A new thought for twins - Compound Turbos? Maybe?

[RotaryProphet]

The point of a compound turbo system like that is that it provides very -very- high boost levels. Let's take an example:

The larger turbo runs at say a 1.5:1 pressure ratio, consuming something on the order of 1200 CFM of air, compressing it down to something on order of two thirds that (somewhat more than that, given heat pollution, but this is example math, so don't crucify me.). So we have 800 CFM at the outlet at 1.5 Bar pressure.

This feeds into the inlet of the smaller charger, which runs at say, 2.5:1 pressure ratio. The turbo doesn't care what pressure the inlet is at, it just compresses it further. So our charge gets compressed to ~320CFM at 3.75 bar, or around 55PSI absolute; think about 40 psi of boost.

It's not really a device to increase response like a twin setup, it's a device to get around the boost/airflow limits of turbochargers. The larger turbo flows enough air for the entire system at a reasonably low boost level, while the smaller turbo, instead of having to worry about massive airflow numbers (The numbers I used are probably par for 1/3 to 1/2 throttle in a big diesel application at that boost level) can compress a smaller charge further.

Heat buildup, however, is pretty intense, since unless you absolutely match the turbos with regard to efficiency ranges, they'll both be running outside of their efficiency ranges most of the time. Even if they are well matched, you're getting heat from two turbochargers in all of the intake air. Hence why they generally run massive intercoolers in diesels.

In short, if your goal is to run > 30psi or so of boost, this might be something to look into, and if you want to run more than about 45, it's almost a necessity, but otherwise, it doesn't do what you're looking for.

[TitaniumTT]

Originally Posted by RotaryProphet

The turbo doesn't care what pressure the inlet is at, it just compresses it further. So our charge gets compressed to ~320CFM at 3.75 bar, or around 55PSI absolute; think about 40 psi of boost.

This right here is what I was unsure of, thank you for the clarification. It was my believe that if the smaller turbo is seeing say 2bar Absol, and running at a 2:1 ratio, that it would be producing 3 bar of BOOST. While it makes perfect sense in theory, I was unsure if the turbo would physically be able to do it. I'm sure it's not simple absolute math as ineffeciancies do exist.

Originally Posted by RotaryProphet

It's not really a device to increase response like a twin setup, it's a device to get around the boost/airflow limits of turbochargers. The larger turbo flows enough air for the entire system at a reasonably low boost level, while the smaller turbo, instead of having to worry about massive airflow numbers (The numbers I used are probably par for 1/3 to 1/2 throttle in a big diesel application at that boost level) can compress a smaller charge further.

In your opinion, is this something that COULD be configured to provide the response that I'm looking for? My main concern here is the response. It's pretty common to see a 450-500rwhp rotary. There are a number of basically bolt-on kits, turbo and mani, that allow for this. None of them however, address the response that I so dearly love in my FC. I understand this is an outside the box kind of thinking as the general rule is you have to give up response for power and vice versa, but Mazda figured it out almost 2 decades ago. Granted they have engineers and foundarys that I don't, but still, anything's possible.

Originally Posted by RotaryProphet

Heat buildup, however, is pretty intense, since unless you absolutely match the turbos with regard to efficiency ranges, they'll both be running outside of their efficiency ranges most of the time. Even if they are well matched, you're getting heat from two turbochargers in all of the intake air. Hence why they generally run massive intercoolers in diesels.

Heat I can deal with. It's a matter of fabrication as far as I'm concerned. Not to sound arrogant but my FC on the dyno on a 65* day can run 4-5 4th gear dyno pulls in 20 minutes and the coolant will stay at or BELOW 185*, oild ~170* and AIT's will start ~60* and end BELOW 80*F. If need be a small inline Water-to-Air could be utilized to lower the charge temp of the smaller turbo's intake back to about ambient before the smaller turbo compresses it, reheating it, then it's just a normal intercooler after that.

Originally Posted by RotaryProphet

In short, if your goal is to run > 30psi or so of boost, this might be something to look into, and if you want to run more than about 45, it's almost a necessity, but otherwise, it doesn't do what you're looking for.

My goal is 360-375 ish torque with it being as early and as flat as possible. A PSI goal hasn't entered my mind yet. I would obviously like it to be as low as possible as I would like to avoid AIT related problems and Meth or water is something that I don't want to run either. Although I think it might come down to two retardedly small for a rotary turbos (GT25ish size) and some meth to keep it under control, although that defeats the really plush easy daily driveably charateristics that I want to keep with this car. I love driving my FC around, but it has become a little raw.