Annular boosters for a QF 750 DP

[BradH]

... 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.

This doesn't surprise me. I like annular boosters, but wouldn't bother with them on a venturi < 1.56" for my application (450-ish ci 7000+ RPM street/strip engine).

[FC-Pilot]

... 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.

This doesn't surprise me. I like annular boosters, but wouldn't bother with them on a venturi < 1.56" for my application (450-ish ci 7000+ RPM street/strip engine).

It caught me off gaurd how much it killed power as there was not much difference on the flow bench. The flow difference was negligible. Yeah, 450” or above I would definitely use a big Venturi like you said. I did like how the EGT’s were much better balanced with the annulars.

Paul

[HQM383]

... 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.

This doesn't surprise me. I like annular boosters, but wouldn't bother with them on a venturi < 1.56" for my application (450-ish ci 7000+ RPM street/strip engine).

It caught me off gaurd how much it killed power as there was not much difference on the flow bench. The flow difference was negligible. Yeah, 450” or above I would definitely use a big Venturi like you said. I did like how the EGT’s were much better balanced with the annulars.

Paul

Did you reduce MAB and emulsion?

I wonder if airspeed over atomized the fuel?

[FC-Pilot]

Yep, best power was with one e-bleed and I think the MAB was .030. It also kept the fuel curve flat smooth. It was just down on power. Anyway, that is just my experience on one engine. I am open to hear what others have experienced in their own testing.

Paul

[ArizonaGuy]

Well, I’m going to give it a shot. Main body w/annulars will be here soon.

Any recommendations on a baseline calibration?
As in: richer ‘here’, or leaner ‘there’, etc.
?

[ClassAct]

Well, I’m going to give it a shot. Main body w/annulars will be here soon.

Any recommendations on a baseline calibration?
As in: richer ‘here’, or leaner ‘there’, etc.
?

You will probably need less emulsion to start with.

[HQM383]

Well, I’m going to give it a shot. Main body w/annulars will be here soon.

Any recommendations on a baseline calibration?
As in: richer ‘here’, or leaner ‘there’, etc.
?

Do you have an AFR gauge?

First thing is to bolt it together and give it a test drive. It will be rich but this establishes ground zero to work from. Adjust jet sizes to where you want AFR then report back if there are corrections that you want to make.

[ArizonaGuy]

I should’ve done this years ago

I didn’t mess with my idle circuits, they were dead-nuts so no need make any changes.
I duplicated the high and low speed ABs from the old down leg main body and calculated 5% reduction in primary and secondary jetting.
It was real close, but actually a touch lean on the top end so I upped the secondary jets two sizes.

WOT AFR is right where I want it (12.8-13.0) but the power circuit was a bit leaner than I wanted.
I calculated total area of the secondary jets and primary PVCRs, calc’ed a reduction in secondary jet and a corresponding increase in PVCR.
It’ll be interesting to see how it behaves (haven’t driven it yet with those^ changes).

It really woke up the low and mid range torque and I’ll be darned if it lost any top end at all.

[HQM383]

I should’ve done this years ago

I didn’t mess with my idle circuits, they were dead-nuts so no need make any changes.
I duplicated the high and low speed ABs from the old down leg main body and calculated 5% reduction in primary and secondary jetting.
It was real close, but actually a touch lean on the top end so I upped the secondary jets two sizes.

WOT AFR is right where I want it (12.8-13.0) but the power circuit was a bit leaner than I wanted.
I calculated total area of the secondary jets and primary PVCRs, calc’ed a reduction in secondary jet and a corresponding increase in PVCR.
It’ll be interesting to see how it behaves (haven’t driven it yet with those^ changes).

It really woke up the low and mid range torque and I’ll be darned if it lost any top end at all.

Welcome to the annular booster fan club

[BOOT]

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.

At what rpm did the downleg overtake the annular?

[BOOT]

I'm a fan of annular boosters for street, vid shows the ones I have atm https://youtu.be/iyLQCWCN1NI

[novafornow]

Another great application for the annular booster is if one runs index races with an under carb stop. ET genie etc. The signal can be goofy in this application. I switched to annular and gained consistency.

[BradH]

I should’ve done this years ago

I didn’t mess with my idle circuits, they were dead-nuts so no need make any changes.
I duplicated the high and low speed ABs from the old down leg main body and calculated 5% reduction in primary and secondary jetting.
It was real close, but actually a touch lean on the top end so I upped the secondary jets two sizes.

WOT AFR is right where I want it (12.8-13.0) but the power circuit was a bit leaner than I wanted.
I calculated total area of the secondary jets and primary PVCRs, calc’ed a reduction in secondary jet and a corresponding increase in PVCR.
It’ll be interesting to see how it behaves (haven’t driven it yet with those^ changes).

It really woke up the low and mid range torque and I’ll be darned if it lost any top end at all.

How was the A-B testing done? Engine dyno? Chassis dyno? Butt dyno and a wideband O2?

[FC-Pilot]

At what rpm did the downleg overtake the annular?

We were starting the pulls at 4000 and the down legs were already ahead on power in hat combo.

Paul

[n2omike]

I think annular boosters would be GREAT on a tunnel ram. Annular boosters in a smaller throttle bore (750) are going to be very restrictive in any semi-serious single carb engine. Annular boosters in a 850 are good for a lot of combinations, but are still not enough for 400+ ci spinning 7K+ rpm.

My 850 annular made 611 hp on my pump gas 363 at 7500 rpm... and pulled to 8k. I would like to try a good down-leg carb to see how it would do.

I made one trip to the dyno, but didn't get to try the nitrous. Who knows, maybe another trip is in order. lol