CM871 vs CM2250 bottom end

[Signature620]

The compressor is not interchangeable. 871 compresor has 4 bolts going into the gear housing, whereas the 2250 has 1 stud and 1 long bolt from the gear housing. If I were you I'd buy a long block and clean, then change over the external components. That way there's no guesswork and the important bits are covered by Cummins.

[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

[Rawze]

(03-16-2017 )Running rough Wrote:  ...
my 2250 is running soi increment at 3 with a 600 2050 cal, with the 871 turbo,
...

I would be very careful of this. Even though the efficiency goes up, I would be very cautious of injector spray angles on the 2250. they are much wider than a 871 and you could cause problems real fast by advancing the timing by very much. The best way to tell where you are at is to pull the injectors out of an engine that has run at a specific place for a while and see what the shadow on the pistons look like. If they are up at the outer edges of the bowl, you are pushing your luck. I.E> you can only get away with so much before high risk sets in. it is NOT just simply soi vs compression ratio vs other factors. All the mechanical traits come into play too.

In this picture, the engine was set for a master SOI offset of 8.0 on a CM871 that had the smaller injectors. The rest of the injection mapping was factory stock. It ran like that for 700k miles and it never caused an issue but it was at risk. On its own it was still safe, but if it had an injector that had gotten clogged, and the spray pattern had changed, it very well could have washed out a liner. The fuel mileage gains were very high from doing this, a solid +2-3 mpg but at what risk?. - Sometimes better is not always better. Too much of a good thing can get you.

(03-16-2017 )Running rough Wrote:  my question is what is the stock soi at low ideal 700 rpm for this engine
...

I am assuming you are still referring to the 2250 you have the 871 turbo on. - if you want to find this out then watch the live fuel flow into the engine and manually adjust the injection mapping. You will find a point where the mg/stroke is minimized and it uses the least amount of fueling. You will also find a place where the valve-train starts to become very loose (you can hear it). - Best place is to be somewhere between these 2 for optimal idle. You can get away with this and not not worry about spray angles etc. because you have no torque load much on the engine. It will not hurt it a bit, but you had better believe as you go up in torque load that it will all change. You also have to start considering all the mechanical aspects like spray angles, lash etc. too once you move away from idle.

BTW: the 2350 is even worse. Spray angles are very high on them and you can't get away with s$it on them to optimize much at all. Those idiots with the big-name tunes are killing these engine with their added +12 degrees bulls$$it to the soi. It is downright ridiculous!.

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it seems stock timing is not as snappy on my 2250, I put soi increment off and torque response is reduced a bit but it is still very drivable, in fact it feels like I can work the engine now without fear of a vibration and such, it will still pull like a mule under full trottle it just seems you have to work the throttle a little more to do the seem thing, I like the quieter ideal as well, but if I really wanted to tune it in and get my response back I would do so above ideal rpm and increase the soi by the seem 3 degrees from low to about 50 percent fueling and leave the heavier fueling stock, that would give a guy a sharper torqueier throotle response but like I said it is pretty good still all on its own, I might leave it, but I am not quite sure what mode my 2250 runs in, but I believe my 871 runs in mode4

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...and it seems my 2250 does as well, I might have over reacted a bit by saying it is not as responsive, it is still Very responsive in what I believe is stock timing, still feels like it would blow a stock 2250 out of the water, got the turbo set where in my opinion it will almost spool as fast as the stock 2250 turbo, however it is clear to me now that this 871 turbo will build you a better overall power band with the right tune, it seems the stock turbo was quick to spool and built good low end but couldn't move enough flow for good mid to top end power, the core 2250 engine when the liner heights are where they should be with the updated xpi pump I think but still not proven, should be a not bad engine, I can tell you the stock tuning for them is nothing special and even cummins admits to that, they should just smarten up and help the poor owner operater that is stuck with the factory lemon flaws, I can promise you if the liners and block fret on my shimed up 2250 that will be the end of my liking for the cummins engine