Well, my old Holley has just about had it. It is a 4150 700 CFM double pumper that leaks like none other along with various other problems. I called Air Flow Research, and they suggested a 650 Speed Demon carb with mechanical secondaries for my setup (in sig). I called up Demon, and they suggested the same thing. I definitely want to go with a Demon because of everything good I've heard about them, but is a 650 going to be enough?? Everyone that has a combo similar to mine is running a 750. Could they just be over-carbing it? I drive the car everyday, so I need it to be streetable. I need to somehow beat the 8 MPG I'm getting now, and that's on a tank without flooring it at all (I'm not looking for much, but 8 MPG is killing me). Despite this, they both still suggested the mechanical secondaries or the vacuum secondaries. ANY help would be greatly appreciated. Thanks in advance.

 

Manufacturers are always going to be overly conservative, and it's best to take anything they say with a grain of salt.

Get a 750 w/mechanical secondaries, 650 is too small.

 

A 650 with mechanical secondaries or a 750 with vacuum secondaries will both work fine.On most vacuum secondary carbs
the secondaries don`t open fully so you don`t get the full flow.

 

i run a 750 demon mech sec. i have almost the exact same combo, cept i'm using trick flow 195's. if it matters at all, i got 23 mpg on the way to the track with 4.10s and a t-56

Eric

 

What would the difference in fuel economy/performance be going from the 700 CFM Holley double pumper to a 750 Speed Demon with mech. secondaries? Thanks.

 

If they are both working as good as possible(tuned correctly), they will be virtually identical.

 

Over carbing is one of the easiest and most common mistakes people make, especially on a small block. On a daily driver, without question I'd go with the 650 vacumn secondaries. That'll still give you plenty of carb, but with good throtle response and reasonable gas mileage.

 

VS or DP shouldn't make any difference in gas mileage if you don't open the secondaries on either one. For some reason, people still say DP's get worse mileage, but I don't understand that. I got 11.5 mpg (almost all highway) with the '57 last weekend driving it to my HS reunion, without getting on it (nor did I attempt to tune it to improve mileage). Gee, wonder how bad my mileage would have been with a 750 DP???

650 vs 750 is a different matter. The smaller carb should increase venturi velocity at the same engine speed, which tends to improve effeciency.

It is also true that over-carbing is easier to live with with a VS carb. So, if you go DP, keep it to the smaller side.

I'd suggest the 650 DP.

 

The only difference I've seen 'Five7kid' is that some of the race DP have a smaller PVCR, and therefore need to have larger main jets. These carb are mainly tuned to run at WOT all the time
Mine is currently running 73's up front with a Proform main body 1 3/8. The orginal main body (830cfm 1 9/16) needed 76's all at 6300 feet ASL.
I still have to pop my ears when I go down to Denver


Ron

 

Yeah I think I'm gonna go with the mechanical secondaries. As for the 650 vs. 750, can't I just jet the 750 a little less to get the same efficiency? Also, what is the physical difference between my Holley 4150 (0-4778) and the Speed Demon? Thanks.

 

I'm assuming this isn't a race carb, so the PVCR shouldn't be an issue. But, I don't know how BG handles that (if Speed Demon are smaller than Road Demon, for instance - makes a little sense, but you can almost be sure the King Demon follows that thinking). Good question to ask of BG.

Physically, the Holley and BG-SD will mount and hook up the same. They look quite different, though.

You can't jet down a 750 and expect the same effeciency/throttle response as a 650 would provide. It's best to match the carb to optimize its primary use.