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!

Anyway, I am pretty certain it has to do with the rpm range of the engine.

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

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.