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Engine basics => Engine set up tips => Topic started by: Alan Resinger on May 31, 2007, 07:33:12 PM
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I got a note from someone recently pertaining to a certain engine. He said that the earlier engines were timed from 130/110 and that later versions had been raised to 140/120. How would that affect the power and the run of the engine?
Just curious.
Alan Resinger
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HI Alan,
It will effect the running of our CLPA engines since we do not have a carb on them. Thst change you referenced raises the rpm where the HP is highest, IIRC. So it would need to be run at a higher rpm for a stunt run with all else considered.
Of course, I could well be wrong, but Randy or someone will nail me if I am! ;D
Anyway, I am pretty certain it has to do with the rpm range of the engine.
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On the Stalkers, this lowers the RPM of the start point of the 2 cycle.
So a higher timed engine will also 4 cycle at a higher RPM.
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I got a note from someone recently pertaining to a certain engine. He said that the earlier engines were timed from 130/110 and that later versions had been raised to 140/120. How would that affect the power and the run of the engine?
Just curious.
Alan Resinger
HI Alan
It changes the point at where the motor makes it torque and horsepower. Typically a higher timed engine with more overlap in the timing will not make power as early as one with low timing. Example a 150 \ 125 degree motor is high timed or some call pipe timing and needs to run higher RPMs to make its horsepower and is not happy at low RPMs.
A low timed motor like 115 / 125 will make its power lower in the RPM band and will not run at high RPMs, as compared to it's counterpart. This is why you can make much more HP out of a well designed 40 than say a ST 60. It is also the reason that a purpose built low timed Stunt engine can make so much torque at a much LOWER RPM. There are other factors involved that affect this such as crank timing, Head shape, compression, etc but in general this is how it works
Randy
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Thanks Randy, That was what I was looking for.
Alan
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Alan and Randy,
...been holding out of this one since it addresses more recent engines than I generally use. Need to fly a lot more to wear out the 'too-many-good-uns" I still have.
Randy mentioned low-timed porting as 115/125? Are those port durations, or opens time/degrees ATDC? If opening time/degrees, durations would be 130° Exhaust, 110° Bypass
Many years ago, I played with a few thoughts which Alan's initial post sounded like it was about.
Port opening (or closing) "lag" is shaft degrees between Exhaust and Bypass(es). "High timed" engines have several degrees more "lag" (which is, per GMA, half of 'blowdown'.) The limit on how close you can have both ports open up may have to do with hot combustion gases torching down the bypass if that port opens too early...
So, how much 'lag' is there on an engine designed to run 18K to 20+K RPM? And, at design or tested peak RPM, how long does that last in fractions of a second? Presume that is the minimum time things can work to vent combustion pressure enough for the bypass to carry the next charge up top?
Time, remember! Actual fractions of a second...
Scale that back to your intended operating RPM for a traditional stunt engine RPM and 4/2 run. It still comes out about 10° "blowdown" - which is 5° shaft rotation lag between EX O / BY O and BY Close /EX Close. For reasons that it is simpler to measure accurately with a micrometer or good vernier caliper than a degree wheel (to me, anyway) I think of degrees before/after Top Dead Center (BTDC; ATDC)
Longer bypass-open time allows more charging to the top end. Later exhaust-opens time extracts more of the combustion energy before venting what's left. For Fox 35s, I reset sleeve timing to 115°X/120° ATDC. That, in duration terms, is 130°X/120°B.
As Randy implies, I doubt I'd see 14,000 on a flywheel, but the torque, break and manners are quite nice. Economy isn't harmed - if anything, most of my Fox35s run longer per ounce than stock, or the great power-modded L&Js (Haven't seen Randy S' Fox 35s, so can't comment. Hear good of them, tho. The Zoot setup handles one of the worst nuisances of the old Fox, so I hear.)
So, the way I've dealt with sleeve timing is part theory, part empiric (using the mfrs quoted power@RPM numbers), and part handwork. Seems to work, within the limited types of engines I've tried it on.
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As Randy implies, I doubt I'd see 14,000 on a flywheel, but the torque, break and manners are quite nice. Economy isn't harmed - if anything, most of my Fox35s run longer per ounce than stock, or the great power-modded L&Js (Haven't seen Randy S' Fox 35s, so can't comment. Hear good of them, tho. The Zoot setup handles one of the worst nuisances of the old Fox, so I hear.)
So, the way I've dealt with sleeve timing is part theory, part empiric (using the mfrs quoted power@RPM numbers), and part handwork. Seems to work, within the limited types of engines I've tried it on.
Hi Lou,
Any chance of posting the mods you do? I have a couple Fox's I'd like to experiment on.
cheers,
Ken