Annular boosters for a QF 750 DP

[ArizonaGuy]

What are the benefits of annular boosters over downleg boosters?
Or are there any?

Street/strip car.

[HQM383]

More sensitivity and therefore faster response to throttle angle changes and better atomized fuel leading to more homogeneous mix. Street/Strip - noticeable increased low to midrange torque and throttle response especially on single plane. Less gains to be had on manifolds that are not airgap as the finer atomized mix can gas too early in the induction tract hampering flow.

Some don’t like annulars due to the booster size impeding WOT flow but the benefits on the street outweigh that concern. The boosters are higher in the venturi than the downleg not impeding as much flow as first impressions give. It is also usually wise to go larger venturi and throttle bore when using annulars over what would be used with a downleg carb, ie an 850cfm annular on 383ci where a 750cfm downleg would normally be used.

There are also complaints of them getting overly rich at WOT but that can often be sorted with emulsion and air bleed tuning. To me the 4150 annular are the most underrated and under used carbs for street/strip vehicles.

Do you have a QF 750 you want to install some annulars in or is it an either/or decision buying a new carb?

What size engine?

[1980RS]

I have run both down leg and annular and it all depends on booster signal for each to work but like others have said, A/D boosters work well on a street engine. Just look at Eddy now they just added them to their AVS2 carb.

[In-Tech]

HQM hit it on the head. The 12 hole and 16 hole annular's are pretty much better all around although you lose a little air flow, due to their size. It's all about fuel curve, as you know. You can steady state on the dyno to make yourself feel better as much as you want, most applications are not steady state but acceleration.

[HQM383]

HQM hit it on the head. The 12 hole and 16 hole annular's are pretty much better all around although you lose a little air flow, due to their size. It's all about fuel curve, as you know. You can steady state on the dyno to make yourself feel better as much as you want, most applications are not steady state but acceleration.

And that’s where they can kick ass over downleg on the street and potentially 60’ better on the 1/4. Peak hp numbers (which a downleg can give a few more) don’t necessarily correlate to acceleration and drivability under that peak in vehicle.

I will also add as I am such an advocate of annulars for street/strip that if they were more widely used and well tuned sales for Holley Sniper systems would be down.

[289nate]

E85???

[1980RS]

HQM hit it on the head. The 12 hole and 16 hole annular's are pretty much better all around although you lose a little air flow, due to their size. It's all about fuel curve, as you know. You can steady state on the dyno to make yourself feel better as much as you want, most applications are not steady state but acceleration.

I put 12 hole billet boosters in my Dominator over the Holley 8 hole boosters, made a huge difference when tuning the engine and the car has a better fuel curve.

[ArizonaGuy]

More sensitivity and therefore faster response to throttle angle changes and better atomized fuel leading to more homogeneous mix. Street/Strip - noticeable increased low to midrange torque and throttle response especially on single plane. Less gains to be had on manifolds that are not airgap as the finer atomized mix can gas too early in the induction tract hampering flow.

Some don’t like annulars due to the booster size impeding WOT flow but the benefits on the street outweigh that concern. The boosters are higher in the venturi than the downleg not impeding as much flow as first impressions give. It is also usually wise to go larger venturi and throttle bore when using annulars over what would be used with a downleg carb, ie an 850cfm annular on 383ci where a 750cfm downleg would normally be used.

There are also complaints of them getting overly rich at WOT but that can often be sorted with emulsion and air bleed tuning. To me the 4150 annular are the most underrated and under used carbs for street/strip vehicles.

Do you have a QF 750 you want to install some annulars in or is it an either/or decision buying a new carb?

What size engine?

Yes, I already have/run a QF 750. I wouldn’t be buying a new carb, but possibly swapping out main bodies (new one with annulars).
331 cubic inch SBF Windsor stroker. Vic Jr intake.

Thank you for your input and for sharing your knowledge

[ArizonaGuy]

E85???

Nope, 91 octane pump gasoline

[n2omike]

I ordered a custom carb from AED for my 363. It went to a Stuska dyno last weekend and made 611 hp at 7500 rpm, and carried to 8k.
The carb has a huge 850 sized Holley center section and annular boosters. It was WAY rich out of the box, and we ended up going down six jet sizes. Ended up with 78/85 jets. Also increased the high speed air bleeds to 35's. Was still going a little rich up top. We made an emulsion change, but it didn't really change anything. Engine is 11:1, is going into a street/strip 1966 mustang and will run on pump gas n/a, even though it will be run with a 100-200 hp nitrous plate and good fuel at the track.

In conclusion... Annular boosters create a very strong signal. Jets will be far smaller than what is ideal with downleg boosters. They can also pull too much fuel as the carb's capacity is being reached. With a traditional 4150 design, they work well on smaller cubic inch engines that don't push the flow capacity of the carb too hard. On larger engines turning very much rpm, the boosters are too much of a restriction. So, use annular boosters in smaller engines and use a larger throttle bore than you would with downleg boosters.

[HQM383]

I ordered a custom carb from AED for my 363. It went to a Stuska dyno last weekend and made 611 hp at 7500 rpm, and carried to 8k.
The carb has a huge 850 sized Holley center section and annular boosters. It was WAY rich out of the box, and we ended up going down six jet sizes. Ended up with 78/85 jets. Also increased the high speed air bleeds to 35's. Was still going a little rich up top. We made an emulsion change, but it didn't really change anything.

Was it ‘engine losing power’ rich or ‘numbers could look better rich’?

PV and pvcr reduced or would that have had little effect?

[levisnteeshirt]

I tried a annular booster 700 ( i changed them ) on a 604 crate motor , the driver really liked it but sold the car before we really got it sorted, , he said it was much better than his old style Stealth ( with holley blocks ) out of the turns

[levisnteeshirt]

I ordered a custom carb from AED for my 363. It went to a Stuska dyno last weekend and made 611 hp at 7500 rpm, and carried to 8k.
The carb has a huge 850 sized Holley center section and annular boosters. It was WAY rich out of the box, and we ended up going down six jet sizes. Ended up with 78/85 jets. Also increased the high speed air bleeds to 35's. Was still going a little rich up top. We made an emulsion change, but it didn't really change anything. Engine is 11:1, is going into a street/strip 1966 mustang and will run on pump gas n/a, even though it will be run with a 100-200 hp nitrous plate and good fuel at the track.

In conclusion... Annular boosters create a very strong signal. Jets will be far smaller than what is ideal with downleg boosters. They can also pull too much fuel as the carb's capacity is being reached. With a traditional 4150 design, they work well on smaller cubic inch engines that don't push the flow capacity of the carb too hard. On larger engines turning very much rpm, the boosters are too much of a restriction. So, use annular boosters in smaller engines and use a larger throttle bore than you would with downleg boosters.

How many emulsion hole blocks ?

[n2omike]

How many emulsion hole blocks ?

They are billet AED blocks, and there were a lot of emulsion bleeds... at least 5, maybe 6 or 7? I didn't count.

[FC-Pilot]

Emulsion is key. We spent all day working out the setup on an AD 750 on the dyno. What we found for the emulsion and MAB surprised me. It was on a 7000 rpm 350. For kicks and giggles we threw one of our standard down leg 750 on and picked up almost 30 HP. I almost cried. we swapped out the boosters and got it to match the power our dyno carb did. What I learned was it was great for having even temps between cylinders, but the standard 750 size with annulars (which these had an .600 inner diameter to flow more air) still choked down a 350. I wish I would have played with an 850 with annular boosters as I think that would have made that engine happy.

YMMV

Paul