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There are roughly two types of turbos, the one type that regulates boost by means of a wastegate, and the other that regulates boost through a series of adjustable vanes. The latter is called a VNT (variable nozzle turbine) and this type is what I use. They have less lag (time-delay between opening the throttle and getting actual power, caused by revving-up of the turbo) and are used in most modern Turbodiesel cars.
There are two types of turbo systems. One is called draw- or suck-through, because the (mostly) single carb is on the inlet of the turbo and the mixture is being sucked through, compressed and blown into the motor through a manifold. This is what for example Mr. Turbo used on olskool bikes. For: Simplicity. Against: Uneven mixture spread for the four carbs, condensing of fuel in the turbo causing poor idling. The second is called blow-through, because the turbo ''blows'' air though the carbs and into the engine. This is what a Turbobusa (i.c.w. fuelinjection) and I too use. For: Good managability of fuelflow. Against : Complexity.
In the same Performance Bikes Magazines I had seen some awesome GSX's that had Mr.Turbo kits on. Even secondhand, these were not in my price category. So, I decided to build a turbo kit myself. I had ridden a couple of Yamaha XJ650 Turbos before, and decided that this "blowthrough carbs'' system was what I wanted for the GSX. I bought a partsbike XJ turbo and so I got most of the external parts, like fuel regulator, fuelpump, boost sensor, knock sensor, electronic ignition etc. The main issue is that I cannot weld myself, so building spaghetti headers and aluminum plenums is a no-no for me. So I came up with the standard 4-1 being linked through the swingarm (á la Honda CB600 Hornet) to the Audi-Garrett VNT turbo fitted between the side panels. The other advantage is that this way the turbo is above oil-sump level so it can be gravity-drained without the need to install a scavenging pump. The plenum is made out of glassfibre/ epoxy/ carbon, but may be replaced later with a stainless one when I get infuriated and want EVEN MORE POWER (it's addictive). I reinforced the engine with heavy-duty studs, upgraded oilpump, backplate clutch with extra steel plate and racing springs, adj. camwheels, a base-spacer to reduce compression, and ofcourse a copper turbo head gasket. In the pics you can see that I went back to stereo shocks because I needed the space to put the turbo in. I also put forks off of an FZR 1000 in RF900 triples, together with the bigger 320mm disks. Because of the shorter FZR forks I lowered the rear end too, what gives it an aggressive dragbike-look.
What were the problems I encountered?
1) The VNT's actuator works on vaccuum, and is controlled by a cars' computer normally. I watched it work on my Ford Mondeo TDCI that has about the same VNT unit, and at idle it pulled the arm in to speed the turbo up. Initially on my bike I connected the actuator to the point between the carbs and the head, expecting the vaccuum to pull in the VNT's arm and so closing the vanes thus speeding the turbo up. This didn't work, and so on my first (adventurous) testride boost came in very late. This problem was solved by fitting a "normal" pressure operated actuator and connect it to the plenum. Now, the vanes are normally closed until it reaches 7 psi and the vanes open to control the boost.
2) During the first testride, the bike badly misbehaved when getting from vaccuum to boost. All of a sudden the fuelling was all over the place, giving a wildly jumping A/F meter and also a wildly jumping bike(!). This was solved by replacing the old and brittle o-rings that were between the T-s that connect to the floatbowls. Apparently they were leaking and the oh-so neccesary dynamic boost compensation never reached the bowls, causing the engine not to be able to create a vaccuum to pull the fuel into the throttle bodies. After replacing the o-rings it was running as sweet as a babies' bottom.
3) Oil light flickering. When you just spent hundreds of euros on a nice, shiny turbo and ditto pistons, you are quite concerned when this happens I can tell you. The solution was simple; the turbo circulates a considerable amount of oil from the sump, lowering the level. When you fill it up to the level you normally do, it will cause the pump to run dry! Solution: Fill it up to the max mark.
4) Rough running at idle. Because the fuel pressure needs to be 3-4 psi higher than the boost pressure, the float-needle valves need to be closing perfectly. On mine they were old and worn. This will cause a too high level in the floatbowls and a too rich fuel mixture. This will cause splutter and backfiring. After replacing these buggers the problems are gone.
What's it like to ride?
After solving the problems, it behaves just as well as the standard carbureted bike. Clean, quiet run, good pickup, LOADS of power from 3,500 rpm onwards. This bike is more powerful and intimidating than ANYTHING I've even ridden. My extensive chassis mods have made it quite well balanced, but it's still very crude compared to todays' bikes.The VNT has the ability to set the point where it starts to boost. I could set the point at just above idle, just so that the vanes don't smother the engine and will let it rev out quickly. It's almost immedeately building boost and it gives you a kick in the pants at 3,500 rpm, running at 7 psi. From that point, it gently increases boost to about 12 psi at 9,500 rpm. This way, you get lots of power when you pull away and accelerate, but not so that you fear the bike will flip over backwards (much). As the revs rise, it's like it says: "So, you want more power? I give you more power!" And it will pull like a demented dentist on acid! Low down, just over tickover and before boost, it seems lazier than the standard 1168cc bike was. That one pulled like a steam locomotive when you opened the throttle. I think this is because of the lower compression and because the engine has to work hard to "suck" it's air through the turbo that isn't spinning fast enough the moment you open the throttle from idle. This will be solved by putting a priority breather valve on the back of the plenum, so it can draw it's air quickly and without resistance. When time will allow, I'll get it dynoed to see how much power it really makes.
The most attracting aspect of this bike I think is the brutal looks, image and the noise it makes. It's not loud, but sounds threatening in a way. The turbo seems to want to suck the whole world in, and whistles as the revs climb, promising even more acceleration and rising the hairs at the back of your neck. The blow-off valve opens when you shut the throttle, like at a gearshift, and provides a fluttering whistle that has dogs running for their mothers and makes you scream in your helmet.
Ooohyes, this is what I wanted!
-Suzuki GS750D frame, tank and panels, Giuliari two-four seat, Wes Cooley replica paint
-Yamaha FZR600 wheels and swingarm, headlights, indicators, footrests, and master-cylinders
-Yamaha FZR1000 forks, calipers and discs, Suzuki RF900R triples, KONI multi-adjustable shocks
-Suzuki GSX1100S Katana engine, carbs, wiring, electrics and clocks. Welded & trued crank. Carbs modified to take boost. GS750 pumpgear.
-Wiseco forged 1166cc pistons, block bored to match, compression ratio lowered to 1:7.9
-Garrett VNT 20 turbo off of an Audi A6 V6-2.5 TDI
-Falicon clutch backplate and HD springs, camwheels and base-spacer
-APE HD studs, Cometic basegaskets and copper head gasket
-Marshall 4-1 header and modified V&H Kawasaki ZX7R supersport silencer & linkpipe
-Dyna coils with Taylor leads and NGK plugs & caps
-Yamaha XJ650 turbo system layout, fuel pump, fuel regulator, electronic ignition, priority breather valve, boost sensor, knock sensor and part of the wiring loom, turbo oil screen-filter
-Zerotec airfilter, blow-off valve and boost gauge, VDO fuel pressure gauge
-Suzuki Bandit switchgear modified to fit kat wiring
-Arias gold 530 pitch chain, 16t Suzuki GSX-R front sprocket, 45t FZR600 rear
-Bridgestone tyres, sizes 120/70-17 front, 160/60-18 rear
-Castrol GPS 10W40 oil
-Weight with oil and fuel, ready to rock-and-roll: 230 kgs.