Annular vs. Down Leg boosters

quote:

Originally posted by Tom@Seal Beach:
Ok who's the carb genius? I'm researching 750 cfm 4 barrels to replace my Edelbrock 600 spreadbore for the new and improved engine. I've noticed some carbs have annular boosters which are said to improve low to mid range power due to better atomization of the fuel. Has anyone experienced the difference between an Annular booster carb and a Down leg booster carb on a 351 C? Not sure if the same carb principles apply with a cleveland port when it comes to boosters?

I have used both but I'm not a carb genious.
I have used a 750 down leg and a 800 Annular.
You are correct the annulars pull sooner and harder. You wont have to cover up a hole in the curve with accelerator pump or power valve timing.
Here my altitude is 4500ft so it might respond differently to where you are.
From what I researched at the time the annulars are better for people who want to run a large carb on a small engine for the reasons I listed above. A carb that is too large will have a big hole in the curve. Some people feel the annulars are a flow restriction but I can't prove that.
At my altitude a straight booster like in a 3310 750 vac secondary works best. Also because of my altitude I tend to run smaller carbs than what might be normally chosen. I woke a cleveland up bigtime going to a 700 dp from a 800 dp on another pantera. Both were downleg carbs.Throttle response change was significant.
The 800 annular worked pretty good after I sorted it but in reality it was too big for what I was doing. It would have been a great Drag race carb.
The Downleg 750 dbl pump was a little better on the bottom but had hic-up I couldn't get out of it.
I ended up not using either carb and went to the 750 vacuum secondary with the straight boosters.

Good luck

Brooke, THANKS that is exactly the questions I need answered...now if you could just move to sea level and do some testing that would be even better.

What I have read is that the annular boosters are bigger and actually impede the shear volume moving through the venturi, but because of the size, shape and larger booster it creates more velocity at lower to mid rpm's and better atomized the fuel. I recall that the total flow drops by 50 cfm +/- on an annular vs. the straight booster.

My thought was to use a 750dp Quick flow, Pro Form or Demon...actual flow probably 700. I was going to follow Dan Jones recipe of .588 lift and 232 duration on a 10.5-1 CR motor. I'd love to drop $$$ for a fuel injection or webbers but that just not in the budget right now with all the other things I'm doing to it.

Tom,

just a reminder, Holley carb ratings are based on dry flow measurements, BG (Demon) carb ratings are based on wet flow measurements, therefore:

750 Holley = 650 Demon
850 Holley = 750 Demon

etc, etc, etc

George,

You should have known I would ask this(why not I'm neck deep already)...What the heck is wet flow vs dry flow?

How about a number that a stupid novice like me can understand....real world, makes sense, a consumer friendly # that you can be compared...NO that would be too easy. SORRY I HAD TO VENT .

If you measure how much air flows through a carburetor at a specified depression (differential pressure) with the fuel bowls empty, that's dry flowing a carburetor. In actual operation a carburetor flows both air and fuel. So if you measure how much air flows through a carburetor at a specified depression with the fuel bowls filled with fuel (or a reasonable substitute) that's wet flow. With fuel being metered into the carburetor during an air flow mearurement there will be less "room" for air, so the air flow measurement will be of decreased volume. Why Barry Grant decided to rate his carburetors based upon wet flow measurements when all other carburetors are rated based on dry flow measurements I can't say.

The important thing to know is that there's a difference in the ratings between Demons & Holleys, for a carb of equivalent air flow capacity the Demon's rating will be approximately 100 CFM smaller than the Holley's. And now that you know that, you are an informed consumer.

George,

Now comes the fun...you've seen the standard calculation for picking CFM on a carb...I assume that would be the dry measurement? Probably dry if you're saying Demons are the only one that measure wet flow...then I'm assuming Quick Flow and Pro Form(which I read make Quick Flow carbs) measure dry? Also the standard calculation on doesn't compensate for valve lift, duration, port and manifold flow, etc. So its hit and miss.

If you wet flow test a Holley I assume it's cfm drops? So are they really flowing the same if flow tested with the same process on the example below?
Dry flow.......wet flow
750 Holley = 650 Demon
850 Holley = 750 Demon

I see that many are running the 750+ Holley DP on the Pantera? That's the size I assumed I should look for, but the snag comes with annular boosters...do I get an 850 Holley with annualar boosters since the flow is reduced and a 1050 Demon with annular boosters since the Demon is rated wet?

Forget the carb sizing calculators. Day in and day out, for motors from mild to wild, I've seen 750 Holleys work flawlessly on the 351C since the early 1970s. This was the carb the knowledgable Ford engine builders recommended back then and it still holds true today. Of course, the caveat is that they must be tuned or jetted properly.

For a SBC that would be considered too big, but not so for the 351C. The 351C likes 2" header primaries too. Unheard of with a SBC. The mighty Clevo breathes big. What other 350 cube american V8 could make 500 streetable BHP in 1973? What other 350 cube American V8 was competing successfully against the big blocks in NASCAR racing in the early 1970s? yeah, I kinda get passionate about the 351C.

I've seen some people having tuning problems drop down to a 700 Holley and have better luck. In days long ago a good choice was the readily available 735 Holley designed for the 428 Cobra Jet. It came out of the box fairly well tuned for the street. Back in those days the 4779 double pumper & the 3310 vacuum secondary were jetted really rich in the low speed circuits. Holley designed those carbs for engines with hot cams and not much intake manifold vacuum, so to compensate for the lack of intake manifold vacuum the lower speed circuits were richer. This resulted in poor drivability and sluggish low speed response to the throttle. Todays choices are much better between the Holley HP series, the Barry Grant carbs & the various "tuner" carbs out there.

Just remember if you're buying a BG carb, you want about a 650, not a 750.

If a carb is rated for 750 CFM, it should flow 750 CFM. If it has fatter boosters in it, the manufacturer has compernsated for that somehow, to make it flow 750 CFM. You shouldn't have to stress over subtracting 50 CFM for the boosters & therefore needing an 800 instead of a 750, etc, etc. The tuners mill away the choke horns, smooth the entrance to the venturis, streamline the butterfly shafts, etc, to make a carb flow more CFM anyway.

When you buy a HP Holley, a BG Demon, or a tuner carb, you're paying a premium price because supposedly some "expert" has worked that carb over so you don't have to worry about it, just bolt it on and have fun. If it doesn't work that way, you've just spent several hundred extra dollars with no results to show for it.

George, Awesome info! Thanks for the carb education. 750 it is.

I am always amazed when I compare the ports on the 351c...no comparison to the SBC. I saw a 427 tunnel port head at my machinist a month ago and yep those ports are big, but when you see the Clevo port you get the reason why they make horse power. One of my friends from high school use to say the ports are so big you could stick your head inside and wiggle your ears.

Can't wait to finish the motor!