The Stovebolt Site Forums The Shop Area Engine Shop 4 barrel conversion

Well, I've started the process of converting my 250 6 cylinder to a 4 barrel manifold. The manifold is from Clifford that I purchased off Craigslist and had sandblasted. I then painted it with some heat resistant paint. The carb that I'm going to use is a Holley 600 CFM that I modified as Tom at 12bolt.com suggested. He blocks off one of the primaries which he says makes it a 450 CFM, which is a better size for the engine. Check out the video on his website or youtube for what he suggests. One picture is of the original manifold and the other is of the 4 barrel test fitted. I hope to make some progress on the conversion this Saturday, if the weather permits.

I used the same manifold but with a 500CFM Edlebrock on the 292.

Did you think about using Clifford's recommended carb? I have been researching both single carbs from Clifford and dual carbs from Langdons. I have dual Rochesters now.

I'm using this carb because Tom at 12bolt.com demonstrated it on the same intake in a Chevelle wagon he has. Plus cost was a consideration as well. Although there is a story about that also.

On a different brand 300 CID 6 cylinder (that brand with the same name as a 50's country music star from Tennessee), I used a 400 CFM Carter AFB on an Offy intake. Worked very well, but was really too much carburetor for a truck on the street.

The van came with a single barrel Holley. The conversion made quite a bit more power and more than doubled the fuel economy! Like having one's cake and eating it as well.

Jon.

Back in the day I put two 650 CFM AFBs on a 347 Pontiac with a hot cam, headers etc. A pair of 600s came stock on superstock 409s. That was way too much. The guy at the speed shop said of the counter weights holding the secondaries closed until there was enough air flow, "Don't take em out." Of course I did, only to put them back in a week later.

Do a calculation on the air requirement for a 235 assuming 100% volumetric efficiency at the maximum anticipated RPM: Just for drill let's say 4,000 RPM. (crunch, crunch, crunch) OK here it is -- you'll need a whopping 272 cu. ft. per minute. I don't know exactly what volumetric efficiency you can pull from a hot 235 but it's way below 100%. So the issue with old inline sixes is not air flow but mixture distribution. Typically the end cylinders get a less dense charge than the center cylinders. That's why hot rodders of old used the two or three single bbl carbs. I'm not dissing AFBs they are an efficient carb and can be tuned to get excellent performance and economy but you still haven't solved the mixture distribution problem. Mopar took a good stab at it with the slant six. Compare its manifolding to a stock I-6 Chev or GMC. Night and day. Four bbls. on a slant six worked very well but the hot dogs still used multiple small carbs on the Chevs and GMCs. A friend from the Wayback Machine had a 53 Chev business coupe with a 1/8 over 270 jimmy sporting 5 Stromberg 97s and tube headers. It would blow the doors off another friends "Duntov" 283 V8 55 Chev coupe. One thing in their favor is that AFBs don't leak, neither do Holleys. By 1965 virtually all Stomberg 97s did.

If your vacuum gauge will hold 0 in. Hg at full throttle at top RPM, that's all the carburetor you need. My truck's 235 will hold about 2" vacuum at full throttle at 3,000 RPM on a hard 3 mi. 6% pull at 4,000 ft. elevation. That takes about 5 minutes and I wouldn't want to push it any harder 'casue there's another hill like that a couple miles further on. I have two little Zienith 228s on an Offy manifold with Fenton Headers. On a warm day (72F) the oil temperature will be about 220F at the top of the hill. Water temperature @ 180.

Tommy - he has a 250 rather than a 235 (crunch, crunch, crunch) so the requirement at 100 percent volumetric efficiency would be 289 CFM.

I would guess maximum V.E. at maybe 85 percent?

So required CFM would be 245.

But your points are quite valid, both as to required CFM and cylinder fill density.

As far as the multiple setups, due to the port configuration of the Chevy 6, three carbs are preferable to two. Size would depend on the manifold style.

Jon.

If your 1 or 2 bbl. carburetor fell to 3.0 inches of mercury at prolonged full throttle, the carburetor size is correct - that's the vacuum their CFM is rated at.
For a 4 bbl. it's 1.5 Inches Hg. Anything below that will not meter correctly, and at zero will not draw fuel from the bowl.
Very complex carburetors such as Weber DCOE, IDF, Dell'Orto DLHA, Mikuni PHH, etc. will continue to meter below that, but IIRC they still need at least .5".

Worked on this today during the lovely weather. I have to complete the wiring on the choke, figure out the return springs, and figure out how to connect the original gas pedal to the throttle arm on the carb. So far so good....

I got it all connected on Saturday and got it started. I've got some bugs to clear up before it runs good. But getting there.

I have an inquiry in with Tom at 12bolt about which intake (Offenhauser or Clifford), and which carburetor, the 750 cfm Holley with his conversion kit knocking it down to 562 cfm, or a Weber 38/38. I like the idea of the 750 running on 1 barrel until the secondaries open, but I have also heard good things about the 38/38. I want to keep my stock exhaust manifold, and will be running 2.25 exhaust with glasspacks on my 292. Eventually the head will get the lump kit installed. Tom is on vacation at the moment and should be able to respond to me next week.

I am very interested in how your conversion turns out Mike.

On a lot of those, they recommend a water source heat for the carb. Tom has the adapter.

I found that there was a vacuum port open yesterday. I plugged that. I also discovered that I had the PCV valve connected to the wrong port. I had it connected to a port on the manifold instead of the port on the carb. I changed that yesterday. It runs better, but seems to have a miss and I can't get the idle down below 1500 without it dying.

The intake that I have does not have water heat. I have the adapter from Tom, but it will not fit on this manifold. I don't know if that is part of my issue or not.

I rebuilt the carb my self, which was a first. So, it is possible that I made a mistake in the process.

Here is a link to my blog that has a video I took yesterday of it running.

It runs...sort of.

Now to get it to run better.

How about Fenton headers and dual exhaust?

Are you sure that the PCV gets hooked to the carb? Any setups that I have seen take vacuum from the intake manifold so it sees vacuum all of the time.

You have an air leak somewhere.

Positive Crankcase Ventilation not Positive Carburetor Ventilation. Hose is connected to the big port on the manifold.🛠

Ron, I have cast iron headers and dual exhaust.

Martin, So, I had the PCV connected correctly the initially, as in a picture earlier in this thread?

Well, I'll have to get more hose and return it to where I had it.

I changed the location of the PCV hose because of pictures I saw like this online. The port that is labeled Power Break or PCV port is where I moved it to.

That port should be just fine, it is doing the same thing as the port on the manifold.

Manifold or timed vacuum spark is a question the answer to which depends on what state of tune the engine is in.
They will run with either, but engines with long overlap and shaky idle quality tend to be "quieter" at idle with timed spark.

I think you may have overlooked some internal passage in the Holley which allow vacuum to find the blanked throttle, such as the idle and transition ports in the wall adjacent to the throttle disc. Have you tried closing the dead primary from above?
BTW: a 600 has all four throttle discs the same size, but the primary contribution is less than half because its venturi is smaller and the booster occupies some of the area. You have perhaps 76-78% of 600.

Mike, I think that port is considered manifold vacuum. Should be fine.

After talking to Tom at 12bolt, he thinks I have a vacuum leak. We discussed several possibilities and I'll begin the process of checking them off until I find the culprit. Here is a link I posted on Facebook of it running.

current running state

I found the intake bolts need to be tightened up more. With that done it is running better.

Glad you found it. When is the maiden voyage?

I'm planning to drive it some this Saturday to see how it works. There is a cruise in the area that evening that I hope to attend.