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Medium Compression K20

Engine Building Fundamentals

Discussion and questions related to the course Engine Building Fundamentals


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So I have a stock K20A2 which is 11:1 which I'd like to build with some new pistons. Today it's got a JRSC kit based around an Eaton M62, that might make way for the much improved TVS1320, however it's taking 10psi today.

A lot of the low compression pistons seem to be 9:1, which feels a little low to me. I was going to ape the later turbo K20 used in the present CTR, and go down the 9.8:1 (or roughly 10:1).

Thoughts?

Your decision on compression ratio is going to be driven by the octane of the fuel you are running on and the boost pressure you want to run. The K series engines do seem to work quite well with slightly higher compression ratios than you'd conventionally choose. If you're running on a pump fuel then I'd be inclined to drop the compression ratio down to approx 10:1 if you're expecting to increase boost/airflow.

yeah so today it's mapped and run on 98 RON (UK & Europe use), and sees about 10psi boost from a old style M62 blower via tiny chargecooler plate. It suffers from IAT temps, as the blower is way out of it's efficiency range in terms of PR to an extent but mainly overspeed. Plan is to move to a TVS1320 blower, so 30% larger and capable of high speed adiabatic efficiency.

I suppose the bulk of the pistons on the market are for people looking to beat on them with big boost via a turbo using street fuel, so 9:1 makes sense there. However I'm looking for about 16psi, and want some headroom down from the stock 11:1 but without killing it.

Key is finding someone reputable piston wise that can get me close to that.

Most of the aftermarket piston manufacturers should be able to supply a piston that has a customised CR, however with such a popular engine you'll likely find what you want or at least something quite close off the shelf. For example JE list compression ratios of 9.0:1, 10.0:1, 11.6:1, 11.7:1, 12.0:1 and 12.5:1 although not all are suited to boosted applications.

Found an arias application with zero dome/dish which will get me to 10:1

just make sure with arias that the piston is rated for boost. they change more than I know to describe with a force fed application. the whole piston design generally changes with exception of the dimensions that make it work in your application.

Im building my block with 9.0 CR CP piston i intended to go 2.5 bar with Garrett GTX3076R .