ESE's works engine tuner

[jonny quest]

Long story short, I put a YZ250 ignition system on a CR250. I have no reference lines.

It ended up being pretty easy to set timing. I initially guessed at it being retarded. I then rode bike and kept advancing it until it stopped accelerating harder off bottom.

I'll dyno it for best performance later, then mark cases, flywheel and stator for future.

It actually runs really good with just seat of pants tuning so far, and starts really easy. I must have done something right.

[TZ350]

.
A hole in one .....



I always kinda thought that the runs on the dyno were to short for any serious trouble but I was wrong, again!

[F5 Dave]

Weight loss program for reciprocating parts.

[wobbly]

Easy to check the CCR now , bonus.

[Wos]

😞...let us learn from 😉 ...what was the reason in your case?

Remember that picture...in our case four mistakes

Dyno load was kept to high, for to long time.

Ignition at peak to early

Powerjet to lean

Thanks! 😉

Wolfgang ✋️

[Frits Overmars]

Dyno load was kept too high, for too long time.
Ignition at peak too early
Powerjet too lean

And in addition, or maybe instead of all of the above:
How thin was that piston crown?
To be safe, it should be 15% of the bore. Judging from the above picture, it was not even half of that.
In a foul-stroke you might have gotten away with it. But not in a two-stroke.

[jonny quest]

And in addition, or maybe instead of all of the above:
How thin was that piston crown?
To be safe, it should be 15% of the bore. Judging from the above picture, it was not even half of that.
In a foul-stroke you might have gotten away with it. But not in a two-stroke.

As someone who works with both 2 and 4 stroke engines... it's amazing to me how much more complicated a 4strk piston is.... and how much lighter they are than a 2 strk piston.

A 2 stroke definitely needs thicker crowns.

[wobbly]

You are onto it as usual Frits , when I first tried the semi flat top piston design in a TM the 4* conical stock piston had an 6mm dome with 3 x 2mm ribs = 15%.
I machined 1.2mm off it to achieve the 38mm Diameter 50% area flat.
That worked on the dyno to test the viability and Hp potential of the head designs, but at the track the dome sunk 1.5mm in the middle the instant it went over 620*C
with 5 full power gear changes ( our normal best power egt at the time ).
Once the com had been automatically reduced it was fine , but now of course - slower.

[lodgernz]

.
A hole in one .....



I always kinda thought that the runs on the dyno were to short for any serious trouble but I was wrong, again!

Jeez that crown is awfully thin.

[husaberg]

Jeez that crown is awfully thin.

I looks like its been nibbled a way a bit by the ignition bunnies

And in addition, or maybe instead of all of the above:
How thin was that piston crown?
To be safe, it should be 15% of the bore. Judging from the above picture, it was not even half of that.
In a foul-stroke you might have gotten away with it. But not in a two-stroke.

That second picture seems to show its a lot thicker than it seems in the first.

[Wos]

I ve forgotten to list our fifth reason for melting the piston.

We converted a yamaha wr 125 enduro to a yz125 cross konfiguration but kept the spring of powervalve of enduro that opend valve later...that leeds to higher compression in high revs

All in all small misstakes

All points fixed now and bike is running like never before 😉😘

Grüße Wolfgang

[Frits Overmars]

You are onto it as usual Frits , when I first tried the semi flat top piston design in a TM the 4* conical stock piston had an 6mm dome with 3 x 2mm ribs = 15%.
I machined 1.2mm off it to achieve the 38mm Diameter 50% area flat. That worked on the dyno to test the viability and Hp potential of the head designs, but at the track the dome sunk 1.5mm in the middle the instant it went over 620*C with 5 full power gear changes ( our normal best power egt at the time ).
Once the com had been automatically reduced it was fine , but now of course - slower.

That's what got me thinking back in the day. In the 1980s the piston crowns of Jan Thiels unbeatable Garelli twins (six straight world titles) adjusted themselves just like you described. But I think it made the bikes faster rather than slower. In any case it made them overrev easier and every rider likes that.

I ve forgotten to list our fifth reason for melting the piston. We converted a yamaha wr 125 enduro to a yz125 cross konfiguration but kept the spring of powervalve of enduro that opened valve later...that leads to higher compression in high revs

To be pedantic: it leads to a higher effective compression ratio but not necessarily to a higher compression pressure because the pipe will have a word or two to say about that, and with the power valve lowering the exhaust port, the pipe will receive less exhaust gas energy to work with.
The late opening of the powervalve may well be a major problem cause, but for a different reason: it reduces the blowdown angle.area, so at high revs hot combustion gases will enter the transfer ducts and drastically raise the cylinder temperature. And when the cylinder gets hotter, so will the piston.

[wobbly]

Right again Frits , in the two projects I have done recently , both using PV that close all 3 ports , it was a real balancing act to adjust the ignition and the PV height to prevent detonation
in the mid range. In the sim you can see the huge trapped pressure in the cylinder created if the PV is too low.
This may in fact make more power at that point, but with the TubMax ( temp of the trapped unburnt squish gases ) thru the roof , destructive deto is inevitable, and difficult to control.
The KZ overeved better as well , but though it gained about 400rpm in the top it lost more than a useable 1000 rpm on the bottom off slow corners.

[Wos]

Dear Frits, dear Wob,

Think diffrence opening of powervalve was only about 1000 reves...yes effektive pressure counts there... but i think that was not MAIN reason for failure

All togther was reason
AND
We tuned ignition peak by riding on the fields...rider had only chainsaw reve measurement , his eyes and feeling at his ass 😉...before we went to dyno ..and he felt peak only about 900 to late... this was the reason we set at powerpeak 3 degree to early ignition ... thats realy a lot to early at this point especially if you have good filling there😮

Grüße Wolfgang

[41juergen]

[QUOTE=wobbly;1131227467]Right again Frits , in the two projects I have done recently , both using PV that close all 3 ports , it was a real balancing act to adjust the ignition and the PV height to prevent detonation
in the mid range. In the sim you can see the huge trapped pressure in the cylinder created if the PV is too low.
This may in fact make more power at that point, but with the TubMax ( temp of the trapped unburnt squish gases ) thru the roof , destructive deto is inevitable, and difficult to control.
@Wob: thank's agin for that feedback, when I played around exactly that happened and after all the leassons learned from you, I went the right pass to cure that...