Annular vs downleg

[Yelcamino]

When would an engine require (or benefit from) an annular carb over a standard downleg carb?

Thanks.

[ProdigyCustoms]

That depends on the motor. Tell me again what it is?

[Yelcamino]

The engine in question is a BBC:

460 cid, 4 bolt main
Brodix Race Rite heads (rectangle port)
Edelbrock Performer RPM Air Gap intake
Comp hyd. roller cam (.566” lift, 224* @ .050”)
Comp Magnum roller lifters
Comp Hi-Tech pushrods
Comp Pro Magnum 1.7 rocker arms
GM forged steel crank
GM forged 7/16 rods w/ ARP Wave-loc rod bolts
Speed Pro forged pistons (10.2:1 CR)
(rotating assembly was balanced)
Total Seal rings
ARP main bolts and head studs
Performance Distributors DUI
MSD 8.5mm Super Conductor wires
Autolite plugs

Trans is a TKO600 with 3.70 rear gears.

Currently there's a questionable Holley Street Avenger 850 on it. I'm considering replacing it and was wondering if there would be any reason/benefit to go with an annular over a downleg carb.

When I ran this engine on the dyno the only carb I had available was my Quick Fuel 850. It worked good but wasn't really calibrated for this engine. The Holley works ok but I'm not happy with it.

[ProdigyCustoms]

I think you need someone to tweak that carb. Those are pretty good.

[Yelcamino]

I think it needs more than tweaking. I went through it and replaced all the gaskets, needles, and both power valves. I also jetted it according to the dyno session reults. The problem is there's a vacuum leak on the primary throttle shaft - you can actually hear it whistle at idle and you can change the tone of the whistle by pushing on the throttle arm. I don't have the ability to bush the throttle shaft and I figure by time I ship it to/from someone that can and pay to have it done, I could almost buy a new carb and bolt it on after I change the jets.

Before I spend any money I thought I should research my options, which is what led me to ask about the annular vs downleg carbs.

[68Formula]

If the cam was more aggressive and/or the compression lower I could see needing annular. The real benefit is when you have a poor carb signal at the lower rpms. The cam is not small, but it's not large either. The compression is decent. The RPM is still a dual plane and provides a good signal. Would annulars hurt?... No. Would they help?...doesn't seem like it in this case.

[Yelcamino]

Ok, so an annular would help solve a poor signal issue, which I'm pretty sure isn't an issue with 15-16" of manifold vacuum.

Thanks.

[Shaker455]

Herb,
Do you run your car at the drags or any type of competition?

Your Street Avenger is a straight leg carb...correct? List # 80870?

If you were to send it to me to boost it's performance I would install new Holley HP series downleg boosters with a shear step for starters.

These are more efficent as they sit down in the airstream and take up less CFM as opposed to a straightleg or annular booster.

Plus the shear step creates better fuel atomization.

Blueprint baseplate for true WOT,Slab the throttleshafts for airflow gain and crisper throttle responce as well as teflon bush the shafts to seal & control vacuum leaks and reduce metal to metal contact.

Also adding 4-corner idle.

I build alot of custom annular & downleg Holley's and I do so with all the info like you posted and with it's intended use.

Then I will set it up and live test it on my own car before I ship it off to the customer for the final stages where I guide thru the entire tuning phase.

Jeff

[ProdigyCustoms]

Exactly what Mike said, he did a excellent job of saying exactly what I was thinking. The annular is kind of a crutch. If you are starting from sctratch, and if the shaft needs bushings, you may be better to replace, I would look at a HP carb.

I just used a couple of the new HP750s (smaller motors) and LOVED them! If you need one, I can probably do a deal for you.

[Yelcamino]

Herb,
Do you run your car at the drags or any type of competition?

Your Street Avenger is a straight leg carb...correct? List # 80870?

If you were to send it to me to boost it's performance I would install new Holley HP series downleg boosters with a shear step for starters.

These are more efficent as they sit down in the airstream and take up less CFM as opposed to a straightleg or annular booster.

Plus the shear step creates better fuel atomization.

Blueprint baseplate for true WOT,Slab the throttleshafts for airflow gain and crisper throttle responce as well as teflon bush the shafts to seal & control vacuum leaks and reduce metal to metal contact.

Also adding 4-corner idle.

I build alot of custom annular & downleg Holley's and I do so with all the info like you posted and with it's intended use.

Then I will set it up and live test it on my own car before I ship it off to the customer for the final stages where I guide thru the entire tuning phase.

Jeff

Hey Jeff,

The car this carb is on has been on the strip and autocrossed here and there, but that's not it's intended purpose; it's really a mild street cruiser with a kickin' stereo and a/c that has been driven 19,000 plus miles since I got it on the road in 2003.

Here's the number on the air horn...


It looks to me like it already has downleg boosters...


Earlier today I spoke with the tech at Barry Grant and he recommended a 750 DP because I have a manual trans. He said the load an auto trans puts on the engine affects the signal to the carb differently than the manual trans which is why the DP is preferred.

How much would all the things you mentioned cost me?

[Yelcamino]

Exactly what Mike said, he did a excellent job of saying exactly what I was thinking. The annular is kind of a crutch. If you are starting from sctratch, and if the shaft needs bushings, you may be better to replace, I would look at a HP carb.

I just used a couple of the new HP750s (smaller motors) and LOVED them! If you need one, I can probably do a deal for you.

Hey Frank, I just saw your post! We must have hit the go button at the same time! I think the HP might be too much of a race oriented carb for this car. I want it to start easy in cold weather (since I live in the colder climate these days), and be a good cruiser.

Let me see what Jeff says.

[ProdigyCustoms]

And I did not read Jeffs post until now, but I agree with what he is saying.

[Shaker455]

Hey Herb,
Thanks for posting the pics and your intended use.

The throttleshaft has to be addressed as that will directly affect your tuning and performance.

I would fix that first and then try to retune as there's many things you can do to that carb.

Seeing it's an 850 mainbody with a rear squiter boss it can be converted to a DP Mech sec and if you wanted a slightly smaller carb it could be achieved by changing the booster to annulars.

Typically with downleg 850's they have a weak signal at part throttle/cruise and I resolve this with jetting the transition circurt.

Annulars have a good signal and do not require this but the trade off is they take up more CFM, if you were drag racing I'd lean towards downlegs.

I haven't had a choke on any of my cars for years and if the carb is tuned properly it will not require it....ran my car into Dec last year until they salted the roads!

More times than not the flap will not be opened fully and cause tuning issues

You can PM or email for prices.

Frank,
I'm glad to hear you had good results with the HP's,

Many come my way overly rich, difficult to tune, and lack of needed vacuum ports for street use.

When I build a new HP from scratch I use a baseplate with vacuum ports and proven Holley metering blocks that feature ported vacuum for engines that want to ultilize vacuum advance.

I add the ported passage into the HP mainbody which has none out of the box.
Jeff

[ProdigyCustoms]

Frank,
I'm glad to hear you had good results with the HP's,

Many come my way overly rich, difficult to tune, and lack of needed vacuum ports for street use.

Rich was a problem with the mechanical we used on Michaels 383, but is attributed that to being a bit big for the motor anyway, but being a nitrous motor............ It runs real sweet now, and I went ahead and put a 50CC pump on it and accelerator pump cam. Gave it a little stronger pump shot of idle with a tight converter.

We just did the vacuum secondary version on a 400 Pontiac that is pretty stock right now, but getting some mods in a few months. We just needed a carb now so we bought to big for later modifications. Thought we may have to play with it, but so far it seems pretty killer. It fired up stone cold, idled right out of the box without touching it, and idles clean. We will drive it tomorrow

[Zee]

Send your carb to Jeff and have him work his magic. I'll have to get some more seat time with my Shaker rodded 800 before revisiting the annular vs. downleg issue. However, since you are running a RPM, I would go with the downlegs.