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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
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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
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