I'm going to be buying a new carburetor pretty soon a holley double pumper I think im going to get the street hp version. I can't decide whether I need a 650 or a 750. I hear varying opinions on both sides. The car is a DD may-october. I don't need a choke i'll just feather it until it warms up. My engine setup:

10:1 355 vortec heads, LT4 hot cam, eldebrock performer rpm intake, hedders. About 375hp.

Car sees occasional track use but is a street car mostly so low end torque and throttle response and gas mileage is important but also max performance as well.

 

For the motor size, it calculates to something lower than a 650. But in all considerations, a 650 would be ideal. I have a 750 and it is way too rich on idle and cruising. All around I think I need a 650. But if I am all out racing and WOT, the 750 has better top-end.

 

How does it run WOT? Also, can you lean down the idle and primary control system?

I would sacrifice running too rich at idle and part throttle for good wide open performance.

I actually don't like to run much leaner than stoichiometric even at idle.

What do you think?

 

I'd go with a 650, unless you plan on upgrading your motor in the next couple of years. The 750 would work and would be the best choice if you plan on doing upgrades in the future. No sense in spending 400 bucks twice for a carb.

 

It pulls hard to the 6500RPM limiter and doesnt hesitate about giving in. Thats with it jetted fat also. But farting with it enough to lean it out on the idle, makes for a bit weaker top end.
I may have better luck with a 675CFM on mine. I dont drive it daily, but when I do drive it on a cruise, the richness is annoying.
Im either going to get a 650, 675 or FI one day.

BTW, its a BG Demon 750, DP, IdleEze, 4 corner, StreetDemon, no choke, Mech Sec. Had it for 4 years. Looks better than a Holley and is nice. But I am thinking of trading it for a 650 (or 675) Holley or BG SDemon.

My cam is a Perf RPM Edel (.488/.510 plus 1.6 rockers, 234/244, 112 LS), Perf RPM intake, full MSD, 355ci, etc
New vid of how the cam sounds at idle -->
http://videos.streetfire.net/video/1...lip_191843.htm

 

wow! u rev the **** out of that engine! I got a bigger cam, and I shift light mine at 6000.

 

LOL....I backed it down 2 6K after I wiped out the original RPM cam. Hehehehe.
Either way, she still pulls hard!

 

whatever you do, make sure you get 4 corner idle. The difference is night and day if you have a big cam.

Ill recommend the 750. Why? A 650 is perfect for a 350, but sometimes its not enough. A 750 will JUST work if you jet it right, but will never come up short.

Had both on my car, i like the 750 much better. But i also have a big-ish cam, and probably the same ball park power figure you have. My 650 DP isnt a 4 corner idle though, which might explain its lack of off the line grunt. They both get the same mileage.... The 750 feels very "explosively powerful". It will launch hard, spin easier, and shift harder. The 650 will do the work, but i REALLY have to get on it, and encourage it along.

As for the no choke. When you get the carb, im assuming you will remove the choke plate? Go to a parts store and get a choke cable after you do this. When the choke is pulled on a holley (mech secondaries anyway) not sure about vac, it will push on its fast idle cam, which opens the secondaries a bit.

My bro has this on his chev 350, i need to do it to mine.

So basically, you have no choke, you pump the gas to start like normal, and you pull the choke to closed position (activating fast idle). The car will start right up, and idle at about 1100 RPM. (you can adjust this speed). Then just leave it, or drive around with it, or whatever, till it warms up and idles on its own.