Warpspeed

Nothing tricky about the management at all, any tunable aftermarket management will work fine. All the Miller cycle does is reduce the effective stroke length of the compression cycle by closing the inlet valve later. This late inlet closing also gives better high RPM breathing as well. Suppose you do a computer simulation of two engines say an RB25 with 10:1 compression ratio, and 8psi boost fitted with a Miller inlet cam, and an RB25 with 8.5:1 compression ratio, 8psi boost, and stock inlet cam. The Miller engine has a bit less less low down torque at say 2,000RPM, but will still be far gruntier than a stock normally aspirated RB25DE down low. Midrange and top end power are absolutely identical in both engines. The Miller engine will have slightly better part throttle cruise fuel economy, so really there is not a lot of difference in the final result either way. Fitting a different inlet cam is all you need to do, and you can always take it out again just as easily. Fitting low comp pistons means you have to remove and completely strip and rebuild the engine, but cost wise, both are going to end up very similar. Funnily enough, you can use similar cams in both engines. In the Miller engine you put the longer duration cam in the inlet and use a stock exhaust cam. With the low compression pistons, you use a stock inlet cam, and a longer duration exhaust cam. The identical pair of cams works out optimum for both engines. If you plan to get a longer duration cam ground to try either approach, make sure it has the drive tang to operate the optical sensor on the exhaust cam. In other words make sure you have two exhaust cams so they will each fit on either side of the engine.

RB25 and RB26 blocks are identical dimensions, crank rods and pistons all interchange (drop straight in) I know this for a fact. The RB26 has I think about 1.3mm extra stroke, so the RB25 and RB26 crank and rods are very slightly different. As far as I know, the RB26 pistons are a straight fit into an RB25 block with RB25 crank and RB25 rods, so all you would need would be the pistons, and a new set of rings to do it. A fair bit of work, but not a lot of cost. (maybe $600 or something for pistons, a quick hone, rings, and gaskets). That would drop your compression down from 10:1 to 8.5:1 I have a set of R34 GTR pistons here (that cost me $200, he he). If you can find an RB25DE piston somewhere, the only thing that is likely to be different is the pin height. We can compare measurements, just to be absolutely certain.

Sounds like a good plan. An Eaton M90 is almost exactly the same size as an SC14, (90 cid = 1475cc) but it is a far better blower though, and should work very well. You should be able to pick one up secondhand at a reasonable cost. There are two ways of getting around the compression problem without stripping and rebuilding the whole motor with low comp pistons. But if you do plan to go that way, a secondhand low mileage set of GTR pistons and pins can be had for about $200. Ask around some of the "name"engine builders, they often throw a set of forgies into a GTR rebuild, and the stock pistons are often in as new condition and unwanted. I am not a great fan of decompression plates because it removes the squish area where the piston crown comes very close to the flat area on the head. It can effect combustion characteristics as well as make the engine more prone to detonation than it otherwise would be at the same reduced compression. So you are really making two changes that work against each other. The lowered compression helps, and less squish hinders the detonation resistance of the engine. A lesser known, but excellent way to tackle the compression problem is to fit a long duration inlet cam, effectively turning the engine into a Miller Cycle engine. To find out more about this type "Mazda Miller Cycle" into Google, and do a bit of research. This has the advantage that you do not have to remove the engine or even the cylinder head to do it. Also you are not going to lose any fuel mileage as you certainly will if you lower the compression ratio in the normal way. The only slight disadvantage is about 10% less low end torque, (under boost) but no midrange loss, and probably a very slight top end power gain. As far as the exhaust goes, I would stay with the stock manifold and fit a slightly larger cat back system. It will save you money and lose hardly any power over a set of extractors. Extractors can cause problems with supercharged engines, and offer none of the benefits that they have with n/a motors. The first problem is that the extractors are going to be restrictive, believe it or not. On an n/a motor the pipe branches are sized to give a pretty high velocity of gas down the pipe, and this DOES have a significant pressure drop. The benefit is that the negative return pressure wave during valve overlap helps scavenge the exhaust from the combustion chamber, and draw in fresh charge from the n/a induction system. So you lose from the initial pressure drop, and gain from the negative reflected pulse. So extractors are able to give more power, because the gain is greater than the loss. Now if you supercharge the engine, there will be a far greater volume of exhaust gas going down the pipe every exhaust pop. The rather long, small diameter extractor branches are really too small, and the back pressure rises to a very high level. You could make yourself a set of big bore extractors, but you cannot buy them, because they would be far too large for any n/a application. So you pay with high pressure drop. The negative reflected pulse is now of no value, because boost pressure is going to sweep out all the exhaust anyway during valve overlap. That is if boost is higher than exhaust back pressure, and it damned well should be. So tuned pipes are a complete waste of time unless you are building an extreme horsepower engine. The other problem with extractors is noise. By concentrating every exhaust pop in the extractor tube, what emerges is a shock wave of frightening intensity. Only an angry rotary engine can do it better. It will destroy your cat, blow the fiberglass or steel wool out of your muffler, and be really objectionable. Tuned pipes are a waste of time on a street blown engine, so what to do? To begin, keep the stock manifold, it will help reduce noise, then just make everything slightly bigger. But it might still be noisy though, noisier than a turbo with an identical exhaust system, and it will probably drone if too large. Once everything else is sorted, there is another way to do it though. The trick is to keep both exhaust back pressure and noise low, without any "pipe tuning", which offers no advantage. Now here is my little secret, that I have developed myself for solving all these problems on a street car. What you need are short exhaust stubs welded onto the head flange, maybe only an inch long. Just enough to get a ring spanner in there. These go straight into an exhaust plenum which might be a length of four inch pipe running along the head, down, and then into a reducing cone just in front of the cat. The secret here is that each exhaust pop empties into a very large plenum volume, maybe thirty times the volume of each exhaust pop. The peak and average exhaust back pressure will be extremely low, and the pulses and the shock waves beat themselves to death in the plenum. The cat (which must be there anyhow), offers some restriction, so the pulsing flow tends to even out. It is fantastic for noise reduction, and pressure drop. After the cat a slightly larger bore pipe and muffler works fine, and it can be as quiet as you want to make it. That is the Warpspeed secret formula, of how to make a quiet low back pressure street blower exhaust system.

You should be able to get a good complete RB26DETT for 6K, a complete but stuffed one, o/k for a rebuild, maybe 3K. I purchased a complete R33 RB26 short motor with scored pistons and bores for $500. It has a perfectly good crank, rods and oil-pump for that amount. I have also seen as new, bare crank and rods going for $500 from someone fitting a stroker kit to an R34 GTR, there are a few of these pop up from time to time on the Forums. But you have to be quick.

What I like about AFMs is that they are far more tolerant to slight changes in engine tune. They actually measure airflow directly, and that is a very important advantage in my opinion. A MAP sensor cannot do this. A map sensor assumes that at a certain given manifold pressure, flow through the engine will be xxx CFM of air. But what if you move the valve timing slightly, or fit a different exhaust turbine housing ? You get to completely remap the engine because the breathing characteristics have now significantly changed. This is fine for a competition car that spends more time on the dyno than on the track. But what if you are just an average joe with a car port at home, and you want to try a different exhaust muffler ? The change in exhaust back pressure might really screw up your mixtures, probably not by much but it will have some effect. With an AFM, your new beaut muffler might change airflow, and the AFM just alters the fuel to suit the measured increase or decrease. So, apart from all the theory, there are also practical considerations as well. As far as restricting power, well, atmospheric pressure is around 410 inches of water, so if your airflow meter has a four inch water pressure drop, you are still getting 99% of atmospheric pressure and density. Not enough to really worry about I think. The induction temperature at your cold air pickup point might be 300 degrees Kelvin (at 27C ambient), if you can lower this by 3C you will pick up 1% in air density which will easily compensate for the drop across your AFM. So some guy fit two huge Q45 AFMs to reduce the imaginary horrible power loss, and then fits a couple of crappy pod filters right behind the radiator. I cannot understand the logic behind any of that.

All the RB26DETTs have 8.5 : 1 stock factory compression ratio. There is no simple single answer, it depends on so many factors including fuel octane, induction temperature, boost level, inlet valve timing, and combustion chamber anti detonation characteristics, and even exhaust back pressure believe it or not. You can juggle so many factors around and get a good result, there is really no fixed rule on what works best. For instance the V6 miller engine that Mazda make has a 10.5 : 1 compression ratio, and runs a supercharger with 15 psi boost, and very inefficient inter-cooling. This is a stock factory engine as well, and detonation is certainly never a problem either. You could probably do something pretty similar with a stock compression RB25DE if you really wanted to, by fitting a different inlet cam and a decent supercharger (not a Toyota blower !).

As far as supporting the front drive-belt goes, there are three cardinal rules. The first is to have the supercharger drive pulley mounted as far onto the crank snout as it will go, not cantilevered right out on the end of all the other assorted stock pulleys. It must get pride of place next to the timing cover. Next, keep both the drive pulleys as large a diameter as is reasonable, this reduces belt tension both static and dynamic, and also the extra belt wrap reduces the chance of slippage. With less belt tension there is less strain on everything. Modern thin multi rib belts can run at fantastic belt speeds without flying apart. Third, run a forged steel crank, no problem on a Skyline, because that is all there is. As to if it is worth it, consider a stock GTR with about 350BHP at stock boost level, and stuff all torque below 4,000 RPM. It will rev cleanly to 8,000 RPM though. Chuck the turbos and fit a sufficiently large screw blower where the airconditioner usually sits, and a good tubular minimum back pressure exhaust manifold. Stock cams, stock intercooler, stock everything else. The power you lose driving the blower will be picked up by the increase you gain by not having any turbine back pressure. this usually works out at about 10% engine power, equal to about 10psi back pressure across the turbines (roughly). It will still make similar power at similar boost, and similar maximum torque as well. The difference will be though that the torque curve will be totally flat from about 2,000 RPM to around 5,000 RPM with zero lag. Standing quarter times probably would be no different, but as an everyday street car it would be far more grunty and easy to drive. It would be consistently quicker from a standing start as well, without having to thrash it. To beat it with a turbo, you would need to do rev limiter launches, and even then it would probably sometimes bog down on you. Only a complete moron drives like that every day on the street. If you fit bigger cams and turbos it gets even worse. With a blower you can increase the boost without losing any drivability at all, and that is its best feature. There is not the same compromise of drivability versus power that a turbo has. People claim turbos give higher power, and its true. You can set up your 1,000+ BHP turbo engine that has a 500 RPM wide power band, and yes it will be more powerful. So what ? You could probably build a similar centrifugally supercharged engine with similar power and minimal power-band, and it will be just as equally useless.

Supercharging CAN theoretically be far better, but it is not necessarily going to be better if is all done on the cheap. For example, you could buy a $300 turbo that came off a smokey old 2.0 litre 150Kw engine that has already seen a million miles and is a bit loose. Just bolt it onto your Skyline with no other changes. should make an easy 400 Kw at the back wheels, right ? Must be true, all turbos are the same and make big power, right ? Or you could buy a $300 supercharger that came off a 2.0 litre 150Kw engine that has also done a million miles and just bolt it on. You will get massive torque and horsepower. All superchargers do this, right ? Sorry about the sarcasm............. But you would be surprised how many people think a very small $300 blower is going to make absolutely huge power numbers when fitted onto just any engine. If you size and fit a decent blower, and tune it properly you can make a far more drivable and powerful package than any turbo. But it is not going to be cheap. The secret is to have the correct size and type blower that works efficiently over the pressure and airflow range you need, just as with a turbo. If you want cheap, and easy, stick with a turbo. With a big turbo you will make big numbers, but it can be a pig to drive every day. I have built, owned, and driven, supercharged cars, turbocharged cars , and a car with both supercharger AND turbo fitted, at different times over the last thirty five years. You only get what you pay for.

Further to Steve's comments on large single versus twins has to do with the relative needs of airflow and boost pressure for various engines. If you look at any flow map it leans to the right, with the most efficient operating islands in the middle somewhere. Now this means a given sized turbo will give you more available flow as the pressure increases, and you operate higher on the flow-map. It also means that at low boost levels, there will be a far lower efficient flow available. Now suppose you have a fairly large capacity engine and you only want to run low boost. You could either fit a huge single, or two smaller turbos, each supplying only half the flow. Ford on the XR6 have gone the huge single way with a monstrous GT40 turbo, which everyone thinks is oversized, but almost certainly is not. Or Nissan with the GTR have two very small turbos to do much the same thing. In each case there is pretty high airflow requirement at fairly low stock type boost levels. On the other hand if you have a small capacity engine and you want to run very high boost, a large single is going to be better because it will fit better onto the flow-map. High flow and high boost go well together with a large single. So as Steve says, if you want 500 BHP it really depends on the boost level you need to make that power. If it is going on an eight litre monster that only needs 4psi boost, go multiple turbos. If its a one litre engine running 50 psi boost or something, a single will get it done best.

I saw the other thread on the same topic first, but rev210 is exactly right. Each turbo will have to spin up to the original shaft RPM to generate the original boost, but with only half the exhaust volume to do it. As a result boost threshold RPM will double, and rate of boost rise will be just about half. It will be pretty awsome at the very top end though. Some smaller a/r exhaust housings would be what I would be looking for, or a couple of stock GTR turbos at a friendly price.

That is the problem, both turbos have to run up to the original speed to generate the original boost, but now there is only half the exhaust gas volume to do it. As a result I would expect the boost threshold to be about twice the RPM it was before. You can fix it by fitting smaller a/r turbine housings, but the top end airflow should be awsome either way.

Hello GTS200X, The RB30 has both longer rods and a taller block height. I cannot remember the exact rod ratios of all these engines, but I do have that information here somewhere. All the RB engines end up in the 1.6 to 1.7 range, so there is no vast difference in this respect. Steve, higher compression ratio does in fact lift the torque and power curves throughout the range by a small but fairly constant amount. At lower engine RPM it does this by raising the compression pressure (obviously). But what is not so obvious is that it also helps the volumetric efficiency at the high RPM end as well. What happens at TDC around the valve overlap period is that some exhaust gas is always trapped in the combustion chamber at whatever the exhaust back pressure happens to be. The larger the combustion space, the higher the volume of trapped exhaust gas. After the exhaust valve closes, and you are on the induction stroke, fresh charge is drawn into the cylinder. But the trapped exhaust expands, and takes up room that could otherwise be filled with fresh charge. It also adds heat to the fresh charge as well, expanding it. This works against the fine job your intercooler has just done. If you can get away with a higher compression ratio, the combustion chamber will be smaller, and the piston can pump out the exhaust and pump in the fresh charge more efficiently. In other words at TDC where there is more piston crown, trapped exhaust cannot remain. This is all particularly true with a turbo engine because compression ratios are lower, and exhaust back pressure higher than with a highly tuned n/a engine. Enrico, yes you definitely need the book to understand the selection menus available. Even then it can be a bit tricky to use.

Yes, I take your point about running taller gearing with the RB30 on a road car. I also agree totally with your comments about valve size (cams) and head work restoring the top end revability of an RB30D conversion. The point I was trying to make was that putting a 20% bigger displacement engine under a stock head is not going to get you 20% extra power. But if you do the extra work on increasing airflow as well as displacement, that is certainly going to give you a bunch of extra power and torque. Some people are unhappy after fitting shorter rear end gears because of the increase in noise and fuel consumption. A good way to find out for yourself if it suits your style of driving is to try driving around for a week or two without using fifth gear. If it drives you totally nuts staying in fourth gear after an hours run down the freeway, forget fitting the shorter diff gears because it is going to be like that all the time afterwards.

You are right. All modern engines that have to pass idle emissions cannot have more than a very few degrees of overlap, and most run zero overlap these days. The way VVT gets over this is to retard the inlet cam at idle, so you get zero overlap and clean idle emissions. At some speed just above idle (1500 RPM?) you advance the inlet cam fully. Because the engine is now under load the inlet manifold vacuum is far lower, and it will stand some overlap without messing up the emissions. The earlier inlet valve closing can significantly increase the low end torque. At higher RPM (>4,000) you again retard the inlet cam. The later inlet closing will help cylinder filling at the top end. I have a sneaking suspicion that the R34 figures for the VVT Neo engine give the fully advanced inlet cam timing figure. this might not actually be unreasonable because the engine will spend most of its life between 1,500 RPM and 4,000 RPM anyway. But I cannot say that is the case for sure. It is important to realize that VVT is put there for emissions, not for performance. If you throw away the VVT and fit a decent inlet cam it will kill the VVT everywhere in the RPM range. Of course if you go totally nuts with the duration it might also just as easily end up worse than stock everywhere. So the tip is go a bit more lift and duration on the inlet cam and dump the VVT. You can go a bit further on the exhaust side without stuffing things up. If you do it right it will be noticeably better everywhere. For people still not convinced, they could easily have fitted VVT to the GTR if it had increased performance in any way. Think about it.

Hi Sydneykid, I thought exactly the same thing after typing that last post, there is something very odd about it. I am thinking that those figures are probably the fully advanced position of the VVT which would then make more sense. Unfortunately those are the only figures I have available and they come without any further clarification. What are your own general ideas on turbo valve timing ?

Steve, The RB25DET has 240 duration cams, and the early non VVT RB25DE have 248 degree cams. I have no idea what the VVT cams are, I would love to know. R33 RB25DET inlet cam 240 duration 7.8mm lift, lobe centre angle 120 degrees, inlet opens TDC (zero degrees) closes 60 ABDC. Exhaust cam 240 duration 7.8mm lift, lobe centre angle 117 degrees, exhaust opens 57 BBDC, closes 3 ATDC. Overlap 3 degrees. R34 RB25DET inlet cam 240 duration 7.8mm lift, lobe centre angle 100 degrees, inlet opens 20 BTDC, closes 40 ABDC. Exhaust cam 240 duration 7.8mm lift, lobe centre angle 117 degrees, exhaust opens 57 BBDC, closes 3 ATDC. Overlap 23 degrees. So they have advanced the inlet cam a fair way on the R34.

Enrico, engine torque is completely independent of bore and stroke or number of cylinders, believe it or not ! Engine capacity is the single main factor that decides torque. Compression ratio will only have a small effect. The curious thing about it, is that the state of tune will alter the shape of the torque curve a lot, but peak torque value always remains about the same for a similar capacity engine. Supercharging or turbocharging makes a big difference to torque, but that is a quite different thing.

Enrico, I first purchased Desktop Dyno a few years back it cost only $45 then. I later purchased the Dyno 2000 package which is far better for $145 (it does turbo and supercharged engines). There is now Dyno 2002 available that I have not tried yet. Motion Software have a website with free demo download programs. The software is distributed worldwide by Mr Gasket Company. Your local speed shop will have the Mr Gasket catalogue, and will order the software in for you. There are better engine software packages available than Dyno, but they also cost a lot more. Dyno is a very good entry level package though, and exceptional value. There is an instruction book that comes on the CD that explains a lot of theory on engine operation that will completely change your perception of how a lot of things actually work. You can simulate any engine, and try out a lot of ideas to see what works and what does not. It is particularly good for choosing camshafts. Joel, I have not experimented with changing the bore size, but will certainly give it a go and get back to you on that one.

Hmmm. Wastegate size, could probably be estimated by calculation of flow area, if you know the pressure drop across the wastegate valve. You could actually measure one on a flow-bench, or estimate the flow coefficient from a similar sized exhaust valve in an engine. Or you could just guess. If you then know that your wastegate flows the same as say a 30mm plain hole, for example, you could then plug this into a formula for calculating the flow/pressure drop across a 30mm orifice plate. I have this formula here somewhere, but need to go and look for it. I have never tried any of this this myself. Another figure that you might find useful is a flow in the induction system of 1.5 CFM per horsepower, and an exhaust flow of 2.2 CFM per horsepower (at atmospheric pressure). As far as response goes, you cannot really do much about it, because it depends on the exhaust energy available, and rotating inertia. But boost threshold can be moved around quite a lot. Exhaust manifold design can also help a lot with this. Valve overlap versus back pressure is also difficult. The relationship above just referred to a general principle without being too precise about it. My approach is to use one of those engine dyno software simulation packages to come up with a good cam combination. If there is nothing really extreme about the engine, it will not be very sensitive to valve timing anyhow. If it is cam sensitive, there is something very wrong with the combination. I like to run minimum overlap that I can without losing lots of power. On my own car I have been experimenting for a couple of years and keep coming back to the same valve timing that works very well. It is 1.6 Litre DOHC 4 valve engine running 15Psi boost, so is probably fairly typical. Inlet cam 245 duration, opens 4BTDC, closes 61ABDC Exhaust 254 duration, opens 57BBDC, closes 17ATDC I am not saying this is the perfect cam for everyone, but it does illustrate a few features. First the exhaust cam is fitted to what would be stock n/a lobe centre angle. Standard timing for a 254 cam is 17/57 57/17, no surprise here. The inlet cam has nine degrees less duration and is also retarded a fair bit. Both of these reduce the valve overlap to only 21 degrees. A pair of stock 254 cams would have 34 degrees of overlap which is far too much. The overlap can be reduced by late inlet opening, which does not seem to hurt things at all. Another interesting feature here about overlap is that most of it is on the exhaust side of TDC. This works best where exhaust back pressure is higher than induction pressure on any engine. Most stock n/a cams are like that as well, with typically about five degrees more overlap on the exhaust side. Stock engines usually have quiet and restrictive exhaust systems. Typical n/a sports racing cams that run extractors and a low back pressure exhaust system seem to like split overlap. Mechanically supercharged engines always have far higher induction pressure than exhaust back pressure, and run best with most of the overlap on the induction side of TDC. The clue here with your turbo engine is to keep most of the overlap on the exhaust side, unless the turbo is truly huge, then run split overlap if you can get the exhaust back pressure down that far. It is a commonly known trick with the GTR to retard the exhaust cam a few degrees, even though the exhaust duration is only 236 degrees. This makes exhaust opening very late, but it still goes a bit better at the extreme top end.

As far as wastegate size goes, it needs to be large enough to bypass all the exhaust the engine can create after the wastegate opens. So there are two variables. If your red-line is 8,000 RPM, and your wastegate opens at 7,000 RPM it does not need to be large because roughly seven eights of the exhaust is going through the turbine, and one eighth through the wastegate. On the other hand, if your wastegate opens at 4,000 RPM, at red-line roughly equal turbine and wastegate flows will be required. This is not strictly true, but you get the idea. The same sized wastegate is going to be able to flow a lot more if the exhaust back pressure is 35 psi than if it is only 12 psi. In order for the wastegate to properly control boost, the area of the control diaphragm needs to be large with respect to the area of the poppet valve area. Otherwise exhaust back pressure may just blow the wastegate open. This will reduce boost, but it is not really what you want. So do not be tempted to make the wastegate too large. Lastly, a long soft spring in the wastegate will control boost better than a short stiff spring because it will vary more in length and open and close the valve further for the same variation in boost pressure. As far as camshafts go, the starting point is knowing how much exhaust back pressure you have, with respect to boost pressure. The higher the exhaust back pressure, the less valve overlap you can get away with. Do not be fooled into thinking that boost is going to blow all your mixture down the exhaust pipe in a turbo engine. If exhaust back pressure is 30 psi, and you run 12 psi boost, this obviously cannot happen. Fitting a big overlap cam to this same engine is not going to help you either. On the other hand, if you run a massive top end only turbo, and an open dump pipe, you might find that you have 20 psi boost, and 18psi exhaust back pressure. A nice bit of overlap might gain you quite a bit of extra power. It will be very peaky though. So the moral is, big turbo, big cam. Small turbo small cam. The exhaust opening and inlet closing points decide where in the RPM range the cam is going to work best, just as it does with an n/a cam. I find a bit of extra duration on the exhaust lobes also helps top end power without killing the bottom end. O/K guys thats more than enough bullshit from me for one session, so flame away.......

There is not a lot of information around, and those that really do know will not tell you anyway. Others are scared to have a go because others may later prove them to be wrong. Anyhow, I am game to give it a try. The first thing is that the exhaust turbine needs to be speed and energy matched to the compressor so that the available exhaust energy can best be used to create boost. Speed and energy matching also brings in the question of exhaust back pressure, and at what flow you want the most efficient operating point. This all assumes operation below the wastegate opening point. Now given a compressor flow map, you can figure out that you need a certain shaft RPM to generate a certain desired boost level. The biggest factor here is compressor wheel diameter. So you come up with a particular shaft RPM that you must reach with as little exhaust back pressure restriction as possible. Now you also need to generate enough shaft torque to turn that compressor wheel, in fact you need surplus torque to accelerate it quickly as well. The amount of exhaust turbine shaft horsepower required to do this might surprise you. Now given a certain volume of exhaust gas out of the engine to work with, there are several ways of attacking this problem. You might for example accelerate the gas through a small a/r housing to very high velocity, and use that to drive a large diameter exhaust wheel at high tip speed. Now a small a/r sounds bad, but remember the wheel is big, and the nozzle area will also be big for a given a/r. The torque developed by the turbine will be high because the wheel diameter is large, but also wheel weight and inertia will be up as well. So you have more turbine power and more weight, these tend to cancel to a certain extent. Or you can go the other way. Use a smaller exhaust wheel diameter with lower tip speed. You can then use a larger a/r housing, but the nozzle size will not be as big as you might think because the whole exhaust housing is a smaller diameter. So you get a whole range of wheel sizes and turbine a/r sizes that will sort of work. With the Garrett T3 and T4 range, people have been playing around with these combinations for over thirty years, and you can usually find something that works tolerably well. Now Garrett have figured it all out, mainly because they now have computer software that can model exactly what is going on in the turbo. The new GT range of ball bearing turbos have the turbines and compressors very well matched for speed and energy. The trend is to smaller diameter turbines with larger a/r. Also the turbine wheel shape has dramatically changed. The old turbines are large diameter, and skinny, with a small exit eye. Also the blades had a lot of swirl near the exit. Newer turbines look more like paddle wheels with the major and minor diameters nearer to being the same. Also the blades are straighter. Another thing, the old turbos were available with a very wide range of exhaust housing a/r. Newer GT turbos usually have only three housings available in each turbine size. Probably the middle size being the most efficient. So if you order your new GT Garrett turbo with a compressor to suit your needs, the turbine will be exactly right for that compressor. If it does not work, you need a different sized turbo, not some extreme sized exhaust housing. As for sizing the old T3 and T4 turbines, that depends on what you want it to do. The most efficient operating point is where you just reach the desired boost level just at the point just before the wastegate opens, this really applies to any turbo. Once the wastegate opens you are throwing away energy, and the power required to drive the compressor all comes from high back pressure. So having a low boost threshold is going to cost you engine horsepower because of high exhaust back pressure. Measure your exhaust manifold pressure before the turbo, and realize that every psi is costing you about 1% of engine power. It is not uncommon to find exhaust pressure to be twice boost pressure, or even more. So measure it, and be prepared for a surprise.

Yes, the disadvantages of a shorter diff are decreased fuel economy, increased noise, and increased engine wear. Fitting a longer stroke crank is also going to decrease fuel economy, and increase engine wear exactly the same. It all has to do with piston speed with relationship to road speed. You can change the relationship to give more piston travel for each meter along the road, you increase the leverage. A longer stroke will do it, and shorter gear ratios will do it as well. It really makes no difference if the engine operates at atmospheric pressure, or some boost level, exactly the same principles apply. But there are other factors that come into it as well. One is rotating inertia, particularly of the flywheel. You are not just accelerating the car, but also the flywheel and the whole drive-train as well. A lower gear ratio is going to make this effect worse. Another factor is piston ring friction in the bore. A long stroke engine is going to add a lot more friction, whereas a shorter gear ratio is not going to add any extra sliding friction. Since yesterday, I have been experimenting with different stroke combinations on my Dyno 2000 software engine simulator. It is really interesting, I can vary the stroke from 70mm to well over 100mm while leaving every other engine parameter exactly the same, including the compression ratio. What happens is that engine torque changes exactly as engine capacity does, but there are absolutely huge differences in the shape of the power curve. A long stroke engine produces very high torque low in the RPM range, and that torque stays fairly flat, then falls off very quickly as RPM rise because the induction system can no longer feed the engine. The power rises to a sharp and narrow peak at mid RPM and then drops like a stone. The identical engine with a short stroke has very poor low RPM torque, that smoothly rises to a shallow hump at mid RPM, and continues on up to quite high RPM, tailing off only gradually. The power curve is much flatter and broader than the long stroke engine, but occurs at much higher RPM, and continues on. It does not drop off anywhere near as fast as with the long stroke engine. Interestingly the short stroke engine also produces about 8% more peak power. This can only be because of reduced ring friction, because I did not change anything else. As I kept reducing the stroke, the power kept rising, but peaking at a higher RPM, with less torque everywhere. The engine I have modeled is a supercharged and inter-cooled RB26 running 15psi boost with special blower cams and ported head. I used that only because that is what I am playing with at the moment. The power maximum I was getting was 499 BHP at 8500RPM, and 405 ft/lb at 4500 RPM. Power drops to 488BHP at 9,500, and 452 BHP at 11,000 to give an idea of how well this thing breathes at the top end. Now all I did was increase the stroke to make it an RB30. Torque went up to 485 ft/lb at 2,000 RPM and fell away after that. Power peaked at 491 BHP at 6,500 with 454 at 8,500, and 353 at 11,000. so at 11,000 RPM the power fell from 452 to 353 just by increasing the stroke by 14mm. Not that you could actually run either engine at that speed, but it does illustrate a trend. While these power figures are not high compared to what some of the turbo guys claim, the supercharger produces far more low and mid range torque, in fact the torque is as flat as a ruler between 2,000 and 5,500 at 400 ft/lb, and without lag. So on road performance should be far better than a peaky high power turbo engine of similar or higher power. Easier to drive as well.

H Joel. I deliberately stayed with n/a motors for the illustration because it is easier to understand. The same holds true with turbo motors as well, BHP per pound of boost will stay the same. It not just sounds true in theory it is true ! But no one does it. When they fit the RB30 bottom end they also change a lot of other things at the same time, I certainly would. Sydneykid is the RB30 turbo king, and It certainly makes a very potent package. The main thing the RB30 bottom end gives you is a lot more off boost torque, and better response from that big bad turbo. All things considered an RB30DET is a very good street package. But it is also true that there is nothing wrong with an RB26DETT modified to the same level. It all just happens further up the RPM range. Personally I would rather have the RB26 and some shorter rear end gears, but others will disagree.

The overwhelming bias is from CAMS, not us here. They banned the GTR because it was just too good for the local pushrod boys. Then they bend over backwards to allow an illegal non existent supposed production car to win. There is no way you can call a mega dollar one off prototype a production car, No way. The whole thing is a bloody joke.

Power depends on airflow through the engine only, engine capacity is irrelevant. So if you take a given induction system, cylinder head, and exhaust system, it virtually fixes the maximum airflow capacity and hence the power. If you then place a larger bore and stroked block under the same head it will still be limited to the same maximum airflow. So an RB30DE will make no more power than an RB25DE if everything else remains exactly the same. Think about it. It will however produce a lot more torque, and the same power at a lower RPM. It will be more grunty and less revy. This might be an advantage, and it might not. So increasing engine capacity by 20% (30/25) is going to increase acceleration and reduce top speed. You can do the exact same thing by lowering the diff ratio by 20% a lot more easily. Just to make it even more interesting, your RB30 with tall diff, will have the same piston speed as an RB25 with the shorter diff at the same road-speed in any gear. They will probably also have very similar fuel economy, acceleration, and top speed capability. Interesting eh !