Why if the Stihl MS310 and the MS390 are the same as the MS290 [other than the bore and pistons] is it that the MS290's maximum rpms are limited to 500 less than the MS310's and the MS390's?
It would seem to me if all 3 share the same crankshaft and bearings then the MS290 should be able to run at the higher RPM as the MS310 and Ms390.
I don't know the answer, but this is quite common with different top ends on the same bottom end.
Is there a larger carb or different coil? I don't have ipls for any of these saws so no way to look myself. Coil difference would be first guess.
Nope the coils and carbs are the same. I always wondered about why the smaller engine was set 500 rpm lower myself. It seems back to front.
http://www.declicmotoculture.com/fs/eclates/6h3gl-029_039.pdf
The main jet is larger (0.058 vs 0.056) in the 310/390 vs 290...look at the ipl.
Quote from: nmurph on October 08, 2013, 08:27:14 AM
The main jet is larger (0.058 vs 0.056) in the 310/390 vs 290...look at the ipl.
I just had a look its 0.58 and 0.54(high altitude) for all three.
That is not the reason for the higher rpm in the 390. Have a look at the Specs here:
(https://forestryforum.com/gallery/albums/userpics/15459/290_310_390_SPECS.jpg)
The only difference in the engines is the size of the bore. So the smaller 46mm bore of the 290 with 34mm stroke would be a long stroked engine when compared to a 390 whose 34mm stroke would be relatively a shorter stroke when considering its 49mm bore.This means the 390 has more rpms than the 290.But I think the 290 with its longer stroke has a more smoother power band.
Joe
Quote from: joe_indi on October 08, 2013, 01:05:13 PM
Quote from: nmurph on October 08, 2013, 08:27:14 AM
The main jet is larger (0.058 vs 0.056) in the 310/390 vs 290...look at the ipl.
I just had a look its 0.58 and 0.54(high altitude) for all three.
That is not the reason for the higher rpm in the 390...Joe
I didn't say it was the reason, and US models have differing jets for the 290 and 310/390
I can't get the IPL PDF to copy, but the PN's are as follows-
11201215600 .056 (used on the 18B/290 US carb)
11271215602 .058 (used on the 21B310-390 US carb AND on the 19B/non US carb)
11301215600 .054 high altitude
The reason I asked was I am wondering if I can set the rpms higher with out harming the MS290.
Quote from: TheTexasRAT on October 08, 2013, 05:02:34 PM
The reason I asked was I am wondering if I can set the rpms higher with out harming the MS290.
You need to tune it according to the four stroking. The bottom end on the 290 will support the higher rpm's.
The fixed main jet runs in parallel with the adjustable needle valve. Both of them combined set the high speed fuel flow.
I expect that the size of the jets is used more to set the position of the HS needle - in other words, get the HS needle around one turn out.
I've got an 029 with a 390 jug on it and the carb has the .54mm main jet. I can still open up the HS needle to get the engine overly rich.
Perhaps there is something about the power curves of the engines that isn't readily apparent. Maybe the 56.5cc engine has a lower torque curve and the carb works better set for the slower rpm.
Interesting ..........
Thanks,
Brett
The other alternative is the main discharge nozzle in the carb. The nozzle sticks into the air flow fairly far and is oriented away from the venturi and into the straight body of the carb. That means the nozzle is less efficient at lower air speeds.
Perhaps the slightly slower airspeed of the smaller engine doesn't allow the carb to pull enough fuel at peak torque and they found that setting it on the rich side cured the problem. Since 12,500 is above the cutting speed of the saw, they didn't worry about it.
If you wanted to test that theory, you can recess the nozzle closer to the edge of the venturi. That should give you the fatter mid-range and allow you to lean it out to 13,000 (should you wish to run the saw at those speeds).
Terry,
your insight is deep. And even though I can follow your train of thought, I think it will be in the best interest of my MS290 to leave things be as they are, as it is way above my pay to be able to operate at your level of technicality. Thanks for the reasoning, as you have thoroughly convinced me if I want the higher rpms to simply get a saw explicitly designed for them (that way me and the engine can be happy).
saw till you drop!!! Over.
Old thread but in case others investigate and find it...port sizes,shapes and timing differences have significant impact on performance specs.