The Stovebolt Site Forums The Shop Area The HiPo Shop GMC 248 4bbl Manifold

Hi Guy's, been away from the Stovebolt for awhile, as well as the truck. I contacted Patricks about their manifold, apparently no longer available from them. I saw Jim Carter has one that mates to a 390 CFM Holley. Definately a pricey upgrade from them. Basically I just wanted to ask the group if anyone has used this setup and how the fit was. I've got a pair of fentons on it now and wanted to make sure the 2 would go together.

Look forward to the insights.

A 4 BBL on a stovebolt is an answer to a nonexistent question. The engine can't flow enough CFM through the restrictive intake and cylinder head ports to make the 4 BBL necessary, or desirable. It would be like putting a single 1/2" link in a 1/4" chain. Unless the air filter, carb, intake, ports, camshaft, valves, exhaust manifold, and pipes are engineered to work together, putting a high-flow carb and exhaust on an otherwise stock engine is just eye candy.
Jerry

I'm no expert on port size differences between the 248/270/302, so this might be irrelevant without some sort of step down rings....

But here's a GMC 302 - 2 barrel manifold on EBay. Seems like a reasonably fair price, too. (Not mine)

https://www.ebay.com/itm/122015849536

Might open up some different carb options as long as you stay CFM matched.

As Jerry said there is no performance value to a larger carburetor on a stock Stovebolt 6. They just look cool and make people at the car shows ask questions.
🤓

To put some numbers to Jerry's comment:

GM used a number of different carburetors (of different sizes) on the 248. Different size carbs were/are used to maximize torque for heavy duty trucks or fuel economy/performance for light duty trucks or.....

The point being, a group of engineers made some decisions.

If we look at one of the larger of these carbs, a Zenith type 228 with a 33 millimeter venturi, we find from Zenith data that this carburetor would flow approximately 230 CFM.

However, quoting from a very obnoxious TV commercial...BUT WAIT!

CFM for 1 and 2 barrel carburetors is measured at a vacuum represented by 3 inches of Hg, whereas the Holley 390 CFM is a 4 barrel which is measured at 1 1/2 inches of Hg. To convert from the 1 barrel scale (the Zenith type 228 is a 1 barrel) simply divide the 1 barrel rating by the square root of 2 (1.414) reference: http://www.thecarburetorshop.com/Carbshop_carbsizesandCFM.htm

So dividing 230 by 1.414 we arrive at the figure of 163 CFM (the figure that the GM engineers determined was on the high side for necessary air flow for a 248.

So the small 4 barrel Holley 390 is more than twice the CFM that the engineers determined was the maximum size needed for a 248!

OK, so if this thing is going to be placed in a trailered race truck, and umpteen thousands of dollars are going to be spent in R & D of special engine parts to either (A) SIGNIFICANTLY improve the volumetric efficiency of the engine or (B) more than DOUBLE the useful RPM limit of the engine then MAYBE the small 4 barrel would be useful.

But of course anything is useful to bench racers.

And Martin, no offense, but must disagree; they DON'T look cool to non-dreamers. They just scream that the owner is descended from SGT Schultz!

Jon.

Jon, no offense taken. Everyone has an opinion.😆

Thanks Gentlemen. I do appreciate the input and the math review on CFM and intake That is some good info to have. Looks like I'll stay with the stock single and save some hard earned $$.

I do appreciate the insight and knowledge this group brings.

Personally I prefer more "go" without much perceivable "show". That is why I went with a T-5 transmission behind a completely rebuilt 235. Those extra gears have a phenomenal impact on what you can do with these old trucks. Every time you shift gears, you are in the middle of the power curve of the 235. No lugging, no screaming RPM.

That's how 216's and splash-oiler 235's managed to pull the big rigs around- - - -Brownie transmissions, 2-speed rear ends, and gear ratios that allowed the engine to stay in its best torque range regardless of road speed. My 55 HP Massey-Ferguson tractor does the same sort of thing with a 3 speed trans, a high and low range, and a fluid coupling between the engine and the manual clutch sort of like an old Dodge Gyromatic. The fluid drive can be locked out for regular direct-geared chores or enabled to give more pulling power.
Jerry

KD - next to having a favorite uncle who is independantly wealthy, when working/modifying older vehicles; math is generally your best friend, closely followed by physics.

Glad we could all help.

Jon.

Even rich uncles get tired of packing sand down a rathole eventually- - - - -they didn't get rich by spending money foolishly!
Jerry

6=8 has an Intake&Headers for the GMC 248.
He also sells an adapter to use a 4 barrel.
http://www.shop.cliffordperformance.net/GMC-270-302_c6.htm

Yes the Intakes & Headers are the same for the 248,270,302

Not to say that no I6 can use a 4bbl but certainly not a Stovebolt. The Chrysler 225 Slant 6 with the Hyper Pack and 4bbl was a hoss BUT it had a 12 port head that could use the increased flow; both intake and exhaust. The Aussies have I6 Fords with 12 port cross flow heads that are real screamers. Our beloved Bolts are super for what they were designed for, rock solid engines with good torque at a low rpm. For years when someone wanted a hot six banger I told them go foreign or to hunt up a Slant 6 or a 240/300 Ford and we would build them one. Now the General has gifted us with the Atlas (or 4200 Vortec) that gives an I6 that is a hot rodders dream and it's ALL Chevy. There is some work being done on manifolds for carburetors, and how to use or replace the VVT cams for these engines to help out the low techies (like myself). The prices are coming down in the salvage yards so expect to see them in the old trucks.

I'm mulling over a low-tech conversion of an Atlas engine involving retro-fitting it to carburetion and a distributor, and finding a place to lock in the VVT cam system for a good compromise between torque and HP. Yes, that's a giant leap backwards, but it could open up all sorts of possibilities for people who don't necessarily want to go with sequential port injection and computer-controlled ignition timing.
Jerry

Jerry,
If you come up with something be sure to share it as there are real possibilities here. MSD has stand alone crank triggered ignitions for a lot of the computer controlled engines and this is good if it hold up well. No timing chain stretch or gear wear to contend with. My 34 Chevy with the 2004 Mercury Marauder modular engine has the MSD set up and runs strong but has very little total time on it so can't give a big thumbs up yet. Have a rolled Trailblazer with a low mile Atlas that is going to find a home in an AD someday but waiting on ones like you to do induction R&D.

Jerry - how about fabbing an intake to use 3 of the Carter type YH carbs. The Corvette versions are out of sight price-wise, but the Dearborn Marine versions, while not cheap, are at least reasonable in price; and they come with automatic (hot air) chokes. And since Dearborn used two of them on a 312, I am certain they would flow well enough for the Atlas engine.

You could convince the uninformed that you have a 235 with the Corvette package that you upgraded to auto choke!

Jon.

That's a definite possibility, but I'm also intrigued by the Keihin vacuum-piston constant-depression side draft carbs from a Harley Davidson that someone mentioned. Since a 1-5-3-6-2-4 firing order only requires one cylinder's worth of volume through each of a 3-carb setup with siamesed ports or a 2-cylinder manifold to each cylinder pair, it should make it pretty simple to tune a carb to the right mixture. That's always been my big gripe about dual carbs on a stovebolt- - - -too much sharing of mixture between cylinders.

For the real P.T. Barnum bunch, I could claim the engine is a 235 with a custom-built 12 port DOHC cross flow head on it!
LOL!

Jerry

Jerry - remember that when working with an IR manifold, or a plenum manifold with plenums feeding less than 4 cylinders; the air flow requirements for the carburetor(s) increases dramatically.

What do the intake ports on the Atlas look like?

I have not run any of the side-draft Keihin carbs. However in a different lifetime (seems like, anyway) we sponsored a 2.0 liter Ford race car with 4 big Mikuni motorcycle carbs. Tuning wasn't that bad (I had been tuning and synchronizing multiple carbs on British and Japanese cars for years); but one racing season, and the slide carbs were totally worn completely out!

Yes, it ran great; but the next year when the racing body outlawed multiple carbs (because we won the championship) it ran quite a bit better with a single Carter WCFB! We never told them we were going to switch anyway when they changed the rules, so we got to complain about the restrictive rules (We won the championship again!).

Jon.

Several years ago and before the eyes stated to tank on me I got a billet dual plug hemi chambered OHC 302 GMC head about 60% done. No water jacket so was a dragster only unit. At that time I was looking at side draft Mikuni's but Jon advised that when new the Mikuni was low priced, easy to tune, and ran well but longevity wasn't a strong point. Since they are low priced maybe using three for development could be a possibility and then upgrade to something better. Without a dyno I would NEVER tangle with Webers again. Man I hope your project once started comes to fruition.

Evan, how does the crankshaft-triggered ignition system handle spark advance? There has to be some sort of computer involved, or so it seems to me. I could use a 3-coil distributorless setup that fires plugs on both the compression and overlap strokes, but getting the proper timing advance would require a bunch of sensors and at least a rudimentary computer, something I'm hoping to avoid.
Jerry

And that, folks, is the way to highjack a thread.

Throw them out cletis. They are a bad influence. They got me thinking about my hotrodding days, and nothing good can come from that! (Oh wait, this isn't cletis' General Truck Talk. Never mind.)
Carl

Carl- - - -a conversation about hotrodding between the forum's most senior pot-stirrers just proves that getting older is inevitable- - - - -but growing up is optional!
LOL!
Jerry

Back to the 248 manifold issue, GMC's used two different size intake port sizes, so, while one port size manifold will fit all, not the best fit. Early(smaller) engines were smaller. Patrick's old catalogue has data.

Ed

I robbed from my 270 to build my GMC302 which has Clifford Headers, Clifford 4barrel Intake, HEI ignition, camwork, 3speed with manual OD. 58GMC 1/2ton truck
I am looking at 12bolt.com for my carb. A step up from the Holley390.He has modified a Holley 4barrel4160 to run as a 3barrel 450cfm. So essentially it's a single barrel carb until you step on the pedal. Go to his site and watch the video.

Danny, a 302 at 6,000 RPM, 100% volumetric efficiency, and zero manifold vacuum would only draw 524 CFM. At 90% VE and 5000 RPM it's only going to need 393. Is there a reason the 390 wouldn't be just about perfect for your application?
Jerry