4bbl on a 250?

[shelby987]

I have a 250 I will be installing on my truck in the near future, the PO built the heck outta this thing, to the point where he felt a 7,000 rpm limit was very feasible......he planned on running a 650cfm 4 bbl on the engine. I am looking for a d.d. with power, but respectible mpg. I was looking for a 390 or 500 cfm ferd 2bbl, but came across a 390 4bbl instead.....would I be looking at similar performance/mpg setup with a 4bbl instead of a 2bbl or am I missing something?
I guess it boils down to this....whats the difference between a 4bbl 390 and a 2bbl 390cfm carb????

[truckdude239]

umm u got 2 extra bbl?

[shelby987]

I was asking more for specifics on performance, mpg, etc....but thanks for clearing that first hurdle for me

[ChevLoRay]

In my opinion, the 4bbl would be preferable, over the 2bbl, with the same cfm rating. The 4bbl would use the primaries until you kicked it into WOT mode. This would give you better mileage and have more fuel available for power, when you needed/wanted it.

I wonder about the 7K rpm desire. I had a '67 Firebird Sprint, with a 230CID OHC 6-cyl. It came from Pontiac with 10.5:1 CR, dual exhaust manifolds feeding a V-8 single exhaust system, a Quadrajet on a 4bbl intake and a hotter cam than the stock OHC-6's. I think it was supposed to be good to 6500 rpms. In '68, Pontiac went with the 250 block, and added the goodies. The horsepower rating didn't change, from the 215HP that was advertised. I don't know what the SAE HP rating would have been, since it wasn't until '71-72 that SAE became the standard rating system. I guess that having 7 main bearings is the key to having higher rpm capability. Older, i.e, earlier, designs used 4 mains and that led to some crankshaft "whip" as rpm's increased. One thing to watch is the harmonic damper. On some 292's used for drag racing, one racer had a problem with his engine when he seemed unable to keep the harmonic balance on the crankshaft....or so he said that was the problem. There could have been other factors that he didn't disclose. But, at least for him, it was a problem.

Sounds like a plan, though. A good split manifold, or headers from Clifford, with their 4bbl intake and you should have a unique-sounding engine. I always liked the sound of a straight six, with duals, especially when running glass-packs. They really sound sweet when you "wind 'em up and let 'em rack off".....having that occur in some old downtown settings where the echo really accentuates your presence to the local gendarmes.....well, now you know what NOT to do. My old TR-3, also caused the plate glass windows to vibrate. Ah, the joys of youth.

Go for it. You only live once, and whatever you have.....well, you can't take it with you.

[tmcgrawlookalike]

i`d give it a try.we had built a ford in-line 6 for a 65 mustang(years ago my buddy had worked for a race engine builder named jimmy nix here in okc and they always tried building something different once)using chevy rods/pistons,howards solid cam,etc.....we used only a 600 holley on a offenhauser intake for it.it worked out good,so a 390 may be ok for yours.just get ready with a jet pack and power valve assortments.good luck!!

[shelby987]

trying to find the best of both worlds I guess.....was debating 390 or 500 cfm carb......thinking since I got the 650 too, maybe I will just swap depending on what I feel like doing....one for race/bogs, one for daily driving......
Just to be clear, the 2bbl I was looking at, also had vac. secondaries....so I don't know if that changes any advice above.
It all comes down to the all mighty dooler.....i mean $$$.
btw, the engine already came with the offenhauser intake and a set of headers, though I might swap them for shorties depending on how far down they hang. Thinking about running glass packs infront of an h-pipe, then flowmaster behind the h-pipe, its a f@rd trick that they used on the old SHO's, and the reviberation when the secondaries opens up is priceless....I will let everyone know how it all turns out!

[LONGHAIR]

Just to be clear, the 2bbl I was looking at, also had vac. secondaries....so I don't know if that changes any advice above.

It has what???????????????

IIRC the CFM ratings are different for 2V Carburetors. I think it has something to do with the level of vacuum at which they are tested. So this is not a linear comparison. (IE, a 500cfm 4V and a 500cfm 2V flow at a different rate)

I would say that to find the ideal carb, you really need to try both. Generally I think the 4V would get better mileage.........if you can keep your foot out of it and worse if you can't.

[Palf70Step]

I think the 650 would be way too much for that engine. The 390 would make a better DD. To set the 4 bbl up right you have to change from a rod throttle linkage to a cable linkage. The 4bbl will give you overall better mpg and performance.

[zeeklu]

I have a 390CFM Holley on my 292 and it works well. Chris

[PanelDeland]

The 650 seems like too much for a DD but it would probably be great for WOT power runs.I would think the venturi signal would be off for part throttle with the 650.JMHO

[LUV2XCLR8]

Originally Posted by zeeklu

I have a 390CFM Holley on my 292 and it works well.

Nice pic I would also say no larger than a 500 cfm myself

[jimfulco]

2-bbl carbs are rated at twice the vacuum as 4-bbls. The flow rating of a 2-bbl must be multiplied by 0.707 to see how it really compares to a 4-bbl carb. A 390cfm 2-bbl will flow like a 276cfm 4-bbl.