Carburetor Annular vs Downleg Boosters

[@vermiciousknid5840]

I used to have an Edelbrock 650 off-road version, but it always ran too rich. I replaced it with an AVS 2, 650 & the difference is impressive. Acceleration is much smoother around town with much better gas mileage & no loss of power. At low throttle it drives more like fuel injection, but still slams me back in the seat when I step down hard.

[@SteelSurgeon]

Your videos are awesome bro! Keep it up. A lot of us newbs need all this info and you are good at dumbing it down for us.

[@MuscleCarSolutions]

Thanks! I just try to keep it simple and put as much information into each video. Glad you found it useful!

[@magdump7380]

I came to appreciate the annular design when I bought my first cheap Summit Racing carb. Haven’t looked back.

[@johnhenderson9173]

This is a fantastic explanation of booster comparison and how they work. Best video I have seen on youtube explaining this. Thank you very much.

[@MuscleCarSolutions]

Awesome! Thank you!

[@bigblockjalopy]

Thats why engines with intakes without a heat crossover and without a working heatriser in the ex manifold MUST have a carb with an anular booster. Best is to have both.

[@Trump985]

Annular boosters create a stronger signal however they create a flow restriction compared to a downleg booster. They are absolutely beneficial in a race application as they allow you to run a lower pressure drop. These is so much confusion over carburetor CFM ratings but to sum it up a 4 barrel carb is rated at 1.5 in hg. With annular boosters, and other modifications it’s possible to get a carb to function below .5 in hg pressure drop. Let’s say your engine needs 750 CFM, well with a 750 CFM carb you will have 1.5 in hg manifold vacuum! Now let’s throw on a 1150 CFM carb. Clearly it won’t flow that much air because the engine isn’t capable of pulling an 1.5 inches of vacuum across it at WOT. However your engine will draw more then 750CFM of air because the pressure drop is lower however with a much lower manifold vacuum. Now we’re getting to a point where we are going to run into problems due to insufficient booster signal with straight leg or down leg boosters and we’re probably going to be around .75 inches of mercury. The engine is going to run like hell and probably going to fall on its face as soon as the extra fuel from the accelerator pump is gone. Well let’s switch to annular boosters and see the difference. The pressure drop across the carb has gone up slightly, maybe we’re seeing .80 in hg now however the carburetor is capable of functioning properly now and you could even size it up to get it back to a .75 in hg pressure drop and still maintain a good signal. Power is up as the restriction is reduced and the airflow is higher. This is why you’ll often see sub 350 ci race engines running dual 1150 CFM carbs. These engines can’t possibly draw 2,300 CFM even running at 10,000 rpm and with pressure wave tuning and VEs of 120 percent, but that’s not the point they are running down below .5 inches of vacuum.

[@kenjarabek9901]

Currently restoring back to being a daily driver; 1980 GMC Caballero with: 305ci/5.0L engine, Carter Quad 4bbl, 350 Trans, 64k on the engine, runs very good. Part of my restorations intentions will include elimination of CarterQuad, Smog sensors, air pump, catalytic converter, etc., and simple additions are hugger headers, dual exhaust (haven’t decided on mufflers yet) and either a 600 or maybe a 650cfm Holley or Edlebrock. Would appreciate your feedback. Cheers and thank you.

[@MuscleCarSolutions]

You’re in a bit of a tough spot with the sub 4” bore engine. Performance parts are extremely limited. What is available is hard to justify when the performance gains are so minimal. I have a video on how to choose the right size carb and the answer might shock you. Given there’s such a gray area with that engine and you are looking to improve performance, I’d go with the AVS2 650. But honestly if you were stating stock, I’d put a 500 CFM carb on that engine all day long.

[@merritts3515]

I would use a 500 cfm at most. A 390 cfm if it's bone stock.

[@Torquemonster440]

I switched from a well optimized Vacuum secondary 750 cfm Holley to an out of the box 850 cfm Holley double pumper with annular boosters. Holy Cow !! Could not believe the difference in the cars ability to accelerate from a dead stop. It was night and day. That carb made my 440 powered B-body pull like I had always thought it should. Why every carb does not use annular boosters is beyond me.. production costs I suppose. ?... excellent video btw. 🍻

[@MuscleCarSolutions]

Wait till you tune it! Even better power and throttle response. Yes cost is the big factor. Carburetors are a dying category. To invest the massive amount of money needed in hopes of recovering it is a big gamble.

[@trailerparkcryptoking5213]

Was your VS a 3310?

[@Torquemonster440]

@@trailerparkcryptoking5213 yes, but I had an aftermarket main body on it.. with down leg boosters, so not truly a 3310.

[@trailerparkcryptoking5213]

@@Torquemonster440 I have a 3310, but it’s been heavily modified by DaVinci Carbs including downleg boosters in the rear and it flow tested at 830 cfm.....I have always wanted to try a little bigger (850) DP on my 454 to see how it responds. I think I have some room for improvement in the carb area.....

[@reggiebarrios3274]

I know you said the older Edelbrock carbs are NOT upgradable to annular discharge but what about the original AVS Series which came in 650CFM and like mine 800CFM. I would Iike to up grade for better low end drivability. I’m sure this won’t hurt my top end considering it’s going on a flat top 350 @ 10.0:1ish with a comp cam somewhere around a 280H which is not that big of a cam. I’d hate to get rid of it since it’s the 800 AVS Endurashine p#19134. 1913 is the part as the the AVS2 800CFM. Any thoughts

[@cutl00senc]

I’m running a tuned 1406 and it runs fine....but I can’t seem to get rid of the slight loss of power as you accelerate. I’m sure the avs will most likely cure the issue, just not ready to shell out the scratch just yet.

[@MuscleCarSolutions]

Sounds like you have more tuning to do, as long as that’s the right size carb for your engine.

[@ricardoflores5555]

Great demonstration. The downleg booster has a curved shape that places the booster lower in the bore and increases the venturi pressure. The term, pressure drop, used on this video is misappropriate. The pressure is drawn downward, but does not drop, it actually increases as it gets deeper into the intake.

[@trailerparkcryptoking5213]

The pressure drop (lower pressure area) happens in the Venturi. Read a fluid dynamics book!

[@kevinwestmoreland2099]

I've read that with an annular booster signal being stronger, fuel flow is quicker and slightly leaner jetting can be used. Is that correct? I have a holley 84010 on my 350 in my truck and the stock jetting of 67 PRI, 75 sec, and 65 over seems a bit RIC.

[@WW226]

I love your carburetor videos! I'm considering a Mighty Demon with the annular boosters myself

[@MuscleCarSolutions]

As long as it’s the correct size, it should make for a responsive street carb.

[@brianfrancis2862]

i have fe360 with 390 intake and heads bored 30 over stock c6 trann with 468 comp cam what be best set up carb put it in galaxie f street

[@MuscleCarSolutions]

Need more info. If you watch my video on how to choose the right CFM, that’s a good place to start. Then you’ll need to define what it’s give to be used for and you’ll be able to select the right carburetor for your application.

[@DanMiller-xw8yz]

I am in the process of installing an Edelbrock AVS2 with the annular rings. My previous carb is an Edelbrock Performer 1406 - 600 cfm. Yes, I have had that nuisance off-idle hesitation from that carb. I did many tunes and mods to help the problem. I am hoping that the new AVS2 will completely solve that problem. Was wondering in the video, you showed the annular booster from the AVS2, removed. Do you know if Edelbrock sells those annular boosters as a possible upgrade to an older Performer ? They boosters look very similar.

[@MuscleCarSolutions]

The last video I did was on this very topic. I cover that question extensively. The short answer is no. The long answer as to why is in the video.

[@triumph928]

I've watched several of your video's today. Seriously considering swapping out my 950 DP Holley for the 800 Edelbrock AVS2. I've got a 468 BBC and while it runs fine I feel the 950 is too much for the street. I want to thank you for helping me make the decision to switch. Always wanted too try a annular discharge carburetor and your video's are very helpful and educational. I now understand why I could never get an Edelbrock carb to run right. Thanks again.

[@MuscleCarSolutions]

It all comes down to three things. Correct fuel system, ignition and timing set properly and the adjustments made to the right size carb. The issue with Edelbrock carburetors isn’t the hardware, it’s that for some reason people don’t seem to be willing to learn how to TUNE. Holleys aren’t any different. It’s just the excessive fuel they push covers up a lot of flaws and people think it’s good enough. Best of luck on your decision!

[@corey_3922]

Nice video bro, I learned a lot. I’m throwing a Mighty Demon 850 w/annular boosters on a LS3 crate engine. Super excited about it

[@zigandarov]

Какой именно внедрять думаешь???

[@mikewilliams-mj9rd]

Hi, I am running a couple of Holley's on a 671 blower that have 4 downleg boosters, could I change the front boosters to the Annular style ? this car run's on the street only.....Thanks

[@MuscleCarSolutions]

It depends. What holleys do you have? Do you have annual boosters laying around?

[@mikewilliams-mj9rd]

They are 600 cfm holley's, and I don't have any boosters I would have to order some...Thanks

[@MuscleCarSolutions]

@@mikewilliams-mj9rd there’s more than one 600 CFM Holley carb. Are they vacuum secondary carburetors?

[@mikewilliams-mj9rd]

Yes, There are two double pumpers....Thanks

[@mikewilliams-mj9rd]

They are 600 cfm, Mechanical secondary square bore...Thanks

[@edsmachine93]

Very good video. Thanks for sharing. Take care, Ed.

[@joearokis8265]

Is there any difference in reliability? It seem the single larger hole in a down leg would be less prone to blockage than the smaller multiple holes in the annular style.

[@MuscleCarSolutions]

Annular boosters aren’t new technology. They’re as reliable as the straight/downleg booster. If it’s THAT bad in the carburetor, it will case issues long before you’d notice it at the booster.

[@joearokis8265]

@@MuscleCarSolutions thanks. The last time I owned a car with a carburetor was in the 80s, before ethanol. I have several motorcycles with carbs and when they sit more than a few weeks, I have to pull the carbs to clean the pilot jets.

[@MuscleCarSolutions]

@@joearokis8265 ethanol fuels still demand some additional maintenance. Keep good filters in the system, drain the crab if you’re not going to drive it for extended periods of time and throw an additive in the tank if it sits for months.

[@steelcitycaprice.899]

I recieved great understanding and appreciated the post.👍😎

[@MuscleCarSolutions]

Glad you enjoyed it.

[@steelcitycaprice.899]

@@MuscleCarSolutions ABSOLUTELY, it put fuel and the air in better PERSPECTIVE as I purchased a avs2 and swapped out cast iron heads for blue print ALUMINIUM and I'm having air fuel issues and I see now how to fix it.👍

[@zigandarov]

Какой бустер лучше?на объём 4.7 литров двс

[@brianfrancis2862]

i got 650 holley on it with downleg booster want something when i hit gas she goes but i want good idel at redlight

[@MuscleCarSolutions]

Boosters don’t meter fuel at idle.

[@ryanpaasch7464]

I have a Carter 625 with a bar that is like the annular? I have a 360 mopar around 400 hp it struggles a little wide open us the carb to small? Any thoughts

[@MuscleCarSolutions]

Maybe? Don’t know. Need a lot more details.

[@britdragster]

Can you swap from a regular straight leg or down leg booster to a annular booster in a 4150 style carb?

[@MuscleCarSolutions]

Some 4150 carbs have that option to purchase an annular booster. Typically the higher priced carburetors.

[@tooljunkie6003]

Great demo!!! I have an AVS Edelbrock carb, and i asked Edelbrock if the Annular booster could be sold separate and if it would would cure the bogg.... Edelbrock said nope , you need to buy the new carb.. Thanks for the video !! As always explained well , and makes it clear to understand

[@MuscleCarSolutions]

The issue is calibrating enough fuel on a non AVSII carb. If you notice, they don’t offer a 600, only a 650. They don’t have a 750, only an 800. Reason being is the booster needs a bigger signal to work properly in its current configuration. Putting the annual boosters on a regular performer series carb would only make it worse. It’s all about the air speed and the pressure drop to pull the fuel from the main fuel circuit.

[@car_ventures]

@@MuscleCarSolutions Thank you! I was wondering the same too. So the issue with retrofitting an annular booster to a performer style edelbrock is that the booster doesn't allow enough air flow through it to pull enough fuel out of the booster?

[@MuscleCarSolutions]

@@car_ventures that is correct.

[@lordsauto]

Hi any thought on the Thompson power blast?? Thanks Bob

[@MuscleCarSolutions]

Never used one. Kind of like the old Tornado for throttle bodies. Doesn’t really shake out that well in actual driving situations. The issue is fuel calibration. There’s a reason why annular carburetors take some tweaks to CFM and fuel delivery. Slapping in a piece of tin might get the atomization, but it’s not going to correct the other areas.

[@ercost60]

Great demo, thanks for this video.

[@mrblanche]

So is the AVS annular booster capable of being fitted to a downleg booster carb?

[@MuscleCarSolutions]

No. Not enough airflow to cause them to operate properly. That’s why a 600 is not offered, only a 650. The 500 you MIGHT get to work, but you’d have to go buy a 500 CFM avs just to take the boosters out. Wouldn’t make any sense financially.

[@mrblanche]

@@MuscleCarSolutions Too bad. I have a brand new 750 Performer that I would like to improve.

[@MuscleCarSolutions]

@@mrblanche wouldn’t work. That’s why the AVS2 is an 800. Needs more airspeed and volume. Sell the 750 and get the 800! Or find one used.

[@mrblanche]

@@MuscleCarSolutions The annoying thing is that I had an AVS2 650, but I don't think that would do the job on my 468 BBC.

[@MuscleCarSolutions]

@@mrblanche probably not near enough CFM.

[@PhilAVega]

Why does my Edelbrock 750 have both? I was cleaning my carb and noticed it. I thought maybe something was broken. Just asking

[@MuscleCarSolutions]

Have both what? Types of discharge nozzles? That should not be the case with the 750. The 750 only comes with a straight leg style booster. The AVSII has a primary annular and a secondary straight leg style. That carburetor comes in an 800 CFM size and not in a 750.

[@PhilAVega]

My carb is the 1407 750 mechanical choke. I noticed the fuel coming out if the right side Venturi like fast drips. Then left side was spraying fuel out in a strong spray at the same time. The carb sits on top of a 454. It would run great. Now I experiencing some hesitation on acceleration. I was thinking maybe the right side needed replacement. I'm new to working on carburetors. Thank for responding back .

[@MuscleCarSolutions]

@@PhilAVega ok so you’re a straight leg style. So if you’ve watched any of my other videos, I always start with fuel system first. What’s your pressure regulator set at? What kind of filters are you running? Can you pop them open and see if you’ve gotten any garage in them? Running an ethanol fuel? E10 pump gas? Then it’s just going to be about popping the top of the carb off and see if there is any trash inside. Little blow through with some carb cleaner to see if you can clear it and see how it runs. Check float levels and needle and seats too. Blow though the jets and the primary squirter as well as the accelerator pump just to check all the bases. See how it does.

[@budhihart8161]

This is just i looked for..are there another booster design that makes atomization better?

[@MuscleCarSolutions]

Not with the limited amount of pressure that a carburetor uses, no. But it’s not boosters alone. Venturi design and sizing are also a factor.

[@budhihart8161]

@@MuscleCarSolutions my carb booster has straight small pipe acroos..and has litle hole at the center of the pipe..that goes through up to down.....is it downlegs type? When i slit a wire inside the hole the car seem runs better and better mpg...what do u think heppens?

[@zombie8294]

where can i buy without buying the whole carb?

[@MuscleCarSolutions]

Buy what? The boosters? They aren’t sold separately. I explain why in the recent booster video I did.

[@psychoholicslag4801]

​@@MuscleCarSolutions actually you can buy boosters seperately but you need a jig to install them in the main body. They're just spin pressed inand can be removed with a punch and hammer.

[@yo.its.michael9425]

Great video as usual

[@MuscleCarSolutions]

Thanks! Very much appreciate that.

[@NightWrencher]

Excellent video 👍

[@MuscleCarSolutions]

Thanks! Glad you enjoyed it!

[@johnmullins2798]

Good stuff!

[@fbombcamaro434]

That's without airflow. With airflow it atomises fuel very well. Annular is better at low rpm as it just has more than 1 hole. Not so good for power top end.

[@zigandarov]

Почему же ??? Много отверстий плохо? Есть замеры?

[@hybridmusclegarage4590]

Another GREAT video!!

[@MuscleCarSolutions]

Thanks man! Appreciate that!

[@direct998]

Great video!

[@MuscleCarSolutions]

Thanks!

[@rono3045]

Those were both straight leg boosters one was an annular straight leg the other one was just a standard straight leg. A down leg they actually bend down just like a Holley

[@MuscleCarSolutions]

Semantics. Ask 100 car guys what that is and they’ll call it a down leg booster. In reality it doesn’t matter why it’s called, the demonstration is what is important. Less know it’s a booster. They’ll call it the venturi before corrected. Is what it is and I needed the name of the video to be search friendly. :)

[@adamballinger1358]

This is not a good test or demonstration ,add airflow over the booster allowing fluid to be drawn in non pressurized and you'll really see what's going on!

[@MuscleCarSolutions]

Unless you’re going to fund that type of expense to create that real life condition for me, it was a perfect demonstration to show the difference between the two. Not the difference between the two in operation. But if you’re interested, my Patreon link is in the description.

[@adamballinger1358]

@@MuscleCarSolutions I did the test I am referring to with some old holley boosters and a piece of 1.5" pvc pipe and my garage air compressor with the air pressure regulated of course and then measured how much fluid was drawn from a Pyrex baking cup like yours.

[@jerr3458]

Is this narrated by Seth Rogan

[@MuscleCarSolutions]

Yes.

[@denniswinston7365]

Looks like I’ve got to dust off the old edelbrock

[@MuscleCarSolutions]

Go build something!

[@talltimbot]

Ring

[@sidecarbod1441]

The design of the Edelbrock annular boosters is really bad, I even had an argument with a guy from Edelbrock about this but I suspect he was just a 'marketing guy', he didn't know anything, they deleted all of my comments! The bore on their boosters is just a parallel hole, it should have a bell-mouth entry and a tapered exit, in fact in cross section the booster should look like an aircraft wing. The consequence of the bad design is a really poor fuel signal, to get around this they have effectively richened up the primary circuits compared to the older straight leg carbs of the same size, the AVS2 500 carb is jetted 30% richer on cruise and 10% richer on the acceleration circuit compared to the old carb, I suspect this is just so it flows the same amount of fuel as the old carb on a wet flow bench when pulling a 'standard' CFM through the carb. The annular booster should be creating a much stronger fuel signal which should mean that the out-of-the-box setup should have been jetted a lot leaner and yet still delivered the same amount of fuel for any given manifold vacuum compared to the older carb. The poor fuel signal can mean that the engine goes lean just as the carb moves off the transition slots because the primary circuit does not start to flow fuel soon enough. Drilling out the tube that forms the pilot jet from 0.7mm to 0.9mm can help allow the transition slots to flow more fuel but this is just masking the real issue. Drilling out the pilot jet means that the idle mixture goes rich but the two fuel screws can lean it back to what the engine requires. Weirdly drilling out the jet to 1mm seem to cause the idle and transition circuit to start to lean off again. I have copied Edelbrocks design and converted an older set of 500 boosters to annular boosters, my boosters suffered the same poor signal, I'm now working on a new set with a proper venturi style bore, with the discharge holes at the narrowest point. Despite the poor design and lean bog issues I have found that the straight bore annular boosters do atomize the fuel well, on the engines that I've worked on the cruise AFR can go from around 15:1 with the straight leg to 16-16.5:1 with the annular boosters before I start to feel a loss in the response of the engine to the throttle.

[@MuscleCarSolutions]

Looking forward to your video of your new prototype carburetor.

[@sidecarbod1441]

@@MuscleCarSolutions I've thought about doing a video on the Edelbrock carbs but the problem is that I'm good with lathes, carbs, that sort of thing, I'm no good with cameras, PC's, that sort of thing!

[@zigandarov]

​@@sidecarbod1441ждём видео 👍