What to do with my Holley 650 DP?

[voodooRunner]

Happy Thanksgiving y'all!

THe Holley I found on my new '65 is a 4777-4 (4150) 650 cfm double pump. I also found an Edelbrock performer, an MSD 8582 (mechanical advance only) dizzy, and old headers. The PO said it has a mild cam but I have no specs on that or on the cylinder bore (engine was rebuilt many moons ago). I still need to check out the gears, as well.

Seems like maybe this stang was being built for the strip? Either way, that seems like WAY too much carb, especially if I'm trying to tune it more towards street application. What do you think?

Also, I'm thinking of going with a vacuum advance distributor, like the MSD 8479, for smoother low RPM performance.

I plan to use this Stang as my driver, but I would also like to eventually attain about 300 hp--without sacrificing my around-town drivability.

Will the 4777 still be too much carb?

 

[woodsnake]

The short answer to your question is, no the carb isn't too big.

If it has a cam, headers and duals already, it's just right. My only concern would be that it is a mechanical secondary, rather than vacuum, for the street.

I Think a vacuum advance distributor will be a good idea, but other wise, I'd leave it alone. Is the car a stick, or an automatic? If it's a stick, then leave the mechanical secondary carb on it!

 

[D.Hearne]

The 650 DP, is marginally on the big side. It will work, but maybe just not as well as a smaller carb. Depends on what the motor's built of. I know for one thing, the intake isn't the best choice, an RPM would do better (as would a Stealth or a Ford A321) If you're going to upgrade the motor more, then stick with the 650. If you're in search of better mileage, then replace it with a smaller vacuum secondary carb, no matter if it's got an automatic or manual trans.

 

[2+2GT]

I have to agree with the above, the 650 is a bit ambitious for a street driver, especially the mechanical secondary. Having driven mechanical secondaries on my daily driver for many years, I can tell you it's not for everybody. The manifold is not my first choice, according to this old chart, even the old Cobra manifold would be better, although I've seen data suggesting the RPM, not shown, is slightly better than the Cobra at the top end. Actually, the old Cobra does surprisingly well, considering it was designed long before computer designs were possible, by guys with slide rules.

 

[voodooRunner]

Good to see that the 4777 isn't completely out of place on my car.

I forgot to mention, I have a 4spd. It looks like most of the discussions I've come across suggest mechanical sec for lightweight cars with manual trans. and lower gears. Seems like for our cars it really comes down to whether or not the carb matches the particular build and (most importantly) gets properly tuned, regardless of ms or vs.

I've also read that mileage on a DP isn't that much of a factor until the secondaries kick in, which depends on how heavy your foot is. Or am I mistaken?

So if I decide to stick with the mechanical secondary, should I work with the 650DP currently on my 289 or maybe move down to the 600DP (4776) which might be more suitable for a mildly built street car?
Or will I be putting the 650cfm back on there a few months down the road when I'm looking for that next "extra kick"?

I was also looking at the Avenger 570 if you think vacuum is definitely the way to go, but not committed to Holleys or anything.

 

[D.Hearne]

The thing about DP carbs on manual transmission cars is really more of a recommendation. It's not something etched in stone. I for one like a vacuum sec carb with a manual, but tuned to where it opens sooner (sec. spring change to a lighter spring) This is what I have on my V8 Ranger, a mildly built (Explorer 5.0 level) roller 5.0, topped with a Ford A321 intake and Holley 570 SA carb. Transmission is a Toploader 4 speed backed by a 3.73 rear. It's an absolute pleasure to drive and gets decent mileage even without overdrive. The thing that kills mileage with a DP carb is having two accellerator pumps squirting fuel into the carb everytime you plant your foot in the pedal. A vacuum sec carb with a lighter sec spring fixes that. A change to a 600 DP, wouldn't be a noticeable improvement, so forget that idea, unless you've got money to toss around. If you really want to try a smaller carb, do it with a 570 SA carb instead. A lighter sec spring will open the secondarys a second or two after you nail the pedal. I think I replaced whatever spring came in the 570 with a purple sec spring.(next to the lightest)

 

[brianj5600]

Another thing that kills mileage on a DP is the small pvcr. Drilling the pvcr to a larger size allows you to run a smaller primary mainjet. Look here Carburetor Tuning the Scientific Way


I have good luck with DPs and automatics too. They are no good with tight converters, but a 3000 stall they work great.

 

[rbohm]

in my opinion the 650 cfm is not too large for a mildy built 302, my issue is with the double pumper for the street. my advice would be to take that carb, tune it for the track, then drop on a 600 cfm edelbrock 1406 tuned for the street. that way you can have the dp for maximum performance at the strip, and the edelbrock for best fuel economy on the street.

 

[woodsnake]

It seems like you are kind of firm in your belief that the 650 is too big.

Why is that?

It isn't too big at all, especially as you have a performance cam in the car.

Even if the car was bone stock, a 650 isn't "too big".

 

[voodooRunner]

It seems like you are kind of firm in your belief that the 650 is too big.

Why is that?

It isn't too big at all, especially as you have a performance cam in the car.

Even if the car was bone stock, a 650 isn't "too big".

I know that the best evidence is hands-on driving, trying out different sized carbs, both vacuum and mechanical. But my car is taking an extended nap right now until I probably end up doing top-end rebuild.

So I was basing that on other discussions and articles I've read and, of course, the Holley site itself. From what I've gathered so far, I understand that the DPs can be very unforgiving in an over-carbed situation, whereas you have more leeway with the vacuum secondary. So a 650 VS would perform well in applications where a 650 DP might be too much.

That make me wonder if maybe the the 50 cfm drop from the 650 to 600 holley dp would make more of a difference than D.Hearne suggests.

Getting back to the Holley site, there were 4 carbs recommended for my engine--289, mildly modified, max 6000rpm. When it came time to select mechanical or vacuum secondary, I met the weight requirement (under 3100), manual tranny, but I don't know if my gears are low enough (haven't checked yet). It DID say that 'the majority of vehicles will use a vacuum secondary carburetor.'
For VS they were: 1850, 81570, and 81670 (570-670cfm)
For MS it was only 4776... the 600cfm DP.

FYI: if I tweak the configuration slightly, i.e. change to highly modified or a 302 with 6500 max rpm, it does also recommend the 650DP.

 

[woodsnake]

I can see what you are saying, but I don't believe you will gain anything by going to a smaller carb, especially if you have plans for more performance in the future.

I say, RUN IT!

(Disclaimer!)

( have you read my disclaimer on the bottom of my sig?)

When the factory puts together a package, they are somewhat concerned about emissions and mileage/economy.

That's why you get a 715 on a 289 from Shelby, and why a 69 351W will come with a 450cfm.

There are several other factors that come into play when doing carb math.
Cam size, exhaust size, valve sizing, gearing, usage, preferred RPM range and more.
The old stand by calculation of rpm x bore size by what ever it is just does not work when you start changing internals.

The engine won't draw more air than it can use. You can over jet, but again, that comes down to internals.

 

[1992MustangGT]

Ok guys, this thread seems to be pretty interesting to me since I'm facing a similar decision; which carb to use. I have a 67 Cougar XR7 that i have just rebuilt and installed a 351 windsor. The engine has a Anderson Ford Motorsport roller cam (.544 lift intake & exhaust), Edelbrock RPM intake, factory heads with extremely ported exhaust ports, bigger valves (1.94 intake, 1.60 exhaust), & shaved .030" for higher compression (running about 10 to 1), headers, automatic, Mallory distributor, and currently an Edelbrock 600 cfm carb (vacuum secondaries).

I am trying to decide which carb would be better to use in terms of power and performance; the Edelbrock 600 cfm I currently have or a Holley Street Avenger 570 cfm, which I also have in my shed that I could throw on and use. It seems that Holleys perform better than Edelbrocks, but since the Holley is 30 cfm smaller, would it still out perform the Edelbrock?

 

[rbohm]

1992MustangGT, in your case i would step up to a 750cfm carb, i prefer the edelbrock, but the holley does well also these days. and fyi, the edelbrock does NOT use a vacuum secondary, it uses an air valve secondary system much like the rochester Qjet.

 

[1992MustangGT]

1992MustangGT, in your case i would step up to a 750cfm carb, i prefer the edelbrock, but the holley does well also these days. and fyi, the edelbrock does NOT use a vacuum secondary, it uses an air valve secondary system much like the rochester Qjet.

Yeah I probably should get a 750, but I don't have the money right now to go buy another carb, so I need to know which carb I should use between the two carbs I have; Edelbrock 600 cfm or Holley Street Avenger 570 cfm??? Which one will give me better performance?

 

[D.Hearne]

Yeah I probably should get a 750, but I don't have the money right now to go buy another carb, so I need to know which carb I should use between the two carbs I have; Edelbrock 600 cfm or Holley Street Avenger 570 cfm??? Which one will give me better performance?

There are other choices as well. Like Holley's 80783 650 VS carb, or a 670 Street Avenger. The 570 will do great on the street, but it'll limit your topend. And if you know a little about working on carbs, there's the used market. This is the stuff I use. I bought the 570 used for $150. I've bought several other used (barely, I steer clear of the ones that look old.)Holleys and with usually just a new powervalve and accellerator pump diaphram, they're good as new. If you already have the 570, I'd run it and keep my eyes peeled for a 650-670 or a 3310 750.

 

[D.Hearne]

When the factory puts together a package, they are somewhat concerned about emissions and mileage/economy.

That's why you get a 715 on a 289 from Shelby, and why a 69 351W will come with a 450cfm.

The engine won't draw more air than it can use. You can over jet, but again, that comes down to internals.

The 715 carb on the 289 was a vacuum secondary carb. Just because it was chosen, doesn't mean it actually used the full flow. Quite the opposite. With a "too big" mech secondary carb, it gets to the point where the vacuum signal disappears and the fuel stops flowing from the bowls. This doesn't happen with a vacuum sec carb. I had at one point a 650DP on the roller 5.0 in my Ranger and I could feel it lose accelleration when the secondaries were open too far. I swapped to an 1850 600 and it made a world of difference in the accelleration. I've used an 850 DP on a 325 horse 390 and it too, was too big when the secondaries opened too far, too fast. Nailing the pedal off the line would always result in killing the engine from the loss of vacuum. If you nailed it on the freeway, it was fine. It is true the engine won't draw more air than it can use, but when the throttle bores are too large, you loose the very thing that makes a carb function........vacuum or as it's also called........pressure differential (in the venturis)

 

[1992MustangGT]

There are other choices as well. Like Holley's 80783 650 VS carb, or a 670 Street Avenger. The 570 will do great on the street, but it'll limit your topend. And if you know a little about working on carbs, there's the used market. This is the stuff I use. I bought the 570 used for $150. I've bought several other used (barely, I steer clear of the ones that look old.)Holleys and with usually just a new powervalve and accellerator pump diaphram, they're good as new. If you already have the 570, I'd run it and keep my eyes peeled for a 650-670 or a 3310 750.

D.Hearne,

Your info has given me some good insight on the Holley carb. Thanks....However, do you know which would be the better street performing carb of the two that I have; Edelbrock 600 or the Holley Street Avenger 570? I've had a Holley Street Avenger 670 in the past (no longer have it) and it had real good power, but since the 570 I have is 30 cfm smaller than the Edelbrock 600, would the Edelbrock out perform the Holley?

 

[D.Hearne]

I'd use the Holley over the Edelbrock everytime, but simply because I prefer Holley's Some like Edlbrocks, some don't, same as with Holleys. If you find a bigger Holley, go for it. The 570 is easily sold on ebay. I had to wait for 6 months to a year before winning the bid on mine. All the ones I missed sold for well over what I paid. Your 570 will work, it will as I said before, just limit the power on the topend and only the topend. And also keep in mind that you cannot compare cfm ratings between the two, each are rated differently. And no matter what it's rated at, what it actually flows on your engine will be different from the rating. No two engines are the same. Holley 4 bbls are flow tested at a 1.5"hg pressure drop. That's what it's going to flow at that unit of measure at WOT. Chances are your engine's not going to duplicate that.

 

[voodooRunner]

The 715 carb on the 289 was a vacuum secondary carb. Just because it was chosen, doesn't mean it actually used the full flow. Quite the opposite. With a "too big" mech secondary carb, it gets to the point where the vacuum signal disappears and the fuel stops flowing from the bowls. This doesn't happen with a vacuum sec carb.

Thanks, D.Hearne. I was going to ask woodsnake about which secondary was on the Shelby 715. And that coincides with what I read, that larger vacuum carbs are more forgiving, whereas your performance will suffer if your mechanical carb is too big.

And I decided that I don't know enough about my car yet (need more info on my gears, cam, cylinder bore) to determine whether my 650DP is "too big" or whether or not a VS or MS carb would be a better fit.

I appreciate all the advice.

 

[woodsnake]

When I posted first about carb sizing, I only wanted to say that the old formula of bore X RPM divided by what ever that number is, isn't gospel.
There are a HUGE number of things that change volumetric efficiency, and all of these factors need to be considered when planning a build.

Yes, certainly the bore, stroke, planned RPM range are considerations, as well as vehicle weight, transmission type and gear ratio. Also, camshaft profile, intake and exhaust valve diameter, roller, solid or hydraulic cam,
and, most importantly HEADS.