Douglasmotorcycles.net
General => General Douglas Discussion => Topic started by: Frank Lyn on 01 Jun 2014 at 00:10
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Hello All,
While getting my 1910 C running for the spring, I measured the compression. The numbers seem rather high but are influenced by the throttle position as only an engine with automatic inlet valves can be.
Throttle Closed: 45 psi
Throttle Open: 90 psi
I scaled the compression height from the drawings in the manual and it comes out to be 25mm, Even though my pistons are not original, they measure 24mm (center of pin to top of piston) so this seems about right (I will yield a few mm plus or minus on these measurement.)
These pressures seem more consistent with mid 30's technology rather than for 1910. I just get the feeling that it would start easier with a lower pressure (easier to turn over and lower voltage on the mag).
Does any one know what it should be for 1910? My method is to pedal on the stand and drop the compression release. Maybe I am spinning too fast when I do this? (Note, I ground the ignition wires to protect the magneto.)
thanks,
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wow 90 lbs sounds pretty good.....for 1910
might check that against something else...another motor... my shovel head...one day went to just see what it was doing for pressure
kicked that thing till I just about blew my own ring out...and I could not get 60 lbs...it wasn't the fastest bike...but ran just fine... no smoke easy start...rev right out real nice...
sooo ....whaddya think was the problem...??worn rings? circlip fall out and score the cylinder?...cracked piston?? .leaky valve..?? blown head gasket???
nope...I had a bad pressure gauge...
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Hi Frank,
I've never measured the compression pressure of a motorbike, but if I did I'd expect it to vary with throttle opening, regardless of whether it had atmospheric inlet valve or not. If the throttle is closed (tight), no/little air will enter the cylinder as the piston falls, there will be no/little air to compress as the piston rises, so the pressure in the cylinder will be relatively small at TDC. With the throttle fully open, there should be good filling of the cylinder, and thus good pressure when compressed.
So let's take 90 psi as the pressure with the engine spinning. To guess the compression ratio, we need to know a couple of things, particularly: when does the inlet valve close? With atmospheric inlet, probably just a little bit after BDC. (If the motor were from a DT or 90-Plus the inlet wouldn't close until well after BDC, so the pressure would be less for the same "theoretical" compression ratio.)
We also need some physics (sorry). If we assume the air in the cylinder is compressed adiabatically (no heat enters or leaves the system during the compression stroke - assumed in an Otto Cycle), 90 psi corresponds to a compression ratio of about 4:1, which seems entirely appropriate for a 1910 Douglas. (I know it's tempting to think that 90 psi means a compression ratio of 6:1 or more, but that's not how it works.)
In practical terms, if you pull the back wheel around you should feel quite strong resistance as each cylinder comes up on compression, but if the belt doesn't slip it should be (just) possible to pull the motor over each compression.
You're a lucky man to own such a fine little motorcycle: hope the riding season goes well for you.
Cheers
Leon