So, here's what's been kicking around my head recently. A bit of a ramble and doubtless I will re-read and edit this many times....
There's the ongoing debate about twin or single turbo upgrades. The question has been asked a million times, and the purists on both sides have made their opinions clear, but to me the arguments are as follows:
Bigger Twins - maintains the characteristic of the car as it was intended. Shorter pipework to the tubs - less lag
Big Single - Scoobs use big singles fine with longer pipework. No custom manifolds or issues with fitment. Easy to upgrade again. Only one turbo to buy.
Given the choice, I'd personally go big single. However, I've been mulling over another option. Compound charging. A bit of background.....
Turbos move volume and compress by ratios. So, a turbo that makes 1 bar of boost is operating at 1:2 (atmostpheric being 1 bar) ie it's taking 1 bar of atmospheric pressure and giving 2 X atmostpheric pressure - and absolute pressure of 2 bar, which is 1 bar of boost. If that turbo had an inlet pressure of 2 bar absolute, then at the same spool would still give 1:2, but the boost it would produce would be 3 bar - ie it takes the inlet of 2 bar absolute, double that which is 4 bar absolute, or 3 bar boost.
This all sounds too much to be true, and in a way it is, for a numbr of reasons, the main one being the heat produced at each stage of charge also increases exponentially. It's generally the preserve of big ass tractors and sled pulling diesels where manifold pressures of 100+PSI are needed. However I don't know that it's entirely without merit on a petrol engine. It has been done with varying degrees of success, using one turbo to feed another as well as using combinations of superchargers and turbo chargers.
It's worth noting that this is not the same as sequential charging. That requires various combinationos of controlled valves etc on the exhaust and inlet systems to remove one (big) turbo from the system whilst the smaller one does the work, and then before the small one runs out of puff, the big one is allowed to spool and ultimately takes over. But these do not typically blow through each other.
A compound setup, in it's simplest form, has a small turbo mounted traditionally on a manifold, feeding it's charge into the plenum (we'll ignore intercoolers for now). The inlet to the small turbo is fed from the charge from a bigger turbo. The bigger turbo receives exhaust from the small turbo.
So, where am I going with this?
If you look at the boost curve of our twins, they make about 18 or 19 PSI and then boost steadily tails off over the rev range. What if we could keep that early spool and and hold the boost to the read line. Maybe even increase the boost and have a steady increase?
What you'd need is a charger before the twins that would raise the pressure that they "see" and it wouldn't have to be by much - the main thing is that whatever is producing the initial charge needs to flow well, even if it's not providing silly boost. For this reason using another TD03 probably isn't going to help, as although it doesn't need to provide bags of boost, it does need to let enough volume through. One interesting notion, as an aside, was to compound charge the existing twins with each other - if the curve boosts to 18 PSI and the trails off to 13PSI as twins, then compounding one into the other would see boost hitting 26PSI before trailing off to 16PSI, but as I say, I'm pretty sure that flow would be the issue.
So what would you put infront of the twins - well one idea is a centrigual supercharger. These seem to be only selected in the event that tubos or a positive displacement supercharger won't fit, but have some merit in that they produce boost which rises linearly with rpm - exactly what we want. At lower rpm the multiplication of boost will be slight, maintaining the cars chracteristic, but it would then give more lift to the stock twins at higher RPM. Given the same curve that boosts to 18 PSI and the trails off to 13PSI, overlaying a centrifugal supercharger that, on it's own, only makes 5PSI at 7000RPM, the resulting boost curve would hit 23PSI and hold a steady 22PSI to the red line. The problem is that superchargers incur significant parasitic power losses to just spin them, compared with a turbo which is "almost" free. That said, centrifugal superchargers are the least parasitic of superchargers, as far as I can tell. The benefit though is that provided it physically fits somewhere, a centrifugal supercharger is as easy to plumb into the inlet as a turbo, but without the need to manage it's boost and no exhaust plumbing.
The other option of course is a third turbo running off the exhaust gasses from the twins. Boost would probably have to be managed independently, monitoring it across the big turbo. There are plumbing questions around whether to use just the wastegate gasses from the small turbos or all of it. I would suspect you'd want the lot, even though some of it has been "spent", as you'd potentially get into difficult situations where you may fall into a valley of death where the twins trail off and fall below their preset boost level (wastegates close) and so no gasses get to the big turbo which doesn't then spool.
Finally, there's an overall issue of how would you control boost on the twins? The whole system relies on the multiplying effect of twin charging, but because it's compounded the control of the waste gates on the twins isn't really based on the total boost above atmospheric, it needs to be based on the pressure on the outlet of the turbos compared with their inlet.
It might be possible to do it arse about face, and have the twins feed thair air into the single charger - their boost would be easy to control then, but they'd be expected to flow more as their outlet is being compressed by the next charger, and I suspect they'd be too restrictive.
This is all conjecture as well, of course. And there are very good reasons why people don't do this, but if you wanted to maintain the characteristics of the car but add top end power, then this might be of interest, and would be more bolt on (certainly the Centrifugal SC would be) than either big single or big twins.
I've been mulling it over so I thought I'd present it to the wide CLubVR4 to pick holes in it and shoot down my balloon before I go too high in it!
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