Covering Compressor Covers

[GeeTR]

Hey All,

Have been wondering for a while now; Quiet possibly the whole world knows this, and i missed the memo buuut...

Do the comp cover slots / holes in certain manufactures products negate the need for BOV's?

I believe they are implemented to help alleviate surge / over pressure, the motor cant ingest.

Would this design feature, allow sufficient air to escape through said slot / hole when a BOV would usually operate?

This thought hasn't been born from need for the function, nor am i looking at any divergent setups, im just interested is the physics of it. Surely if it did negate it, OEM's would try it.

Idea's, theory's, suggestions, 2cents appreciated frm anyone

Please no, Uleh omg dose, stock is fine, Goose sex is gr8 responses

Edited February 17, 2008 by GeeTR

[discopotato03]

I think the answer to that one would be no . Port shrouded compressor covers move the surge line on a compressors map usually higher and to the left but this is to avoid compressor surge when the engine can't take all the air the compressor is trying to pump - low volume high pressure low engine speed situation .

A "BOV" is really just a throttle closed bypass valve to vent the air moved by the compressor that has nowhere to go when the throttle valve suddenly closes .

The way I see it the air downstream of the compressor would have to fight is way back through the revolving blades to get anywhere near the comp covers ports and by then the rotating assemblies slowed down so no function gain here .

IMO the GTR's ganged throttle closed recirc valve (2) system is the way to go .

Cheers A .

[GeeTR]

Hey Disco,

Cheers for the reply. Im well versed though, on how basic engine physics. It just seems that...

Port shrouded compressor covers move the surge line on a compressors map usually higher and to the left but this is to avoid compressor surge when the engine can't take all the air the compressor is trying to pump

Is functional similar if not the same as...

A "BOV" is really just a throttle closed bypass valve to vent the air moved by the compressor that has nowhere to go when the throttle valve suddenly closes .

I was thinking maybe the surge slots/holes being positioned right at the comp wheels tips, which allow some leeway with surge, could assist with reversion when the throttle blade closes? Kinda seems like the same effect to me.

I dont intend to change ne thing, nor do I posses a divergent turbo, I'm just interested in the theory. I don't intend to sell my stock bov's ne time soon

Edit:

The way I see it the air downstream of the compressor would have to fight is way back through the revolving blades to get anywhere near the comp covers ports and by then the rotating assemblies slowed down so no function gain here .

My understanding was reversion is exactly that, air thats coming back out the comp wheel blades... hence why surge slots/holes might render a bov unnecessary? you dont think?

Edited February 18, 2008 by GeeTR

[R31Nismoid]

Nah it wont stop the use/function.

What its going to do is alter the surge point, its more low RPM based when a turbo builds boost.

Sometimes due to the size of the comp wheel/exhaust the front is pumping too much air for the motor and you get that bit of surge... its barely noticeable sometimes but you can see it on the dyno sheet or hear it if you have ya beaver in the engine bay to listen.

BOV's are a full release of pressure, there the shroud is working as the buildup of boost begins

[GeeTR]

Yup, I understand the premise of having a comp / turbine wheel mismatch; benefits of surge slots and could imagine the bov works dramatically better. It just.... dunno.

Haha, maybe I'm asking a Q for which i wont accept an answer for lol.

[R31Nismoid]

Well think of the BOV as a full-throttle --> off throttle item

The shroud as a getting back onto the throttle item

Certainly not my best explaination that one lol

[GeeTR]

Ur a regular Martha Stewart lol.

Yah, I think ill just leave the thinking there. Slots are for a more finite over pressure (wheel mismatch)

Cheers

[discopotato03]

If you look closely at a turbo with a port shrouded comp cover you'll notice the lower set of blade tips is just behind the slot in the covers nose or snout . It was explained to me that higher set of blades tips sweep over the slot and so these outer blades don't draw air as well if the pressure difference is significant between the front of the inner and outer set of blades .

I have seen instances where turbos such as GT3082R's (AKA GT3040R's) which are a bit of a mis match have been made to work better with port shrouded compressor covers . I suppose the logic is that if the large compressor (in relation to the turbine) tries to pump too much air and surges , the ported shroud bleeds off some air which goes "around the block" .

Ever noticed how HKS use port shrouds on some of their custom comp covers - particularly for six cylinder apps ?

Some examples are Nissan RB specific GT-RS's/GT2530 KAI/2835 and 3037 Pro S turbos . works for them .

Cheers A .

[R31Nismoid]

Yeah a 3040 was a mismatch (600hp) albeit a very slight one.

Wish i had a shrouded cover to test with 3-4 years ago