Wanted: Advice on 258 build-up

Farna will my bottom end torque suffer with this big of a carb a 600cfm?

Yes, it will suffer some. If you build a racing only six you can run the 600 cfm carb fine. That wouldn't be an easy street driver though. For a mild six (say around a 250-260 hp 258) it's too big. You can tune it to run, but you'll hardly ever use the secondaries. I've run a Holley 390 and the engine felt like it could use more carb, but about 500 cfm on a 4V (350 on a 2V) is about it. I think the formula John is using for carbs is based on 3" of vacuum, which is the 2V standard. Engineers found that a 4V carb wouldn't hold 3" of vacuum on a flow bench with all four barrels open, so they use 1.5" for them.

To convert a two-barrel rating into a four-barrel rating, divide the two-barrel rating by 1.414. So a 500 cfm 2V is equal to a 350 (353.6) 4V.  The 390 4V = 275 cfm 2V, which is a bit small for max power, but great for economy and then power when you need it.

To convert the four-barrel rating to a two-barrel rating, multiply the four-barrel rating by 1.414. So a 600 cfm 4V = 850 (848.4)  2V.  NOW you can see why a 600 cfm 4V is too big! The 465 cfm Holley Truck Avenger = 657 cfm 3V, snd is the largest carb I'd recommend for the 258. To run that you need something like the Isky 26x Supercam and a good 2.25" exhaust with turbo muffler, or 1.75" duals. Much larger on the exhaust will hurt instead of help also.

I did catch some chatter on here about a hydraulic cam with lift at or above .500 being too much for today's lubricants. Well, maybe...but...

My current 232 runs a Crane H-272-2 hydraulic cam with 216 & 228 degrees duration (@.050 lift) and .484 & .512 lift. I once talked to a guy at Crane (the old Crane - before it was reconstituted) and he told me the only way I was getting away with it with the current set-up is because my cranking compression is so low due to the stock pistons and head. Anyway, this is a stock and original bottom end with currently about 70,000 miles on it, rebuilt head with Chevy valve springs to match the cam (machinist didn't tell me which ones he used), stock valve train otherwise, Offy dual plane intake, Holley 390 carb, Clifford full length headers (got 'em just before they switched to shorties only), dual 2 1/4" exhaust terminating just before driver's side rear wheel, and turbo mufflers. I'm not actually sure what the stock rear gear ratio is as I haven't been able to research what was available for this car, but I once compared wheel to differential revolutions and calculated it to be somewhere around 2.8:1. Anybody know?

The point of all this is that I assembled all this 14 years ago. You could make an argument that there is less zinc in today's lubricants than back then, but I doubt that the difference is that great. The motor still runs sweet and I've had no problems. I installed the cam with Crane's assembly lube, followed correct break-in procedures, and have always used 4 quarts of synthetic blend oil and two bottles of STP at every oil change which I do once every spring (car sits all winter). Have I been good or lucky? You decide - I'm just sayin'...

And Ive read Smokey Yunicks book. I also read the edelbrock book, and Ive also read when the auto jounalist would interview him..........I'll grant that Yunick knew some things but what he did is now somewhat out of date, and on certain things he was wrong.

1986cj7 wrote: purple72Gremlin wrote: uncljohn wrote: If you run the numbers on Carb Size for a 258 it comes out a little over 375 CFM,  A 390 Holley 4bbl or a 400 Carter (or edelbrock) AFB is a good size and can be tuned well. A 600 is just flat too big.  If you do not know how to tune it and take advantage of it, everything will suffer. I've run in line 6's with 650's.  They run richer than a cob and with bog and flat spots.  But the bragging rights were great They ran lousy. Mores law towhit: If a little is good more is better does not work for cam shafts and carburetors. but your choice. I rode in a 74 Hornet that had a stock 4.0 in it, it had a 600 Holley DP on a 4bbl intake, it had the 1974 points distributor in it, and it ran very well. It all comes down to tuning. Ive seen other set ups thats not supposed to work and they do. The carb I'm using is a DP mechanical secondaries with offey dual plane intake.  The shop doing the work recommended the carb for the tuneablity.  And yes I still thing the 600 is to much carb for a 258.  Now is it the best I don't know and bragging rights I could care less.  But what I do want is the best performance in this motor.  With $2000 in just parts and machine work, I want the most out of this motor I can get.  And this is why I want all opinions.  And thank you all for your posts.  Sorry to the op I didn't want to hijack your post but I'm in the process of building my 258 at the same time.  And have my block and head being machined now.

was just mentioning it. You can run whatever you want. I just have a stock YF on my 258