Warpspeed

Just expanding a bit on WetGTR's comments. The ignition system needs a bit of thought and planning, because the Autronic ECU offers a number of alternatives on how it can be used. The SMC is the basic ECU, the SM2 and SM4 offer additional features, but I will describe here the ignition of the SMC. First, the SMC has eight individual ignition outputs, (none of which provide any dwell). They are simply on/off outputs, where the on/off transition is the ignition firing point. The software allows many different ways to set all this up, but basically for a six cylinder engine there are three alternatives. 1/ Use one ignition output to drive a single ignition coil via an external Bosch igniter module that provides the necessary dwell angle. This one ignition output could also trigger an external CDI unit. Obviously a distributor cap and rotor button will be required to fire six plugs. 2/ Use three ignition outputs to drive three external Bosch igniters, and use three double ended waste spark ignition coils. The Autronic CDI can also be used with this system as it can drive up to four coils. 3/ Use six ignition outputs to drive the six standard Nissan coil on plug igniters. That would be my preferred option with a Skyline. Use of the waste spark system is perfectly o/k if you ARE RUNNING THE ORIGINAL CAMS. All modern emissions cams have zero (or almost zero) valve overlap. The ignition system fires two plugs at once, and the inlet valve of the other cylinder MUST be shut when you simultaneously fire both plugs. Now suppose your ignition fires 15 degrees BTDC in cylinder 1. Cylinder 6 also fires, but the inlet valve will be shut, and that cylinder will be at the end of the exhaust stroke with only exhaust gas in cylinder six. But suppose you have a set of 262 cams where the inlet valve opens at 22 degrees BTDC ? Cylinder six will have its inlet valve open, and the combustion chamber will be open to the intake runner and full boost pressure. The injector will also be spraying. What do you suppose might happen if the plug fires !!!!!! Waste spark is great for stock emission engines with standard cams. Even boosted engines. The Commodore supercharged 3.8L V6 uses waste spark. But if you fitted some decent cams to it, expect trouble. The other issue is that CDI ignitions give vastly higher output energy, but the spark duration is very short. Straight coil/igniter ignitions can also give fairly high energy outputs, but the spark burns for much longer, rather like a welder. For competition it probably doesn't matter much. But for a road car, if you plan on running fairly weak mixtures (for economy) a longer plug burn time is likely to give less mis firing at very light throttle. CDI units can sometimes be more susceptible to cross firing unless great care is taken with plug lead layout. But if done properly, it should work fine. My choice would be coils and igniters for a road car, either one, three, or six. It all depends on the application. The Autronic CDI is around $1000 The Bosch igniters (with dwell) are $175 each. So six would cost about the same as a CDI. Lots of alternative ways to do the ignition with an Autronic ECU.

2BNVS, I bought some Autronic parts a couple of months ago. Basic SMC ECU with loom $1758 Top of the range SM4 with loom $2200 The Autronic air fuel meter is absolutely the best unit available, comes with various options, and two different types of sensor, but prices range from $1900 to $2000 For a Skyline probably all you need extra would be a different optical disc for the CAS. Not sure but I think that is about $50. On a Skyline you would probably use the factory igniter modules, so ignition is taken care of. Best contact your nearest Autronic dealer for firm prices. I bought this one from Ray Hall in Queensland. HE is the biggest dealer, and runs the Autronic Forum. Ray was extremely helpful, and I recommend him highly.

Just did a quick computer simulation on Dyno 2000. RB30, 11:1 compression, extractors, std cams = 200 Hp at 5,500 Rpm Estimated 126 RWKw, an absolute torque monster 210 Ft/lb RB30, 11:1 compression, extractors, 262 cams = 266 Hp at 6,500 Rpm Estimated 168 RWKw, 231 Ft/lb at 5,000 Rpm, but it looses a lot of torque below 4,000 Rpm. That would be my guess if it is properly tuned.

Have to agree, the Autronic is extremely easy to get going and is very user friendly. What really sets the Autronic apart is the superior software. It may not be the cheapest ECU, but it will save you a lot of dyno time, and in the end it probably costs no more by the time everything has been fully sorted out.

Your best bet with an ECU is to not stuff about, go straight to a power FC. The results you get will be highly dependant on proper engine tuning, and a stock ECU (whatever type) is going to be a definite handicap.

If you use the standard flat top pistons in the RB30 you end up with around 8.5:1 compression. But you can fit the RB25DE pistons to your RB30 bottom end. these pistons are domed and will give you a compression in the range you want. I am not sure of the exact figure, but it will definitely be greater than 10:1

I have an SMC fitted to a Ford Laser turbo. It went in over Christmas and had no problems with it at all. The engine started and ran first attempt, and the self tune feature using the Autronic air fuel meter works extremely well. The best thing you can do is join the Autronic Forum as mentioned above.

I am familiar with those actuators. They use two diaphragms, a small one driven by boost pressure, and a larger one in front operated from manifold vacuum. Each diaphragm has its own spring, and there is a push rod inside to decouple the rear diaphragm from the front one. The system holds the vanes in the fully open position at small throttle openings (high vacuum) and closes the vanes when you floor it. The vanes then open again when the desired boost pressure is reached. I had a homemade system exactly like that, but it still left a lot to be desired. A much better idea is to use a stepper motor, and fully map the vane position against Rpm and load. The biggest problem I had was that moving the vanes into the fully open position would certainly limit turbine Rpm and control boost, but it did nothing to reduce increasing turbine back pressure with flow. Fitting a larger turbine wheel would then unacceptably reduce the boost threshold and response. I played around with this VATN for four years, and it was NEVER anywhere near as good as a decent modern ball bearing turbo. It was better in some respects, and a lot worse in others, but on balance the whole thing was a mess. When HKS bring out a VATN with a suitable control system for petrol applications, that is superior to a straight high performance ball bearing turbo, that would really be something. But it just won't happen. There is far more scope for twincharging petrol engines. Use a supercharger and turbo together. The latest 2006 VW 1.3 litre Golf GTI just released in Europe sets new standards for power, engine flexibility and fuel efficiency. That is the way I see things progressing if you want something better than a ball bearing turbo. I had a twincharge system fitted to my car before the VATN. My satisfaction score out of a hundred: Twincharge system 100 points VATN 30 points Ball bearing turbo 75 points My next car will definitely be either supercharged with a screw supercharger or twincharged. Either should easily beat a straight turbo for on the road performance.

Discopotato sure knows his stuff ! Yes indeed, diesel engines are fundamentally quite different. Diesel engines use no throttle body, and the flow through the compressor and turbine is always fairly high. Diesels also produce massive low end torque that always rapidly falls off as Rpm rises, and they never develop high top end horsepower (for their engine size). As a result diesel engines don't have the high top end air flow requirements that petrol engines have. Have you ever noticed that large diesel engines never use wastegates ? The TO4 range of turbos were all originally designed for large diesels, which is why none of them have internal wastegates. VATN turbos work really well over limited airflow ranges, and as Disco says, much lower exhaust gas temperatures. Turbo applications for diesel engines are totally different to turbo applications for petrol engines. Garrett have developed the VATN to the point now where any new high output diesel engine is almost certain to use a VATN turbo, they are that good. But the problems of applying these same turbos to petrol engines are fairly formidable and not to be underestimated. Porsche have made it work, and good luck to them. But don't think you can just buy a Porsche VATN turbo and bolt it onto your Skyline and get similar results, not without the sophisticated fully mapped software control system to control the movable vanes anyhow. I am not trying to put Porsche down, just let you guys know that this is not some magic new turbo that is about make everything else obsolete. Not for us petrol heads anyway. But if you are a trucker, they are absolutely great.

I am sure it works great. But that probably has much more to do with engine management, and control and integration of the movable vanes and wastegate (if used), rather than the turbo itself. Don't expect to get similar performance from just a bare VATN turbo, without the fancy software control system to go with it. Garrett are pushing their GT series ball bearing turbos for high performance OEM petrol applications, and their VATN for high performance OEM diesel applications. There are reasons for that...........

VATN turbos have been around for over twenty years. They work wonderfully well on diesel engines, but are not terribly good on petrol engines. I know because I have run one on my DOHC Laser turbo engine for about four years. A VATN will give you very low lag, a very low boost threshold, and very low top end power. The problem is that ALL the exhaust has to go through the turbine, there is no wastegate. That means very high exhaust back pressure which limits top end power. The only way around this is to use a wastegate as well as adjust the exhaust vanes but it is very complicated to control. I tried for four years and eventually gave up. It is a bit like sequential turbos, a brilliant idea that does not work terribly well in practice. VATN turbos have been available for twenty years, where are the really fast drag cars and fast street cars using these turbos ? I bought a brand new VATN from Nissan as a spare for a Nissan Patrol diesel. It was rated at 150Kw on a four liter diesel, so figured I should be able to get 150Kw from my 1.6 litre petrol Laser. I could never get it working to my satisfaction. If these things are so good, why did Nissan not use a pair of them on the GTR ? Nissan used them on their diesels around that time, so why not on the GTR ? Porsche do a lot of funny things, and they have obviously made it work somehow. But I very much doubt if the technology is readily transferable to other engines.

Well yes, I suppose you could look at it that way. But actually the wastegate flows from the supercharger outlet, back to the inlet. After all, it is a bypass system. There are basically four requirements. 1/ The supercharger boost pressure absolutely MUST be in a direction to force the poppet valve off its seat if there is a boost pressure spike, so the wastegate MUST flow in the same direction as it originally did when used as a turbo wastegate. Mounting it reverse way around so it flows in the wrong direction it is not going to work smoothly. 2/ The diameter of the control diaphragm absolutely must be at least twice the rated flow diameter of the poppet valve. An even bigger difference in diameter would be much better. Anything less than twice is not going to work unless the desired final boost pressure is unusually low. 3/ There will be two hoses coming from the control diaphragm on the wastegate, one from either side. These go to either side of the throttle body so the diaphragm sees the pressure drop across the throttle body. Idle vacuum should fully open the wastegate against the internal spring force. That tells you which way around the hoses connect. 4/ Spring pressure should be such that about 2.5 psi just lifts the wastegate valve off it's seat. 5psi should fully open the wastegate against spring pressure. These are only rough guide pressures to initially selecting a spring. Those pressures will definitely work, but testing may decide you to change things slightly. But those spring figures will definitely get you a working system.

Dave, I am not sure exactly how the system in the Mini functions. But the Supercharged 3.8 Commodore also uses an Eaton supercharger with a rather similar looking bypass built right into the blower casing. Now the Commodore uses a diaphragm actuator that looks just like the one in that picture. The whole thing is operated by the electronics in the engine management unit. For instance in the Commodore, the ECU restricts boost until the engine is up to full operating temperature, and it NEVER allows any boost in reverse gear. But otherwise, the ECU fully controls the bypass system under all modes of driving. Unfortunately that system needs the "brains" of the ECU to operate the supercharger bypass actuator. You cannot just connect a hose from the actuator to somewhere on the engine and expect it to operate properly. I have tried a bypass throttle body and actuator myself, and my experience with it was that it would just flop open, and flop shut. There was no proportionality at all, it was either open or shut with nothing inbetween. A microprocessor based system using PWM to progressively bleed air pressure into and out of the actuator with some sort of feedback may be possible, but it is hardly a do it yourself project. It would probably work something like one of those very expensive electronic boost controllers, but the software would need to be very different. I am an electronic engineer, and although possible, it is just all too complicated. I have also experimented with various home made bypass valves, but the best system is still a commercial external wastegate modified with a lighter spring. The main requirement is for the control diaphragm to be at least twice the diameter of the poppet valve. Yes ynhrgt, a 60mm wastegate would work fine, but is the control diaphragm LARGER than 120mm in diameter ? Probably not. I have never seen any wastegate with a control diaphragm larger than 80mm diameter. They may exist but I have never seen one, 120mm would be enormous.

I very strongly suggest you get it all working and sorted out before spending big money on cosmetics. Once something is ceramic coated, it is not possible to cut and weld it, or modify it in any way. Any external wastegate will work provided the control diaphragm is AT LEAST twice the diameter of the wastegate poppet valve. Garret, Teal, or Turbonetics wastegates are very good. The Chinese ones you need to be a bit careful. Nothing you can buy will work straight off out of the box, you need to find a suitable much lighter wastegate spring. That will take a bit of experimentation to get it exactly how you want it.

Well, I have never tried to fit an RB26 cam to an RB20, but I have a pretty fair idea of why you guys are having so many problems, so here goes: First thing is that there will be a production variation between the sizes of the cam bearings. Not much, but then the bearing clearance is not much either, maybe a couple of thou. You cannot just swap cams without checking the bearing clearances. Well you can, but it is NOT a very good idea. You will never notice a sloppy bearing, but a tight one is going to be a real drama. So buy some plastigauge from Repco and measure the clearance of each cam journal with each cap properly torqued down. The extra lift is so small it is hardly likely to bind the coils, but check it anyway. Turn the cam so each lobe is at maximum lift, and see how much further you can push down the follower. You should be able to get a 2mm gap at least before the valve spring goes solid. If not, you risk serious engine damage. The cam drive gears ARE different between Skylines, so you cannot just bolt any old gears onto your cam and expect to get the exact factory timing. If in doubt get some adjustable cam gears, and take the time to set them up to Sydneykids figures. The next problem is what has really defeated you guys and why your cars will not idle. A hydraulic cam has no tappet clearance ramps. The base circle of the cam is flat, then the valve opens instantly at the specified opening point. Likewise it closes pretty suddenly back down to the base circle. Because the hydraulic lifters take up all the clearance, there will be no tappet noises. A solid lifter cam is VERY different. From the flat base circle, there is a very shallow clearance ramp that begins very early and SLOWLY closes up the tappet clearance. Once the clearance has been taken up, the valve really starts to lift quickly. Likewise the valve drops fairly fast with the tappet clearance still all taken up, and there is an exit ramp that slowly opens up the tappet clearance. These starting and ending ramps are very long, but only amount to perhaps half a millimeter of cam lobe lift. If you fit a solid lifter cam like the GTR cam to a hydraulic lifter, like in an RB20, the valve timing at the beginning and ending is changed radically. The hydraulic lifter goes solid, and the valve starts lifting off it's seat very early, much earlier than it should. The initial valve lift is extremely low, but the valve will still leak. Likewise during valve closing the valve is held just off its seat for an extended time. What this means is that during engine idle, both valves will be open together for quite an extended time, but only just open. At 7,000 Rpm there is not enough time for any serious leakage. But at idle Rpm the valves will leak badly, because of the very large valve overlap this leak creates. It will sound fairly lumpy at idle, assuming you can actually get it to idle. A lumpy idle may sound really cool to some people. Now the problem is, that at idle there is a fairly high vacuum in the inlet plenum. If both the inlet and exhaust valve are open together, exhaust will be sucked a fair way back up each intake runner. This exhaust will be drawn back into the engine causing the lumpy idle. The air fuel ratios will need some fairly drastic changes too from what they originally were. So don't expect it to work too well with a standard ECU. Even with a Power FC it will probably be quite difficult, if not impossible to get a smooth idle. I have never done this modification to an RB20, so I don't know how difficult it would actually be. Your best bet might be to either go back to a proper hydraulic aftermarket cam, or fit the GTR solid lifters. I don't even know if that is possible on an RB20 head. It is quite possible that a few guys have done this modification, and really like the rough idle, and say it makes the car sound mean. It probably also goes better at higher Rpm. Myself I prefer to have things working properly, and am not surprised some of you cannot get the engine to idle at all. I wish you guys luck, but really what you are doing is not a terribly good idea unless the whole thing is approached properly.

It probably does not matter much if the bypassed air goes back into the supercharger inlet, or the turbo inlet, as long as the air can circulate freely through the supercharger with a closed, or almost closed throttle. Back to the turbo inlet has the advantage that it will eliminate any possibility of compressor surge, so that might be better. Just make sure that all that volume of recirculating air cannot cause any turbulence back at the airflow meter at small throttle openings, or the ECU may not be terribly happy. After two years of testing and experimenting with many alternative bypass methods, I can only suggest that a modified wastegate works extremely well, and a modified turbo blowoff valve will give absolutely horrible drivability. But do it any way you want, it is your car. We can only advise you of the things we have found not to work, and perhaps save you from a few problems. The same goes for engineering a good reliable belt drive for the supercharger. You can listen to the experiences of people that have already succeeded after having had some real dramas, or you can learn it all yourself from scratch the hard way.

I agree totally with disco, thermal management and low under bonnet temperatures are all part of having an efficient and trouble free package. Thermal wrap is one way, probably best for guys with tube manifolds. If you are still running the cast iron factory manifold, the best way I have found is to construct a fairly close fitting mild steel sheet metal box right around the entire manifold. It must be as air tight as you can make it. There should be a jacket of completely still dead air in there, which will insulate surprisingly well. If the sheetmetal cannot curl right up behind the manifold, it should seal tight against the engine block surface. The surface temperature of the steel will probably be in the region of 90C to 120C, a lot cooler than a 1,000C+ manifold. HPC coating is supposed to be excellent, but I have never tried it myself.

For some strange reason, I have not been receiving e-mail notification of additions to this thread ?? Anyhow, I will attempt to explain why the supercharger can never be restrictive. Any supercharger that is set up to make some positive boost, only does so because it pumps a larger volume of air than the engine would normally flow. The engine acts like a restriction to the supercharger, and the pressure builds up between the supercharger outlet, and the engine induction. This positive "boost" pressure will rise, until the flow rate through the supercharger and engine are exactly the same. Because the supercharger and engine are directly coupled, they will rise and fall in speed together, and the boost pressure may vary slightly with Rpm, but the supercharger is never going to be a restriction to the engine while it is producing some positive boost pressure. So there is never the worry that a supercharger is going to become a restriction and limit engine power. It is there to do exactly the opposite. By placing a turbo compressor ahead of the supercharger, nothing has really changed, except the supercharger now sees some already compressed air at it's inlet. It can still never be a restriction, the supercharger will always further increase the boost pressure coming out of the turbo to something higher. Yes, the supercharger will absorb some power, and so will the turbo. But they give back far more power than they consume. In fact the supercharger will be pumping denser air, and the drive power is even higher than it would be if the intake was just open to atmospheric pressure. Believe me, none of that really matters. Your piping diagram would work fine the way it is drawn, provided no airflow meters are being used ahead of where the wastegate is. The airflow meters need to measure exact engine airflow, and that cannot happen if you are venting some air that will never reach the engine. Although just venting air from the wastegate will work, it would be very noisy whenever the wastegate is open, and that will be almost all of the time except at full throttle. Much better to feed the air back either directly into the turbo intake, or directly into the supercharger inlet, (after the turbo). It will vastly reduce the noise problem. It would also allow you to use an airfolw meter in the usual location before the turbo. That would work because all of the air going through the airflow meter also goes through the engine, no matter if the wastegate is bypassing air or not. I fully agree with stocky that a single large turbo has many advantages in a twincharge application. Cost and packaging being the main ones. I have already done all of this myself, and had a twincharged system running on my car for over eighteen months. Stocky is in the final stages of getting his twincharge system on the road. Stocky having previously had a supercharged engine fitted with this bypass system, we both know and understand the requirements pretty well. I have experimented with all types of bypass systems and the wastegate idea works wonderfully well. If you fit a turbo blowoff valve the car will be a jerky pig to drive. Try it and see. The bypass has to be proportional, smooth, and progressive in action. A blowoff valve is sudden and violent. Anyway once the whole system is fitted to the car, minor pipework changes and bypass systems changes are fairly easy to experiment with. The big drama is mounting and driving the supercharger, and locating the turbo and two wastegates around everything else in the engine compartment.

Excellent description Neil, Ynhrgt, I think you misunderstood me. All this is a fairly simple idea, but horribly difficult to explain, and put into few words. The wastegate is shut tight only during near full throttle, go back and read exactly what I said. It must be shut (obviously) or there could not be any boost pressure at all. Yes, the bypass will remain open pretty much all of the time while driving, because you will be driving around at light throttle most of the time, at least on the street you will be if you want to keep your licence. The bypass only closes full above perhaps 3/4 throttle, or whenever you see any boost, and that will only be for a few seconds in each gear during acceleration. Don't go too big on the bypass. Try to imagine how much boost pressure you would lose if you drilled a 32mm hole in your supercharger pipework ! Believe me, even a 25mm hole would be a MASSIVE boost leak. A very effective bypass need be no larger. The bypass only needs to dump most of the pressure, which even a small wastegate will do. Look how big the holes are in internal wastegates, some are surprisingly small. Another very important aspect of this, is that the control diaphragm and spring must be proportioned to open and close the wastegate at the desired throttle opening. Somewhere about five to ten inches of mercury vacuum upstream of the throttle is where you want the wastegate to open/close. That is around 2.5psi to 5psi pressure differential across the throttle. So the spring will need to be fairly soft to work in that range. Now the spring still needs to be stiff enough to hold the poppet valve shut against full boost pressure without leaking, and the spring HAS to be suitably soft to get the correct control operating pressure range. As a result the poppet valve must be made a lot smaller than the diaphragm. If the diaphragm is twice the diameter of the poppet valve, the area ratios will be 4:1 . That means the maximum boost pressure that would force the wastegate open uncontrollably would be four times the control pressure. Perhaps 10psi to 20psi boost if the control pressure is 2.5psi to 5psi. So make absolutely sure that the wastegate diaphragm is at least twice the diameter of the poppet valve, the bigger the difference the better. Good quality wastegates will be like that, Garret, Turbonetics, and some of the better Japanese gates. Avoid the cheap and nasty Chinese gates. Some of those have like a 60mm poppet valve, and a 60mm diaphragm, with a 42mm outlet. That is exactly what not to use. Something like a 32mm gate with a 65mm diaphragm, or a 42mm gate with an 80mm diaphragm would be pretty good. Even better would be a homemade hybrid wastegate, 80mm diaphragm with a 32mm poppet valve.

Thanks Stocky, if you can dig up your explanation it will save me a lot of typing and answer ynhrgt's supercharger bypass question. Disco, I take issue with you on a few points. You say that boost pressure is higher than turbine inlet pressure on a four valve street engine. That might be true if a maximum sized turbine is used, so full boost pressure is not reached until almost at redline Rpm. If you want a reasonably low boost threshold and a flat boost curve the exhaust turbine will need to be made smaller than that. On my own turbo engine, full boost is reached at about half of maximum RPM, and turbine inlet pressure is exactly TWICE boost pressure. Oh, and it is a four valve DOHC engine too. That is fairly typical of a reasonably sized modern ball bearing turbo. The other point is that turbos only actually reach their maximum efficiency over a very limited range of flows and pressure. Any compressor flow map will quickly tell you that it will be operating well away from the peak island on the flow map for much of the time. A supercharger has a MUCH wider operating range, and although the peak efficiency may be less, the average efficiency can be a lot higher. Screw supercargoes operate at 70+ percent over perhaps a 10:1 flow range. Turbo may operate at 74 percent over a 1.5 : 1 flow range if you are lucky and well below that everywhere else. I will not mention compressor surge, or lag. There is not the slightest doubt which is going to be more efficient. If you have a reasonable intercooler, the actual discharge temperature of the supercharger or turbo becomes much less important. What causes detonation is excessive charge temperature at the end of the compression stroke causing unstable combustion. The temperature of the charge at the beginning of the compression stroke is vitally important, (as is the compression ratio) in determining the final temperature reached. Low induction temperatures coming out of a good intercooler are not going to be worth a damn, if the combustion chamber is filled with very high pressure exhaust residuals at up to 1,000 degrees celsius. A turbo engine will have a lot of trapped very hot, high pressure exhaust gas, that simply cannot get out past the exhaust turbine. That is going to mix with the cooler incoming induction air, raising it's temperature VERY significantly. If your exhaust manifold is glowing red hot, the exhaust gas going through that manifold is red hot too. Your low compression turbo engine, with the dished pistons is going to hold a lot of trapped exhaust gas that cannot escape. THAT is why turbo engines detonate so readily. Now with a supercharged engine, there is essentially minimum exhaust back pressure after the engine. If the engine has tuned pipes, the exhaust port pressure may even be negative during valve overlap. All that hot gas gets sucked right out. And if not, a higher boost pressure will drive it out during valve overlap. A turbo engine that has a higher exhaust manifold pressure than boost pressure cannot use valve overlap in the same way to scavenge the engine. Only a turbo race engine can. Your turbo race engine with the big turbine and high overlap cam can make massive top end power, but it will be a very peaky laggy engine. A supercharged engine can run milder cams, and have a very broad flat torque curve. I know which I would prefer in a road car. It is a fact that given an identical engine, a turbo version will be far more prone to detonation than a supercharged version. There is also the myth that turbo engines are more powerful. If that is true, why are all the fastest drag race cars supercharged ? Where are all the four second turbo cars ? Don't get me wrong, I like turbos, my own road car has a turbo, but they are not as perfect as some people like to believe.

The first thing is that if you add a supercharger, the engine is going to flow a lot more air. The original twin turbos will be far too small on both the compressor side and the exhaust side. They could be replaced with larger twin turbos, but a big single turbo would probably be a much better alternative. The purpose of the bypass system around the supercharger is to open up a flow path direct between supercharger intake, and supercharger outlet. That does two things. First it removes all supercharger contributed boost when open (obviously) by allowing the supercharger to recirculate air around through the open bypass valve back to the supercharger inlet. When you go to change up a gear at redline Rpm, you quickly close the throttle. Without a bypass of some sort, the supercharger would continue to pump air up against the completely closed throttle, causing a MASSIVE boost spike. It would blow a hose, or burst your intercooler core for sure. So you want the bypass to open fully whenever you suddenly back off on the throttle. But it must also close fully when you want some acceleration, to allow the supercharger to work normally. When driving around at fairly light throttle, (or at constant highway speeds), the bypass can be held open. This has the effect of removing all back pressure from the supercharger, and it removes almost all the drive torque required to turn the supercharger rotors. It is like having the rotors spin around in free air without any outer casing. So there is essentially no power loss, no air heating, no noise, and it will improve fuel economy significantly. This bypass must operate SMOOTHLY and PROGRESSIVELY. The usual (noisy) turbo blowoff valve is very sudden and violent, and totally unsuitable for this application. A much better way is to adapt a turbo external wastegate to perform the bypass function. All it needs is a suitably light spring fitted, and it will work fine. In order to make any boost pressure at all, the supercharger must pump more air volume than the engine would normally flow by itself. It can never therefore be a flow restriction to the turbo. If there is a higher boost pressure after the supercharger than before, it is certainly not restricting flow. So don't ever think that the supercharger will hold back the turbo at the top end by being a restriction. What DOES restrict top end horsepower is the exhaust turbine back pressure. Now nobody ever talks about how total exhaust back pressure costs horsepower. You never hear turbo people say my turbo engine makes 400Kw but if it had zero total exhaust back pressure it would be making 500Kw. Unfortunately that 100 lost Kw is the power needed to drive the exhaust turbine to make the boost in the first place. People just look at the cherry red exhaust pipes and the glowing full moon exhaust housing on the dyno and marvel at how powerful the engine is. But the same people point at a supercharger and look at the big pulleys and say how awful, that supercharger is sucking 100 Kw out of my engine. My 500 Kw engine only makes 400 Kw because of that damned INEFFICIENT supercharger. The truth is, that 500 Kw engine needs 100Kw to compress the air. You can do it with a turbo, or you can do it with a supercharger, but that 100 Kw still has to come from somewhere. A further truth is that a supercharged engine runs with high boost and very low exhaust back pressure, the heat can get out. You never see red hot exhaust pipes even on an 8,000 Hp top fuel dragster. They run cool. Supercharged engines are relatively detonation free because the heat can get out. A turbo engine is the exact opposite. The heat is trapped by the high total exhaust back pressure, and as a result turbo engines are horribly susceptible to detonation problems. Everyone knows that. With twincharging it is the ideal compromise. Boost pressure can be kept just above exhaust back pressure, even with a full street exhaust system, and detonation is much less of a problem. It will also be much more responsive than a turbo engine down low. And it will have better top end airflow than a straight supercharged engine would have. The Opcon Autorotor screw supercharger is the best and most efficient supercharger available. A Whipple screw supercharger (made by exactly the same company) are a cheaper mass produced version of the same thing. A new Eaton roots supercharger costs about the same as a Whipple but is nowhere near as efficient. A secondhand Eaton off e-bay is a budget way to get started. Toyota superchargers are just complete crap, and are hardly worth the very low cost they can be bought for.

Not a good idea. If you just de-clutch the blower, air pressure will spin the rotors o/k but the pressure drop is fairly high. The engine will lose a LOT of power that way and it will be really gutless. Toyota use a blower bypass as well as the clutch. With the bypass open it does not really matter if the rotors are spinning or not. A clutch is a nuisance, and it wears out. Better not to have a clutch at all. A clutch also rather limits you when selecting pulley sizes.

Hi ynhrgt, have only just found this thread. I e-mailed you yesterday, and pm'd you today. It may be best if we just talk here, so others can join in. Stocky (Neill) is the guy in Queensland I mentioned building a twincharge system right now. As he is already here on this thread, no need to go over to the Commodore Forum to reach him. A twincharge system is really just a supercharged engine with a turbo fitted to it as well. The turbo compressor feeds straight into the supercharger intake, there is no need to turn off the supercharger. A bypass system around the supercharger is essential if you plan to leave the throttle body in the original location. As you have an RB26, leave the six throttle bodies right where they are. That is definitely the best way to go about it. It is fairly obvious that you cannot just fit a positive displacement supercharger up stream of the throttle body. When you close the throttle at high engine Rpm (when changing gear), either something will burst, or the drive belt must slip or break. A bypass arranged around the supercharger will eliminate that problem. A bypass will also effectively unload the supercharger at small throttle openings. Although the rotors still spin, the supercharger effectively draws no drive power from the crank. It will improve fuel economy by about 10% and reduce noise and heat buildup in the supercharger. Floor it, the bypass slams shut, and away you go. Back off, and the bypass opens. It works wonderfully well.

Yes, I know exactly what the exhaust flange is. Let's just wait and see if another single turbo buyer turns up, I am in absolutely no hurry.

I fully realise you really don't want to split this up, but I may be interested in one of the turbos if someone else is interested in the other ???