I bought the Nov issue of Hot Rod Deluxe for the Yblock build-up article. They show dyno charts for the same engine with two size carbs and with and without headers and a step-up the the Blue Thunder manifolds. Unfortunately the dyno charts start at 3000 RPM and they do not give the compression ratio of the engine or a manifold vacuum value. The TAKE AWAY for me was going from a 465CFM Hollry to a 710CFM Holley gave SIGNIFICANT IMPROVEMENT at 3000 RPM and up. All of you that keep repeating advice of using a 390CFM Holley should read this article. If you have a performance cam then if you don`t use a carb that will support it, you are wasting your money. AS always my opinion. Pete

There’s still a group that inaccurately equates larger carbs to flooding an engine with fuel or automatically running richer.  As far as the article goes, it did still leave some questions as to how much tuning was actually performed with each combination.  But I’m in total agreement that with improved air flow in an engine through porting, cubic inches, better intake, etc. comes the need for a larger carb if going for higher rpms.  But in defense of smaller carbs, I recently dynoed a 308 inch Y that had a Blue Thunder intake with a 750 cfm Holley carb and ended up picking up an additional 12 horsepower by simply downsizing to a Holley 600 carb.  Both combinations were optimally jetted.  It all depends upon the combination and the goals of the customer in what the ultimate use for the engine will be.  If it’s simply destined to be a cruiser, then a larger carburetor that optimizes top end performance may not be the best combination.

Lorena, Texas (South of Waco)

As has previously been pointed out there are several glitches with the H.R. Deluxe article. The initial 10 h.p. increase with the carb change is described briefly but not shown in the Dyno results. And we may be talking apples & oranges here re the 390 cfm carbs vs the 710 carb in the test. The test engine isn't a 292 or so, it's a 34 inch bigger stroker of 326 inches. I believe that makes a fair difference. And the 2nd test gains (shown) includes Headers & more timing... As for Brodie's question about the original H.P. of the std 312 engine, it was rated at 245 in 1957. However, we need to keep in mind that BHP ratings prior to I think 1971 or 1972, were gross ratings, done devoid of generators & water pumps etc. Someone here can maybe be more specific about net H.P. ratings.

Paul

Ted; I need some insite. Your engine that picked up HP with the smaller CFM carb. What was the cam? Duration, lift, lobe seperation and at what RPM? Any reading on manifold vacuum? Thanks Pete

The specs on the camshaft in question are 264°/272° adv duration, 228°/238° dur at 0.050”, ground on 110° lobe centers, and .480”/.512” lift.  Have included a pic of the cam card but may be downsized too much to read.  Don’t recall the manifold vacuum or the rpm ranges off hand but it was good enough that there were no issues with power valves opening prematurely and the engine idled just fine with all carbs that were used during the session.  This cam was designed to be used with an automatic trans with a small stall on the converter.  The vacuum sensor was hooked up on the dyno so that information was available at the time of the test but the saved data on the computer will be full throttle data which will not be representative of idle or cruise situations.

 

Doing a search for “Hot Rod Deluxe” on this site, also brings up some prior threads of which here’s one of interest.

http://www.y-blocksforever.com/forums/Topic16849-6-1.aspx

 

Lorena, Texas (South of Waco)

I ran Dyno tests on my 301 cid Y it definitely made more power with a 600CFM 4176 Holley than a 750. The tests were described in an article in YBM towards the end of 1998.  It may be that a 390 CFM carb is too small optimum power on well modified Y's but most of 'em aren't big enough or hot enough to run 750CFM on the street.

Doug T

The Highlands, Louisville, Ky.

I did a 390 on my 272 but it was mostly just for looks since the engine, other than a mallory electronic ignition, is stock as a rock and I just wanted a 4bbl. There is no performance involved, however, I will say it sure perked the old 272 up. Another reason was as decent fuel mileage as I could get. Back in the day, when I was racin' the old '57 all of the time I used a Carter AFB off of a '62 Pontiac. Don't know what the cfm was but it worked great! Had a manifold with 2 WCFB's and it worked fairly well too but I think maybe not as good as the one AFB. That old '57 was a Chevy eatin' SOB though ..

Doing Fords for 45 years. '56 Customline Victoria

E.J. in Havana FL