The Stovebolt Site Forums The Parking Lot Big Bolts 4 barrel intake manifold

Ok, has anyone tried putting on a 4 barrel intake on a larger truck? I know that the largest carb recommended is a 390. What kind of advice can you all give me on this subject?

What are you calling a larger truck? 1955 and up 3 ton with a V8 came with a 4 barrel. GMC used a Holley 4000. Chev a 4GC Rochester. If you are talking about a 6 cy, a 4 barrel is the absolute largest you can use unless its a really wild engine. 4 barrels are a lot less hassle than multiple carb setups.

Sorry about that...I meant to ask about a 1951 1 1/2 ton truck with a GM 270 inline six.

I'm not sure what your objective is with a ton and a half. A 270 will run pretty good with the correct Zenith.

My wondering is if it will improve acceleration on the uphill pull or not. I don't want a speed demon, just an improvement that is affordable. Any suggestions?

More carburation will help at higher rpm, but it will usually hurt throttle response at lower rpm. I think having everything tuned up just right would do more good than anything. There is a guy in New York who is very good at rebuilding the old Zeniths. http://www.cargurus.com/Cars/m-Larry-Isgro-Carburetors-Incorporated-sp188936

I have a lot more gear in my 2-ton (& a lot more booty to haul too!), but my 270 pulls real nice uphill in original trim.
I have a stock manifold and the factory spec Holley single barrel, and it pulls pretty well in mid to high RPMs. Myself, I dont think the 270 breathes hard enough for more than a 2bbl, the volume of air going into the carb needs to be enough to keep the air velocity high enough for the jets to function properly.
Having over carbed an engine or two, I can say that typicaly you only see the improvement at high RPM, and the rest of the time you're just sucking fuel down like mad. On my particular application (desert racing motor) I was running in high range most of the time so it worked out OK, but for day to day driving not so much,...

I'll have to agree with most of the posts here. A larger carb is not going to to help much accept with higher RPM. Low RPM may be lost, and that is where you need it for that truck. The original setup should perform reasonably well. I can't see spending the money on a new/bigger carb for your truck. You'll continue spending money for more gas if you do, and possibly have a power loss at low RPMs. Spend the money on a good tuneup and carb adjustment and/or carb overhaul. Remember that your truck is nearly 60 years old too.

Yeah, it is old. Some of these things I just don't know yet, which makes this website the perfect place to find out info on whatever I need to! Thanks for all the advice. I think I'll be going the original route and keeping it the way it was or is. Would putting in cast iron headers be a good idea? You know, for split exhaust?

Swap out the 270 I-6 for a 302 I-6 ?

Jump up to the 60'-74' GMC Big Block V-6's 305ci -478ci ?

Not sure just adding a larger carb will make the 270 do as you want . . . Its also a 60 year old 1.5 ton truck with a 6 cylinder Gas engine, So its not going to get up and go like 6 year old 1.5 ton truck with V-8 turbo diesel . . .

Mike

I have not placed a four barrel on a 270 however:

My shop truck for more than 400,000 miles was a non-GM 300 CID six. Offenhauser produced a 4 barrel intake for this engine.

Removal of the Holley 1 barrel (stock) and replacement with a Carter 9400 (400 CFM) resulted in better low end torque, much better power under WOT conditions, and surprisingly to me, about 50 percent better fuel economy on the highway. After the change, got 22 MPG at 70 MPH in a van.

(Opinion) Zenith made a much better carburetor than Holley, thus results will probably not be so dramatic.

Now, playing with some numbers:

The Zenith for the 270 is rated 215 CFM (2 barrel scale). This converts to 152 CFM on the 4 barrel scale.

The Carter 9400 is rated 400 CFM (4 barrel scale). This can be thought of as a fixed 200 CFM (primary side) and an as-needed 200 additional CFM (secondary side). Remember that Carter has an auxiliary secondary weighted airvalve. Even though you open the throttle, the secondary won't provide additional air/fuel until the engine actually needs it.

Thus the fixed portion of the 4 barrel is approximately 50 CFM larger than the Zenith. However, the primary side on the 4 barrel has two smaller barrels than the one larger barrel on the Zenith. This increases air velocity around the throttle plates at lower RPM making the small 4 barrel (with slightly more CFM) at least as efficient as the large 1 barrel.

Will it pay off financially? I cannot answer that, as I have no idea how many miles you will drive; nor how much you will spend for an intake. The 400 CFM Carters are out of production, and you will pay a collector price for one. With my 300 CID and 400,000 miles, the conversion paid off many times over.

And this post should not be contrued as a recommendation; simply the other side of the coin from actual experience on a similar engine.

Jon.