King Lear

I have the 383, 9.9 CR, 284/292 adv dur cam, .543/.563 lift, RPM Air-gap intake, hilborn scoop, hooker super comp side pipes, RHS 200cc heads.
Ok, I have the Quick Fuel 750-PV vacuum sec down leg boosters on it. After reading Vizard's book, I think I should be running annular boosters not down leg. So I am thinking of going over to the Speed Demon 850 vac sec annular booster carb or the Mighty Demon 825 Cfm carb vac sec annular boosters. Or possibly the Quick Fuel Super Street 750 cfm mech sec annular boosters.
So opinions from all you carb guru's am I going to benefit from switching to annular boosters?

TimAT

Read this. If you're any less confused than I was, good. If not, do what I did. Call them and ask!!

http://www.gnetworks.com/v4files/bar...0Questions.pdf

ikwhite

King not sure if you are aware but Barry grant now makes a 750 annular speed demon as well, that is of course if you did not want to step up to the 850.

750 Anunular Speed Demon
#1402020 750 mechanical
#1402020VE VS

ikwhite

King Lear

Any more opinions on annular vs down leg? I just got off the phone with BG Demon and they told me to run the down leg booster Mighty Demon 650 vacuum secondary carb. He claimed it will flow up and over 800 cfm as is. He also stated that since the vette is a lighter vehicle it would benefit more from a down leg then an annular booster carb. Lars I need you buddy!

gkull

What is wrong with your quick fuel carb? Change carb type does not fix problems magically. You always have to do some tweaking.

I have been running a modified Speed demon from day one on my 383. IMO - VS carbs have some advantages and a sub 6500 rpm 383 would run fine with 650 cfm.

IMO - annular dist carbs

King Lear

Nothing is wrong with it. I have been through 2.5 to 7.5 pv, up and down the jet range. I just cant get rid of the off idle bog hesitation. I have went up and down sq sizes. It feels like I just cant get enough air speed in it at low RPM. Once I am past 2000 rpm it is fine. I am putting new RHS heads on next week and bumping the CR up to 9.9, right now it is at 9.2. I am going to try the QF with the new heads and see if I can work it out. I am hoping the increase in CR will eliminate my low rpm drag. Right now it seems to run best with 31 sq, 74 prim, 82 sec, 3.5 pwr vlv. I am thinking that my since cam calls for 9.5 to 10.75CR and I am not in that range right now, I hope that is my problem and not the carb. Using the vacuum gauge in gear I am pulling 7-9psi at the manifold. Seems to run a little rich at idle but if I lean it out anymore it falls on its face when you punch it. The plugs look good.

gkull

your vacuum would require a 6.5 PV. The secondary air bleed will determine the jet size for the secondaries, but 82 IMO is probably on the rich side.

The below 2000 bog has alot to do with the initial position of setting the throttle blade with .020 of the x-fer slot uncovered and you have to have the squirter arm set so tight that even the smallest throttle movement causesgas dribble.

Is you cam an h-flat?

Gordonm

Interesting. When I talked to them about my combo he like the 825 Might demon which I am running and said that the annular booster would be more driveable. I might give up 5 HP on top but the mid range and driveability would be better. Also I asked about the flow numbers and said an 850 flows 850 and a 650 flows 650. If I converted my 825 to annular boosters it might knock off 25 cfm due to the different boosters. A lot depends on who you talk to there. Call a different person and you might get a different answer.

King Lear

I have the pump arm pretty tight, I have a hydro roller cam. Crane HR284 with 1.6 roller rockers.
I am going to wait until I get it back from the builder with the new heads. Right now it is at 9.2 CR and 6.7 dcr(wow) with a .057 quench with 160cc Pro Comp heads.
When I get it back I will be at 9.92 CR and 7.79 DCR and a .042 quench with 200cc RHS Pro Action heads. I think the adjustments I am making are greatly going to improve the performance of the car and change the whole booster signal.

King Lear

Did it, the new guy said to run a 650 Mighty with mech sec or a 750 Speed D with mech sec. Said he wouldn't run a vacuum carb even on a TH400 with 3.08 gears.

gkull

HR-284-2S-12 IG

Your cam is extreemly mild for a 383. I would expect 14+ inches of vacuum at a 800-900 idle rpm. I have bouncy 11 inches with 14 degrees more intake duration than you on a solid roller with a single plane intake

I would set the timing to 16 initial and if those heads have modern fast burn chambers recourve the dist. to a max of 32-34 degrees total all in by 3000 rpm.

King Lear

I plan on bumping the timing up once I get it back. He has it a 10 initial which I thought was way too low. My old XE 268 cam I had at 19 initial and it ran like a champ. I do have that kind of vacuum at idle in park, not in gear. I put in a 6.5pv and it ran like crap, 3.5 made it run much smoother in the transition.

gkull

there is a little formula for a power valve and it is basically two inches less than your lowest idle reading. Like when you put it in gear with the brake on. The higher initial timing really brings up the vacuum.

I also had a 3500+ stall convert when I had an auto tranny so i didn't really ever have an rpm drop in gear.

You might try driving with the vac. gauge taped to window so you can see how the vacuum runs. You don't want the power valve to dump fuel on small throttle changes while driving. Just when you hit it. I would think that it would be hard to 3.5 inches unless you really open it up.

King Lear

I did that with the gauge, I actually ran the hose inside and had my wife drive while I monitored the gauge. the lowest it dropped to at a stop in idle was 6psi. So then I went with the 3.5 and it turned out good. I still think the timing is too low myself, so I will bump it to around 15 and change the MSD bushing in the dist to get around 34 total. I also have the quick springs in it right now.

Gordonm

Bump it up to 15? What are you at now. I run my initial at 18 to 20 With my cam I can barely to get it to run at 16 or below. Mine idles at about 950 plus or minus 100 rpm. I am running a stick though.

King Lear

He has it set at 10 initial.

King Lear

Thanks, that is what I am leading toward the Speed Demon 750 annular, I think I am going to stick with mech sec. I seem to get better performance out of them even on an automatic.

Gordonm

10 initial is very low. 14 to 16 is normal. 18 to 20 if it is a high duration cam.

I have always had better luck with the double pumper series carbs than vacuum. If the yare setup correctly they should perform very well in your car.

MotorHead

Don't know who you were talking to at BG but I have talked to a reliable source there and he told me BG carbs are rated different than Holley carbs. My 825 Race Demon flows 975cfm according to him.

Anyway annular discharge boosters was one of the best mods I have ever done. Much crisper throttle response and more torque, I would recommend anyone to do it. The new 750 Demon with annular boosters would work great

King Lear

Speed Demon or Mighty Demon, I am thinking the 750 Speed Demon mec sec annular is more practical then the Mighty Demon 750 vac sec, since I wont be on a strip anytime soon. Too bad they dont make a Mighty Demon annular 650 vac sec. I would go with the 750 speed demon annular vac sec but I just hate chokes especially electric.