The Stovebolt Site Forums The Shop Area Engine Shop Chev. 250-6 w/ offy intake and Holley 600 cfm

I hope this is the correct forum, but I am sure some of you can help.
I have a 1948 3600 that I have owned for about 6 yrs. It can with the current 250-6, with Fenton headers, and an Offy intake and a 600 cfm carb. Otherwise the engine is stock. I switched a TH350 for a 200R4 and it retains the original 3/4 ton rear. With overdrive, lock up torque converter and tall 265/70/15 tires I can run about 65 mph at 2200 rpm.
I know that the carb is too big for the engine. My mechanic (I am not the skillz guy) has jetted the carb as far as he can and overall it runs "ok". I have resisted the urge to throw money at the carb situation, but I really think a smaller (390 cfm)carb would help the engine run better. I realize this may be a general question, but is it worth $500+ labor to address the carb? Will I really get better response/mileage/etc from the change?

What say y'all?

What would it cost to go back to the original carburetor and manifold? A lot less than buying another incorrect carb, I'll bet!

At 5000 RPM and 100% volumetric efficiency (both of which are never going to happen) that engine will only be flowing 361 CFM. A 4 barrel of any sort on a Chevy inline six is a solution to a nonexistent problem.
Jerry

You are approaching this from the wrong angle. The 600 Holley is not too big; your engine is too small.

G.M.C. had a 503 CID 6 in the early 1950's. Change the engine, the 600 Holley ought to be fairly close Maybe just a little large for the 503!

All joking aside, Jerry has the right idea although I would probably suggest a Zenith from a G.M.C. 270 on your 250 and the original 250 manifold would be just right.

Jon.

$500 labor to jet a carburetor? You need a different mechanic.
You can completely disable the secondaries, install a 4.0 power valve in the primary, and run it as a 2 bbl. for about $20.

There's a program in the public school system known as "R.I.F." ("Reading Is Fundamental") It supposedly works wonders helping inner-city kids learn to read. The OP indicated "$500.00 PLUS labor", which tells me he's considering buying the wrong carb for $500.00 and then paying somebody to install it. P.T. Barnum had something to say about people like that, didn't he?
Jerry

How much have you already paid him for it to run "ok"? The price of the 390 Holley is quite a bit higher than the average 600 Holley, but is the correct remedy for your problem like it or not. It'll be money well spent.

Have you thought about something other then a Holley? One of the mid 60's Rochester 4Jets would make a good choice. There are some pretty small CFM units out there, and you probably will never run into another 250 inline 6 with one on it.

Just for grins

G.M.C. used, for the most part, Zenith carburetors; and Zenith was a company that actually posted CFM ratings.

So just what size carburetors did actual GM engineers think was necessary for various 1952 G.M.C. 6 cylinder engines?

Since these are all 1 barrel carbs, the standard rating scale at 3 inches mercury (Hg) was used.

228 CID - 188 CFM
248 CID - 210 CFM
270 CID - 214 CFM
302 CID - 224 CFM

But wait, as the obnoxious TV commercials burn into our brains; we are now speaking of a 4-barrel conversion, and by convention, 4-barrels are rated at 1 1/2 inches of mercury (Hg).

So doing a conversion by dividing the 1 barrel rating by the square root of 2 (1.414) we obtain:

228 CID 188 -> 133 CFM
248 CID 210 -> 148 CFM
270 CID 214 -> 151 CFM
302 CID 224 -> 158 CFM

More difficult to deal with Rochester, as venturi sizes are not specified, but the largest Rochester monojet was rated at 250, converting, this would be 177 CFM on the 4-barrel scale. I do not know if this large monojet was used on the normal 250 CID Chevrolet engine. I would guess this venturi was used on the governed truck engines.

So a 390 CFM Holley is more than double the very largest Monojet, and 2 1/2 times the size of the carburetor GM engineers thought was adequate for a 270 CID. And the 600, to paraphrase my college calculus instructor from so many years ago "the proof is intuitively obvious, and the solution is left to the reader".
Jon.

Jon, you're wasting valuable electrons, just like I do when I start preaching common sense to people who think "bigger is better" where carburetors are concerned. Aren't they the same guys who are sure that the next lottery ticket they buy is going to make them a multi-millionaire?

Either way, they're lousy at math!
Jerry

Jerry - I actually enjoy doing the math. It was one of my favorite subjects all through school. And when teaching comp sci, I used to push my students to take more math classes.

One of my favorite math problems, and anyone who cannot get the answer in less than 30 seconds, needs more math; is to sum all of the integer numbers from 1 to 100. That is 1 + 2 + 3......+ 100.

Quick, what is the answer?

Math, other than having a very rich relative, or being independantly wealthy, is a car/truck enthusiast's BEST friend!

Jon.

HINT:
1 + 100 = 101
2 + 99 = 101
3 + 98 = 101

Evan - REALLY good hint

Solution:

1 + 99 = 100
2 + 98 = 100
.
.
.
49 + 51 = 100

49 times 100 = 4900 + 50 + 100 = 5050

And for you math majors (that can remember equations)

Sum = n times the quantity low plus high all divided by 2

or S = n times ( low plus high ) / 2

where

S = the sum
n = the number of terms in the series
low = the low number in the series
high = the high number in the series

So sum = 100 ( 1 + 100) / 2
or 100 times 101 / 2
or 10100 / 2
or 5050

But I would never consider letting my students use the equation until they understood its derivation.

And to the OP, my apologies for the diversion, but a little application of mathematics shows why your vehicle will run better with something other than your existing unit and your proposed possible purchase.

Jon.

So, I agree the simplest and most effective solution it to put the stock intake and carb back on the engine.

However, just for sake of discussion....GM had a factory 4 barrel for the 265 v8, correct? I know different number of cylinders, four barrels not one, and totally different RPM capabilities. But the displacement would be similar-ish. Particularly if the 250 was rebuilt over-bore.

If one was hellbent on a 4 barrel for a 250....would the 265 carb be about the closest shade of lipstick for the 6 cyl pig?

Jerry - are you there? Your turn!

Jon.

If you couldn't tell by all the wiggle words, I have no intention of trying it. And I completely understand it's a fool's errand. Just curios if there's any carb on earth that could sort of work at some sort of acceptable level.

Please don't hurt me!

JW - I saw the wiggle words.

Internal combustion engines are amazingly adaptable.

A good friend, that was a Carter engineer, to prove a point, put a 750 Carter Competition Series carb WITH MANUAL CHOKE on a 2000 cc pinto! And made it work, after a fashion. He had to control the choke with one hand, the steering wheel with his other hand, and shift with his third hand!

So it WOULD run!

Another example: if you owned a hayfield in southern Illinois, of several thousand acres, you COULD haul in the hay using a Toyota pickup! But a semi with flatbed might work better.

If your 250 was a fully-modified race engine in a trailered race car, and you left the line at 5k RPM on the dragstrip, the 390 Holley would probably work. But this is the engine forum, not the high performance forum and especially not a race-only forum.

And please take no offense, but "sort of work at some sort of acceptable level" is not in my world. No recommendation.

But to partially answer your question: as far as I am aware, the smallest 4-barrel produced in the USA was a Rochester for the 195 CID Pontiac Tempest. They are quite scarce, and very pricey; and I certainly would NOT recommend one for the 250.

Jon.

Thanks for playing along, Jon. One more silly hypothetical question and I will let it go.

If one was hellbent on putting a 4 barrel on a GM 250 and determined to make it work....what would be the least-terrible carb match?

JW51, Jon said it's not in his world. Let it go man. You are chasing 🦄

If you just want eye candy, take another look at panic's earlier post about disabling the secondaries.

If you just want eye candy, take another look at panic's earlier post about disabling the secondaries.

Ummm...necessary? If we just accepted what was "necessary" in life.....