ORIGINAL: proptop
FWIW, CP's got an interesting thread going in the 1/2A forum...he's checking the timing figures of several engines.
I'm just wondering now if maybe it's the carb and/or the size of the venturi that would limit the r.p.m. in a dive? (pumping losses or words to that effect? )...the engine couldn't suck in enough air to allow it to spin any faster? That in conjunction with the timing.
I think that's what keeps some engines from blowing when they have a shaft run...the venturi size is such that it gets to a certain r.p.m. then chokes. (I know there are other considerations such as cross sectional area of the port runners, runner saturation, crank's I. passage, the pipe, etc. etc. )
Just out of curiosity, does anyone have the timing #'s for the Rossi .53? Being a Rossi, I don't think it's outside the realm of possability that it could turn to 20K (give or take a little ) in a prolonged WOT dive and not fly apart?
Maybe Speedboy could do another video, and do some dive tests...gradually increasing the angle of the dive, and you could do the analysis of the r.p.m. (right now, I'm picturing Clark Gable in "Test Pilot" or maybe Fred McMurray in "Dive Bomber"
)
From another post.
The Rossi 53 has very mild exhaust timing. I also used a degree wheel to document the timings (by the way, TDC is 0, BDC is 180 on a 360 degree wheel). Here are the results.
Exhaust timing 160 (open at 100 closes at 260) changed this to 170 (open at 95 closes at 265) I opened up the top center middle third of exhaust port - rounded.
Intake timing 134 (open at 113 closes at 247) left this alone
Crank intake 208 (open at 212 closes at 60)
Blowdown 13 degrees, after exhaust timing change it increased to 18 degrees
Since the exhaust mod alters the compression ratio, I decided to document this as well. I used the timings to position the piston in the cylinder and measured the before and after compression strokes. I also used a buret to measure the volume of the combustion chamber. Here are the specifics on that:
Button volume (combustion chamber) .5 CC
Bore size .876
stock compression stroke .585
new compression stroke .545
shim .005
quench area (area between the piston and the button at TDC) .018
I put all of this data into a compression calculator and this is what I got:
Stock compression 8.94:1
New compression with shim installed 8.4:1
New compression with no shim installed 8.94:1
I opted for the no shim installed configuration since it put the compression about where it was before the exhaust timing mod. I also changed the glow plug to an OS #8 and kept the 5% nitro 18% oil Power Master fuel. I decided to see just how fast the motor would spin after the mods, so I installed a small (sorry no markings) prop and tached it at 22,500 RPM
I then installed the 10x7 and tached it at 16,300 RPM . Much better! ThrustHP calculates HP at 2.165, not 2.3 but I can live with that number for now. I wonder how much a full pipe will help?
Rick
< Message edited by 5.0Rick -- 12/8/2007 12:39:30 AM >