annular vs downleg bs straight leg

[kalli]

Hi all, I was just wondering about all those booster options. I fully understand on how they work and what might be a better or worse placement (i studied aircraft engineering, that gave me a bit of insight about air/velocity/pressure) I once have read that the downleg boosters are basically the same as the straight legs just that due to the downleg they're sitting a bit lower so that the exit of the booster is bang on the smallest part in the venturi for a better signal. So that obviously is great if you have bought a few sizes to big a carb :-) Annular boosters are even better at giving signal, but they're huge, so they might restrict the flow in the venturi So what I'm wondering is: - if you have a 650cfm carb and you would (not that I'm going to) change from straight leg to annular. wouldn't that reduce the cfm rating by a lot? - if you buy a 650cfm annular wouldn't that flow worse than a 650 downleg? - in what situation would you actually run annular boosters instead of downlegs (money not being an option). Are annular boosters only for people with such mad overlap cams that don't get any signal at idle with any other carb or is it for people who want to run 850cfm on a 289 engine? any comments appreciated ! Kalli

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[cprstreetmachines]

A higher signal is usually caused be more of a restriction. Annulars though, are larger in the center, drawing more of the air through the center, creating a better signal without the restriction.

fwiw though, signal or no at idle doesn't matter. Not flow through the booster. They just move more fuel, easier. I noticed a ton more fuel at the same rpm with them

[kalli]

so on what applications would you use them (annulars)? preferrably all or just something specialised ?
and yes. my mistake. no relation between boosters and idle alright. just thought it'll help on low signal situations

[67mustang302]

I'd run them on anything personally. Since they make better signal faster, the main circuit will come online more easily and help with throttle response, you also won't need as much pump shot to cover it. They help to atomize fuel better which can help power, but downlegs with good signal atomize pretty well too. Annulars will reduce carb cfm not so much because they're bigger, but because they spray a larger fuel cone that gets in the way of air flow. How much cfm drops depends on the design of the boosters. As for cfm rating it depends, a 650 that has annulars added as an afterthought may drop to a lower cfm rating, but one that's designed for annulars may be flow tested with the annulars, so the main body flows a tad more than one for a downleg.

I noticed on my annular 650 that it also did move a ton more fuel. I had to jet it way down and go quite a bit bigger on the high speed air bleeds in an attempt to reduce the signal...the boosters were coming on and drowning the engine with fuel.

[kalli]

thanks mate. I was just wondering as I stillhave the QF650 on the cards. It's going to be a few months, but this way I can already gather information. The price difference is about 50$ (don't quote me on that) between the downlegs and the annulars on those. So I was mainly wondering if that is a racing thing only and just a waste of money on my engine or no.
The bad thing is I'm still undecided. I just figured out how the PV restrictors in their metering blocks work and I'm working my way up. I'm currently checking on all documantation i can find for the Idle restrictors and high speed air bleeds.

I like the way that where every holley tuner would pull a drill out of the box they have screw in restrictors for it :-)

Kalli