Carburator 101: Downleg vs Annular boosters on Holley and QFT carbs

[mcwho]

I wish there was a section on engine builds in here.

I have been doing research on which of these are best for Street Roadsters with a mild 33i cu in engine.

Specifically 302 Stroker @ 331 cu in, Edelbrock Performer Rpm Heads, RPM Air Gap intake, MSD Distributor, current carb is a Edelbrock Thunder 1806 AVS carb, 10-1 comp ratio, and Cam spec is: Hydraulic Roller 224 intake / 234 exhaust* @ 050 duration.* .542 / . 562 Valve Lift

For all the research I have read a few items such as Annular does not always work better than a downleg, I read that if you had a "cold intake" the Annular was better. I am building what I consider a daily street driver. I want reliability.

Looking at the Holley an Quick Fuel carbs I have come up with more opinions to stay with downleg. I also have read that the Annular Hardware takes about 30 cfm away from a carb because of the hardware restriction.

Based on what I have read so far, a QFT SS-650 is just a tad big for this engine but should work fine. This one had Mechanical Secondaried, Electric choke and downleg Boosers.

Commpens? Opinions?

[edwardb]

My Mk3 had a 306 with AFR 165 heads, RPM Air Gap intake, MSD distributor and box, COMP Cams 35-440-8 Magnum hydraulic roller camshaft (similar to yours but a little less lift) and it ran fantastic on a Quick Fuel SS-650 carb. I don't own it now, but the new owner reports it's still running great. I do not think the SS-650 is too big for your engine. Like all carbs, it needs to be tuned. But should be a good match for your engine.

I have the -AN (annular ring) version on my Mk4 #7750. Wayne Pressley recommends them. First I've heard it reduces the cfm. Real happy with that one too. That engine is a DART 347 with bigger heads and a little more cam. I have the 750 version on that engine. Probably a little big. But it's wideband tuned and hits all the numbers. Runs really great as well.

I don't think you'd go wrong with either version.

[NAZ]

A 331 CI turning 6000 RPM with 80% VE would require a 673.4 CFM carb at .7" of manifold pressure @ WOT. Your 650 CFM carb should work well. If you are looking to replace it go with a 4150 style 650 CFM with annual boosters as they provide a stronger signal starting at mid RPM and will give better atomization of the fuel (read that more efficient burn / better power). When it comes to carbs if you can't find the exact CFM rating pick the next closest one down -- bigger is not better here.

[mcwho]

What I read was that the Booster ring was larger on the -AN versions, and that causes more of a restriction, that's what causes you to loose 30 CFM or so.

[NAZ]

Yes, if you take a carb that is flow rated with down leg boosters and replace them with annular boosters you will be reducing the CFM flow at 1.5” that the original carb was flow tested at. The bigger the banjo, the more reduction. But if you buy a carb designed with annular boosters it should flow what the manufacturer rated it at. For a street car that operates at a wide variety of RPM and load conditions there is an advantage to using annular boosters. Before you purchase a carb check out QuickFuel part number SS-650-AN. This may be the ultimate 650 CFM double pumper for a street car with an engine your size. Some also say you can get away with upsizing a carb and still have good mid-range mixture if you use annular boosters. However, there are plenty of drivers happy with their Holley double-pumpers running down leg boosters on their hot street cars. If your 650 CFM carb seems to work for you then stay with it. If you want lots of adjustability and have the knowledge to tune a carb, go with QuickFuel and consider the annular booster option.