I have two carburetors in my possession: 1) Speed Demon 750 Mech. 2) Holley 750 Vac Sec.
This is a weekend warrior that is used for cruising during the summer, however I would like to have maximum performance for the street and maybe a few runs at the drag strips with the joy of breaking into the mid 11 sec. This engine dyno with two different carbs and the 4779 won hands down. How would you compare the Speed Demon 750 Mech. VS Holley 3310VS

Holley 4779- jets:70/80 timing: 34
Torque Horsepower
502@ 4300 464@ 5800

Holley 4779- jets:70/80 timing: 32
Torque Horsepower
495@ 4300 455@ 5600

Holley 3310VS jets: 72/75 timing: 36
Torque Horsepower
484@ 4600 467@ 5900

Holley 3310VS jets: 72/75 timing: 34
Torque Horsepower
479@ 4700 459 @ 5600

Holley 3310VS jets: 72/75 timing: 37
Torque Horsepower
481@ 4600 457@ 5700

Here ar the specs of my engine:
My SBC 409

Engine Block: 400 SBC ID# 330817/14y139664/ T0508CSU
(4.125 bore X 3.750 stroke) after .040 bore (4.165 bore x 3.75 stroke =409)

Deck Plate Bored, Mains Align Honed, Block Deck .002, Deck Plug Holes & Tap w/3/16 Holes, Starter Bolt Drilled, Front Three Gallery Plugs Tap for ¼ Pipe Plugs.


Crank: Scat 4340 Crank 3.750 6.00, Internally Balance, Part# 4-400-3750-6000

Bearing: Clevite– MS1038H Mains and CB663H Rods

Piston: Wiseco Pistons, .040 over, 5.4cc; 430 grams, Part# PT020H4

Rods: Eagle ESP 4340 H-Rods 6.0, Part# CRS6000B3D

Block: Zero Deck: 0.002

Compression Ratio: Static-10:44 Dynamic-8:27

Cam: Grind: Solid Lunati/ (Recommended by Harold Brookshire)
280/284F10 .525/.530 Lift, 247/251@.050, 159/162 at .200, .500/ 504 valve lift, 2500-7000 RPM, Installed Straight up, .026" Valve Lash Hot, .026 Adjustment Range: .018" to .030" Hot.

Valve Springs: Lunati Part#73124 130 Seat 330 Open.

Heads: AFR 195 Part# 1035 w/23º valve angle, 74cc, drilled steam holes.

Head Gasket: Fel-Pro-1014 / .039 in. compressed thickness, 4.200 in. bore, 9.0 cc compressed volume

Intake: Edelbrock RPM #7101

Carb: Speed Demon 750 Mechnical

Rear End Gear: 3:73

Transmission: Pro-Built Street/Strip 700R4

Stall Converter: A.C.T. 3000 Stall

Exhaust: Hooker Super Comp headers 1 5/8 Headers part # 2460-1 ceramic coated;

Hooker Super Competition high flow cat-back exhaust system, Intermediate pipe: 3 in.
* Tail pipe diameter: 2 1/2 in.
* Dual tail pipes out the rear under the bumper

Looking at this combination do you think will be the "right" carburetor for my application or what would you recommend.

Thanks: David

I'd be running the double pumper, whatever brand name it had on it. Holleys and Demons work equally well.

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A Canadian, EH? (with a 10 second street car)

What really bother me on the dyno was that a 4779 double pumper out performed the 3310VS by 30-40 TQ/HP. I don't have any idea how that happen.
Hopefully some carb experts can chime in here. both carbs had orginal jets & power valve.
Look at the dyno results between the 4779 and 3310:
http://community.webshots.com/user/dtl504

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DTL504
SBC 409
84 Z28 Camaro,Scat 4340 Int Bal Crank, Wiseco Flattop Pistons, Eagle H-Beam 6" Rods, Lunati
Solid Cam: 280/284F10, 247/251@.050, 525/530 lift, 110LSA, Edelbrock RPM Intake, Demon 750, AFR 195 Street Heads, Hooker Ceramic Coated 1 5/8 Headers, Pro-Built 700R4 A.C.T. 3000 T.C., 3:73 Limited Slip.

[This message has been edited by DTL504 (edited December 26, 2004).]

A vac sec will keep the secondaries from opening fully until fairly/very high RPMs (especially if you have a stiff spring in the secondary vacuum chamber). That'll knock down your torque curve throughout, as well as hurt peak HP.

Did you make any attempt to put a lighter secondary spring in the 3310? Even the stock (plain) spring supplied in the carb is usually too stiff for best performance. I haven't found a vac sec Holley yet that didn't work better with a much lighter yellow spring in it, and most will take the white (lightest) spring available without complaint.

If you're running a 3000+ stall converter and a farily steep rear gear in a weekend fun car forget about vac sec carbs and go with double pumper. Vac Sec carbs are better for modest street vehicles that can see the engine being "lugged" down in a fairly low RPM range from time to time due to a low stall converter or a manual that is street driven and often turning very modest RPMs putting around town.


[This message has been edited by Damon (edited December 26, 2004).]

I don't see much of a comparison except at 34° total timing. In the other tests the total timing was different and could make a large difference in the power. Giving only peak numbers is also misleading - what was the average torque and horsepower over the range you desired?.

I did not do any internal changes to the 3310. But the DP out performed that 3310 straight out the box, so that what im banking on at this moment. After that I will still have both carbs to do some apples to apples testing at the track.

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DTL504
SBC 409
84 Z28 Camaro,Scat 4340 Int Bal Crank, Wiseco Flattop Pistons, Eagle H-Beam 6" Rods, Lunati
Solid Cam: 280/284F10, 247/251@.050, 525/530 lift, 110LSA, Edelbrock RPM Intake, Demon 750, AFR 195 Street Heads, Hooker Ceramic Coated 1 5/8 Headers, Pro-Built 700R4 A.C.T. 3000 T.C., 3:73 Limited Slip.

Here is the dyno sheet, they did not have the air/fuel ratio data for whatever reason:
http://community.webshots.com/user/dtl504

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DTL504
SBC 409
84 Z28 Camaro,Scat 4340 Int Bal Crank, Wiseco Flattop Pistons, Eagle H-Beam 6" Rods, Lunati
Solid Cam: 280/284F10, 247/251@.050, 525/530 lift, 110LSA, Edelbrock RPM Intake, Demon 750, AFR 195 Street Heads, Hooker Ceramic Coated 1 5/8 Headers, Pro-Built 700R4 A.C.T. 3000 T.C., 3:73 Limited Slip.

I wouldn't read too much into the carb tests. The mechanical secondary carb has a higher power potential because it has a lower pressure under the carb. Your dyno sheets show no jetting changes in either carb, and you use more timing with the 3310 than you do with the 4779. The average power difference is nowhere near the 30-/40 you allude to, more like 7 or 8. Now that you're off the dyno and in to the car with the accessories, car headers, heated intale air and such your carb and timing requirements may be slightly different.

David,

We replied to this question on hotrodders.com

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