What are the pros and cons of these two carbs when running with an Offy intake, 2.76 ratio, dual Clifford headers, mild rebuild, 244 cam (mileage, performance):

Holley 4 bbl 395 cfm and the 2 bbl 500 cfm

How much of a difference will there be in fuel economy as compaired to my Carter BBD 2 bbl.

The 500 cfm 2 barrel is overkill for a mild engine, the pressure drop with the big throttle plates won't be nice (in the city, you may never get out of the transition circuit and use your main circuit...)...

The 390 Holley 4 barrel (#8007 with vac secondaries), works OK, you may have to do a bit of 'tweaking with the secondary spring and jets to wring out everything you want from it, but if you keep your lead foot out of it...you will see similar results to the carter 2 barrel....

The better choice on the two barrel would be a Mr. Gasket #1937 and a Holley 2300 2 barrel (like # 80179 off a Mercury Monarch/etc...)

The other thing that can help a bit is shedding the clutch ruining gearing and get something more kind to your mill and setup, like 2.94/3.23's that way you expend less gas trying to get the car motivated (although you may expend more gas because the car is more 'fun' with those gears and you may be tempted to tromp the gas at every green light...)...

-D.Idiot

Thanks.

What is the CFM rating on the Holley 2300?

The 'normal' 2300 is 350 cfm @ 3" Hg, the two 'monster' versions are 500 and 625-650 cfm (for those 2 barrel only classes of racing)....

Most Ford small blocks in the 70's got the 2300 in 350 cfm... the 80179 is particularly neat since it also has the 'economizer' venturies that will help create better 'signal' for the carb to respond to demand and throttle changes (better for smaller engines). I just finished rebuilding one of these to try on either a super six intake with the 1937 adaptor, or to plate off my Hpak intake and see what I can get for max MPG....

-D.Idiot

So, your saying my best bet would be the 2 bbl Holley 2300 instead of the 4 bbl 395 cfm.

Let's break it down:

The cam is almost 'stock' except for a smidge more lift than what you have right now. The small 2 barrel will give good economy and only has two jets to change/easy to tune. The 4 barrel can give good economy, but you have to tear it apart and replace springs and jets for a week while driving around with a vacuum gauge and checking your plug color on and off the highway (you'll also have to 'lock out' the secondaries to tune the primaries), for best results. (unless you just plug and play it and post a couple days later about the 'bog' off the line and the black puff of smoke and bad mpg you're getting...but the power is good once you get up to 2000 rpm...)

Another problem exists with the rear ratio you listed. The ratio does well as long as you're on open highway doing 80mph....off the line it's not great... the 'grunt' off the line may cause enough load (and vacuum drop) for the power valve and maybe the secondaries to open after each stop light (9-10 mpg in town at best).

If you have everything 'jiving well' then the 4 barrel can be tuned to give you the best performance with good economy, but to get there it won't be 'plug and play'.... The best I've seen from the 390cfm and the items you're putting on was in a mid-60's A-body wagon that was dyno tuned with 3.23's and a 3 speed manual he was getting 26 mpg, but his dyno run was only 80hp at the rear wheels, and his actual 1/4 mile time was in the high 17's... With what you have, you should be shooting for what your Carter is giving you currently... (21-23 mpg? highway)

before making a final decision, get a Holley tuning book and read it over, not only will it help you figure out 'why' it works but what to modify and how to rebuild it (even why to select the carb).


Take your pick on either carb and what your technical ability to 'tune it'. Lucky you have a mild build, read over my hyperpak posts and see how much fun it is to try to get the carb to fit the mill....

-D.Idiot

p.s. I have slightly more cam than you, slightly more compression than stock, and am running a 600 cfm Holley and getting 14 mpg in town and 19 on the highway (but 3.55's with an overdrive gear really helps...)

Thanks.

I did not realize that the 4 bbl would require so much work. I am used to my very low maintenance Carter BBD. City and highway I get about 25 MPG, the car weighs 3200-3300 lbs.

If I were to get a 4 bbl Offy intake - would it be possible to put a 2 bbl on it ? Would it be difficult to fabricate or obtain a kickdown etc for the 2 or 4 bbl.?

If you already have a 2bbl intake and want to try a different 2bbl carb, you will be going to a lot of effort to make that new 2bbl carb work on an Offy intake. The Offy 4bbl intake is drilled specifically for a 4bbl carb with a Holley square flange base. There are 4 venturi holes on the Offy so you will either have to cut one big one in the intake or pick a pair to use for your carb.

Since you already have a Chrysler 2bbl intake, you are much better off continuing to use it as everything already works on it. If you really want a bigger 2bbl carb, you could also try using a Carter BBD from a V8.

In short, use your current 2bbl intake to try a different 2bbl carb. Change to the Offy 4bbl intake only if you want to try using a 4bbl carb. If you want to change the orientation of the carb so that the throttle shaft is parallel to the engine, you could look at the Clifford intake but that would also require going to headers or Dutra Duals.