Tom Wilson
December 20, 2006

While Jim Bell's Kenne Bell started the whole twin-screw blower scene in the United States back in the Buick Grand National and 5.0 H.O. days, the pivotal event in his modern era was the five-day '03 Mustang Cobra test we reported on a couple of years ago. By swapping the Cobra's old-tech Roots supercharger for a far more efficient twin-screw, Jim treated us all to 617 rwhp in an otherwise frighteningly stock Terminator.

It was crazy stuff. With just an open air inlet, open exhaust, electronic tuning, 20 pounds of boost, and a splash of race gas, the Cobra was making race power with a totally internally stock engine, stock exhaust manifolds, and stock cats. At the time, it was unnervingly easy to make such power, and even after these antics have become almost commonplace it's still a wonder how the modern engines make such big power. We chalked it up to the fancy Four-Valve cylinder heads, good manifolding, and all the rest that comes with a top-dog Cobra.

Kenne Bell's Two-Valve project is sano, with a minimum of extraneous flash for the confidently powerful look. The polished canister next to the alternator is the water-to-air charge-cooler reservoir. It fills through the small plug in its top, so there is no icing it down. The reversed alternator shows how the supercharger and other accessories share the same serpentine belt.

Perhaps even more amazing is Kenne Bell has repeated, actually surpassed, the performance with the yellow Two-Valve smoking its way across these pages. Granted, it took a prepped short-block and ported cylinder heads to reach the power stratosphere set by the Cobra, but still it's an amazing thing to see what the popular '99-'04 GTs can do with a bit o' boost.

About 642 rear-wheel horsepower is what it can do, but what it takes to get there is a slightly longer story. To make three times stock power in a Two-Valve GT engine, Jim Bell began with a '99 Mustang GT, a "program" car from Ford that Bell used for development of his Two-Valve supercharger and even glamour duty at the SEMA show. Once the basic Two-Valve kit was developed, Jim, electronics expert Ken Christley, and dyno technician, track driver, and all-around wrench Brent Morris kept right on going building power.

Getting enough fuel to the engine is often trouble with the newer Mustangs when reaching the mid-500-rwhp mark. Then it's easiest to exchange the stock GT fuel tank, pumps, and controllers for the Cobra version. Electronic control issues will also arise, although the '99-'04 Mustangs were still given cable-activated throttles, so the electronic throttle plague is not a factor here.

There are real physical limitations of the Two-Valve engine. The good news is these engines boast strong blocks, crankshafts, heads, sport a good oiling system, and generally are surprisingly durable. The bad news is the stock pistons and powdered-metal connecting rods are good for 450 rear-wheel horsepower, according to Jim. After that, you need an all-forged aftermarket short-block.

Only a few pressures and temperatures are needed to monitor the underhood goings-on. Of these, boost literally gets top billing.

Kenne Bell's Two-Valve goal was to reach the mid-600-rwhp level, enough to match the Mustang Cobra. If the sun and moon aligned, they also wanted a 9-second timeslip. Furthermore, we haven't mentioned that the big power was being mated to an automatic transmission, making the performance goals more difficult due to the reduced efficiency of the automatic transmission.

For performance and longevity, KB had the 4R70W automatic authoritatively prepped by Jerry Wroblewski and Precision Industries of Oakland, Tennessee. Besides the usual high-strength internal modifications, this transmission was wearing an approximately 2,400- stall torque converter and was rigged to manually lock the torque converter. This was done for performance reasons-it posted 6.5 percent more power when locked up-and to provide repeatability on the dyno.

As we've previously detailed Kenne Bell's standard Two-Valve supercharger kits, we'll only review them here as stepping stones to the big power. The first step is KB's 1.7-liter supercharger kit at low boost and no charge cooling, followed by the same kit with charge cooling and 9 pounds of boost. Supporting pieces at the 9 pounds of boost level are a 90mm mass air meter, a 75mm throttle body, and a 75mm cold-air inlet. Power is 431 rear-wheel horsepower on the stock short-block and using a manual transmission.

A drag slick is always a good idea, and absolutely the only way to deal with more than 600 hp at the tires. Even so, the best short time to date is a 1.54-second squirt, which could be improved with more chassis work given this much power. The wheel is a 15x10-inch Colorado Custom Yuma.

While well past the 1.7-liter blower's best efficiency, stepping this kit all the way up to 15 pounds of boost with a simple pulley change gives 507 rwhp with the manual transmission or 476 rwhp with the automatic. It also triggers the need for a prepped short-block-meaning KB's stocker went bang on the dyno-and Kenne Bell therefore repaired its yellow tester with a Sean Hyland unit featuring 8.1:1 forged pistons.

In fact, the engine and chassis were prepped at this time to support the expected 650-plus rear-wheel horsepower. Renegade Racing ported the Performance Improved cylinder heads rather extensively, and Comp Cams XE274HR cams and matching springs were fitted. The fuel system was upgraded to '03 Cobra returnless specs. That would be the internally baffled Cobra fuel tank and its dual fuel pumps, plus an extra 1?4-inch fuel line and filter plumbed into the stock fuel rails, making both rails plumbed directly to a fuel pump. The injectors are 60-lb/hr units, and the stock rails are large enough and are thus retained. Initially the ignition was stock, but at high power one of KB's Boost-A-Sparks is necessary. Both Denso and narrowly gapped NGK spark plugs were used, and the exhaust benefited from long-tube Hooker headers, a Bassani non-cat X-shape exhaust crossover, and an after-cat system.

Naturally, the induction centered around a Kenne Bell supercharger, which was moved up to a 2.2-liter Blowzilla to regain the necessary blower efficiency. Kenne Bell's 5-inch Cool Air kit and a Kenne Bell Big Oval throttle body-a Cobra throttle body with the twin-bores machined out and a single, large oval throttle plate fitted-assured no induction loss on the naturally aspirated side of the blower. The 90mm mass air meter remained.

It's another hot-rod interior until you notice the automatic transmission shifter. Even that has remained stock, as the prepped 4R70W doesn't need any help shifting.

A couple of unusual requirements arise at this state of tune. For starters, a taller Cobra R hood is required to clear the sewer-sized Cool Air plumbing, and the big Cobra fuel system needs a modified pump driver. The Two-Valve GTs come with a single pump driver-a compact circuit board that controls fuel pump output that's in the trunk-that soon can't handle the Cobra dual pumps' amperage needs. Kenne Bell's answer was to fit a second driver. This is an absolute must if SVT Focus fuel pumps are substituted for the stock Cobra pumps, which is a common upgrade, as stock Cobra pumps support up to 650 rwhp and the SVT Focus pumps 750 rwhp. All of this fuel pumpery is in addition to a Kenne Bell Boost-A-Pump which steps up the pump voltage as required. The good news is the entire 700hp returnless fuel system, drivers and all, is now available exclusively through Earl's Automotive [(818) 838-7154]. They can give you the details on the pump fusing requirements and other details.

Electronic tuning was done in-house at Kenne Bell's by Ken Christley. While hundreds of dyno runs were made with about as many different calibrations, it all boiled down to one of Kenne Bell's switch chips. Anyone hoping to duplicate this car would be forced to custom-tune it, although KB could get you close right off the bat because they have so much development tuning done already.

Jim Bell admits he's found plenty of horsepower on the inlet side of superchargers, and he has the low-restriction intakes to prove it. The cool-air intake leading to the 90mm Cobra mass air meter is his breathing aid for big-power applications. It mounts a large conical air filter in the inner fender, then connects it to the air meter with giant 5-inch-diameter flex hosing.

The front-engine dress is a more mechanical concern. The stock Mustang GT uses a six-rib, 6.5-inch-diameter pulley, and there are several options to deal with the blower-ratio and belt-wrap issues. What's needed is a large-diameter crankshaft pulley. KB makes these by machining and then sleeving a 4.6-liter Triton truck crank damper/pulley to arrive at an 8.5-inch-diameter crank pulley. This large pulley fowls the water-pump pulley, so Kenne Bell tried going down an inch to a 4-inch-diameter water-pump pulley, which proved a disaster. The water-pump speed was much too fast, and the resulting out-of-control water pump resistance cost 26 rwhp before the water hoses blew off. KB then went to an electric Meziere water pump, which cured the issue. The end result for ultimate power is a six-rib system with the electric pump and an 8.5-inch-diameter crank pulley. KB says the six-rib pulleys give adequate belt wrap given the large pulley diameters. Alternately, a 7.5-inch-diameter crank pulley with a 5-inch water-pump pulley on the conventional water pump is fine for 500 to 600 rear-wheel horsepower, with only a 1hp loss attributable to the overspeeded water pump.

Chassis-wise, Kenne Bell tried a fair number of bolt-on solutions before reverting to a nearly stock chassis fitted with Mickey Thompson drag slicks on the strip and Nitto Extremes for street duty. Lakewood drag struts up front provide the necessary weight transfer, while a D&D K-member reduces weight and provides header/working clearance.

So, how did it all work out? We're showing the stock and highest-power dyno results. Even given the big boost, prepped engine, non-emissions configuration, and custom electronics tuning, the 431-rwhp gain is still mighty impressive. The final tally was 642 rwhp at 20 pounds of boost on 100-octane fuel. This was with the low, 8.1:1 compression engine and through the automatic transmission. Jim Bell says the same configuration with a manual trans is worth an additional 40 rwhp, or 682 rwhp. That gives this Two-Valve the edge over the earlier Four-Valve test by approximately 20 hp when both are tuned way out to the edge.

Yes, the Colorado Custom wheels in front are larger at 18x10 inches. They mount Nitto 255/35AR18 Extremes and do their best to obscure a set of Baer brakes underneath.

More real-world is an intermediate set of numbers at 579 rwhp from 15 pounds of boost and 91-octane gas. Again, that's with the prepped engine, 8:1 compression, and automatic trans. KB reports the car behaves nicely on the street in this configuration. We didn't drive it, but there's no reason why it shouldn't, although the low compression means it won't be super snappy or return stellar fuel economy when off boost. But it shouldn't be a dog, either, and what the heck-you can always step on the throttle.At the Fontana dragstrip, Kenne Bell reported a 10.32-second/134-mph pass with 20 pounds of boost and 118-octane C-16 race fuel. Running at the limit of 91-octane pump gas-15 pounds of boost-netted a 10.71/127 timeslip. Not bad for a full-weight Mustang GT.

And what about those hoped-for 9-second passes? Traction was an issue, and when KB tried for 9s with 24 pounds of boost, ignition breakdown was a problem. After trying some exotic arc-welder aftermarket ignitions, Jim Bell says the best spark was the stocker amped by his Boost-A-Spark.

So, the 9-second pass is still to be had, and considering this yellow Cobra-killer has left Kenne Bell, along with Brent Morris, the single-digit track heroics are awaiting work on the short times and likely some weight reduction. But isn't it amazing to think it's even possible?

Unusually peaky data came off Kenne Bell's Dynojet during this test, and our 100-rpm resolution doesn't show the absolute peak figures, but you get the idea. So, if peak numbers in the text don't align exactly with the figures in these charts, you can still believe the text numbers.

Furthermore, these are just the two highlights of hundreds of tests on this car. Kenne Bell has all of its tests on file if you're seriously running one of their blowers and want tuning assistance. To show the highlights of the Two-Valve development, see the accompanying "Two-Valve Dyno Summary."

 Naturally Aspirated Kenne Bell, 20 psi Difference 
RPMPOWERTORQUEPOWERTORQUEPOWERTORQUE
2,500110232n/an/an/an/a
2,600115231n/an/an/an/a
2,700119231n/an/an/an/a
2,800124233254477130244
2,900131237322583191346
3,000138241342599204358
3,100144243359608215365
3,200149244378621229377
3,300153244387616234372
3,400158243395610237367
3,500163244406609243365
3,600168245424618256373
3,700173246444630271384
3,800178246465643287397
3,900184248483651299403
4,000189248497653308405
4,100192246506648314402
4,200195244514643319399
4,300199243525642326399
4,400203242538643335401
4,500206241547638341397
4,600209239562642353403
4,700212236573641361405
4,800214234583638369404
4,900217232596639379407
5,000218229603634385405
5,100215221610628395407
5,200213215618624405409
5,300211209622616411407
5,400210204626609416405
5,500209200631603422403
5,600208196636596428400
5,700207191641591434400
5,800206186643582437396
5,900202180644573442393
6,000197172644563447391
6,100182157649559467402
6,200n/an/a643545n/an/a
6,300n/an/a606505n/an/a

This chart highlights the Two-Valve Mustang GT development story. Some of the manual transmission power figures are calculated from the automatic numbers. All figures are supplied by Kenne Bell, and each test uses all the hardware, such as throttle bodies and mass air, as previous tests, plus changes as noted.

AutoManualDescription
217245Stock '99-'04 GT
N/A431KB 1.7 blower, 9 lbs, 90mm mass air, 75mm TB,
  Cool Air Intake
476507KB 1.7 blower, 15 lbs boost. Inefficient blower,
   undersized throttle body.
530564KB 2.2 blower, 15 lbs boost, 5-in Cool Air Kit
508541As above, w/8:1 compression, 15 lbs boost
579616Camshafts, ported PI heads, headers, X-shape
  exhaust crossover, and 15 lbs boost
64268220 lbs boost, electric water pump, underdrive
  alternator pulley