Can somebody explain the difference and pros/cons.

I see most Holley carbs, even the new double pumpers 700 cfm and below, are straight. My 770 Avenger is straight, but the 870 version is downleg. 750 dp's and up and the hi-po carbs have the downlegs.

I have a feeling that the straight design may limit top-end power?

This is in Popular Hot Rodding this month (I think)

The straight leg boosters must be cheap, 'cause they seem to have no redeeming features otherwise.

Downleg booster provides better signal, so it works better than a straight leg at LOW rpm.

I can't tell you about WFO flow. I've heard annular boosters are more restrictive of air flow at high rpm, but I'm not sure there's a difference between straight leg and downleg.

My theory...

It is really all dependent on the shape of the outer venturi in the carb. You want the booster placed at the highest velocity point in the outer venturi. This could be high (straight leg) or low (down leg).

One advantage of designing this to be low is there is likely somewhat less turbulence in the flow further down the carb throat than closer to the entry. Moving the booster/velocity point further down also allows a more optimal carb throat entry design for better airflow.

I read that article in Hot Rodding, but man, is too long!

I wish they would just sum up whats good and bad in one paragraph. I don't need anymore theory in my life!

Didn't the first Holley 3310-1's come out with down leg's and are rated at 780 cfm??

Vs the later, 3310-2, -3 and -? having straight, but only rated at 750 cfm??

pdq67

Anybody else...come on!!!

What are the nascar boys running?

downleg boosters are less restrictive than straight leg boosters. like pdq mentioned the original 3310-1 had downleg boosters and is rated at 780cfm, the 3310-2 and up have straight leg boosters and are rated at 750cfm.

both carbs have the same venturi and throttle plate diameter, the 3310-1 gets its extra 30cfm of airflow from the downleg boosters.

and i respectfully disagree that a downleg booster is more responsive than a straight leg booster at low rpm. a more restrictive booster design is going to be more responsive at low rpm because it will have a stronger signal due to the more restrictive design, which is exactly why an annular booster carb is more responsive than either the downleg or straight leg boosters. it only makes sense that a straight leg booster will be more responsive than a downleg booster.

having said that, the downleg booster is more suited to higher rpm usage. because of the stronger signal of the annular and the straight leg booster, they will tend to run richer in the upper rpm ranges than a downleg booster (with the same jetting)because of the stronger signal they produce. i have read that annular boosters can present some tuning issues at high rpm due to the very responsive design, how high of rpm i dont know and i dont know if the straight leg would present the same type of issues or not.

i think that given the proper tuning, 3 carbs with each of the 3 booster designs will run pretty close to each other(like splitting hairs), but all three will probably like different jetting because of the different signal each type of booster has.

i think that given the proper tuning, 3 carbs with each of the 3 booster designs will run pretty close to each other(like splitting hairs), but all three will probably like different jetting because of the different signal each type of booster has.

Good write-up Brandon! This paragraph explains why the 770 SA I'm running only uses a 72/75 setup w/a .25 squirter. My 750 liked 74/82 and .31/.35 nozzles.