670 vacuum secondary vs 650 double pumper

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duster360

duster360

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What would be the difference going from a holley 670 street avenger carb to a 650 double pumper. Would a larger double pumper be better? Let me know what you think. This is a street car 99% of the time and not a daily driver.

Combo:
340
Big valve J heads
Roller rockers
Eddie air gap intake
Lunati 268 voodoo cam
Hooker headers
4 speed
3.91 gears
 
20 cfm.

Seriously, I think the double pumper is a better match. 4 speed, good gears. It's a no brainer.
 
I agree with Rusty.
The DP can be tweaked...with a little tuning...to outperform a vacuum secondary carb in most cases. Especially with the setup you're running.
 
If you decide 750, I have a DP for sale. Bareley used. I may be mistaken, but thought 750 is what came with 340s originally.
 
UOP said:
Another vote for the double pumper. How high do you rev it?
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Engine is still fresh at the moment. The cam card says it peaks at 6200 and I am not afraid to turn it up there.
 
I used to recommend the 670 on this site, but I've seen issues with them posted across the net, so I'm reserving promoting them...at least for now.

With the 4spd and the gear you have, a 650/750 DP is a no brainer at this point :thumbrig:. Good luck if you decide to swap...I don't think you'll be disappointed.
 
Would a 750 DP run well on this combo or would I be better off all around going with the 650 DP.
 
Either will run well! If you find a decent used 650 (List# 4777) for a reasonable price, I surely would consider that. But, if you're looking to buy new, I'd do the 750.....mainly because it will be usable down the road for just about any engine combo you'd like to put it on.....not to mention resale value if you ever went that route?
 
750 will allow for more room later on. 650 is good for a cruiser. The Hp I have is fulley adjustable HP center and corner idle. Flow tested by BiGS carbs
 
I was one of those that had the 670 and had issues with it. I dont recommend them to anyone. If you only want those two options I vote the 650 DP. Otherwise like said jump up to a 750. You wont be disappointed going to a larger carb.
 
One of the problems with a vacuum secondary on a 4-speed, is that it takes a moment for the back barrels to open. So, every time you let off the gas to shift, your back barrels are going shut and then you are waiting for them to re-open again. Because it's a 4-speed, you would want the dp. If you already have the 650 dp, go with it. If you haven't bought one yet, I'd go with the 750 dp.
 
No DP yet. Looks like I will be getting one based on the info I have read here so far. Just gotta decide which one.
 
straightlinespeed said:
I was one of those that had the 670 and had issues with it. I dont recommend them to anyone. If you only want those two options I vote the 650 DP. Otherwise like said jump up to a 750. You wont be disappointed going to a larger carb.
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When you say those two options, sounds like you may have another carb you might want to suggest.
 
Looking at your intended usage again, I think 650 DP would be a better choice for the street. 750 might be over-carbing it.
I chose a 650 DP for my last build and it worked great. Never felt under-carbed.
 
Im never going back to a vacuum sec Holley. the TQ is another story. Im a sucker for the moan that baby makes. Small primaries are nice on a stick car if, like me, you like to romp on it at low revs.But the bigger 750DP can be tuned to respond almost as well, and it pays bigger dividends up top, at the track,and my a$$dyno says midrange too.
-Having said that,I think my next carb will be one with metering rods.A metering rod carb can be tuned to post some good gas mileage numbers. Numbers a Holley type carb cant usually touch.
With the size cam youve got, gas mileage is still on the table. Unfortunately those 3.91s are past borderline, Taking your revs to 3150ish@65. However, a pumpkin swap is only 90 minutes. Then on a cross country trip, the money you save on gas can almost buy you a room at the other end.And the reduction in NVH is most welcome.
-Holley and holley-type carbs can be excellent carbs, especially at the track.
-But the metering-rod carbs,on engines with street-friendly cams,in the hands of a sharp tuner,can do so much more. Especially one with multiple boosters and a spread bore.
-Given the two listed options;as to size, its a coin toss.Ima DP fan .Stick cars like DPs
JMHO
 
The difference, power wise, between the 650 and 750 would be minimal. According to my calculations (cu/in x rpm x .9 / 2820), using a 6500 rpm redline, it comes out to right about 705 cfm, so you're right in the middle. Here's how I'm looking at it. If you're planning on buying a carb without a choke, go with a nice 650. If you want a choke, then maybe step up to a 750 as the choke tower will be a slight restriction. Either way, a Quick Fuel SS series (with choke) or Q Series (without choke) would be my recommendation.
 
RustyRatRod said:
20 cfm.

Seriously, I think the double pumper is a better match. 4 speed, good gears. It's a no brainer.
Click to expand...

Agree,

99% Street Car, with a 4-Speed Manual Transmission and 3.91 Gears.

* Holley 0-4777 '650' Double-Pumper.
Series #4150
Throttle Body Sizes ... 1.688" x 1.688"
Primary Jets ............ .071"
Secondary Jets ........ .076"
Power-Valve ............ 6.5
Primary Nozzle ......... 25
Secondary Nozzle ..... 25

Easy to change Jets,
 
Looks as if the 650 is going to be the way to go. Thanks for the help fellas.
 
I picked up a 700 DP for my 340, but I have 4.30 gears.
650 DP will give you good throttle response.
 
Just for clarification - the 670 is a later, wet flowed carb. Tested in the same manner (dry flowed) as the older 650DP it flows over 700cfm. Personally I like the DP for 4speeds too, and I'd go with the 650cfm 4150 version.
 
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