Garrett Turbocharger Speed Sensor

[prophecymiller]

I just saw this the other day and it is supposedly new from Garrett...I'm sure these have been out for a long time, but this is the first time I have seen one. Here (http://www.turbobygarrett.com/turbobygarrett/products/Accessories_continue_speed_sensor.html)

I think this could be a cool thing to have, might help you monitor how sealed your intake system is...I did the math on my turbo at 100,000rpms, and at the outer edge of the compressor inducer, it is moving about 670mph...this means that at 113,695rpms, the turbo would break the sound barrier at 761.2mph at sea level, which would obviously cause it to grenade:p

I suppose if I wanted to get really accurate, I would have to measure the exducer diameter to get the highest speed of the wheel, but I don't know what the diameter is, so this is close enough to get an idea...I found this interesting though, I figured the speed would be lower, and didn't expect to hit mach1 at only 113,695rpms...and especially at our elevation, where mach speed is lower than at sea level...you would not be able to spin the turbo up as high, so we are getting kinda screwed up here.

So are really small turbos the only ones that can spin upwards of 100,000rpms?

[thiazole]

I agree that they spin ridiculously fast, but I think grenade speed is more like 200,000+rpm, but it depends on the turbo I guess.

[prophecymiller]

Oops, I was thinking the turbo would fail if the wheel breaks the sound barrier...with that in mind, there aren't any effects from the wheel going mach1 are there? Like any sudden noise like a small sonic boom?

[Cloud]

Oops, I was thinking the turbo would fail if the wheel breaks the sound barrier...with that in mind, there aren't any effects from the wheel going mach1 are there? Like any sudden noise like a small sonic boom?

There is of course the massive brick wall in airflow and enormous drop in efficiency with skyrocketing shaft speeds ;)

[JackM]

I had a car where at 9+k RPM the water pump would break the sound barrier under water. What happened was the water stopped moving, it didn't bother the pump or wheel in anyway. I would think it would be similar with air. One thing to think about is the turbine wheels in most turbos are larger than the compressor, so if your compressor will hit the sound barrier at 113k RPM the turbine wheel was probably there far before that. If I remember right- Jim at TEC used to run just about every turbo at 150-200K RPM when testing at his shop. Pretty neat watching a turbo spin up to that RPM on the VSR.

Jack

[Cloud]

I had a car where at 9+k RPM the water pump would break the sound barrier under water. What happened was the water stopped moving, it didn't bother the pump or wheel in anyway. I would think it would be similar with air. One thing to think about is the turbine wheels in most turbos are larger than the compressor, so if your compressor will hit the sound barrier at 113k RPM the turbine wheel was probably there far before that. If I remember right- Jim at TEC used to run just about every turbo at 150-200K RPM when testing at his shop. Pretty neat watching a turbo spin up to that RPM on the VSR.

Jack

The speed of sound of exhaust gases is significantly higher than ambient air. As a result chances are very good you will still hit mach 1 on the compressor side first unless you have a really strangely setup cold and hotside combo.

[Kibo]

FP used to sell a turbo tach, but it was prohibitively expensive for what would be primarily a novelty for most of us.

[prophecymiller]

FP used to sell a turbo tach, but it was prohibitively expensive for what would be primarily a novelty for most of us.

That's what I kinda figured with this one too...It would be cool to have, but it's probably like 300$+ which is a little pricey for something that will be looked at once every six months like a boost gauge:p

[prophecymiller]

I had a car where at 9+k RPM the water pump would break the sound barrier under water. What happened was the water stopped moving, it didn't bother the pump or wheel in anyway.

Man that's strange...I wonder why the fluid stops, maybe something to do with a barrier created by the speed of the rotation???

[Cloud]

Man that's strange...I wonder why the fluid stops, maybe something to do with a barrier created by the speed of the rotation???

It doesn't 'stop' per se, you just can't really move fluid faster than the speed of sound of that fluid. You can move through a fluid at super-sonic speeds but you really can't force a fluid to efficiently travel beyond the sound barrier. Super-sonic flows generally mean the fluid just completely cavitates and the molecular bonds in the fluid are not strong enough to maintain the flow, instead the molecules comprising the fluid actually separate from one another (which would look like aeration in the fluid except the space in between the molecules is filled with nothing instead of a gas).

In a liquid based flow cavitation usually means the local pressure in the liquid is decreased so much that the liquid becomes a gas and dramatically decreases the amount of mass flow as a result. You can get this on water pumps, fuel pumps, anything really if you try to move enough of it.

[thiazole]

the local pressure in the liquid is decreased so much that the liquid becomes a gas and dramatically decreases the amount of mass flow as a result. You can get this on water pumps, fuel pumps, anything really if you try to move enough of it.

That is exactly the reason. It has to do with the fact that on the "exhaust side" (for lack of a better term) of the water pump there is a lot of pressure - by the same token, there is a tremendous vacuum that is produced on the other side. All liquids (and even solids for that matter) have a vacuum point where the molecules can no longer cling to each other at a specific temperature and they will spontaneously boil (or sublimate for a solid). That is why water boils at a higher temp at sea level than here. There is a point where water will easily boil at room temperature. As chemists, we use vacuum pumps along with a super cold condensor to remove solvents from a reaction to a separate collection container without heating it up and it works very effectively and very fast. When this happens in your coolant system, because a water pump is made to pump liquids and not gases, the flow would almost come to a complete stop when you reach this critical vacuum. You could push the envelope a little further with a higher pressure radiator cap, but the best solution would probably just be a bigger water pump pully (or smaller crank pully) to slow things down.

[prophecymiller]

That's some cool information...so this is probably most of the reason why a turbo will hit it's "max" airflow and you can't really extract much more out of it at that point because the air is trying to be forced to move beyond the sound barrier...after the air goes through the turbo at that point, do the atoms form molecular bonds again since the flow slows down post turbo? But at that point, your mass flow would be very small and the air would be very hot like was previously stated...

What could you do to slow down the wheels if you are hitting mach1? Would you just have to turn down the boost or just get a bigger turbo?

[Mirage]

Best way to do it is to get a turbo capable of flowing the amount of air you need.

Next best is to turn down the boost, but typically that will decrease overall power output.

When you end up "maxing" out a turbo, this is typically what happens along with the wheels typically being out of balance enough at those speeds to make contact with the housing.

Anyone ever figure out whether a compressor wheel "expands" or not at it's upper rpm limits? Or are these wheels typically stable up until they explode?

Marcus

[Cloud]

With conventional turbos no, there is really no way to slow down the turbo. If you had a shaft speed sensor and a circuit to speed limit it at the wastegate then yes you could do that but chances are good you would have a better time just getting a larger turbo that could handle what you were throwing at it.

Wheels are typically stable till they explode yes. Actually people 'max' the e316g constantly. I have seen many dyno charts of people maxing them and the boost linearly falls off as the rpm goes up with no increase in airflow or power. Generally speaking when you get boost drop at high rpm and it isn't from the wastegate blowing open it means you hit the limit of the turbo and it can't flow enough air to maintain a constant manifold pressure anymore.