CM871 vs CM2250 bottom end

[Running rough]

(03-14-2017 )Rawze Wrote:  

(03-14-2017 )Running rough Wrote:  it would be interesting to know the compression ratio of my cm2250 cpl3606, i have heard the 2350 engines run at 18.9 to 1 ratio and i heard the new x15 effiecty series runs at 19.7 to 1, that is quite high, i belive 870 and 871 engines run at arount 17 to 1 while the austrailian isx e5 cm2250 runs around 17 to 1 compresion as well

2250 is 17.4:1 (some documents say 17.3:1). blanket statements simply 17:1 - They respond like they are 17.4:1 so that is what I go by.


871 and older compression ratios have never been widely publicized. It is close to the 2250 but definitely responds like it is a bit less overall, hence the low-end torque response is not quite as good on them. I would bet it is 16 something, and this is what the tuning on them reflects too. Newer documentation says 17:1 but it is mostly a blanket statement. - I was told it is more like 16.3 from an ex-instructor but am not sure I trust that 100%.

I did an soi table with stock soi at ideal to 1000 rpm to keep it quite and then just advanced it a bit in the low to 3/4 fueling and then basically stock at full torque, NO soi increment, this is on the 871, but my 2250 is running soi increment at 3 with a 600 2050 cal, with the 871 turbo, my question is what is the stock soi at low ideal 700 rpm for this engine, with soi increment at zero it reads around 4 and with soi increment at 3 it reads 7 and I get that, but what generally would a stock tune for this engine run at ideal, don't get me wrong I think it sounds throatier with the 3 degree timing advance but have not driven it with it at 0, your thoughts