750 or 830 cfm Quick Fuel Carb

[EBSTANG]

351 C (stroked to 408), hydraulic, full roller, 10 3/4:1,

572 hp @ 5900 rpm/ 598 ft lbs @ 4000 rpm on engine dyno stand (at the flywheel)

425 hp @ 5500 rpm/ 448 ft lbs @ 4100 rpm on chassis dyno (at the rear wheels- inertia type)

These runs were performed with a Holley 4150 tuned by AED to these specs...

 Jets Primary-79, Jets Secondary-90, Primary Idle Bleed-.078, Primary High Speed-.028, Secondary Idle Bleed-.070, Secondary  High Speed-.025, Power Valve Primary-4.5, Squirter Primary-.031, Squirter Secondary-.036, Air/Fuel Primary-13.2, Air/Fuel Secondary-13.0

This carb was way rich apparently as it let a good amount of deposits on valves after 4,000 miles.

Holley uses formula 408 x 6000 rpm (my use) = 2448000 divided by 3456= 708 cfm for 100% efficiency.  Street use can be less than 100% but let's base it on the high side and assume 708 cfm is good.  That would leave me with the 750 as my choice.

Would someone who knows more than me tell me if the Quick Fuel BD750 would be a good choice.

 

[johnwanzel]

My engine will have similar numbers and I plan on using a quick fuel 750 with annular discharge boosters. I did a lot of research on my engine combination and it seems that the 750 would be a lot more Street friendly than the 850 and the annular discharge boosters will react better on the street for low-end throttle response than with regular downleg boosters.

 

[Don C]

A 351C with stock stroke is very happy with a 750cfm carburetor, so a 408 shouldn't have any problems with one.

I think of those formulae as being suitable for Chevy engines, Clevelands like more.

 

[EBSTANG]

A 351C with stock stroke is very happy with a 750cfm carburetor, so a 408 shouldn't have any problems with one.

I think of those formulae as being suitable for Chevy engines, Clevelands like more.

In case it matters...I forgot to mention it has Trick Flow PowerPort 190 Aluminum Heads- 195cc intake port/112cc exhaust port (TFS-5161B620-C00), combustion chamber volume- 62cc- profiled. Surface & port match heads to Air Gap intake.

Holley has that formula, just want to be sure the 750 won't be undersized although we now know the 850 was oversized.  Thanks for your input.

 

[4Vforever]

Depends on what you really want from the engine as to which carb you really need. Also do you want a choke or the better air flow of a HP style throttle body. No matter what though performance engines want fuel and tuning a carb to be 100% street friendly on a high performance engine is near impossible. A choice needs to be made, tune it for all out street or all out performance there is very little in between. This is why most guys that drive engines like this on the street tune their carbs at the track to suit what they're after and after they've finished, tune them back for street duties again. This is the reason why I go for things like quick change fuel bowls to make things easier. Some people I know of are lazy and run the same tune for the street and track, but it's about the only time the car is driven on the street though is to the track. With modern carbs though tuning can be made a lot closer than it could be previously, but still it's either still tuned for all out power or street duties and even trying to find a happy medium will still be rich as no matter what, the engine still wants that fuel for the top end. Tuning for pure street duties then don't keep the boot into it at high revs as leaning out kills parts real quick. 

I've actually found AED carbs to be very good, in fact I put one on a 347 Windsor a few months ago and it was near spot on out of the box. Of course it needed a bit of tuning still, but there's not a carb yet I've pulled out of the box and it was absolutely spot on. So personally I'd be tuning your existing carb to what you really want from it. 

I've also used a couple of the QF Q series on street strip engines pretty successfully as well, but as I said they're tuned to suit where they're running also, either street tune or strip duties. The first incarnation of my new build is not going to make the power levels of your engine, but I'm going to be using an 850 Q series that's been heavily modified and probably flows over 900 CFM, but will be tuned to suit. With things like multi stage emulsion and changeable air bleeds, tuning it to where I want it is a whole lot easier than it was years ago. Yes its finicky and a pain in the rear, but in the end will be worth it. In fact instead of changing out emulsion jets at the drags, I'll just change out the entire metering block to make life easier. 

So as to your question which one, it's all up to what you exactly want from your engine. Something in the 800-850 CFM range is what I think in a engine like this, but if you want it purely as a street engine then a 750 or even a 780 CFM carb is what I'd be looking at. As I said you need to decide exactly what you want from the engine.

 

[EBSTANG]

My engine will have similar numbers and I plan on using a quick fuel 750 with annular discharge boosters. I did a lot of research on my engine combination and it seems that the 750 would be a lot more Street friendly than the 850 and the annular discharge boosters will react better on the street for low-end throttle response than with regular downleg boosters.

Interesting, could you explain a little more about the annular discharge boosters...maybe share a link?  Thanks...

 

[Jeff73Mach1]

The volumetric efficiency on a well built cleveland can exceed 100% The jetting you listed looks like there are no power valves in the carb.

My 393 spins much higher than 6000, but an 850 has been fine for me with a manual and a 3.89 gear

Cam specs, compression ratio, transmission and gearing would make it easier to advise you

 

[tony-muscle]

What cam specs are you using?

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[EBSTANG]

The volumetric efficiency on a well built cleveland can exceed 100%  The jetting you listed looks like there are no power valves in the carb.  

My 393 spins much higher than 6000, but an 850 has been fine for me with a manual and a 3.89 gear

Cam specs, compression ratio, transmission and gearing would make it easier to advise you

Comp Cams hydraulic roller cam, brass lifter bushings .040" drain orifice, Lunati lifters (#72335-16)/ roller rockers (#15380-16), Manley Rods (Intake #25715-8, Exhaust #25715-8)




                              Cam intake duration @ .050= 234 actual

                              Cam exhaust duration @ .050=  236 actual

                              Hydraulic Intake/ Exhaust valve- gross lift  .588/.600

 


TKO 600 5 speed trans.-


2.87- 1st gear

1.89- 2nd gear

1.28- 3rd gear

1.0- 4th gear


.82- 5th gear (OD)

 


10 3/4 : 1 Compression

 


3.50 final with 28" diameter rear tire

To answer the other contributor's (4Vforever) question as far as my intent for the car...I don't race it, so prolonged high rpm runs are very rare. I just like to run around town and go for pleasure drives, mostly in the 4-5k range, sometimes hittin' it. I drive the car 3500-4000 miles per year. (dry weather only)

 

[johnwanzel]

Annular booster venturis atomize fuel better and provide a stronger fuel metering signal at low air velocity. In other words, annular booster venturis benefit the low rpm and mid-rpm performance of a motor in the same manner as the smaller primary throttle bores of a spread bore carburetor. These attributes make annular booster venturis popular for improving the low rpm operation of performance engines, where they have earned a reputation for improving torque, horsepower and throttle response at low engine speeds. However the improvement in fuel atomization distributes fuel more consistently throughout an intake manifold, resulting in more consistent fuel/air ratio from cylinder to cylinder, therefore annular booster venturis actually improve torque and horsepower across a motor's entire power band; and they improve fuel economy too! The only drawbacks of annular booster venturis include their larger physical size (which reduces the airflow capability of a carburetor by a relatively small amount) and their greater cost of manufacture.

 

[EBSTANG]

Annular booster venturis atomize fuel better and provide a stronger fuel metering signal at low air velocity. In other words, annular booster venturis benefit the low rpm and mid-rpm performance of a motor in the same manner as the smaller primary throttle bores of a spread bore carburetor. These attributes make annular booster venturis popular for improving the low rpm operation of performance engines, where they have earned a reputation for improving torque, horsepower and throttle response at low engine speeds. However the improvement in fuel atomization distributes fuel more consistently throughout an intake manifold, resulting in more consistent fuel/air ratio from cylinder to cylinder, therefore annular booster venturis actually improve torque and horsepower across a motor's entire power band; and they improve fuel economy too! The only drawbacks of annular booster venturis include their larger physical size (which reduces the airflow capability of a carburetor by a relatively small amount) and their greater cost of manufacture.

...doesn't look like they offer annular boosters in the BD 750. This engine has gobs of low end and although who wouldn't want more, I really like the Black Diamond coating. I'm really interested in making sure the 750 isn't too small (I don't think it is) and I'm looking for reassurement. They do have an 830 but I was told that my 850 was way too rich for my engine AND IT DID leave a good amount of deposits on the valves upon disassembly this winter. It only had 4,000 miles on a complete rebuild. So that, along with Holley's formula which netted a 708 cfm result is what is leading me to the 750 cfm choice.  I was hoping all you guys would say the 750 sounds right... :chin:

 

[johnwanzel]

Hahaha yes the 750 sounds right. I know you want to stick to the black diamond look and it will perform well and look great for your application. I wanted to give you another option with the annular boosters since it's a street car but I don't think it's available with the black diamond finish. That thing should scream with the new carb. Your setup is almost identical as mine. Same trans but I'm running 4.11 gears.

 

[EBSTANG]

Hahaha yes the 750 sounds right. I know you want to stick to the black diamond look and it will perform well and look great for your application. I wanted to give you another option with the annular boosters since it's a street car but I don't think it's available with the black diamond finish. That thing should scream with the new carb. Your setup is almost identical as mine. Same trans but I'm running 4.11 gears.

::thumb::  You're right about the black diamond look but if they offered the boosters with the black I'd get them for the few extra bucks...why not?

I'm just waiting for Jeff, 4V and Tony to reaffirm my decision before I make my move. I know Jeff knows his sh*t on this stuff and I value his opinion although he's a little more serious than me regarding getting everything he can out of his stroker. I assume he races it as he just added a cage to his this winter. I just like to pretend I have a race car.   I've got a week to make a decision. :whistling:

 

[tony-muscle]

I dont have the experience to opine what would work best for you. My guess would be that the 750 would give you better low end versus a small loss at the higher RPM. As also said, the annular discharge improves lower end.

In my case i just completed the installation of a FiTech EFI with the idea to simplify fuel adjustments. Also, the EFI should make it easy to grow with my plans of stroking the engine next winter. I am planning a similar setup to yours but keeping the 4V heads.

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[TommyK]

Unless you have your heart set on a new carb I would tune what you have.

You already know from your dyno pulls that the A/F ratio is good at wide open throttle. It sounds like it is rich every where else though. A different carb will not automatically fix this. It will just require a different tune.

The QF carbs are somewhat notorious for rich part throttle. Its a bit of work but that can be fixed by playing with the ifr/iab combo and possibly adding a transition slot restrictor.

Also a 79 main jet may be a bit large for best lean cruise. Do you have a wideband?

 

[c9zx]

Do you have the Brake Specific Fuel Consumption (BSFC) numbers from the Dyno Pulls? If so, they would be very helpful in tuning the carb you have, +1 on Tommy K's post. As Tommy said, the jetting sounds like it may be too much. Chuck

EDIT: What 850 do you now have and what style boosters are installed, down leg or down leg with a step?

 

[EBSTANG]

Unless you have your heart set on a new carb I would tune what you have.

You already know from your dyno pulls that the A/F ratio is good at wide open throttle. It sounds like it is rich every where else though. A different carb will not automatically fix this. It will just require a different tune.

The QF carbs are somewhat notorious for rich part throttle. Its a bit of work but that can be fixed by playing with the ifr/iab combo and possibly adding a transition slot restrictor.

Also a 79 main jet may be a bit large for best lean cruise. Do you have a wideband?

I don't know what a wideband is...

 

[EBSTANG]

Do you have the Brake Specific Fuel Consumption (BSFC) numbers from the Dyno Pulls? If so, they would be very helpful in tuning the carb you have, +1 on Tommy K's post. As Tommy said, the jetting sounds like it may be too much. Chuck

EDIT: What 850 do you now have and what style boosters are installed, down leg or down leg with a step?

I don't think I was given the BSFC numbers...I don't know what they are even if I had them.

I have a Model  # 4150 (4781-8 LIST) 850 cfm, double pumper, 4 corner idle carb w/ mechanical secondaries

As I stated earlier I sent this carb to AED and they supposedly tuned it to match my build. (that is what they were told to do) AED sent a sheet which stated they rebuilt with these specs...

Jets Primary-79, Jets Secondary-90, Primary Idle Bleed-.078, Primary High Speed-.028, Secondary Idle Bleed-.070, Secondary High Speed-.025, Power Valve Primary-4.5, Squirter Primary-.031, Squirter Secondary-.036, Air/Fuel Primary-13.2, Air/Fuel Secondary-13.0

They did a "Wet Flow," "Rebuild," and "Test Run." I don't know what that really means.

If this sounds good to any of you, I will be selling this carb once the new one is installed and running good. It is my impression that the Quick Flow carbs are much easier to tune (cost me less if I am not the tuner) or would be much easier to learn if I decide over the next few years to learn how to tune myself. The advancements made with new tech in carbs, regarding tuning, and the art of tuning should make this much easier for the hobbyist. Thoughts...?

If I could get $300-$500 for this rebuilt carb it would certainly offset the cost of the new and someone else would get a perfectly good Holley 850 which may be better suited for their build. My dyno pull video on this site was done with the Holley.

 

[tony-muscle]

Unless you have your heart set on a new carb I would tune what you have.

You already know from your dyno pulls that the A/F ratio is good at wide open throttle. It sounds like it is rich every where else though. A different carb will not automatically fix this. It will just require a different tune.

The QF carbs are somewhat notorious for rich part throttle. Its a bit of work but that can be fixed by playing with the ifr/iab combo and possibly adding a transition slot restrictor.

Also a 79 main jet may be a bit large for best lean cruise. Do you have a wideband?

I don't know what a wideband is...

Wideband O2 sensor. With it you can measure the AFR of the exahust and help adjusting the carb to run the ideal ratio.

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