Turbo Restrictors and Air Cavitation....

[takai]

Ok, this would normally be quite an obvious question, but im getting some wildly conflicting information on it. So i thought i might put together a bit of a compendium of information on the topic.

Basically, for those who may not understand, a turbo which is producing 20psi of boost without a restrictor may be starved for air at higher engine RPM with a restrictor. Hence, as most fluids do, the air will cavitate due to the compressor blades spinning without being able to compress air (due to the lack of air, see aforesaid restrictor).

However, several workshops who have been often proven to know their stuff, and several high running competitors have said that they have never had any issue with cavitation at high boost pressures, with high rpm. For example, an SR20DET running a 7750 redline, and a medium sized turbo (2540 i think) seems to have no problems with cavitation right up to redline. The people who hold this view seem to argue that as the system is not a closed system (i.e. the engine is taking air in, and as such there isnt the "backpressure" to cause the system to cavitate.
But i had always thought, and modelled, that cavitation didnt need an overall pressure, or back pressure, but rather just needed there to be not enough fluid to support the compressor spinning.

Opinions, and ideas?

[kingmick]

if your involved your with IP you will know that the magic figure is around 380hp with a properly shaped restrictor. cavitation will start after that as you cannot suck any more air into the restrictor.
so they are making less than 380hp or there abouts with there setup and redline or there cheating..lol
with a restrictor you try and get the best possible powerband out of the engine. a broader band will win you races as the max HP is capped.
The trouble with the restricors is they are not nice on the turbo. the cap seems to work, that is until someone with money comes along with a very well setup car and a big dollar chev and knows how to use there tyres properly.

[takai]

Yeah, problem with big dollar Chevs is in most cases they need to have big holes cut in the bonnet. Not quite so good for the class regs.

Anyway, what i was more getting at is that the 360hp (most figures ive heard have been around 360hp rather than 380hp, but it always varies) is restricted specifically by the amount of air that the turbo can flow. Now, theoretically the turbo could flow that amount of air at a lower RPM and therefore have more boost. However, as the engine demands more boost it tends to try and pull more air.
From some further thinking i have done the solution seems to be to start bleeding off exhaust through the wastegate as the RPM rises, but let the poundage fall, so that the shaft speed of the turbo stays the same.

Man thats going to be a pain in the arse to figure out.

[kingmick]

Yeah, problem with big dollar Chevs is in most cases they need to have big holes cut in the bonnet. Not quite so good for the class regs.

Anyway, what i was more getting at is that the 360hp (most figures ive heard have been around 360hp rather than 380hp, but it always varies) is restricted specifically by the amount of air that the turbo can flow. Now, theoretically the turbo could flow that amount of air at a lower RPM and therefore have more boost. However, as the engine demands more boost it tends to try and pull more air.
From some further thinking i have done the solution seems to be to start bleeding off exhaust through the wastegate as the RPM rises, but let the poundage fall, so that the shaft speed of the turbo stays the same.

Man thats going to be a pain in the arse to figure out.

exactly right. keep the shaft speed at a the max it can be constantly. the turbo will draw air threw the restrictor till it reachs the max pressure it can draw threw. diffrent blade designs will work better than others at holding onto the last bit of pressure that can be pulled threw. talk to on of the turbo companies that do turbo's up that use restrictors! there advice can be a good start.

lol and the engine mounts a free so a drysumped chev slots in nicely under the bonnet.

[takai]

Yeah, ive fired off an email or two to Garrett, and waiting on a response.

Strangely enough with the Chevs, they shoudl be quite fast around Mallala, but it seems that the weight over the front wheels gets them down in the times. Eldo was making some quick times on the weekend, but was let down by a chunk of piston skirt jamming up the dry sump pump....

[kingmick]

yes the extra weight hammers the tyres and they go off quick! hence why it makes the field more even, as the lighter cars tires last longer before going off. when i said big dollar i ment like an $80k 700hp chev! be funny to watch trying to save the tyres,fast down the straight and slow cornering.

[Billzilla]

if your involved your with IP you will know that the magic figure is around 380hp with a properly shaped restrictor. cavitation will start after that as you cannot suck any more air into the restrictor.

To get all technical, it isn't cavitation it's choking because the local flow goes super-sonic. If you have the restrictor far enough away from the compressor inlet so as to let the airflow settle down again it'll work fine. The closer the diameter is to the compressor inlet, the closer you can have the restrictor.

[takai]

Interesting, all those NA flow principles do come in handy after all. So if i make the restriction on the gentlest taper as possible, and the extremity of it 50mm away from the compressor wheel i should see some better gains?

[Norbie]

From some further thinking i have done the solution seems to be to start bleeding off exhaust through the wastegate as the RPM rises, but let the poundage fall, so that the shaft speed of the turbo stays the same.

Man thats going to be a pain in the arse to figure out.

I remember someone posting a link (not sure if it was here or on another forum) to an optical device which can measure the shaft speed of a turbo. Perhaps you could design a boost controller which uses this as an input, thus guaranteeing the turbo never spins faster than your pre-determined rev limit?

[takai]

Hm, interesting, ill go have a look.

[SL666]

i thought 'cavitation' was to do with water.. specifically it boiling?

i would asusume that turbo's might surge pretty badly if they run out of air.. but im not even sure that would happen.. NFI....

[Ben Wilson]

Anyway, what i was more getting at is that the 360hp (most figures ive heard have been around 360hp rather than 380hp, but it always varies) is restricted specifically by the amount of air that the turbo can flow. Now, theoretically the turbo could flow that amount of air at a lower RPM and therefore have more boost. However, as the engine demands more boost it tends to try and pull more air.
From some further thinking i have done the solution seems to be to start bleeding off exhaust through the wastegate as the RPM rises, but let the poundage fall, so that the shaft speed of the turbo stays the same.

Man thats going to be a pain in the arse to figure out.

Exactly - I don't have any dyno plots with me, but most of the restricted EVO charts I've looked at hit around 20-22lbs by about 3000RPM and steadily drop off as the revs increase. (peak torque around 3500-4000, all over by 6000).

To get all technical, it isn't cavitation it's choking because the local flow goes super-sonic. If you have the restrictor far enough away from the compressor inlet so as to let the airflow settle down again it'll work fine. The closer the diameter is to the compressor inlet, the closer you can have the restrictor.

You also get a simple vacuum between the turbo and the restrictor. The big problem is once you get a vacuum, there is less load on the turbo so it gets faster generating more vacuum and so on - things get ugly very quickly. You can easily hear a car running too much boost by the turbo whistling as is overspeeds....

[Pube]

Heres a bit of useless info, gas turbine engines can cavitate and destroy engines. However in the aircraft industry we call it a surge. I have seen it happen during engine testing where the aicraft is stationary on the ground and max takeoff power is applied and the aircraft is not facing directly into the wind. As the engine pulls air into the intake, the air can simply run out as seen by the intake begining the cloud up. As there is no more air in the front of the engine, the fan blades draw air fom behind the blades. This can cause an immediate stall and damage to the compressor blades and also a loud bang. Thats why it is important to face a gas turbine into the wind when running. Anyway thats my story for the day.

[takai]

Ah didnt know that, i always thought that they launched into the wind to allow for extra lift of the wind speed adding to the takeoff speed.

[SL666]

if the plane is moving, its making its own wind so its not an issue.. only for static testing..

big turbo's get compressor surge when they are trying to pump air at a pressure/flow thats off their effeciency island..