650dp or 750dp on 347?

[bmf347]

An AFR on 20 requires BIGGER jets to richen the mixture. 16:1 is ok for cruise but is still rather lean for these old engines/ignitions.

WOT AFR should be around a 13:1 plus or minus depending on the engine.

Sorry for the confusion. it was a while back, i was trying to remember as best i could what my numbers were. Was purely for arguments sake anyhow.

[Ethyl Cat]

Do you have flow numbers for the heads?

What are the valve sizes?

Runner volume?

[JungleJetDriver]

I have seen 40hp improvement with an out of the box conventional 750DP over a 670 avenger on a mild 393W.

This was with the afr corrected on both!!

Same day same dyno tuning session.

I picked up several hp with this switch as well, but its was a worked 750dp and I also changed the intake. That said, I think it would be hard to run out of air with a properly tuned 670 on a mild build.

[n2omaverick]

I have proven over and over again that any size holley can be run on any size engine or combination. Holleys are infinitely adjustable. Difference in sizes and secondary function effect certain driving conditions, so it's all on what the end user wants.
Saying a carb that is too big will idle or run rich is a prehistoric statement.
Carbs that are too big flow more air than the engine can handle creating a weak signal to the boosters and create a very lean condition. This is the bog you feel and hear! A carb that is too small has the opposite effect!
And a rich idling carb needs adjustment to the throttle shaft, power valve, or idle mixture screws! A carb has an idle circuit! If it's not idling on this circuit then you have made a wrong adjustment!
To the original poster, my recommendation would be to start on the smaller side and be ready to morf the carb into what ever you think your engine needs or is growing into. Different metering blocks, bodies, baseplates, allow your carb to keep up with whatever you throw at it.
If this were not a street car my recommendation would be a 950hp! I ran one on my 347 in 1997 and it ran a best of 10.18@134 naturally aspirated. With a biggs 750 it went a best of 10.31@131 same day same track no other changes! Wide band data logger showed an afr of 12.8 at wot with both carbs! So what was the difference??? Air flow and fuel flow or the afr would have changed!

[Handlebar Moustache]

Learn how to drive with a DP carb and you won't see a significant gas mileage difference vs a VS carb. Dial in the DP properly and you won't have stumbling, bogging, or hesitation.

Like Ethyl Cat said, don't try to floor it in 5th when you're going 30 mph and you won't have a problem. It's really not a trade off...just tune the damn thing right and you are good to go. I'm not saying there's no place for vacuum secondary carbs, but I'll take a well-tuned DP any day.

[4idfox]

If you are going to go with a 750. Buy one of these and replace your 650 main body.

http://www.summitracing.com/parts/hl...300c/overview/

This will give you changeable air bleads. Be carefull with those though.

[brianj]

I'll be the first to say I don't know carbs very well.

That is the best argument for a vacuum secondary carb right there.While you can get more out of a mechanical secondary carb, you need to really know how to tune one. I know I have trouble, and I have been playing with carbs for quite a while. You will probably, in my opinion, be better off with the ease and simplicity of a vacuum secondary carb unless you really want to try to learn to tune. They are just more forgiving on a street car. That being said, I do not claim to know all-just what I have dealt with personally. Good luck.

[ryanws6]

Well, I going with a DP. I did have a VW Ghia I built with a 2180cc and 2 dual Webs 44s. This was back in mid 90s. I say this because I know there are so many others that know so much more. I feel I have the enough skills to tune it. Biggest key is having the right tools and I have to buy the wide band.

I do want to buy the 750 quick fuel but I may do an couple other upgrades before I buy it. I'm going to buy head studs and a new bigger cam. Still though like I thought many say 750 or 650.

[FM2NOTCH]

it's not rocket science and the owner isn't racing nhra stock eliminator,
slap a 600 v.s. holley on it, maybe change the jets a couple of times if you are drag racing, possibly change a squirter if it stumbles off idle, ad a adjustable v.s. pod or just buy the secondary spring kit and be done. or use a 650 dp and do the same,except the secondaries are manual, no need for a spring kit.

[brianj]

Well, I going with a DP. I did have a VW Ghia I built with a 2180cc and 2 dual Webs 44s..

I had an M.G.B. with dual S.U. carbs on it in the early Ninety's. Gawd I hated those things. I almost replaced them with the Webbers- supposed to be a far better carb. Keep us posted- I am curious how you make out. We have quite a few of these "which is better" threads, but not many where people come back and say how they made out with their choice.

[4idfox]

I had an M.G.B. with dual S.U. carbs on it in the early Ninety's. Gawd I hated those things. I almost replaced them with the Webbers- supposed to be a far better carb. Keep us posted- I am curious how you make out. We have quite a few of these "which is better" threads, but not many where people come back and say how they made out with their choice.

I know what you mean about the su's. my wife had a 240z back in the mid eighties that would vapor lock in the summer. Put dual downdraft webers on it and tossed the su's in the trash.

[ryanws6]

I will for sure post up what happens. I've yet to get the short block together. I'm still working on the engine bay. I should get all the engine parts on Friday. So I hope to have the bay done before Friday.

[wgt500]

Well, I going with a DP. I did have a VW Ghia I built with a 2180cc and 2 dual Webs 44s. This was back in mid 90s. I say this because I know there are so many others that know so much more. I feel I have the enough skills to tune it. Biggest key is having the right tools and I have to buy the wide band.

I do want to buy the 750 quick fuel but I may do an couple other upgrades before I buy it. I'm going to buy head studs and a new bigger cam. Still though like I thought many say 750 or 650.

You better stick with a "smaller cam" and buy "bigger gears" if you're going with a "double pumper" carb. You won't have any trouble tuning the fuel map with a "double pumper". In fact you will get decent fuel economy if you keep your foot out of it. They're just like any other carb until you put the pedal to the floor. Just don't expect crisp full throttle performance below about 2500rpm. 3.55 gears aren't even close to what that carb needs in order to be a decent street performer. Anyone who thinks vac. secondaries are slow to open on the street, really doesn't have much experience tuning carburetors. I like an engine to produce max torque and acceleration when needed, whether it be 1200rpm or 4000rpm. I don't like the carb to be the "bottle neck" for nearly one third the operating range of a street engine. With the "double pumper" and 3.55 gears you will lose most of your bottom end performance. I can't tell you how many of these carbs. I've owned, worked on, or thrown away for customers over the last 40yrs.! Whatever you do, all the best!

[ryanws6]

Thanks WGT, however, I have a steeda T-5 with close ratio gears. Like I said in another post, at 70 I'm at 2800 rpms. So it's like 4.11 gears in the rear.

[ChrisPerry]

Learn how to drive with a DP carb and you won't see a significant gas mileage difference vs a VS carb. Dial in the DP properly and you won't have stumbling, bogging, or hesitation.

Like Ethyl Cat said, don't try to floor it in 5th when you're going 30 mph and you won't have a problem. It's really not a trade off...just tune the damn thing right and you are good to go. I'm not saying there's no place for vacuum secondary carbs, but I'll take a well-tuned DP any day.

This^^. i love my 650 double pumper on my 302. doesnt, bog run like a bag of you know what. its adjusted to the engine, and runs great.

[cb84capri]

I have a steeda T-5 with close ratio gears.

so you have a t5 with a steeda tri-ax shifter?

cale

[ryanws6]

Whats a tri-ax shifter?

I have a hurst shifter in it and LOVE IT.

When I did get on it especially from 2nd to 3rd you could hear the gear wine up in the trans and yes it had syn lube in it. Shifting was real tight as tight or more than my t-56 viper internal on my TA. Only thing for me is for me its geared too high. I don't like 70 @ 2800. I'm sure with the 347 I could spin 3rd.

[Ethyl Cat]

I do want to buy the 750 quick fuel but I may do an couple other upgrades before I buy it. I'm going to buy head studs and a new bigger cam. Still though like I thought many say 750 or 650.

I would put together what you have and see what you like/dislike before you go changing things now. You might be surprised how it runs.

As an example, it only takes a 240@.050 duration cam for a pump gas 509 BBC to go 9.60's in the quarter in a 3700 lb car. Well built as that engine is, you are still only 20 degrees different for 162 less cubes. A lot of it is done with the cylinder heads.

By the way, you clearanced that block before it went to the machine shop, right?

Steve

[ryanws6]

I had the machine shop clearance the block.

[synthartist69]

With a double pumper carburetor, the secondary throttle plates open starting when the primary throttle plates are about 2/3's of the way open. When the primaries are fully open, the secondaries are fully open regardless of the engine's demand for air. When the secondaries are forced open at low rpm by mechanical linkage on a street engine, there will be a hesitation followed by slow acceleration. The hesitation takes place because the engine isn't pulling enough air volume at low speeds to cause the secondary venturi to flow fuel. The addition of a second accelerator pump, (double pumper) on the secondary side of the carburetor is used to "inject" additional fuel to mix with the big gulp of air diverted to the secondaries from the primary side of the carb.when they were forced open before the engine demanded the extra air. This secondary accelerator pump shot of fuel is the only fuel the air flowing through the secondaries will get until air flow rises to the point the venturi will flow fuel. This takes place after a lean condition which actually slows acceleration. Yes, you will feel the secondaries "kick in", but what you are feeling is a hesitation. On the other hand, with a light drag car and very low,(high numerical) gearing that launches at 3000rpm, the engine will have the demand for secondary air flow almost instantly after leaving the line, long before a vacuum diaphram could open the secondaries. With this car the secondaries need to be forced open very quickly by mechanical actuation. The secondary accelerator pump is simply preventing a lean condition until the secondary venturi start to flow, which is almost instantly. As you can see, the mechanical secondary carb,(double pumper) is not designed for low rpm use, such as seen by a street car.
A vacuum secondary carb. uses air flow through the primary venturi to open the secondaries. As air flow increases in the primary venturi to where the engine needs additional air, a vacuum diaphram opens the secondary throttle plates. Since the engine is demanding more air and the secondaries open slowly there is no hesitation, just a smooth transition to all four venturi operation. This type of "on demand" air/fuel flow can take place at 2500rpm or 4000rpm whenever the engine needs the additional volume of air. You can mash the pedal to the floor and the secondaries will not "kick in" until the engine actually needs the additional air/fuel flow, regardless of rpm.
Remember, the engine is just a big air pump and it determines how much air it needs; the driver doesn't. On the street with a mechanical secondary, (double pumper) carb. all the driver can do by mashing the pedal to the floor at low rpm, is slow down the air flow through the carb to the point of inefficient fuel metering and hesitation.
Hope this helps!

This is very useful for me. I have a quickfuel 650 slayer DP mechanical secondaries on my mild 347. The car has always had a hesitation at low rpm, basically anything below 1800 to 2000. Almost feels like a misfire but it runs great at high rpm. Also, the car always runs too rich but if I lower the fuel level in the bowl it doesn't have quite the kick when I jump on the gas. I have a Ford racing 5 speed with 3.55 rear end in my 65 mustang. City driving is very irritating because of this. Do you think I should go with VAC secondaries to solve this?

[Walking-Tall]

LMFAO @ the QFT MALfunction... gee, what a surprise. All moot... would I suggest a vacuum secondary in most all cases? Yes, I would, because they can be dialed in to administer exactly what the engine wants. The bottom line is this: drive-ability and cruise efficiency is completely about the idle/transition circuit... and so, double pumper or scary named newer avenger or slayer BS or what have you, dialing in the primary idle feed restrictions and idle air bleeds (and maybe adding transfer slot restrictions if the camshaft is ridiculous radical) is where perfect function is at, NO MATTER. Besides giant Dominators, there is NO SUCH THING as too much carburetor... the only danger there is is making too much torque and horsepower, AND average torque, area under the curve. Anybody needing help in this department, don't hesitate to PM me.

Sometimes... QFT base plates have far too wide or long transfer slots... transfer slot restrictions installed is a CURE.

[kingsnake]

Lots of good info here, but I will try to make this process of carb selection simple.
I have over 30 years of experience in engine development gained at the Ford Dynamometer Lab.
Put your carb on and a vacuum gauge. Mash the throttle and watch the gauge.
At WOT (Wide open Throttle) if the vacuum reads 0" go to a smaller carb. If it pulls more than 1" of vacuum go to a larger carb.
AS far as DP Vs. Vacuum secondaries, both can be tuned to run well on light cars with tall gears.
My preference on street cars is Vacuum sec.
No matter what size carb you end up with proper tuning will make a big difference.
(Accelerator pump cams, Jets , Power valve, Secondary diaphram spring, ect. )
Good luck!

[Hissing Cobra]

This is very useful for me. I have a quickfuel 650 slayer DP mechanical secondaries on my mild 347. The car has always had a hesitation at low rpm, basically anything below 1800 to 2000. Almost feels like a misfire but it runs great at high rpm. Also, the car always runs too rich but if I lower the fuel level in the bowl it doesn't have quite the kick when I jump on the gas. I have a Ford racing 5 speed with 3.55 rear end in my 65 mustang. City driving is very irritating because of this. Do you think I should go with VAC secondaries to solve this?

Did you tune your current carb on a dyno to see what is actually going on? If not, buy a jet kit, some gaskets, squirters, and anything else that you may need and tune it. I don't think a new vacuum secondary carb is going to solve your issue. Why? Because that one will need to be tuned too. It's a lot cheaper to tune what you have than it is to buy a new carb. Tune the QF 650 and you should be able to get rid of that slight hesitation.

For what it's worth, I have a Holley 650 double pumper on my 347 and it runs perfect. There's no hesitations, stumbles, bogs, etc... It idles at 1,000 rpms, and the gas mileage is ok at 14 or 15 per gallon.

[brianj]

Lots of good info here, but I will try to make this process of carb selection simple.
I have over 30 years of experience in engine development gained at the Ford Dynamometer Lab.
Put your carb on and a vacuum gauge. Mash the throttle and watch the gauge.
At WOT (Wide open Throttle) if the vacuum reads 0" go to a smaller carb. If it pulls more than 1" of vacuum go to a larger carb.
AS far as DP Vs. Vacuum secondaries, both can be tuned to run well on light cars with tall gears.
My preference on street cars is Vacuum sec.
No matter what size carb you end up with proper tuning will make a big difference.
(Accelerator pump cams, Jets , Power valve, Secondary diaphram spring, ect. )
Good luck!

Mine reads 9 pounds of pressure. Now what?
Lol. Sorry, couldn't help it. That is actually good advice, although I've always gone with 2" as a guideline.

[kingsnake]

I'm LOL with ya brianj.
I had a feeling when I posted it some boosted guy would probably chime in with a funny.
Truth is I work with an old timer who worked at Holley R & D in Warren Mi. back in the day. He claims Holley actully developed most of their carbs on the flow bench at -1.5" Hg.
The Extra pull at -1.5 - -2.0 Hg (increased velocity) usually shows a little better low end torque on the dyno.
I have found the slightly better mechanical effiancy at -1.0Hg normally gives a small increase in HP throughout the rest of the torque curve on most small block setups.
Pick the number that works best for you. Its the old torque vs. HP game.
We are really splitting hairs on this one. No major increases either way.
Some where between -1 and -2Hg is probably a pretty good rule of thumb to base carb sizing selection on.
Much less than -1Hg and the carb is to big.
To small and your pretty much running a restricror plate. Might help a guy feel like a NASCAR guy.
Do ya wanna be a NASCAR guy or an unrestricted NHRA guy.
Your choice. I'm going NHRA. LOL