The Stovebolt Site Forums The Shop Area Engine Shop Proffit or Hot Rod Lincoln

Or who ever has experience.On this 302 GMC I'm building,I've decided to build a sheetmetal intake for the hell of it.I have the tools and reasonable skills to to do the job,looked at some designs,mostly on drag engines.Anyhoo,a plenum manifold like the vintage cast ones,3 carbs on a log,3 short tubes feeding the 3 round intake ports seems ok.Some guys say a manifold with balance tubes or plenumn style isn't necessary for street use.I say the plenum or balance tubes serves to smooth the engine and allows the engine to breath through all 3 carbs at higher speeds.The carbs are Holley 1904 single barrels, about 130 cfm each.I have experience with these carbs and 3 matched ones on hand,so that's why I'm using them.
I'll start the fresh engine on one stock carb, don't wanna be fussing with jetting until it's run in.

the chevy 6 cylinder power manual book by Leo Satucci has a good chapter on these type of intakes. It also has a blue print of a triple carb intake and pics of some homemade intakes They are for 250 and 292 but it could give you some ideas.I can scan the blue print and send it to you if you don't have it.

Might try a post over on inliners and see if you can smoke out that Mitch character from Ideeho who claims to have the fastest farm truck in the world at Bonneville--he had some kinda homebrew intake on that 302 but was so covered up in the pics that I couldn't tell what he'd done--spect he has some experience with this tho. :p

Definitely enlist the help of somebody with some experience! A log-type manifold can and does develop isolated rich and lean spots at certain RPM's due to harmonics, so it's possible for a multi-carb setup to go off mixture at oddball times and speeds. Good luck!
Jerry

Inliner help is all racing stuff,like Hotrod says,a street engine may encounter problems at part throttle that's of no concern for a limited powerband racing engine.A top Inliner told me I can't build one properly, good enough reason to make one then
I know Leo Santucci pretty well,he really doesn't have experience other than drag racing.But he thinks it's possible with some thought.
Looking at Offenhauser,and the old manifolds from Thickson etc, they are simple castings,a common log plenum, short runner for the intake ports and 2 or 3 or 5 carb mounting flanges.Doesn't look like rocket science to me.Lots of guys run these manifolds, I had multiple carb experience on a 261,was no problem.
I can buy a 3 carb cast manifold from the newly formed Wayne Co for a nifty 450 bucks, or I can weld one up for 50 bucks.The worst it can be is wrong,right?A good experience either way.

Since you are welding anyway you could always try baffles between the carbs to cut down the harmonics. I've been thinking of a fabricated manifold too, I'd like to see how you do with it. Are you going to provide a heat source?

Your DOOMED

Why don't you mod an XKE three SU carb set-up

Buy a two pot and enjoy the 302's torque

I've watched that AD tub @B'ville....runs like hell!!!

Doomed? Scott was doomed in Antartica,but he wasn't building intakes.
I checked a 3 carb Jag intake, short runners, common plenum.I looked at SU's, the 1-3/4 and 2 inch carbs are 200 bucks plus each for good ones.Looked at Harley S&S sidedrafts, thought about it, probably hit the steering column on the rear carb with side drafts.
Could go for a 4bbl intake like on my 261,but I'm not convinced it's the best setup for an inle,ya know different length runners.May not matter on a street engine though.Besides, 3 carbs seem proper on a 6 cylinder.

Well--I haven't seen a homebrew intake constructed before--sounds like few here have either--been thinking about it this afternoon--why use sheetmetal? Ur an elecrical guy -- why not use aluminum tubing--conduit--mock it up and bend it-- or buy the bends and weld em up-- get flat aluminum stock for the ports-- and carb flanges-- drill the pipe(log) out with a drill press for the additional runners--rig up some heat plate to the botttom to run water or exhaust--need to borrow a tig or get some buddy to weld it up for you. Gotta flow better than some twisted up sheetmetal--just my thoughts--I'm sure we'll see sumthin Pescarellaesque.

Why steel??? Steel is inexpensive,I can buy mandrel bent tubing with short radii,can't make bends like that myself,I have 18 gauge sheet steel on hand,and access to a brake to bend it.I have some experience working with steel.I own a MIG welder.
If I use aluminum,I have to buy equipment to weld it,the aluminum is more expensive,harder to find and doesn't bend as easily as steel,and i never really worked with it much.Now the project becomes more expensive and I have to rely on others to do some of the work.
What I learned from years as a construction wireman;to keep on schedule for a completion date and to keep costs inline,never work yourself into a corner

Well I can't say that Barry Weeks didn't try to warn me about this--sumthin about u were seen openin ur wallet once and moths flew out.

I was thinkin that somethin a little more substantial wud be desireable but hey ur heart's plainly set on sheet metal --glad to see that you've got some gud flat metal and ur not usin corrugated stuff u pulled off the barn roof. Braise that puppy up--plow new ground--take pics as u go--we're gonna need pics of this masterpiece--tech tip time--inquirin minds may want to copy you with thier own enhancements.

I'm in favor of the steel, particularly while you're in the "cut and try" stage. Once you've got a good prototype in progress, you might think about duplicating it in aluminum, maybe with a piece of large-diameter conduit for the main runner, and smaller pipe for the connectors to the head. It would be lighter, and look better than welded steel, once the design is finalized. I think I'd use exhaust tubing of different diameters for the prototype.
Jerry

Buzz, I hear ya, but like Hot Rod Lincoln says,it's a one off design. If it functions nice, I may duplicate it in alloy or stainless. But hey it's just a engine, a coat of paint may be good enough
I'm familar with aluminum conduit, don't know if it welds nice however,electrial fittings are all mechanical.But ya can buy aluminum tubing and elbows made for industrial purposes that can be welded.
Aluminum has one disadvantage, it tends to run cool, making manifold heating necessary for slow speed fuel mixture vaporization.

Truckedup,
Go for it, and take lots of pics. Hey guys, the '53-'54 Vettes ran with three sidedrafts and they did alright. I think three are right for a six, I have a new Offy 3x1 manifold sitting on the living room floor just waiting for cruisin' season to slow down for the winter.
I just hate to cut the firewall flange to clear the back carb and linkage. Anybody had any experience with this problem? Oh, by the way, right now I'm running a 2x1 Offy intake and a set of cast iron Fentons (14,000 miles and climbing) so I know a little about multiple carb problems.

You could always make a manifold that had smaller balance tubes (smaller than the main tubes between the carb and the head) running between the carbs, that would make the low speed running smooth but not have the harmonics of a full plenum manifold.

Tony Go down to Homeless depot!
Get some plastic pipe and glue.
That will work!!!! Cheap

The JAG/SU thing was a joke!!!!
Two pots work fine on the street

You must just have too much time on your hands
HONESTLY

There are many good used manifolds around designed proven and cheap

I like Nicson but I understand....you may like
Stevenson......

With that lump on your pistons you are not going to go fast anyhow.....Why the fuss

Let's see, about 6 hours to fab up the manifold, about 50 bucks worth of tubing and 3 carbs I allready have. If it runs great,I learned something,if it runs like a pig, I learned something
Lumps? they are .125 high.flat top pistons yield 7-1 to 7.5 actual compression depending on overbore with a 121 cc combustion chamber 302 D shape head.
Dare to be different,build a manifold.

Tony I'm not worried you are building a GMC so you already know the taste of crow

The lump should actually be a slight dish,deck + piston and fly cut head You don't have a dyno....I do.....You get more compression and mileage along with more useable power.

All joking aside buy a nice old two pot....honest
this isn't a Chevy

or learn to OINK

Something else you need to make a bypass hose from the rear of the head to the otherside of the
thermo....this allows the water to syphon when you shut the motor off and also gets over the air pocket created at first fill and after stopping hot GMC's tend to steam at the rear of the head resulting in air bubbles trapped around the exhaust seat this push more water on and on.....

Use 'Water wetter' AND keep the motor cool GMC's make more power under from 160-180 and far less above

Jim,you're up early or late
I do have access to a dyno,the shop doing the machine work has a modern unit. When I complete the engine assembly,it's going on the dynometer for break in ,then jetting and timing power adjustments.I do electrical work for the shop owner in exchange for some of the machining and dyno time.
Slight dish?
120 cc head,12.8 cc's for the .060 head gasket,piston at zero deck,4.030 bore,4 inch stroke,do the math.Bring the piston up .020 above deck,do that math,just barely 7-1 actual compression.Yes, I did cc the combustion chamber.This ain't no low compression blower motor.
The GMC 302 D head is far better than an old Chevy 6 but nothing special in stock form,very similar to a Mopar B block head.
The rest of the engine is basically idential to a Chevy in design, 4 main bearings,coolant flow,bearing sizes.
I'm no expert,I build engines in my clay 140 year old clay floor cellar on an old engine stand,haul them up the cellar walk out and out to my dimly lit barn on a pallet dragged by a Jeep.
My 261 runs darn good for what it is,I have no doubt this GMC will very well thank you.

Tony I'm six hour ahead of you CET you can do the math
Access to a dyno doesn't mean you've done any development

WHO SAID ANYTHING ABOUT .020 UP more like .200 or .302 or .350 I'm not givin you any real numbers for free........ you can do the math

I have nodoubt that the GMC will run

It will run because it has more hp stock than your modded 261 and bags more torque

You do as you wish....I know you are hard headed
I was only suggesting stuff that'd work
Don't get your panties in a twist just cause you know I'm right

Do you know how many heads fit that motor?

Like I said SIX HOURS ahead..........

I'm starting to think we all need the nurse with the meds.

Here's a shot of a 250 with triple sidedrafts, owner said his is figuring it's over 300 hp. Just something to look at.
http://www.myoldtruck.com/photopost/showphoto.php?photo=2399&size=big&sort=1&cat=500

Heh heh heh. I wud pay some attention to that steam bypass idea. These GMC motors are notorious head crackers--might be a good idea to find a water jacket back there and run some pipe to the stat. Heck put a little coil in the line throw a kettle in the back--prolly be able to boil up a spot o' tea on a hot day.

I've heard some say that GMC had so many head designs(Ive seen four) cause they were tryin to invent one that wudn't crack--apparently never succeeded---maybe that 3/4 inch longer motor causes it-- can't say.

Chevy--we may sometimes be a little slower gettin there but at least we arrive with our head in one piece. :rolleyes:

Hey Tony, sounds like a fun experiment. Heck, whether it works or not I'm sure you will be on the version 2,3,.. Like I say, sounds like cheap fun. One thing I would do though is put a known setup on it to for breakin of cam/lifters then let the games begin.
Dennis

We put triple strombergs on a homemade intake, on a 302 in the early 70's , 0.100" off the head , 0.070 off the block , strange camshaft , 2" I.D seamless pipe in the intake ports , something odd like daimler inlet valves ,, memory is getting vague here ,, headers ,, in a '46 pickup .... GOBBS of torque

Jim, I'm listening to what you say,believe me,and will take your info into consideration.Hmm, I'm thinking we are both hardheaded
Buzz, old Chevy 6's are also crack heads.

Hey Tony

I know your listening and everyone else here should know we are friends

Yup I'm hard headed too....for most of my life Drivers trusted me with their lives. If you don't honestly believe in what you are doing and how you do it then you are sending someone out to get hurt or worse.Do remember the worst any of my real race cars finished was 5th once.And I never had a DNF. I'm proud of that but not too proud to continue to learn.

Bizzin I think the heads crack from abuse and overheatin' also from poor or old fuel...I've cut these all up they are pretty good castings

They do steam around the exhaust seats inside. Air
doesn't cool well and that maybe the problem
When the old guys tell you to slow the water down what they are really saying is increase the block pressure....this is the difference between the system/cap pressure and that assisted by the water pump and varing water passage size

Six hours ahead It's time for lunch

I built a single carb. downdraft intake out of exhaust pipe once to replace the stock updraft intake on a guy's '28 Chev. Tig welded up real nice. Used a real small Holley 1-bbl. from a early '60's 170 CI Ford 6. Worked OK until the building the car was stored in burned to the ground. (Fire was not caused by the car.) Intake pic at: www.stovebolt.com/weeks

6 hours? You must still be in Poland buying old train stations Jim? You and Z ever coming home? The hot tub must be getting cold by now.

Tony--

I've never seen a study on whose the worst head cracker. My personal experience is that I owned one GMC 228 and three chevys. Replaced 2 GMC cracked heads and no Chevy heads. Will admit that I worked the jimmi harder than the three chevies combined. Always thought the jimmi ran a whole lot hotter than the chevs--but the 228 was a lot older.

Buzzin, my 235 head has now been welded in 3 places that I know of. And the newer generation of sixes aren't any better.

Dang it, Jim. I was just getting ready to sit back and watch you an Tony duke it out. Thought maybe if I joined in we could all three sit down for a cold one afterwards. [img]http://www.myoldtruck.com/forums/images/smilies/beers.gif[/img]

Barry .......today we brought a 100 y.o. house onthe main square of this little town we are in
with a granite foundation and a brick workshop behind. It was a blacksmith shop. I have all the equipment stored away for years...the bellows is 6 ft long
BIG arched windows and doors

We made an offer on a flour mill built in 1922
all brick ant three stories with a tower
When the guys quit work the swept the floor and locked the door
It has a central system of pullys and big flat belts...even the tools are hung on the wall

Total time warp

The station went upside down...long story

Fred--

Heh heh heh; I swear ur worse than me--three places--hell; worse than Tony . I've at least gotta spare 848 head (uncracked by the way) under my bench.

What u guys are doin to these engines is beyond me--Kansas ain't even hot--I've never busted a Chevy head--old or 250 generation. Got the spare anyway--if I do crack mine--I'm gonna drill the new head for a bypass line. I've read that the military used to make GMC install those bypasses on military 302s.

Fred, no worries, Jim is just telling me who's boss of the GMC's
I like to argue engines even if I have to lie.I really don't know much, but I'm a good reader,own lots of technical books, so I sound like I know something.

Never learned to weld, so have to have this done by others.

Have done (or designed) several manifolds such as the one being discussed. Our solution is: aluminum conduit (thick wall) as has already been suggested; conduit bends for the angles; junk carbs for the flanges (Carter BBS and BBD made for Chrysler give a number of different 1 and 2 barrel flanges).

Have never had to do any bending. Just saw (or mill) the various pieces to fit, and take them to my local welding shop.

Manifold heat is not generally necessary with ambient temperatures above 50 degrees F. For lower temperature, fab a sheet metal aircleaner, and have two inlets: one from a fabricated sheet metal "stove" on the exhaust manifold or headers; the other to open air.

You need the equilization tube on the street!

Sounds like a fun project. Just do it!

Jon.

Hey--

Using junk carb bases is pretty crafty. Slick. :p

Conduit bends? I'm am electrician,1-1/2 conduit(1-3/4 actual OD) has a large radias when bent,the elbow would be way too long, and the ID is 1-5/8,slightly small for the GMC ports.2 inch pipe is the next size,2-1/4 OD ,too large.Buying short radias elbows in steel,stainless or ally is no problem.Mild steel is easy to work into shape if the ends need to be flared or swedged for airflow or fit
I don't want to put a lot of money into it,it's a prototype,and as Jim says, it may not work properly in end
I have a Mig Welder and torches, so it's gonna be steel,Mig or gas welded or even brazed if necessary

If you use steel, then the junk carb bases can be used for flanges. A very legitimate use for the Rochester "B" if you use one barrels. If using two barrels, Carter made a WCD for AMC that has a cast iron flange, and is relatively inexpensive at swap meets.

Maybe I wasn't clear in my description of "conduit bends". We use the preformed aluminum angles and then cut them to fit.

See no reason why steel wouldn't work, aluminum was just easier for us. Like I posted earlier, never did learn to weld; last time I tried, the result looked like the under side of 2nd grade school desks (second hand bubble gum)

Jon.

AND A HOT GLUE GUN....Tony you got some glitter?

Reading? Reading is good.Please remember....You can read a lot about fishin'Be a complete fishin' expert

AND STILL NEVER CATCH A FISH!!!!