650 vs 750 on 347 stroker

Which one would be better 650 double pumper. Or 750 double pump both with secondary vacum its going on 347 stroker the intake is a performer rpm air gap

 

Unless it's built to the nuts I'd run the 650 DP.

 

I'm pretty happy with the Holley 750HP on my 308. One of these days I'm going to throw a 650 on it and see if there is any difference. Of course, I only run it flat out, but the 750 seemed to waken it up.

 

It will be i just dropped off the block to get it decked bored and everything ran me $308 i think it was a good deal plus i just put stainless steel valves and got the heads back from ported and polish

 

750 All day! I'm with Don. I have a Holley 750 on my 308 as well and runs great! People freak out a lot over this but truth be told, there is no such thing as a carb that's too big. Only one that has the wrong jets. The engine is only gonna draw as much air as it needs. You have to just balance out the fuel. Go with the 750. You won't regret it. Especially on a 347! ON my 347 I'm going with a 1050 dominator!

 

Well, lets see.

If someone on here can give you better advise then try both and see which one works the best, then he/she is a MUCH better engine builder then I've ever been or will be.

If you can get both then try them it won't take long to see which works best. By the way the 750 wouldn't be a double pumper if it has vac secondarys.


From Holley: For street cars the vacuum secondary carburetor works best on midweight or heavyweight cars with an automatic transmission. They are more forgiving than a Double Pumper because they work by sensing engine load. The mechanical secondary carburetor is best on a lighter car with radical camshaft and a lower gear and manual transmission or on a car that is going to be used for racing purposes.

 

I agree.... I ran a 650 then a 750 with my set up, the 750 runs the best. I also talked to Demon (carb that I have now in a 750), I was told with my set up from compression to gears that a 750 was the best carb for me. I haven't touch a screw or idle on this carb.

 

People often feel larger carbs work better because your getting more motor response per gas pedal movement. In other words if your motor only draws 500cfm then a 1000CFM carb is going to give you at 1/2 throttle what you would feel at full throttle on a 500cfm carb. This makes the first 1/4 inch of pedal movement feel like you have a rocket. However, everything from 1/2 throttle to full throttle is wasted. Same situation by using a 750 if its too big. Carburators need enough air velocity for the fuel to be pulled through. Running a carb too big usually means lean spots in places that are harder to fix because the air signal is too low. I would find out what your motor needs for cfm and make a choice then.

 

last motor i built was a 408 I only had a 750 aed holley. I forget the math on it but a 650 will be plenty for a 347. Might go for mech secondaries tho. You should have enough motor for it. My motor was up around 550-575 and drove it daily.

 

i run a 830 on my 306

 

The 400 in my GTO was quicker by 1-2 tenths with a 600 DP over a 750 DP. With the 455 it was different though!

 

I ran both the 600 and 750 edelbrocks on my 347. I know its a different situation. I stayed with the 750 as it seemed to run better and didnt hurt ETs, didnt seem to really help either. According to Edelbrocks flow chart a 350 c.i. motor needs 600 CFM of air at 6000rpm. So if you are going to shift at 6000rpm or higher then you will be at the carbs max capability and close to it to for some time before you shift. with the 750 you are good to 7500rpm so you wont be missing any potential power.

 

My New 347will have a total of 900 cfm feeding it (Two 450`s) But that is ALSO "different".

My 308 has always been very Happy with them.

 

Im still confused as to if its a vacume secondary or double pumper but my take would be if its a double pumper a 650 is plenty but if its a vacume secondary id go 750. The vacume secondary carb would be a better choice for the street also.

 

I run a 600 cfm Holley on my 351 W - a street car that has a 5500 redline.
If I was goingto race it I might go as high as 720 cfm but even at 100% VE it can't flow 720 cfm. Your 347 will be fine using a 650 as long as you keep the rpm below 6500.