VIDEO: Why the FP Red needs the 80mm cover!
Jun 1, 2010, 06:51 PM
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I think there is a lot of merit for using port shrouded compressor housings on turbochargers that are expected to work over a wide turbo and engine speed range .
To get a turbocharger to spin up (spool if you like) early enough in an engines speed range to make some low end torque has some compromises . Generally sufficient gas speed through the turbine blades at lowish engine speeds is the achilles heel because to git it here sometimes means having an exhaust gas restriction (pressure rise) in the turbine housing .
We are very lucky that Mitsy went to the trouble and expense to give us twin scroll twin integral wastegate turbos systems since Evo 4s because it makes a significant difference in the spool low exhaust restriction stakes .
What I'm about to say is my interpretations of port shrouded compressor housings and none of this is from Roberts or FPs mouths .
Garrett at turbobygarrett.com has a useful cross sectional diagram of a port shrouded compressor end .
If look you can see that the compressor wheel has two groups of blades sometimes called the full height blades and the lower splitter blades .
You'll also see that the leading edges of the splitter blades is just below the inner wall of the radial slot - surge slot .
From this you can work out that all the air that the splitter blades can pump is going to go through the housing and downstream to the throttlebody .
Not so with the full height blades because the air they are churning has an alternate escape rout through the slot and back to the front of the housings snout or boss .
Compressor surge is a situation where the compressor is turning at sufficient speed to pump more air than the engine can ingest at a given engine speed .
At low engine speeds the inlet tract itself is more than capable of charging the cylinders but they can't use all the air being pumped in . Usually you get pressure fluctuations in surge conditions and if they get fast enough and violent enough they can very easily damage compressor blades to the point of failure/fragmentation if you don't do anything about it .
If you think about it you get a pressure differential between the heights of the full and splitter blades when a turbo surges and this is where the slot can allow some of the air being pumped out through the slot if sufficient pressure exists in front of the splitter blades . I see it as a means of partially unloading the compressor wheel which is a bonus in itself because it helps prevent compressor surge and allows the wheel to turn faster at this stage so improving "spool up" - less wasted turbine energy or slip losses through the turbine blades .
I have read that when a turbo is passed the stage it wants to surge the slot can offer an extra feed path to the full height compressor wheel blades which is important because all you have at best is atmospheric pressure to feed the compressor and its housing with .
Also the boss size on PS housings is often larger than the plain ones meaning you get to use a larger diameter higher volume duct - big path for low atmospheric pressure to feed the turbo with .
As per the Mack Symphony , compressor blades travel at high speeds and like to give off a sonic shriek at the tips a lot of which is muffled by non PS housings .
HKS often fitted custom PS compressor housings to Garrett turbos for certain car/engine specific apps such as Nissan RB six cylinders . Examples are 2530KAi GTRS GT2835 Pro S GT3037S and Pro S TO4Z and their T51/R KAi and SPL .
Some of you would have seen the bell mouthed insert HKS use in the GT3037S PS'd turbo and that may have in part cut down the whistling sound PS comp housings give off .
I think essentially port shrouded compressor housings do more good than bad and don't worry too much about the odd percentage point of compressor efficient lost . I mean you wouldn't use a port shrouded comp housing on a turbo that wasn't going to surge but if it did other methods of getting around it have worse side effects IMO .
Bottom line is that I reckon the ported housing lets you use a larger/higher flowing comp wheel necessary to make power up high and still let you have some response and torque from boost in the lower engine speed ranges - particularly with a twin scroll turbocharger/split pulsed exhaust manifold .
Cheers Adrian .
To get a turbocharger to spin up (spool if you like) early enough in an engines speed range to make some low end torque has some compromises . Generally sufficient gas speed through the turbine blades at lowish engine speeds is the achilles heel because to git it here sometimes means having an exhaust gas restriction (pressure rise) in the turbine housing .
We are very lucky that Mitsy went to the trouble and expense to give us twin scroll twin integral wastegate turbos systems since Evo 4s because it makes a significant difference in the spool low exhaust restriction stakes .
What I'm about to say is my interpretations of port shrouded compressor housings and none of this is from Roberts or FPs mouths .
Garrett at turbobygarrett.com has a useful cross sectional diagram of a port shrouded compressor end .
If look you can see that the compressor wheel has two groups of blades sometimes called the full height blades and the lower splitter blades .
You'll also see that the leading edges of the splitter blades is just below the inner wall of the radial slot - surge slot .
From this you can work out that all the air that the splitter blades can pump is going to go through the housing and downstream to the throttlebody .
Not so with the full height blades because the air they are churning has an alternate escape rout through the slot and back to the front of the housings snout or boss .
Compressor surge is a situation where the compressor is turning at sufficient speed to pump more air than the engine can ingest at a given engine speed .
At low engine speeds the inlet tract itself is more than capable of charging the cylinders but they can't use all the air being pumped in . Usually you get pressure fluctuations in surge conditions and if they get fast enough and violent enough they can very easily damage compressor blades to the point of failure/fragmentation if you don't do anything about it .
If you think about it you get a pressure differential between the heights of the full and splitter blades when a turbo surges and this is where the slot can allow some of the air being pumped out through the slot if sufficient pressure exists in front of the splitter blades . I see it as a means of partially unloading the compressor wheel which is a bonus in itself because it helps prevent compressor surge and allows the wheel to turn faster at this stage so improving "spool up" - less wasted turbine energy or slip losses through the turbine blades .
I have read that when a turbo is passed the stage it wants to surge the slot can offer an extra feed path to the full height compressor wheel blades which is important because all you have at best is atmospheric pressure to feed the compressor and its housing with .
Also the boss size on PS housings is often larger than the plain ones meaning you get to use a larger diameter higher volume duct - big path for low atmospheric pressure to feed the turbo with .
As per the Mack Symphony , compressor blades travel at high speeds and like to give off a sonic shriek at the tips a lot of which is muffled by non PS housings .
HKS often fitted custom PS compressor housings to Garrett turbos for certain car/engine specific apps such as Nissan RB six cylinders . Examples are 2530KAi GTRS GT2835 Pro S GT3037S and Pro S TO4Z and their T51/R KAi and SPL .
Some of you would have seen the bell mouthed insert HKS use in the GT3037S PS'd turbo and that may have in part cut down the whistling sound PS comp housings give off .
I think essentially port shrouded compressor housings do more good than bad and don't worry too much about the odd percentage point of compressor efficient lost . I mean you wouldn't use a port shrouded comp housing on a turbo that wasn't going to surge but if it did other methods of getting around it have worse side effects IMO .
Bottom line is that I reckon the ported housing lets you use a larger/higher flowing comp wheel necessary to make power up high and still let you have some response and torque from boost in the lower engine speed ranges - particularly with a twin scroll turbocharger/split pulsed exhaust manifold .
Cheers Adrian .
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