SourceDemon Carburetion270.438.2039Demoncarbs.com

VORTECHENCLOSURE

NOVEMBER.2013DRAG RACER 50

PROCHARGERBONNET

TESTING A

AGAINSTA

Text by Archie BosmanPhotos by Moore Good Ink

HOWCHANGING

CARBURETORBOOSTERS

GAINED OVER50 HP

DR_1311-50-56-BLOW 8/13/13 12:47 AM Page 50

DRAGRACERMAG.COMDRAG RACER 51

In Lamar Walden’s engine shop, thedynamometer hurtled around to5,997 rpm and recorded 627.5 hp.The engine, a 38-ci small-block

Chevrolet, empowered a Vortech S-Trim supercharger that generated 10pounds of boost and directed itscharge through a ProCharger bonnetand Demon Carburetion’s new 750-cfm blow-thru carburetor. This bonnet-vs.-enclosure test was

instigated by Demon’s engineering staff,who had requested data to complete thespecifications for their new blow-thru car-buretor. Particularly they needed to knowhow its performance varied between apressurized bonnet and a pressurized enclosure. When asked to predict whichone would win Lamar replied wryly, “Theone that atomizes the fuel best and deliv-ers it equally to each cylinder.”Fifteen minutes later the 750 blow-thru

and bonnet were replaced by a Demon750 GC (General Competition) carburetorneatly installed in a Vortech enclosure.Pressurized by the same 10 pounds ofboost, this combination yielded 655 hp at5,975 rpm, an increase of 28 hp. When Ken Duttweiler, the engineering

brains behind the boosted Bonnevillerecord-breaking Speed Demon, was askedhis views on these two styles of boosting hesaid, “We’ve prepared only a small numberof engines with blow-thru carburetors andof those we noticed they ran pretty well atidle and at full throttle, but around halfthrottle they could easily become disori-ented—they didn’t know whether they wereArthur or Martha! I suspect it would havebeen less of an issue if the carburetor hadbeen placed in an enclosure.” The next day the same group of Demon

engineers gathered at Walden’s shop in Doraville, Georgia, just outside I-285, thering road that encircles Atlanta. Overnightinspiration had intervened and they had decided to rerun the same tests, but thistime using annular boosters instead ofdown-legs. Suddenly the test expandedfrom two contenders to four: bonnetagainst enclosure, down-leg boost venturiiagainst annular. The results were startling.For the third test, the lid was removed

//Suddenly the test expanded from two contenders to four:bonnet against enclosure, down-leg boost venturii againstannular. The results were startling.//

01In the first of four tests a 750-cfm Demon blow-thrucarburetor with down-leg boosters recorded 627.50 hp at

5,997 rpm. The engine, a 388-ci small-block, generated 10pounds of boost pressure via a Vortech S-trim superchargerdirected through a ProCharger bonnet.

01

from the Vortec enclosure and annular boosters were substituted for down-legs. The dynamometer was started and after normal operating temperatures were established thetest was run, resulting in 658 hp at 5,978 rpm. The conclusion revealed a negligible gainof 3 hp over the down-leg test.

But for the fourth test, using the 750 blow-thru with annular boosters and a ProChargerbonnet, the carburetor suddenly hit its stride, producing 680.80 hp at 5,979 hp. Using thesame blow-thru carburetor and bonnet, the engine had gained 53 hp by simply switching boosters!

Lamar’s son Rob said, “For peak horsepower I expected the bonnet to have the edge—although the extent of the horsepower gain surprised me. But for driveability on the streetand around town I still prefer the enclosure.” DR

700.0

600.0

500.0

400.0

300.0

200.0

4.000 6.000 8.000 10.000 12.000 14.000 16.000

2000

7000

6000

5000

4000

3000

Torque* and Hp* vs. Time

Run Time (Sec)

Torq

ue a

nd H

p

RPM

* Correction Method: StandardNotes: VORTEC BONNET

750 Mighty Demon Blow Thruwith 90-degree ProCharger Bonnet

627.5 hp @ 5,997 rpmPeak Power

Torque Curve 548.5 ft-lb tq @ 4,030rpm574.3 ft-lb tq @ 4,763rpm549.5 ft-lb tq @ 5,997rpm

starts at climbs to

ends at

DR_1311-50-56-BLOW 8/13/13 12:48 AM Page 51

700.0

600.0

500.0

400.0

300.0

200.0

4.000 6.000 8.000 10.000 12.000 14.000 16.000

2000

7000

6000

5000

4000

3000

Torque* and Hp* vs. Time

Run Time (Sec)

Torq

ue a

nd H

p

RPM

* Correction Method: StandardNotes: SECOND PULL STOCK WITH BOX

750 Mighty Demon Down Leg GC Vortech Enclosure

655.0 hp @Torque Curve

5,975 rpmPeak Power

574.2 ft-lb tq @ 3,874rpm598.7 ft-lb tq @ 4,782rpm575.8 ft-lb tq @ 5,975rpm

starts at climbs to

ends at

NOVEMBER.2013DRAG RACER 52

02

700.0

600.0

500.0

400.0

300.0

200.0

4.000 6.000 8.000 10.000 12.000 14.000 16.000

2000

7000

6000

5000

4000

3000

Torque* and Hp* vs. Time

Run Time (Sec)

Torq

ue a

nd H

p

RPM

* Correction Method: StandardNotes: MDA ENC

750 Annular Mighty Demon GC Vortech Enclosure

658.5 hp @ 5,978 rpmPeak Power

576.5 ft-lb tq @ 4,051rpm606.8 ft-lb tq @ 5,003rpm578.5 ft-lb tq @ 5,978rpm

starts at climbs to

ends at

Torque Curve

03

02 In the second test a 750-cfm Demon General Competition carburetor with down-legboosters operating in a Vortech enclosure generated 655 hp at 5,975 rpm.

03 Still using an enclosure but substituting annular boosters for the down-legs, the thirdtest produced 658.5 at 5,978 rpm, a negligible gain of 3 hp.

700.0

600.0

500.0

400.0

300.0

200.0

4.000 6.000 8.000 10.000 12.000 14.000 16.000

1750

6125

5250

4375

3500

2625

Torque* and Hp* vs. Time

Run Time (Sec)

Torq

ue a

nd H

p

RPM

* Correction Method: StandardNotes: 750 ANNULAR 83 P/90 with 35 H/SPEED PROCHARGER HAT

750 Annular Mighty Demon Blow Thruwith ProCharger Bonnet

680.8 hp @

598.8 ft-lb tq @

5,979 rpm

4,040rpm

Peak Power

616.5 ft-lb tq @ 4,780rpm598.1 ft-lb tq @ 5,979rpm

Torque Curve starts at

climbs to ends at

DR_1311-50-56-BLOW 8/20/13 12:49 AM Page 52

NOVEMBER.2013DRAG RACER 54

04But in test four the power increase wasas impressive as it was unexpected. By

simply switching the boosters from down-legto annular, the ProCharger bonnet recorded680.80 hp at 5,979 rpm, revealing animpressive gain of 53 hp.

05 The most visible difference between theDemon blow-thru (left) used with the

bonnet and the GC used with the enclosure liesin the base plates.

06 The machined channels in the blow-thru base plate introduce a source of

boost pressure to four machined grooves on thethrottle shafts. This innovation seals theshafts and prevents pressure escaping from thecarburetor. For security, screw-in vacuumfittings replace the press-in type. 04

05

06

DR_1311-50-56-BLOW 8/13/13 12:48 AM Page 54

NOVEMBER.2013DRAG RACER 56

07 Further differences between the blow-thru and the GCare visible in the metering blocks. The orifice in the blow-

thru power valve channel restrictor measures .078 inch, whilethe GC measures .05 inch.

08 Pressurized by the supercharger, the enclosure is asealed box subjected to boost pressures usually

between 8 and 18 psi.

09 The GC carburetor operating in the enclosure uses astandard .110-inch needle and seat, while the blow-thru

uses .130 inch, a much bigger valve providing greater fuel delivery.

07

08

09

DR_1311-50-56-BLOW 8/13/13 12:48 AM Page 56