on siamesed exhaust ports on sixes.
Is Clifford  Research still going? Maybe them. How about Don Ferguson at Wilmington Forge- those folks have multiple records  with inline sixes.

No doubt cam timing on the siamesed ports would need a look at.

I'd see if the Iskenderian folks might be cooperative. Too bad Howard Johanson is no longer with us.

Those motors made big power in their day, regardless

[panic]

Never mind.

[power58]

Found another good old text.  How to hop up Chevrolet & GMC engines
by Roger Huntington ISBN 1-931128-07-3 reprint of 1951 book List $19.95
covers 228,248, 270 GMC engines 158 pages 51/2 X 81/2 size

[panic]

Good luck.

[Ron Golden]

Guys,
I appreciate all the help since I'm shooting in the dark on this one. I dyno all the engines for a local high performance oriented machine shop and have seen just how effective header tuning can be. I've seen a torque increase of 96 lb/ft at 4500 rpm on a 406 SB Chevy when collector extensions were used.

I'm trying to build a top of the line engine: aluminum rods, custom light weight JE pistons, aluminum roller rocker arm assy, external oil pump, etc. I'm not cutting any corners and think the correct exhaust can make or break this engine.

David Vizard and I go way back (he's my guru) and he didn't think his knowledge of the 4 cylinder engines would be of much help on the GMC. I have both of Philip H. Smith's books on exhaust systems. I also have Roger Huntingtons book on the GMC. I gleaned some info from Smith's book but not enough to be 100% certain what to do.

I'm thinking I may have to fabricate a universal header flange that will accept different header configurations and dyno the engine with each configuration, then tune the one that looks best.

Damn, why didn't I take up ceramics or knitting instead of cars.

Thanks guys, Ron

[Rex Schimmer]

Ron,
I am not much of an aluminum rod fan, they can have a "life" problem and I would certainly go for a good set of Carillo steel rod instead. Maybe at the lower rpms that your Jimmy will be turning they would be OK but when you run them in a drag motor that can turn 10 grand you need to throw them away every 75-100 runs or they will make "windows". 75 runs would be 750 seconds on a drag car, that's 12.5 minutes or about 3-4 runs on a Bonneville car. I am sure that there are people running them at Bonneville I just think steel is a better choice.

Rex

[RichFox]

I saw what Doug Robertson had to do to get aluminum rods in his GMC and decided not to wast money that way. I bought Crower steel rods, +5/8 inch to put the wrist pin close to the rings and at that time I had them made for Pontiac rod bearings because I thought they would be better. Engine ran good in the Jack's Bar fuel dragster for years and as far as I know is still running somewhere. Nothing wrong with Carrilo's. A little pricey if you don't need the very best.

[desotoman]

I'm building a 321 inch Jimmy for 1950's type drag racing. (HA/GR dragster)

Ron

Nothing wrong with Aluminum Rods for this application.

Tom G.

[RichFox]

Turn a GMC upside down and look at how close the stock rods come to the cam. If the cam is not timed right the lobes will hit the rod bolt. Even with a special reduced circle base cam you will grind so much material off of the rod to clear that it becomes a questionable rod. That's my opinion for what ever it's worth.