Which piston to use in my 85 - Saab Performance Bulletin Board - Saabnet.com


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Which piston to use in my 85
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Posted by 85turbosled [Email] (more from 85turbosled) on Sat, 17 May 2014 12:34:59 Share Post by Email
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Ive been slowly gathering parts for my 85T from trips to wreckers and such. Ive been tuning with this 85 for a few years and have done fmic, injectors, 3" downpipe mated to ATP turbo wastegate housing, volvo management, ported out manifold, cams, big oil cooler, blah blah...anyways makes 58 more at the wheels from when I started. I havent changed APC the boost pressure as of yet.

...time for my 2.3 head now, as ive now acquired a 2.1 manifold/fuel rail as well.

problem is that installing this 2.3 head onto my 85 with 11cc piston dish will lower compression to around 8.15:1 as we all know and this isnt going to fly. I like to have good off boost power and this would be disappointing...

so what piston to use to counteract the 2.3 head bigger combustion cc?

based on some basic (and maybe wrong) math, assuming the piston compression height stayed the same:
>86 turbo piston: 7 or 8cc, yields 8.7:1 CR
>86 NA/S piston: 1 or 2cc (almost flat??) yields 9.7:1
91-93 turbo piston came out to 17cc assuming same compression height so that cant be right...cant remember seeing THAT much dish in that piston...

so anyone have experience with this? my thoughts at this point is to maybe use the >86 turbo pistons at 8.7:1 and possibly take 1 or maybe 2mm off the head surface to bring compression ratio back to 9.

another possibility would be to take NA pistons to a shop and have them machine dish into the crown to achieve what I want, bit I dont know if the crown has adequate thickness to do this however.

posted by 24.207.15...

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