There are some misconceptions about what the larger cover does. It is easy to jump to conclusions that "bigger is better"

The area inside the compressor cover has to be compressed for a turbo to make boost. Increasing the compressor cover area can have a negative affect on HP. It is easier to compress a smaller area than a larger one. I did some back to back tests on a 57trim turbo years ago. Tested the difference between the B cover and E cover.
The smaller B cover made 25ft lbs more torque. The larger cover may have made a slight increase in power at high boost/high-rpm, but low rpm power was definately softened. In case of Mitsubishi turbo I think the only reason they went to a larger cover was to reduce compressor surge. The MIVEC increased boost response to a point where a reduction in compressor surge was needed.

Obviuosly at some point the size of the cover becomes a horsepower restriction. I don't think the 8 cover is at that point. This could be proven if someone installed a EVO8 turbo on a MIVEC, but I doubt anyone will do that.

 

Good subject. Me myself i think i have a bigger cover on my turbo. It has a 2 inch air flow path into the LICP. I dont think stock is 2 inches. Probably more like 1 1/2 inches.

 

HMM, and in the case of bigger turbo's, example a to4s cover vs a to4e?

 

The part I don't get is that you compress a volume of air, not an area. So if you have a smaller compressor orifice, I imagine that would allow you to compress the air within the compressor section more quickly, but what about everything after? My physics is failing me here, but I would imagine that a smaller opening in the compressor outlet won't necessarily help you build boost more quickly.

However, I'm sure that there is a point of diminishing return, where an opening can be too large... it's just a question of how large that ideal opening should be.

 

Must keep in mind that the compression of air results in a temperature increase and turbulence. It is often this temperature increase and turbulence that results in a decrease of actual air-mass regardless of the actual pressure of the gas.

We just finished working with IHI to develop a new 20G "large" compressor housing for our STI series turbochargers. The end result was 300RPM increase in spoolup with no loss in top end power over the existing Mitsubishi design, no changes in compressor wheel, no changes to inlet size.

Cheers,

Gary
Gruppe-S

 

the increase size of the compressor cover was due to the increase size fo the diffuser right? and they made the diffuser larger to build up boost faster... the increased size of the cover is only an ancillary effect...