Confuzed1

iTrader: (4)

Hello all! - Just wondering which type of carb is best for a roots-type application?

I've done some reading, and I've seen more than one recommendation to use a vacuum secondary type carb such as a Q-jet or Edelbrock carb. But I happen to have 2 double pumper carbs that are practically new - and I'd like to use them of course. One is a Speed Demon 650 DP (I know, too small) , and the other is a Pro-Systems Holley 750 DP. I was looking at modifying the PV reference on the Holley and give it a try. But if a vacuum secondary carb will overall work better on a street and occasional strip basis, I guess I'll end up buying one if I need to.

Your thoughts?

xpndbl3

iTrader: (5)

draw thru or blow thru are you talking about?

Confuzed1

iTrader: (4)

Sorry, I abbreviate too much at times...
I meant Vacuum secondaries VS a double pumper. Both being a draw thru setup.

I've read in a few places that a VS carb is easier to tune on a blower than a DP in a draw thru blower setup, but is it just hearsay? I've ran a Q-jet in the past on a N/A setup, and yeah - if tuned right they're great. But so is any carb if tuned right.

Aside from having to switch the power valve porting (Holley type DP carb), does it really act differently if it sits on top of a blower? Any advantage or disadvantage to either? Which is more "streetable" on a blower and why???

Hope this makes sense......

V8Astro Captain

iTrader: (1)

If you look around you'll see that most blower carbs are mechanical secondary. I'd use on of the DP's for draw thru. The VS vacuum reference is right under the carb which won't do you any good on a roots type blower.

five7kid

iTrader: (14)

That's not how a vacuum secondary system works. You can get that information from the Holley website. The vacuum source for the secondary diaphragm is in the venturi - the more flow thru the venturi, the stronger the vacuum signal, and the more the secondaries open.

The good ol' boy racer here uses VS carbs on his roots 383. Multiple season track champion, must be doing something right.

V8Astro Captain

iTrader: (1)

I had QJet power piston in mind. I have been thankfully corrected

Sonix

iTrader: (1)

Who'd that be?

leeperryracing

or his competition must be doing something wrong...

five7kid

iTrader: (14)

I resemble that remark. . .

Actually, he finished 2nd to me in the 2006 season. He missed a couple of races last year which put him out of the running.

Confuzed1

iTrader: (4)

OK guys, here's the deal:

I have a 750 Holley double pumper carb from my N/A setup. I got it from Pro-Systems and it has less than 3K on it. Still bolted to my intake which is sitting on the workbench. If I use it on the blower, from what I understand, I'll have to change the power valve reference port by plugging the port on the bottom of the carb and drilling into the port from the side and installing a vacuum nipple. I then run a hose from that nipple to the intake. Then I would have to figure the tuning from there.

My other choice: Edelbrock also offers a 800 cfm Edelbrock AVS carb "Calibrated for the blower" in a kit along with the supercharger as an option. Costs around 400 bucks more than the "blower only" kit, so you're paying for it.

No power valve referencing changes needed, and it's supposedly "calibrated" already. I'm taking Edelbrock's statement with a grain of salt...I somehow doubt this carb will work properly on my 400 even though they say it's "calibrated".

I've read that running lean is a bad thing for any forced induction engine - I gotta feeling that Edelbrock "calibrates" their carb to go on a mild 350 to go with their blower. But if it'll truly be easier to tune with a blower than my existing Holley would be, then I may get the kit + carb.

So - stick with the Holley or get the Edelbrock?

And yes, I've seen mostly Holleys sitting on top of blowers, so yeah, I know they'll work....but I've seen vacuum secondary carbs atop blowers too. Doesn't tell me which would be easier to tune...on a draw-thru setup.

I'm thinking the secondary flaps on a Edelbrock carb might be more responsive when it sits on top of a blower, therefore making it more adaptable to a stick shift car. Could be totally wrong...

lukn4trbl

From a tuning stanpoint, the Holley would be the better choice.

Carburetors are tricky to understand, but once you get the hang of how all
of the circuits work, and how they interact you will appreciate the flexibility
of the Holley.

As for Vacuum vs. Mechanical, you can save headaches using a vacuum
secondary on the street. Mechanicals are good for wide open racing
when manifold pressure changes are not an issue. If you know when to
give the engine more throttle driving around the street, you will be fine
with a mechanical system. It's just a pain when using an auto transmission
and there are load changes. You have to be on the ball.

As for which works better on top of a blower, the science is all the same.
The difference comes in the design of the boosters, flanges, venturis, etc.
Some booster combos with a well made venturi will respond to pressure
changes faster. No two carbs are alike! Even from the factory, you would
be surprised at the differences in casting.

Another thing to remember is that you need to change the bleeds and jets
if you swap carburetors with different CFM ratings.

You could have a properly tuned 750 carb on the engine, then throw on
a diffrent MFG carb and go totally lean/rich because of the settings.

The best thing you can do is try both carburetors. Set them up and dirve
around for a while. See which one suits your cruising style best.

F-BIRD'88

use your 750DP. Modify the PV reference as you plan to.
start with 75 primary and 88sec jetting as a base line. (rich and safe)
You'll need larger accelerator pump shooters and a selection of accelerator cams + maybe a rear 50cc accel pump kit . You can rob one of these off a spread bore DP holley (650 or 800cfm 4165 model) (pump and pump linkage)
Your DP will work very well on the blower with a typical 11" 2800-3000 stall converter.

The blower and blower intake manifold add quit a bit of effective manifold plenum volume (the carb sees the whole thing as plenum volume). This increased effective plenum volume requires more accelerator pump shot volume and capacity. The engine will also want a modified spark advance curve with more initial at idle (throttle response and idle quality)+ a smooth curve to max advance (3000-3500rpm) and boost regulated reduced spark timing as the boost comes in. "boost retard" to prevent detonation. {or water injection}
Vacuum advance should be sourced off the intake manifold and will need to be custom dialed in.

Confuzed1

iTrader: (4)

Allright! Thx.

OK, I'll stick with my Holley then. Supposedly, Pro-Sytems wet flow tested it to 790 cfm. I have all the specs. I know it has 2-30cc pump/cam kits installed with 74/84 jets from the N/A setup.

I just might call the guy to see if he'll just tell me how to set it up without having to send it in = mo $$....

I'll also be buying a WBO2 tester to help dail the carb in (if I learn how to use it!) lol. No stall convertor needed, I have a stick.

Also getting a MSD boost timing master to retard 1-3 degrees during boost. For the cam, I called Comp cams, and he recommended part# 12-415-8, it has a 113 LSA, 224/236 at .050 with a 502/520 lift. I think I'm going with it.
I want the powerband from 2-5500, so it should do nicely!

Also getting a Summit small cap HEI distributor - thinking of adding an adjustable advance vacuum can, depnds how well the MSD works I guess.

Ordering the blower tonight, so there's no turning back!!

leeperryracing

that cam will work really nice with your blown combo...you should have great throttle response(if carb, timing etc) is spot on, not choppy but noticable idle with that duration and bucket loads of torque

Confuzed1

iTrader: (4)

Thx Leepyear!

I'm hoping the cam will get me where I want. I'm expecting a fairly smooth idle, some great torque , but mid range should be it's strong point I hope. Just hope this small blower is up to it, but it seems to have decent reviews and I like the 1/3 of a HP drag at 60 MPH claim!

Never owned a blown car, but since I was 16 but I've always wanted one...even if it is old school carbed. Thx for the replies everyone.

Hotrodrobert

Done several draw thru setups and the Holley is the easiest. I like the vacuum secondaries better but most guys insist on DPs. No matte which, your REALLY NEED TO INDEX THE POWER VALVE!!!!!!!!!!
If you need to know how, pm me and I will help-done it many times and it is a snap.
F-Bird'88 had good ignition info.
I have also done the Q-Jet and the Eddys and they both are a lot less trouble than the Holley types, just terrible to set up the first time and a lot more work to index the power valves, though they work great and do not blow the valves once done.
Factory Buick Turbos used draw through Q-Jets in the early days and they were indexed

Confuzed1

iTrader: (4)

Huh? Hotrodrobert - When you say "indexing" do you mean modifying the power valve reference port? Because I've never heard of indexing it. I DO know I can't seem to find a part number for the vacuum fitting I'll need to stick in the hole when I modify the power valve - and I don't wanna steal one off another good carb unless I absolutely have to.

I didn't know Edelbrocks and Q-jets used power valves, but I've mainly only dealt with Holleys.

Hotrodrobert

Holley has new carbs with indexed power valves built in for blower use. Check their new catalogues.
There are several ways to do it, but the easiest is to use the ported vacuum fitting on the metering block. You have to plug the vacuum hole in the main body, the vacuum hole now connedted to the ported vacuum, and cut or drill some passages to the ported vacuum fitting.
In a car, this is important but for marine, it is disaster without it.
Certain loads and throtle positions can cause no vacuum or even boost in the engine with vacuum enough to close the power valve under the carb-the engine is past WOT and the power valve is turning down the mixture!!!
If the carb you are working on doesn't have a fitting there, it is easy to add.
PM if you need details.
All the street carbs I know of have some sort of power valve to lean the mixture for cruise.