Thoughts on annular vs down leg for street truck

Need to buy a carb for my new build.

472
9.5:1
Edel heads
Performer and jomar powercone
218/224 HFT 112 Lsa 108 ICL
1 3/4” headers

Street truck, 79 F250 4x4, 3.55’s, 35” tires, wide ratio c6, 5500lbs with driver

90% 500lbs in bed or less, 5% 500-3000, 5% towing 6k

Would like vacuum secondaries, electric choke, good street manners in transient.

Seems like a lot of ppl like annular on the street for low rpm torque?

I prefer annular boosters in most applications, especially street. Not sure if anyone makes them in a carb like you described though.

Downlegs are less costly to manufacture. Small CFM carbs are best suited for downlegs due to size of the opening some what to say that the space is not practical for a tiny annular.
Keeping that in mind... any carb over 750 CFM will like an annular over a downleg. Then also if a carb is modified from downleg to annular there is a CFM reduction because the booster occupies more space.

I have Quick fuel 750 q series with annular boosters on a mild 502 and absolutely love it. It's probably a little small, but on the street it's super crisp and responsive.

I recently picked up a Holley 950 hp for it and look forward to a back to back drag test on both carbs.

I have used a List #9380 850 on many different combos for everyday street use and it works great. Very responsive & very "tractable"................

Annular booster provide more booster signal at lower venturi airspeed however, give up some signal at higher venturi airspeed along with some cfm of airflow.
Most every combination of engine and carburetor size will require different things to be optimum; venturi boosters included.

Sometimes for whatever reason annular boosters especially in a 1 3/8 venturi can be VERY combination sensitive. I've had an engine on the dyno that would not run on all cylinders with a 750 annular Holley. Change the intake it was ok, run the carburetor on another engine it was ok. But run it on the high compression 350 Chevy with a good cylinder head and it was terrible. I don't have a good explanation but I've seen it happen whereas a down leg stepped booster worked just fine.

Sometimes it depends on the booster itself. Many stock carbs come with an annular booster that has fewer but larger holes, and they tend to stumble or not completely atomize the fuel because its pulling larger droplets of fuel.
Many of the carb builders put in a booster that has many more holes, and smaller in size, that helps the mixture/atomization process be closer to something that would rival a fuel injector for the quality of mixture.
Some will even machine a small step just under the holes, that creates a 'shear' in the airflow passing thru the booster, and thus it helps create an even stronger vacuum "pull" through the discharge holes.
I've personally seen a nicely prepped carb like this gain 31 horsepower and 18 lbs. ft torque over the non-modified carb of the same size.

A friend of mine who builds carburetors had similar problems on some Nascar Craftsman or whatever they called them trucks carburetors. Back then the rules were unrestricted induction low compression and some teams experimented with using the annular boosters in a body with a 1-3/8 venturi. What was strange was that on some engines they ran great but on other similar combinations they were terrible, sometimes on the dyno at wide open but other times under on track conditions. I don't think that they ever figured out exactly why this was so but it was. When I asked him about the one that was giving me trouble he immediately said, "let me guess, you've got a high compression engine with a nice intake and a good set of cylinder heads. The engine seems to run fine at part throttle, idle etc but as soon as you open it up it falls apart"? This was exactly the case. Very weird.

I have had good results using an annular booster in 4500's as well as in a 1000HP body. They can work well in some cases.

I reached out to Patrick at prosystems who said he felt this combo would benefit greatly from annular boosters however in a nutshell, he wasn’t interested in building me a carb for a street application. (Vacuum secondaries and choke)

DaveMcLain wrote:A friend of mine who builds carburetors had similar problems on some Nascar Craftsman or whatever they called them trucks carburetors.  Back then the rules were unrestricted induction low compression and some teams experimented with using the annular boosters in a body with a 1-3/8 venturi.  What was strange was that on some engines they ran great but on other similar combinations they were terrible, sometimes on the dyno at wide open but other times under on track conditions.  I don't think that they ever figured out exactly why this was so but it was.  When I asked him about the one that was giving me trouble he immediately said, "let me guess, you've got a high compression engine with a nice intake and a good set of cylinder heads.  The engine seems to run fine at part throttle, idle etc but as soon as you open it up it falls apart"?  This was exactly the case.  Very weird.  

I have had good results using an annular booster in 4500's as well as in a 1000HP body.  They can work well in some cases.

X2 on the Annular Booster's on the 1000 Hp Body I have the 80514 1000 cfm HP annular booster carb on my 466 works very good and has nice street manners!

I have read annular shines in street application yet no one makes street carb (vacuum secondaries and electric choke) with them?

Seems odd.


Maybe that should tell me for some reason it doesn’t work well?

somethingclever wrote:I have read annular shines in street application yet no one makes street carb (vacuum secondaries and electric choke) with them?

Seems odd.


Maybe that should tell me for some reason it doesn’t work well?

BLP (Bo Laws Carbs) will make any combination you want.

If you even want annular boosters in the primaries and stepped-downleg in the secondaries of any size carb ... you got it.

somethingclever wrote:I have read annular shines in street application yet no one makes street carb (vacuum secondaries and electric choke) with them?

Seems odd.


Maybe that should tell me for some reason it doesn’t work well?

Annular boosters were common in factory carbs with VS and choke. I think the reason you don't see them in the aftermarket is that people who want VS and a choke are more interested in price than performance.



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somethingclever wrote:I have read annular shines in street application yet no one makes street carb (vacuum secondaries and electric choke) with them?

Seems odd.


Maybe that should tell me for some reason it doesn’t work well?

The stock Holley vacuum secondary carburetor that came on the 460 in my '79 Ford F350 has annular boosters in the primary and I believe a straight leg booster in the secondary. It has a choke etc. That carburetor worked ok but tended to be a little too rich in the transition slot. Finding one of those carburetors would be very easy, rebuild it and give it a try it might work well for your application.

Edelbrock now has a carburetor which has annular boosters in the primaries and regular boosters in the secondaties ... The Holley truck "Avenger" carburetors have had boosters that way for years.

I can Swage in annular boosters into any carb you want.  You'll want the air correctors and fuel restrictions threaded so you can tinker and recalibrate it to your setup tho.
A cheap ole swap meet 3310 would probably be best.

(Or as Randy mentioned, so can BLP)

The Summit VS carbs also have annular boosters.

TommyK wrote:The Summit VS carbs also have annular boosters.

YES ... that is an updated Autolite "Flat-top" carb, updated with Holley parts. Holley used to sell those carbs years ago.

Thanks everyone for your feedback.

I will reach out to Bo Law Carbs and go from there.

I am not sure if a properly sized/setup annular outperforms a properly sized downleg/straightleg on the street or not...hoping the carb builders can shed some light on it.

If i don't go custom built, then I may try that Holly Truck Avenger that Randy suggested.

It's interesting that the OEM's were putting annular on truck engines back in the 70's and also that the only carbs built today like what I am looking for are generally labeled "truck" or atleast "street" carbs.

i also may be overthinking this with 472" and a 218 deg duration cam drawing through the venturi's.

I don't think Randy "suggested" the use of the Holly Avenger carb as much as he "announced" it's existence.

I believe HOLLY's Street Demon 750 CFM carburetor would be a good choice for your build. It has a "triple stack" booster and many very good features.

somethingclever wrote:Thanks everyone for your feedback.

I will reach out to Bo Law Carbs and go from there.

I am not sure if a properly sized/setup annular outperforms a properly sized downleg/straightleg on the street or not...hoping the carb builders can shed some light on it.

If i don't go custom built, then I may try that Holly Truck Avenger that Randy suggested.

It's interesting that the OEM's were putting annular on truck engines back in the 70's and also that the only carbs built today like what I am looking for are generally labeled "truck" or atleast "street" carbs.

i also may be overthinking this with 472" and a 218 deg duration cam drawing through the venturi's.

man don't take this the wrong way as I'm not trying to be offensive, but I think you are over thinking this. I think the truck avenger has some bowl tricks inside for extreme off roading also. in all honesty if you throw a reg old 750 on it I don't think you'll feel a difference in what type of booster the carb came with. now of course you can tell if you need some pump work or squirter nozzle sise jets ect. I like the avenger carbs but not $100-$150 more than a reg 750 holley. if you were out for every last drop of horse power, the go custom. but to me its over kill. get what you need elec choke...? ck... vac secondaries...? ck...750 cfm...?ck... you have a pretty basic street build. just something to ponder...

torino501 wrote:man don't take this the wrong way as I'm not trying to be offensive, but I think you are over thinking this. I think the truck avenger has some bowl tricks inside for extreme off roading also. in all honesty if you throw a reg old 750 on it I don't think you'll feel a difference in what type of booster the carb came with. now of course you can tell if you need some pump work or squirter nozzle sise jets ect. I like the avenger carbs but not $100-$150 more than a reg 750 holley. if you were out for every last drop of horse power, the go custom. but to me its over kill. get what you need elec choke...? ck... vac secondaries...? ck...750 cfm...?ck... you have a pretty basic street build. just something to ponder...

Pay particular attention to the difference in torque production below 3000 rpm.

https://www.hotrod.com/articles/carburetor-showdown/

torino501 wrote:

man don't take this the wrong way as I'm not trying to be offensive, but I think you are over thinking this. I think the truck avenger has some bowl tricks inside for extreme off roading also. in all honesty if you throw a reg old 750 on it I don't think you'll feel a difference in what type of booster the carb came with. now of course you can tell if you need some pump work or squirter nozzle sise jets ect. I like the avenger carbs but not $100-$150 more than a reg 750 holley. if you were out for every last drop of horse power, the go custom. but to me its over kill. get what you need elec choke...? ck... vac secondaries...? ck...750 cfm...?ck... you have a pretty basic street build. just something to ponder...

No offense taken at all. It's a discussion board and it's not the first time I have over-thought something.

I read this older article and this particular test found seat of the pants gains with annular. https://www.hotrod.com/articles/carburetor-showdown/

But each deal is different and I don't know what gains, if any, i would have in my combo listed above.

with so much dyno-proven and track proven experience on the board, I thought someone else may be able to chime in with an affirmative, or negative, to the validity in booster differences on this particular application.

Seemingly with only 218 deg intake duration and 472" sucking through the carb, i would not expect to see 80 ft-lbs difference at 2000 rpm where the combo will live 90% of the time....but if it's worth a little power, and a little economy too, then why not do it.

DanE wrote:I don't think Randy "suggested" the use of the Holly Avenger carb as much as he "announced" it's existence.

I believe HOLLY's  Street Demon 750 CFM carburetor would be a good choice for your build. It has a "triple stack" booster and many very good features.

Thanks Dan...I looked into those carbs last night after you suggested (ok..announced) it.

This, along with the off-road truck avenger, could be good off the shelf solution for me.