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Many years ago I obtained a copy of "Maximum Boost" by Corky Bell after reading about its release in a copy of "Fast Fours & Rotaries" back in about 1996 or so.
Since then I have used this book to understand compressor maps and how flow, pressure ratio and compressor RPM all work together. These days I have noticed some drag cars and various Turbocharger compressor covers being provisioned with a compressor RPM sensor (which I assume is some kind of optical arrangement).
What I would like to know is, how is the RPM information used in such a setup? Is it just used for data logging as an extra channel of information, or is it used as an input to the ECU for fuel/ign calculations at all? I know you have a huge roadmap planned, but would you be able to consider this as a potential HPA course?
Most speed sensors are hall effect or reluctor style. Optical require a Light to strike a sensor which is interrupted via a trigger disc or blade. So in order to use an optical you would need the compressor blade to interrupt an LED - trigger.
The turbo speed sensors are used to see where in the compressor map you're running.
On a compressor map there is "turbo speed lines" these lines show at what RPM the turbo is operating at X pressure ratio and/or mass air flow. It will help you determine when your are starting to reach the choke line and over spin the turbo.
Thanks for your answer, I understand that the sensor is used to figure out where on the compressor map you are, but I don't understand what the limiting factors are - I have only seen turbo speed sensors on top end drag cars, and from what I know about compressor sizing, the peak compressor efficiency "island" at the centre of the map is targeted towards peak power, rather than running the turbo hard up against the right hand side "choked" region... In any case, you'd know when the turbo is up against the choke line anyway, even without a speed sensor, since power would flat line on the dyno..
Out of interest, I saw that some Garrett turbos now have a factory speed sensor port (such as the new version of the GTX42) which I think is cool! The idea of monitoring turbo RPM seems to be catching on!
After a little searching around the web, I found the info I needed on the Garrett Honeywell website. The new GTX3584RS has the factory port and some info on the sensor used and how to install etc.
It sounds like it really is only of use on a maximum-effort turbo system, so I won't be needing one any time soon :)
There's a few uses for a turbo speed sensor. As has already been mentioned, between turbo speed and pressure ratio you can pin point where in the compressor map you're operating. This can be useful in validating turbo selection. You can also utilise turbo speed in the ECU control algorithms - Say for example in some cases you could use turbo speed as an input to boost control or as a safety parameter to prevent over speeding the turbo.
I am running an EFR 7064 turbo myself with the speed sensor. Really nice feature to have. Not exactly necessary for most use cases but I like seeing the turbo RPM to assess the effect of certain changes on spool time for example.
The new Bentley W12 engine uses 2 turbos with speed sensors as well in an OEM application.
A speed sensor isn't a bad idea if you live and travel to higher altitudes regularly. I live at ~8,000 feet above sea level, nearest track (road course) is in the 5,xxx ft ASL range, and I travel to tracks that are sub 1,000 ft ASL. I know EFR turbos do not like to be regularly overspun, so I can use the sensor to see how much harder the turbo is working and adjust compensations or add safeties based on this, especially in cases where I am on track for 20+ minutes at a time. There is a huge difference between drastic altitude changes and how much harder the turbo has to work to provide similar boost levels.