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2007 Ford Mustang GT 500 - Twin Screw Supercharger Kit - Too Much!
We Take A Detour On The Wild Side As Kenne Bell Pushes A GT 500 Past 800 Hp With A Blower, A Throttle Body, And Tuning
Horse Sense: We aren't showing an interior photo of Kenne Bell's GT 500 prototype because it's completely stock. But maybe we should have, as the car was delivered with one chrome air vent on the driver side, and three black air vents at dash center and on the passenger side. There's a fix from Ford, but we'd keep this snafu as a mark of individuality-and give MCA judges something to argue about.
Too much is just right, and now we know what it feels like. It's the constant nag that traction must always be managed, even at part throttle with sticky new tires on a dry day. It's ferocious wheelspin while casually accelerating in Second gear. It's the realization that if someone pulls alongside in a good bolt-on car, you'd better be sharp; even then, they'll likely beat you to 50 mph. It's knowing that once past 70 mph, nothing can dream of keeping you in sight. It's the joy of considering Fourth as your primary acceleration gear. It's eight cylinders producing 1,000 hp. It's a special kind of cool.
Even more chill is the way this power is made using a laughably small amount of hardware, about $6,000 in social lubricant, and not even a full day of installation. But that's what you get when Ford does it right and the aftermarket is ready to build on that solid foundation.
The foundation, of course, is the '07 Ford Shelby GT 500. It packages Ford's largest passenger-car V-8, 5.4 (331ci), DOHC, Four-Valve with a six-speed manual transmission and a stout 8.8-inch live axle. But what we learned-as we'll detail in a minute-is the GT 500 is blessed with numerous valuable supporting parts, which make hypercharging it a breeze.
Well, it's a breeze if you're Kenne Bell and have already sorted through this process with a massive amount of in-house brainpower. Ford already prepped the GT 500 with all the good parts. Kenne Bell was ready with Twin Screw blower kits for all the modular-engined Mustangs, as well as kits for the similar Ford GT supercar. KB recently stepped up its blower program with larger Twin Screw superchargers ("Home Screwed," Nov. '06, p. 48). The company also has Jim Bell's engineering talent and Ken Christely, the in-house electronic tuner who designed this project.
The project has spawned the expected Kenne Bell blower kits (Stages 1 through 3) for the GT 500. There's also a pile of research about the supercharging stratosphere that's so fun to read about, not to mention pedal down the road.
Following a well-established development procedure, KB borrowed a new, completely stock GT 500 from Earl Moorehead of Earl's Automotive. Strapping it to the Dynojet, KB optimized the stock Eaton-supercharged, charge-cooled engine, and then removed the Eaton Roots-style. KB's latest 2.8 big-bore Twin Screw supercharger was bolted on next, and the car began climbing the boost ladder. As expected, the H-version (high internal-pressure ratio) of the 2.8 proved more efficient at around 15 pounds of boost, so that's what was used to take the combination up to an all-out, everything-fresh 24 pounds of boost. And to relieve any suspense, its best-ever number was 810 rwhp at that boost.
Some relief. Now you really want one.
Getting the 2.8H Kenne Bell onto the GT 500 was mechanically simple, although hood clearance was tight. The underhood padding had to be removed, and a small notch in the reinforcing web structure was required. The Twin Screw supercharger was bolted to the stock Ford intake manifold using a thin adapter, and no changes were required in the front engine dress. Kenne Bell incorporated the large bypass into the right rear of the installation. It wasn't the most compact bypass placement, but Ken wanted it in an easily accessible location for no-hassle servicing.
Driving the long 2.8 supercharger required a new, more compact front blower pulley and drive to retain the stock drivebelt location. The blower pulley was redesigned to fit over the blower's drive snout, not in front of it as with previous KB superchargers. Kenne Bell developed this drive in 31mm and 77mm versions. The GT 500 fitment uses the 31mm drive.
As rapid blower-pulley changes are an important part of Kenne Bell's blower strategy, the quick-change KB pulley capability was preserved. The only practical change is the new pulley uses four small bolts instead of two larger ones.
Minimum blower-pulley diameter is limited to 2.5 inches by the blower drive. As this delivers 24 pounds of boost, we doubt anyone will mind. Other available pulley diameters are 2.75, 3.00, 3.25, and 3.50 inches. When the higher-boost 2.500 and 2.750-inch pulleys are run, the stock blower belt is used. The 3-inch-and-larger pulleys require a longer blower belt. Ken says this longer belt might be supplied with all Kenne Bell GT 500 kits, although the final details were still being decided at press time.
No changes were contemplated regarding the GT 500's complex, difficult-to-access harmonic damper and blower belt pulley arrangement. Being that it's a two-belt engine, changing the crank pulley for higher supercharger rpm isn't a practical option.
From the beginning, it was assumed the stock Ford intake manifold and charge cooling would be retained. KB's experience with the '03-'04 Terminator Mustang Cobra engines, as well as testing the GT 500, has shown Ford's charge cooling is the best there is. With the GT 500 mounting the blower atop the intake manifold-which flowed 1,700 cfm on Kenne Bell's flow bench-there wasn't a need for the expense or bother of a new intake-manifold casting.
Mounting the KB Twin Screw to the Ford intake manifold required the thinnest possible adapter plate, due to the tight hood clearance. Ken fabricated one from aluminum for the prototype. The photos show the tightly packaged bypass passages, mechanical-locking registration groove, and other adapter-plate details.
To follow up on the charge cooling, Ken says that he copied more than 12,000 numbers during the kit's development tests, and in that time, "I never saw the intercooler discharge temp go higher than 138 degrees. To an engine, that's freezing-cold air; anything less than 150 degrees is like candy to the engine. I made runs all day long and hot swapped a pulley [A two-minute job.-TW]; I'd run it and run it and run it. In August, it was 110 degrees and [the air-charge temperature] would stay 138 degrees or lower. It's amazing."
Ken isn't kidding about the volume of data recorded during development. The dyno-run sheets number in the several hundreds and fill two large three-ring notebooks. The charge-air temperatures were taken from the OEM sensor, and the temperature probes Ken added so he could see the air temperature entering the mass air meter, exiting the supercharger, and inside the manifold downstream of the charge cooler. Boost loss across the charge cooler was also monitored. It never exceeded 1.7 pounds.
Kenne Bell also knew from the beginning they would focus on the inlet side of the supercharger. Because Twin Screw blowers ingest air at the firewall end of the supercharger, it has always been a trick to get the incoming air turned around from the air filter and mass air meter into the supercharger. Sometimes this has meant a slightly restrictive air-inlet casting because there wasn't enough room between the supercharger and firewall. On S197 Mustangs, this area is not so cramped. And given the sizeable, air-hungry Four-Valve V-8 and long 2.8 blower, the KB crew was determined to not starve the blower with the inlet-air casting.
With adequate room in front of the GT 500's firewall, the result is a huge blower inlet that easily flows 1,700 cfm, which is the intake manifold's maximum flow rate. Its oval opening is so large the GT 500's 2x60mm throttle body can't mate to its flange. A separate adapter plate had to be used to neck between the throttle body and inlet.
Not surprisingly with computer modeling, the stock throttle body easily supports more than stock horsepower. Drawing on previous Ford GT experience where a 2x70mm throttle body is used, KB knew a larger throttle body would help but would also be expensive. The exotic GT uses a cable-actuated throttle body; the GT 500's is electronic, skunking what would have otherwise been the easy option. Furthermore, even a 2x70mm throttle body wouldn't be enough with the 700 rwhp the KB GT 500 was going to make at high boosts. Something larger was going to be needed, so Kenne Bell produced a prototype 2x75mm billet monster to table concerns about airflow.
The electronic throttle body took considerable tuning for Ken to make it work. The new version, available only through Kenne Bell, measures 2x77mm in final production. It's expensive, so the kits make do with the 2x60mm stock throttle body as long as possible; the 2x77mm unit isn't used until the final step. In fact, the big throttle body singularly takes the power from the low 700s to 800 rwhp.
Jim points out that people used to think a single 70mm throttle body was good for the 5.0 H.O. engine and a 75mm was too large. Now we need twin 77mm throttles.
As development continued, the first stock-breathing item to give up was the mass air meter. It ran out of electronic capacity at 525 rwhp, but this was cured by adding a Diablo MAFia mass air extender. Airflow through the stock mass air meter was an issue. The plastic meter flows a lot of air with no problems, but it's molded as part of the restrictive Ford air-filter box. KB decided Ford had a great meter, so they duplicated its housing section separately from the airbox. The stock electronic mass air element was transferred from the stock airbox/air meter to the KB mass air tubing. The MAFia Extender was fitted and that was the end for mass air concerns.
On the prototype shown here, the air filter is a flat oval with a washable, cotton-mesh filter made for Kenne Bell. It has generous airflow capacity and shouldn't change in the production kits. An air dam encircling the filter will be developed to quarantine as much hot underhood air as possible.
If getting air into the supercharging system required work, getting it out was hardly a challenge. The stock cast-iron exhaust manifolds and catalytic converter sections of piping work fine. The prototype GT 500 had a Bassani after-cat section that likely helped, but it wasn't tested separately so we're not sure how much it contributed.
The GT 500 fuel system required little to support the Kenne Bell airflow. Similar to Terminator Cobras, the GT 500 uses dual fuel pumps, but the Terminators use a single fuel-pump control module for both. This module electronically varies voltage to the pumps in response to engine-management commands and is the brains of the returnless fuel system. KB has found the Terminator controller overheats when juiced to the max by a Kenne Bell Boost-A-Pump. The high voltage is too much for the controller's electronics at high outputs; they shut down until cooled.
The GT 500 uses dual fuel-pump controllers, and they have no trouble handling the voltage that the KB Boost-A-Spark zaps at them. And while many said it wouldn't work, the GT 500 has done great with one BAP over two controllers. It's a good thing, too, as a GT 500 with stock injectors needs 92 psi of fuel pressure at full chat.
More good news in the fuel system is the stock 52-lb/hr injectors. Granted, they're maxed at 800 rwhp, but they do get the 5.4 there and don't need to be changed. Fuel pressure is jacked to the moon via the BAP. From the stock 60 psi, it needs to reach 82 psi at 625 rwhp and 92 psi at 700 rwhp or greater. Alternately, fuel pressure could be left at 82 psi and 60-lb/hr fuel injectors used starting at 700 rwhp.
If Ford did a good job on the GT 500's exhaust and fuel systems, the company did even better on the ignition. Ken doesn't know what hardware Ford upgraded on the GT 500 ignition compared to earlier Mustangs, but he figures it could weld pig iron underwater. The only ignition upgrade required turned out to be a swap from Ford's platinum sparkplugs to conventional NGK TR6s gapped to 0.025-inch. Fortunately, the GT 500's 5.4 employs conventionally shaped sparkplugs, not the Three-Valve Mustang GT's stirrup-style ground electrode sparklers.
The minimal amount of parts needed to reach 800 rwhp is amazing. The stock cast-iron exhaust manifolds; stock catalytic converters; the stock engine, including everything inside and the intake manifold; stock front engine assembly dress (belts and tensioners, among other things); the stock clutch; and the stock fuel system, including pumps, controllers, fuel lines, fuel rails, and injectors, were all in place at 810 rwhp.
Bone stock at 8.5 pounds of boost with its 3-inch blower pulley, the Roots-style- blown GT 500 made 444 hp and 428 lb-ft of torque at the wheels on Kenne Bell's Dynojet. Ken electronically tuned the stock combination, making 481 rwhp/462 lb-ft at 8.7 pounds of boost. KB always tunes the stock application to ensure its hardware-and not the tuning-provides the power gains.The ignition timing is locked at 23 degrees for all testing so the computer can't add or subtract it, which would have a huge impact on power. Such control of variables, along with extensive instrumentation and datalogging of temperatures and pressures, is a hallmark of Kenne Bell testing. It's some of the most thorough, scientific testing we've seen in the aftermarket.
A maximum Eaton-boost combination was also tried using a 2.59-inch aftermarket blower pulley and the stock crank pulley. Thus far, you can't go larger on the lower pulley without fouling the cam sensor. This gave 12.3 pounds of boost and 517 rwhp/518 lb-ft on the KB dyno. That's not bad, but the Eaton is hanging out with nothing left to give at this point. Don't forget: You need a custom tune to duplicate these numbers, especially below 3,000 rpm where Ford's GT 500 tune is grandmotherly muted. Ken confirmed our suspicions that the lame response below 3,000 rpm on a showroom-stock GT 500 is from ultra-conservative tuning.
Fitting their 2.8H supercharger, KB started with a 3.00-inch blower pulley and an all-stock air-inlet path-stock air filter, mass air meter, rubber hose inlet, and throttle body. This is a larger supercharger turning more slowly than the ultimate Eaton combination. Boost was 12.6 pounds, just 0.3-pound better than the hot-rodded Eaton, but power was a resounding 606 rwhp. That's an 89-rwhp gain over the Eaton's best; all of it is attributable to the larger Twin Screw's superior efficiency.
As testing continued from the summer through the fall of 2006, we can't detail the hundreds of dyno runs made, but hitting the highlights is instructive. One of our favorites came early. Using the plain Stage 1 KB kit-which is simply the 2.8H blower and stock air filter, inlet, and throttle body, a romping 657 rwhp/625 lb-ft was made using a 2.750-inch pulley and pump gas. What's intriguing is the bang-for-the-buck. The Stage 1 kit is the most affordable at $4,599, pump gas is an attraction, and you can't use much more than 650 hp at the tires on the street anyway. Should more be desired at the track, the 2.500-inch pulley could be fitted for a small bump to 19 pounds of boost and 664 rwhp/677 lb-ft using race gas.
If the mid-600-rwhp level is achievable using the stock inlet, it's still not necessarily the most efficient way to get there. By 12 pounds of boost, the Stage 2 KB kit is the way to go. This adds the KB Cool-Air Kit, which is the big air filter and rubber tubing, as well as the Big Oval mass air meter. The stock 2x60mm throttle body is maintained. The limit with pump gas is the 3.00-inch pulley, yielding 14 pounds of boost and 666 rwhp/596 lb-ft of torque. Using the 3.250-inch pulley, it's also well ahead of the Stage 1 kit at 12 pounds of boost-632 rwhp to the Stage 1's 576 rwhp, a 56 rwhp gain.
When reaching for 700 rwhp, the Stage 2 kit shines. With the 2.750-inch pulley, it blows 18 pounds of boost and 708 rwhp/671 lb-ft. That's on race fuel, but it's a great combination for street/strip duty-666 rwhp on pump gas during the week and 708 rwhp at the track with good gas on Saturday night. Best of all, the Stage 2 kit is only $300 more than Stage 1, and the pulley change is simple.
With the boost turned up with a 2.500-inch pulley, the big air filter, mass air, and other parts of the Stage 2 kit installed, it became clear that the major choke was the stock 2x60mm throttle body. A jury-rigged 2x70mm throttle body was tried and found to be not enough, so KB developed the previously mentioned 2x75mm unit. It's a key to the power kingdom, especially at high boost. Given the smallest 2.500-inch pulley of the Stage 2 kit is good for 20 pounds of boost and 727 rwhp, adding the throttle body makes it a Stage 3 and bumps boost to 23 pounds and power to 800 rwhp/738 lb-ft.
The downside to Stage 3 is the cost. The billet electronic throttle body is $900, making the complete Stage 3 kit $5,799. But 700 rwhp won't be passed without it.
All Stages are supplied with an SCT flash tuner with a well-developed, intelligently aggressive tune appropriate to the hardware. For the record, we haven't witnessed all the dyno runs, but Kenne Bell showed us 801 rwhp using the 2.500-inch pulley, so we're convinced the power is there.
Ironically, the most powerful Kenne Bell kit is the easiest to install. Removing the Eaton takes less than a half-hour, Ken says. Then the KB adapter plate is screwed to the intake manifold, the blower to it, and finally the throttle body, mass air, and the other parts-if you opt for them. The Boost-A-Pump attaches to the fuel-pump modules in the trunk and is a simple wiring job. The prototype GT 500 didn't have a mounting for the BAP, but it can be zip-tied to any convenient spot, as it's a small, lightweight box. There's also the removal of the underhood insulation pad and a quick cut of the support structure to attend to. That might burn time if you need to find a whiz wheel for the cutting.
Ken says the install takes less than three hours, but we'll go with his "less than eight hours if you're anything close to being competent." Remember, he said it. Ken also noted the whole installation can be reversed to stock, as there is no drilling, bending, or other butchery, other than the small spot on the bottom of the hood. Even that can be covered by the hood pad.
As for the power, it's transformational. If you're hankering to see what too much feels like, don't say we didn't warn you.
Sometimes short and sweet info paints the big picture best. Here's a summary of Kenne Bell GT 500 dyno data to put the kits in perspective.
To avoid data overload, we're only showing two dyno runs from hundreds. The first is the showroom-stock GT 500 readout before any electronic tuning. The second is the 801-rwhp pass made when we visited at the end of development. As it always seems to work out, the true peak number doesn't show in the 100-rpm resolution, but we assure you, it was at 6,320 rpm.
Kenne Bell also showed us the best test, an 810-rwhp blast using the same configuration that ran 801 rwhp for us. The difference was attributed to a slightly slipping blower belt and heat-soaking during our visit. KB notes that after several hundred runs, an otherwise-good blower belt will lose enough grip to slip slightly, hence the missing 0.5 pounds of boost during the 801-rwhp run.
As all the dyno graphs have essentially the same shape torque and horsepower curves (more boost means the same curves at higher numbers), these two runs give an accurate idea of how the KB runs on the GT 500. To address the many points in between, see the Dyno Summary sidebar.
|RPM||Baseline||Kenne Bell Stage 3||Difference|
When reviewing any dyno data, note the correction factor used. On chassis dynos, the two correction factors commonly seen are SAE and STD. The SAE is from the Society of Automotive Engineers; It's the more widely accepted correction factor in engineering and serious tuning circles. The STD, or Standard, reads a percent or so more power, so it's more fun for casual tuning and more helpful to the average dyno shop engaged in the endless knife-fight of the unregulated, self-promoting hot-rod world. At the 800hp level, the difference between SAE and STD is around 15 hp, with the STD number larger. All Kenne Bell tests use the SAE correction factor and WinPEP 6.03 data acquisition software on their '96 Dynojet 248.
|STAGE 1||STAGE 2||STAGE 3|
Note: Stage 1 is the blower; Stage 2 adds an air filter and a MAF; Stage 3 adds a 2x75mm throttle body. Bold denotes highest power available with 91-octane pump gas. Best power with 93 octane and Stage 3 calculates to more than 700 rwhp, assuming the electronics are tuned for it; KB can provide this tune. Power increases on equal fuel among kits are due to increased blower efficiency because of freer-flowing inlet air. Boost is measured after the charge cooler. All power and torque figures are at the rear tires as measured on Kenne Bell's Dynojet. An SAE correction factor is used. Kits will likely come with 3.00-inch pulleys, others can be substituted or additionally purchased
So what was that we mentioned about 1,000 hp? It's what the engine is actually producing to make a Dynojet read 810 rwhp.
To put numbers to it, 810 rwhp is 932 hp at the flywheel if you use the industry standard 15 percent driveline-loss figure. Jim Bell further guesstimates-and he's surprisingly accurate in his guesstimates, as we've learned throughout the years-the 2.8H Twin Screw needs 70 hp to drive it at 24 pounds of boost. Adding it to the 932 flywheel horsepower, we get 1,002 hp. That's what the crank, rods, and pistons inside the GT 500 engine are experiencing-3 hp per cubic inch. Incredible!
We'd all love to try megapower, but it's different. We've hinted at the driving technique required with such huge power and how a less-enthusiastic combination could easily be faster in a typical zero-to-hero street blast that tops out at 65 mph or so. Something else to noodle is all those driveline parts downstream of the engine.
One consideration Kenne Bell ran into was what to do with a bent two-piece Ford driveshaft after it relieved itself under the stress of being caught between the underhood gorilla and the Dynojet drum. That's right: The driveshaft didn't hit anything, and it wasn't shocked with dragstrip gear changes-it simply yielded under the torque it was transmitting on the dyno. A single-piece Ford Racing driveshaft was the answer.
But there'll be plenty more parts quivering when this monster hits the asphalt in anger. For example, the transmission wasn't designed to input 600 lb-ft of torque at 2,000 rpm; we wouldn't bet on axles or U-joints.
In the short term, wheelspin is the savior. As long as the tires go up in smoke, almost everything else ought to live. But when the slicks roll onto a glued track, the budget needs to provide for a fully prepped rear axle and chassis reinforcements at the least.
Max power on the dyno was made with the air filter laying up and out of the engine compartment. It seemed to gain horsepower, maybe a single digit. Production kits will have an air dam or fence built around the filter, but getting cool air to S197 engines remains an issue. Ford didn't leave large enough air paths to the left-front corner of the engine compartment. Car owner Earl Moorehead is planning on cutting a scoop into the side of the front fender-just outboard of the headlight-to feed his orange monster. It ought to help.