The 'Stealth' Z - Part 3

by Steve 'Stealth' Webb

(Click on each image for a larger view)
Well, itís been a while since my last article, and a lot has happened in the interim. The good news is I still havenít blown up anything yet, and it (and Julie) survived the trip to Denver and back with no problems. The bad news is that it is still not painted yet... At 03:00 AM the night before leaving for the convention I did put the sealer coat on, so now itís a rusty red color. In the pictures from the convention car show, it even had some shine to it - go figure?! I did find out at the convention that BFG R1ís last about 2k mi. of street driving and around 5 full days of racing. I found this out as cord started to show through the center of my rear tires during the morning practice in Denver at Stapleton Airport! After a frantic search, I arrived hours later with some Goodyear "Gaterback" street tires TWO sizes smaller than the 275/40/17 R1ís they replaced. They werenít very competitive, but it made for some great sideways action! (and I needed something to drive back to Dallas on).

Ok, enough rambling on. To cover the latest events first; at Alamo Autosports we just finished setting up our state-of-the-art Dynojet Chassis Dyno. The Dynojet is a revolutionary new "inertial wheel" dyno that can give a complete Horsepower and Torque graph from one 7 second run. Itís all computerized and will correct for altitude, temp, barometric pressure and humidity. You can even overlay up to 3 graphs on top of each other to compare runs. If you canít tell, I am slightly excited about our new ëtoyí. The accuracy is amazing, and you wouldnít believe the amount of information one can learn with the ability of producing back to back runs after making changes to the vehicle (yes, the ëStealthí is going to see some ëextendedí time on it!).

surge tank
Custom welded aluminum "surge tank" has inlet and overflow on top, and main fuel feed at bottom of "V".
port-matched turbine inlet
Notice the port matched turbine inlet (arrow on L) compared to a stock unit (on R). It may be hard to see, but the porting extends approx 3½" down the throat to the integral wastegate.
ported outlet/wastegate
The outlet and wastegate (arrows on L) have been ported and enlarged to match the exhaust downpipe (compared to stock unit on R).
'20G' Compressor
Upgraded '20G' compressor (on R) has a larger wheel with a more aggressive blade design
In the last issue, I think the two gremlins I mentioned were fuel starvation and boost creep. The fuel starvation problem will be fixed with the addition of a ësurge tankí. A surge tank is a separate container of about 1 gallon capacity that is mounted just in front of the gas tank and just behind the mustache bar. It is constantly fed by a ëscavenge pumpí from the main fuel tank. The supply for the main pump comes from a large fitting at the bottom of the surge tank. Due to the small volume, and the design of the tank, there is always fuel available to the main pump. This is nothing new, almost all road race cars have a similar setup - that is what enables them to run under high ëg-forceí conditions and utilize the maximum fuel in the tank without starving. Now keep in mind, this fix is for a high demand application and the remainder of the fuel system has already been upgraded with a high-pressure pump and 1/2" fuel lines from the pump to the engine.

The boost creep problem has almost been solved by some extensive porting of the turbine housing around the inlet and wastegate (see picís). The wastegate can almost hold boost now in first and second gear. It still tends to creep in third and fourth, but itís not as dramatic and much more controllable (by throttle position) than before. An added bonus is the extra power found after the porting. Subjectively, the car feels as quick and eager at 6psi now as it did at 10+ psi before. Itís a very dramatic difference! I think the next step will be to disassemble the turbo again, perform additional porting around the wastegate opening and machine a new larger wastegate valve. In addition, a full 3" mandrel bent exhaust will be fabricated to fit in place of the 2.5" system now. I believe the further reduction in backpressure will do a lot towards totally stabilizing the boost.

The next upgrade (to be done in conjunction with the turbine work) will be the addition of a sport compressor housing (designated a "20G", stock is a "17C"). Dyno runs will be made before and after all mods if possible. While everything is apart, the exhaust manifold will receive some careful porting and polishing. Iíll probably wait to match the intake runners until I have a reason to pull the head and do it right. Everything possible will get ceramic coated though. This includes turbo, intake and exhaust manifolds, heat shields, and intercooler to throttle body inlet piping. Heat is energy, and itís proven the more heat (and thus energy) you can deliver to the turbo, the quicker the spool-up will be and the more power it will make.

With the above mods, I hope to gain around 40 to 50 hp at the flywheel, at the same boost pressure. The dyno will tell the real story, so weíll see how close I am. As I have said before, a key to the horsepower gains I expect to see, is the ability to custom tailor and modify the fuel and spark events with the Accel DFI fuel management computer. Being off the fuel curve just a ëlittleí can mean a LOT in horsepower - if not roasting an engine.

Expect lots of dyno graphs next month!

Go To The Stealth Z Part 4