Twin-screw
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On this page:

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Historical Note

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Supporting Modifications

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Twin-screw Installation Project

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Compressor Rebuild

 

In 2004, I put Taz through some major performance changes with the installation of a twin-screw compressor kit from Kenne Bell. This put the car's power level north of 430 RWHP (525 FWHP). Even more importantly for a mostly street-driven car, the Cobra finally exhibited some real low end grunt in addition to more top end power. I can now allow the engine to loaf along at 1500 - 1800 RPM and still enjoy the option of squirting into an opening in traffic without downshifting. No more having to keep the motor up on the cam to prevent it's response from going flat. Wow, a "virtual" big block! How cool is that?

 

Historical Note

Kenne Bell has been a respected name in automotive forced induction for decades. Although the price of admission to KB ownership is somewhat higher than it is for any of the available centrifugal supercharger kits, the KB kit provides one thing none of the hairdryers can offer at any price - a torque curve nearly as flat as Nebraska. Fat torque is exactly why some of us are willing to pay a premium for a KB over anything else. Well, maybe that and the fact that the quality of a KB kit is as close as you're going to find to a cost-no-object product anywhere in today's disposable society. KB kits are thoroughly engineered, painstakingly tested, and backed by some of the best automotive technical support around. Every kit comes complete with everything you need and an installation manual that's second to none. Anyone with a reasonable mechanical aptitude and a decent assortment of hand tools can install a KB kit.

I knew that I wanted a KB for Taz even before I ever signed the sales contract for the car. I had already been to KB's website and knew the company had a twin-screw kit available for the '96 - '98 Cobras, so I didn't expect it to be too long before a '99/'01 version hit the market. During my early research into F/I options, I had been to every supercharger manufacturer's website and evaluated the product offerings. I had looked at all the centrifugal kits, but I knew I wasn't going to be happy with one of those - great for the track, but not so great for the street. I had even looked casually at the turbo kits, although I really wasn't interested in a turbo. For me, it had to be a KB twin-screw or nothing at all.

For a long time, it was nothing at all. Years, actually. In fact, the '99/'01 Cobra kit came very close to becoming the KB kit that never was. Somewhere during the kit's development, KB had shelved the project, and the company had no real plans of reviving it. Another exasperated prospective buyer, who had finally become weary of waiting in silence, was informed of this when he contacted the company about a projected release date. Refusing to take "No" for an answer, he somehow convinced KB management to resume development work if a sufficient number of New Edge Cobra owners demonstrated sincere interest in the product by providing advance deposits. 

As soon as Kenne Bell agreed, KB aficionados who owned Cobras of the model years in question began stumping for the kit. Being a member of that group, I immediately went to work trying to generate interest - and deposits. I really wanted this kit to happen, but it turned into much more of an uphill battle than I had expected. I was somewhat dismayed by the number of naysayers and detractors who began flooding the recruiting threads we had started on various Mustang forums with all sorts of hate talk. Many were impoverished ratchet heads trying to achieve their performance goals on shoestring budgets, who realized they'd never in their wildest fantasies be able to buy KB kits for themselves and wanted to ensure that no one else could enjoy one either. (Misery certainly does love company, doesn't it?)

But a few of the haters - and the ones that really irked me the most - were the individuals who had originally planned to install KB twin-screws on their own '99/'01 Cobras, but who had eventually succumbed to their impatience and settled for one of the centrifugal kits that had been available for quite a while. Afterward, these people had discovered that their centrifugals not only failed to address the little mod motor's one glaring fault - poor low end torque, but typically provided only mediocre performance under all but wide open, high RPM conditions. Although very few of them would admit to being anything less than completely satisfied with their alternate choices, they still maintained that KB was a villain for not having the product available when they had wanted it. These were the most vicious KB detractors, and their attacks were disturbingly relentless. 

More than once I was almost persuaded that all the haters were going to succeed in nullifying our efforts - and our dreams - by quashing our kit for good. But somehow, despite their poison pen posts in nearly every forum thread we had started, we managed to amass enough deposits before our deadline to persuade KB to put the '99/'01 Cobra kit back into play. The product for which some of us had waited years was finally going to become available to us. I was genuinely surprised. Ah, but I love it when a plan finally comes together.

Editorial Comment:  Where are all those loudmouth detractors now? Where are all those doomsayers who spent endless hours prognosticating the terrible fates that awaited us and our cars if we were foolish enough to install positive displacement superchargers? Where are all those self-proclaimed experts who admonished us that our stock bottom ends would be grenading left and right from the unbearable stresses introduced by the evil twin-screw devices after which we lusted? Well, they all had to sit down and SHUT UP, didn't they? Over a decade later, with none of their dire predictions having materialized, what else could they do without looking even more retarded than they obviously are?

Consider this an object lesson. The world is full of self-proclaimed experts on every topic imaginable, and it's often difficult to separate the wheat from the chaff when it comes to "expert" advice, especially if that advice is motivated by a hidden agenda. I'm reasonably certain that most of the KB haters I encountered early in 2004 knew perfectly well that the fear and loathing they were disseminating was complete nonsense. Nonetheless, there they were, standing on their respective soap boxes trying to sell their drivel to anyone gullible enough to buy it. Why? Only they can answer that question. The rest of us can only guess. In any event, question "authority" and beware the hidden agenda. (I assure you that I expect no personal dispensation in this regard.)

 

The Supporting Cast

As soon as Kenne Bell announced that enough of us had submitted deposits for development of the '99/'01 Cobra kits to resume, and that the retail versions were only a few short months away, I began acquiring and installing the various ancillary parts I believed necessary to support Taz's twin-screw. First, he needed additional instrumentation to enable monitoring of his fuel pressure and engine vacuum/boost conditions. I purchased and installed the gauges even before the twin-screw kit's development had been completed. You can find details about the instrumentation upgrades on this website's Instrumentation page.

Also, the more I thought about all the additional heat the twin-screw would be radiating into the engine bay, the more I liked the idea of replacing the stock hood with one that provided some decent ventilation. Ideally, air would flow smoothly from the front clip, over the engine and compressor, and then out the top rear of the engine bay. One of the Ford Racing 2000 Cobra R hoods would have provided an ideal airflow path if its top louvers were opened up, and I nearly bought one of those. In retrospect, I probably should have bought a Ford Racing hood and had it modified to my specifications. (Read about my aftermarket hood odyssey on this site's Body & Trim page.)

About the time the Kenne Bell kit's resurrection was beginning to unfold,  Mark Chiappetta of Zone-5 Performance began soliciting pre-production deposits for a brace he was developing to prevent the Cobra's 8.8" differential cover from shattering. (Although Zone 5 is now defunct, the brace is still available from the folks at BilletFlow.) Eaton-blown '03/'04 snakes had begun experiencing catastrophic cover failures, and cars that had been pumped up with additional power were especially prone to fragging the brittle cast aluminum part. Considering that I planned on taking Taz's power level somewhat beyond that of a stock Terminator, I was quick (number six on the sign-up list) to join the pre-release group purchase that Mark made available. You can read more about the brace on the site's Drivetrain page. 

The final member of the KB compressor kit's supporting cast didn't join the troupe until after the opening curtain had risen, while the blower installation was underway. During my engine teardown, in preparation of the twin-screw kit's installation, I noticed that a significant amount of goop had accumulated - again - in the lower intake. Although this had been an ongoing situation, I hadn't been concerned previously about the oil being siphoned up through the PCV valve. There had, after all, never been much in its path for the oil to gum up, and the ingested oil hadn't resulted in any pinging, so why bother fixing what wasn't broken? But with the car's intake path architecture about to change, the siphoned up oil could potentially result in a catastrophic failure by reducing the intercooler's efficiency. It was time to find a solution. Jump to the Intake System page of this site for details on the oil catch tank that I added to avoid the problem.

 

Taz Gets Blown

Finally! I'll bet you thought we were never going to get to this point. Me too. Sometimes, I'm such a windbag. (... Sometimes?)

The photo below is a shot of the way the twin-screw installation turned out. Finally. However, there were a few bumps along the road that led Taz - and me - to this destination. This is not to say that a Kenne Bell kit isn't the highest quality, most complete supercharging solution on the market. It most assuredly is. That's my opinion, anyway. And since this is my website, it's only my opinion that counts around here.

 

Nevertheless, being among the first in the entire known universe at anything can have its drawbacks. In this particular instance, the newly released '99/'01 KB Cobra kits had barely emerged from prototyping, and the preliminary release of the manual was still hot off the presses. Couple these facts with my anal retentive, perfectionist nature, and you've got to expect a few ripples. I ran into my share, but if I had to do it all over again, I wouldn't hesitate for a second.

In any event, as long as you know one end of a wrench from the other, own or have access to a reasonable assortment of hand tools, and possess the financial means to purchase one of these kits, you will be able to install it on your own vehicle by simply following the absolutely first rate installation manual. Guaranteed. (Presuming, obviously, that KB makes a kit for your particular application.)

Let's have a quick look at the major parts that arrived in the two enormous cartons that contained my own kit. In the upper left panel of the composite graphic below, you can see the compressor pre-installed on its base plate, along with its snout, drive pulley, and intake manifold (with blue tape over the throttle body mounting location). Clockwise from the upper right of the same composite, you can see the intercooler, the discharge manifold, and a view of the compressor's boost bypass assembly.

 

 

Looks like a piece of cake, eh? Now, let's have a look at the array of supporting parts that came with the kit and also required installation before the car was ready to roll. The heat exchanger, at the upper left, is the only part in this group that I'll bother to mention by name. There are far too many others to detail every one.

 

Naturally, several engine parts must be removed before installation of those provided with the kit can begin. Now, removing parts is no big deal, but I had barely begun the process before I was instructed to begin sawing and grinding on my motor. Folks, that means my "point of no return" was all over me like a cheap suit before I felt like I had even gotten started. Oh well, no guts no glory! Besides, I didn't yet own a Saws-All, and this was a perfect excuse to add one to my tool crib.

After purchasing a couple excellent Milwaukee power tools, I was ready to begin removing the aluminum "ears" from the heads (the passenger side location is circled in red below) and clearancing the eight bosses that were located in the valley of the block (the four passenger side bosses, including the knock sensor boss, are circled in blue). There would be no reconsidering this thing, now. I had burned my proverbial bridges behind me.

 

Having sealed my own fate in a rooster tail of aluminum flecks, I pressed onward with the installation, only occasionally wailing and gnashing my teeth at the thought of having whacked chunks off a perfectly good engine that had never given me a moment's grief. The composite graphic below contains shots taken during the teardown and early installation phases of this project.

 

The photos in the next composite were taken later during the installation. In some of the shots, you also can see the billet Ford Racing coil covers - still covered with protective plastic - that I added to complement the polished billet compressor. What can I say? I'm half Italian, and I like a certain amount of bling with my zing.

 

Lastly, you can see several shots below that I shot during the final stages of the installation, including shots of the KB daughter card being installed in the PCM, the new 90-mm MAF meter, the aluminum intercooler fluid reservoir, the bottom heat exchanger brackets, and the KB Boost-A-Pump.

 

I spent weeks on this project. For a time, I was certain that I had somehow drifted into my own little corner of the Twilight Zone, and I would never finish it. But the day eventually arrived when I began wiping off the tools and putting them away. I was finally ready to wake the beast!

 

Murphy Rears His Ugly Head 

Alright, I'm a plodder and a paranoid, but when I'm finished everything is right. And it was, too. The first time I keyed that ignition after completing the installation, the engine fired right up and purred like a contented kitten, albeit a very large kitten. While completing the engine and intercooler fluid fill procedures, I started and shut down the engine three more times, and each time it ran smooth as silk. Hot stuff!!! We were ready to take Taz out for a shakedown cruise.

Hillie and I jumped into the car, and we eased out of the garage. Since, I had run the tank nearly dry to facilitate the fuel pump installation, I was careful to stay out of boost on the way to our first stop, the local Texaco station. After we had filled up and motored off again, everything was still peachy. Or so it seemed.

When we finally reached a point in our drive where the road was excellent and the traffic was sparse, I decided to roll into the boost for the first time. Going in, the boost felt and sounded great, but when I let off to coast up to the next intersection, I heard the sound of metal rubbing against metal. I immediately pulled off the road for a look under the hood, but couldn't see anything amiss, and the sound had subsided by then. I knew that there was almost no clearance between one of the idler pulleys and the alternator case, and KB had suggested that clearancing the alternator might be necessary, so I figured that's what was causing the sound. I'd deal with it when we got home. Or so I thought.

We set off again, and I rolled into the boost a second time. This time the metal on metal whizzing sound was much more pronounced and followed by a second, much more sinister sound: that of a giant winch pulling a huge steel cable taut. Then, when it sounded as if the cable had reached its limit, the engine just stopped. No, it didn't stall; it stopped instantly. Dead. As if a giant invisible hand had reached out and grabbed the crank pulley. I tried to restart the car a couple times, but the starter couldn't budge the motor through even a single degree of rotation. Fantasies of having seized the motor began dancing in my mind's eye, along with the usual sugar plum fairies. I dialed AAA, and we made the ride home in the cab of a flatbed, with Taz strapped down in back, and me fretting the worst all the way.

After we got Taz back into the garage, and he had cooled a bit, I performed a couple quick tests primarily designed to determine if it was time to start searching for a new motor. Everything indicated that my motor was still intact, so I shot off an email to Brent at KB technical support and turned in for the night.

The next morning, I received a reply from Brent suggesting a couple of possibilities for me to investigate. I also mentioned my situation to my boss, who told me one of his son's cars had exhibited exactly the same symptoms when its alternator had seized. He reminded me that it's possible that the engine will refuse to crank if any one of the accessories run by the serpentine belt seizes up.

At lunchtime, I drove home, released the serpentine belt, and began twisting pulleys by hand, looking for one that wouldn't turn. I found it when I reached the compressor pulley. Frozen solid. I was extremely relieved to confirm that it was in fact, not the motor, but the compressor, that had seized. KB was very good about replacing the failed unit, and I didn't consider the infant mortality to be in any way a reflection of KB quality, since I've been around the track enough times to realize that every manufacturer has his share of crib deaths. This time, I just happened to be the unlucky slob holding the dead baby.

I had to uninstall what seemed like half the kit's parts in order to extract the bum head unit - and then reinstall them all when the replacement arrived - but that all went fairly quickly and without incident. (Experience really is an excellent teacher.) The good news was that further investigation revealed Taz had somehow dodged a major bullet. None of the aluminum chips that the dying twin-screw had shed while it was still spinning down in failure mode had made its way into a cylinder - the intercooler's core had channeled all the little metal fragments down into the bottom of the discharge manifold, where they had remained. But I still shudder when I think of how different the outcome might have been if I hadn't opted for an intercooled kit. Oh, well. Any landing you can walk away from, right?

 

Running Strong 

Following my installation of the replacement compressor, the car ran great and delivered just about all the power I cared to take from its little 280 cid stock bottom end. During his second trip to the dyno in 2004, Taz put down 433 SAE-corrected rear wheel horsepower, and that was with bad gas (a story for another time). Kent Kim, at Intense Motorsports opined that the car would have been good for at least 450, on decent gas, an estimate with which Ken Christley at KB concurred. I figured this was plenty good for a small V8 equipped with hypereutectic pistons and powdered metal rods running the 91-octane swill that passes for premium gasoline around here. Besides, the car's acceleration was already somewhat traction-limited on street tires.

 

 

The chart below contains Taz's best pull from his December, 2004 dyno session. Click on the chart to see a larger image. This pull was made on a tank of gas consisting entirely of tree-hugger 91, and the PCM was pulling timing to counteract a little pinging. (Yes, the Kenne Bell tune monitors the knock sensor signals to protect the motor, and I like it that way.)

 

Let's use this documented 433 SAE-corrected HP to crunch a couple numbers. Allowing for the widely accepted 18% driveline loss (the loss in power between the crankshaft and the rear tires in these cars), this puts Taz's net flywheel horsepower right at 528. (433 / (1.00 - 0.18) = 433 / 0.82 = 528) Yikes! Considering this power was being produced by a motor displacing only 280 cubic inches, this works out to a specific power output of a little better than 1.89 hp/cid. Geez, I must be getting old, because I can still remember when the specific outputs of really radical street machines were hovering around 1.0 hp/cid, and mom's daily driver was weighing in at about 0.5. We've come a long way, baby!

By the same token, horsepower indicates only a vehicle's top speed capabilities. Horsepower numbers may sells cars, but it's torque that gets you to the finish line, so let's not overlook the real beauty in the above chart. It lies in that broad, fat torque curve. Yes! Finally, a solution for the little mod motor's glaring lack of low end grunt! Finally, this car is a breeze to drive in stop-and-go traffic. In fact, after living with the KB for several years now, I'm amused when I recall that I was initially concerned about making Taz less streetable by installing the Kenne Bell kit. The fact is, the car is now considerably more tractable, more forgiving, and more capable in every respect. Believe it or not, Taz is even more docile now (when I'm not flogging him) than he ever was bone stock. And his reliability is as solid as it ever was. Engine cooling has never been an issue at all, and I haven't been required to touch a single thing to keep the car running smoothly. That's what I call quality engineering! Thank you, Kenne Bell.

2011 Update:  After having my ass handed to me early in 2011 during a casual street face-off with a new Cadillac CTS-V coupe, I swapped out my compressor's snout pulley for a smaller one. I doubt that I put the motor in jeopardy by doing so, because I had been running a Torco Unleaded Fuel Accelerator blend for quite some time that bumped my tank octane to about 96. But just to be sure, I fattened up the car's open loop air/fuel ratio by an additional 10% with the MAFterburner to compensate for the higher boost. After all, Taz can't have some silly Cadillac kicking sand in his face.

Editorial Comment:  A fresh dyno pull with the Torco blend, in conjunction with the smaller snout pulley that I'm now running and various driveline upgrades that I've installed since that 2004 dyno session, such as an aluminum flywheel and a Torsen differential, would likely put the car close to 500 RWHP. So, why haven't I put Taz back on the rollers? Well, basically because I'm not concerned enough about playing the numbers game to drop another C-note merely to post a more impressive chart. The car feels great, and that's what I care about most. Taz's power level certainly feels ample for my personal wants and needs, and my own satisfaction is all that matters to me.

 

Compressor Rebuild

After the Cobra spent more than two months on jack stands for extensive driveline and suspension upgrades from early December '06 well into February 07, its Kenne Bell compressor began to require fairly regular additions of oil, not often enough to present a reliability issue, but frequently enough to be a Class-A nuisance. After putting up with this aggravation for 18 months, I decided at the end of October 2008 to remove the head unit and send it out for a rebuild. Luckily, I had saved the detailed notes I'd made while removing the defective original KB compressor back in 2004, because those notes proved to be invaluable in facilitating the supercharger's removal and reinstallation.

I had read some genuinely glowing testimonials by folks who had used Embree Specialty Machine to rebuild their compressors. Considering the shop's excellent reputation, I elected to send Taz's compressor off to ESM for servicing. Dealing with Wade was a pleasure, as he maintained an excellent line of communication throughout the entire process. For example, after receiving my compressor, Wade phoned just to let me know that the unit had arrived undamaged. Then, after tearing into the compressor, Wade again phoned to let me know exactly what he had found: as I had suspected, the seals had simply dried out from disuse, and the next engine restart had torn up the dry seals.

Note to self:  periodically clock the compressor's snout pulley when the car is laid up for any length of time.

As a matter of policy, Wade returned the old seals and bearings to me with my rebuilt compressor, so it was easy to visually confirm his assessment. He even sent me a box that had contained one of the new seals, so I could verify that he did, indeed, use high-quality Viton seals. I'd like to also note that my compressor appeared to have been handled very carefully while it was away. Since my KB head unit is polished billet, I was a little apprehensive about what its cosmetic condition would be when I got it back, but was relieved and happy to see that Wade had obviously been careful not to mar the finish while the unit was at ESM.

After I received the rebuilt compressor and reinstalled the unit, it became immediately apparent that Wade had retimed the rotors to perfection. Not only did the rebuilt compressor sound better, but I also noticed a couple of undeniable improvements that were almost certainly attributable to better rotor timing. Not the least of which was an undeniable increase in the Cobra's torque production. People will tell you that seat-of-the-pants evaluations are worthless. And I agree that for small, incremental gains of a few HP, that's generally true. But I doubt anyone would dispute the fact that large gains are readily apparent without the benefit of instrumentation. While living with the Cobra as a supercharged vehicle for over four years, I had become intimately familiar with exactly what conditions were required to break the car's rear end loose. After the compressor rebuild, those requirements changed dramatically. The Cobra's demeanor was much more aggressive, and rear end breakaway was significantly easier to induce. In fact, despite being more interested in the slashing through the twisties than in straight-line acceleration, I'm now wearing the back tires out faster than the fronts. This says one thing to me quite clearly: more torque. Alot more torque, and I like it!

Editorial Comment:  As most regular visitors to this website know, I like to alert others when I have a particularly good or particularly bad experience with a company. I believe that if more folks did this, there would be far fewer bad companies around. From the preceding paragraphs, it should already be obvious that my experience with Embree Specialty Machine falls into the particularly good category. For those of you with superchargers in need of servicing, I strongly recommend that you consider having ESM perform the work. Although my compressor is a KB twin-screw, ESM provides services for most types and brands of superchargers. If you decide to trust your blower to ESM, and Wade does as good a job on your compressor as he did on mine, you'll be very happy that you did.

 

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Embree Specialty Machine Click this link to visit ESM's company Website.

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Intense Motorsports Click this link to visit the Intense Motorsports company Website.

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Kenne Bell's Website Click this link to visit the Kenne Bell products Website.

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Torco Click this link to visit Torco's company Website.