Hi. I'm new here and found the site via DEF and reading his excellent article over on bimmerforums on reading compressor maps.

I have a redtop SR20DET in a Peugeot 505 and recently bought a rebuilt GT2871 52 trim/.64. Then I read an article over on boostcruising where the author stated that the 2871 is basically a mismatched turbo and that the 71mm comp wheel is too big for the 53.8mm exhaust wheel and slows the spool of the exh wheel too much.

He mentions Garrett's recommended comp/exh wheel ratio of between 1.1:1 and 1.25:1 and points out that the 2871 has a wheel ratio of 1.32:1. Going by this formula the max comp wheel size that would suit a 53.8 exh wheel would be a 67.25mm wheel.

Sorry for being long-winded but I'd like DEF's and other's opinions of the idea of changing the 71mm comp wheel on the 2871 for a 67mm Garrett GT wheel. I tried to source a 69mm wheel as on the HKS2535 but have been told they are not available. I've posted my calculations for the SR20 below using DEF's calculator and drawn them onto the flow charts of the above wheels as well as the GT28RS disco.

I should say that I don't want to go to cams at the moment and only use the car on the street and am more interested in response than max hp but as the 67mm wheel flows 40lb it should be able to flow 400hp or so. From the maps would the 67mm wheel be a good midway point between the disco's response and the 2871's topend?

I have a redtop SR20DET in a Peugeot 505 and recently bought a rebuilt GT2871 52 trim/.64. Then I read an article over on boostcruising where the author stated that the 2871 is basically a mismatched turbo and that the 71mm comp wheel is too big for the 53.8mm exhaust wheel and slows the spool of the exh wheel too much.

He mentions Garrett's recommended comp/exh wheel ratio of between 1.1:1 and 1.25:1 and points out that the 2871 has a wheel ratio of 1.32:1. Going by this formula the max comp wheel size that would suit a 53.8 exh wheel would be a 67.25mm wheel.

Sorry for being long-winded but I'd like DEF's and other's opinions of the idea of changing the 71mm comp wheel on the 2871 for a 67mm Garrett GT wheel. I tried to source a 69mm wheel as on the HKS2535 but have been told they are not available. I've posted my calculations for the SR20 below using DEF's calculator and drawn them onto the flow charts of the above wheels as well as the GT28RS disco.

I should say that I don't want to go to cams at the moment and only use the car on the street and am more interested in response than max hp but as the 67mm wheel flows 40lb it should be able to flow 400hp or so. From the maps would the 67mm wheel be a good midway point between the disco's response and the 2871's topend?

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