ESE's works engine tuner

[Frits Overmars]

I machined a head with a roof that matched the slight dome of the piston crown, so that the chamber was a simply a slightly dished disc of constant thickness. I made it the same volume as the two heads I'd previously run, the first one having a traditional 50% squish band and a roughly hemispherical central bowl. The second had about a 35% area band and a low flat roof and performed slightly better.
On the dyno the new, open chambered head was surprising. It wanted exactly the same spark timing as the other heads and torque and peak hp was about 3% or so higher. Also surprising was the fact that it wanted significantly less fuel - about 4% or 5% less. However it wouldn't tolerate richness like the other heads would. Where they would run with jetting in excess of the optimum with little loss of power the open chambers output would drop much more rapidly with excess fuel. I don't know if these results are a weird anomaly or not but I was pretty happy with it overall.

How did you establish the fuel consumption Johnno? By jet size, by measuring the fuel flow velocity, by weighing the consumed quantity of fuel per minute?

[oldjohnno]

How did you establish the fuel consumption Johnno? By jet size, by measuring the fuel flow velocity, by weighing the consumed quantity of fuel per minute?

Ah, you got me. It was by jet size, so I can't really say it used a lesser mass of fuel. Still, it'd take a pretty significant increase in venturi signal to produce the same flow with the smaller jet. I should retest with better measurements.

[Wos]

I've long been interested in chamber design. In the four-stroke world modern engines with shallow pent-roof chambers utilise tumble to promote fast flame propagation, with little or no squish/quench. Spark timing is typically around 20deg for these, about half of that used in many older closed-chamber engines.

I always thought the transfer flow in a 2T would promote as much motion as these current 4Ts, possibly more. As an experiment I machined a head with a roof that matched the slight dome of the piston crown, so that the chamber was a simply a slightly dished disc of constant thickness. I made it the same volume as the two heads I'd previously run, the first one having a traditional 50% squish band and a roughly hemispherical central bowl. The second had about a 35% area band and a low flat roof and performed slightly better.

On the dyno the new, open chambered head was surprising. It wanted exactly the same spark timing as the other heads and torque and peak hp was about 3% or so higher. Also surprising was the fact that it wanted significantly less fuel - about 4% or 5% less. However it wouldn't tolerate richness like the other heads would. Where they would run with jetting in excess of the optimum with little loss of power the open chambers output would drop much more rapidly with excess fuel. I don't know if these results are a weird anomaly or not but I was pretty happy with it overall.

In the 4T world the trend has long been away from squish and towards shallow, open chambers. Mainly driven by emissions requirements I think and the need to reduce UHCs. With these it's not squish that's the problem, it's the quench that goes along with it. And because the two are inseparable it's hard to tell what causes what. I kinda suspect the benefits of tight clearances are as often as not because of the reduced quench volume rather than mixture motion.

My understanding

Mixture motion or call it turbulez...the higher the better.

Made some reserch at fraunhofer to burning behavior of coal dust in air

Result was exactly same

One coal particle somewere is beginning the process of burning...the higher its "SPEED" the more other particles crossing its way and are ignited too...and so on.

small number of particles need more turbulez...

Lot of particles and high turbulenz was leading to explosion ...

Reason for research was to avoid accidents in coal mines


Grüße Wolfgang

[jonny quest]

Why did the oems use cl8se to 2mm of squish on YZ250 and CR250?

[philou]

reliability.

accepts any fuel. even expired

[Wos]

Why did the oems use cl8se to 2mm of squish on YZ250 and CR250?

Thats a good question

Yz has a huge squish gap...
after reducing to about 1mm same compression the Charakter was very aggressiv...difficult to handle that power for amateur user

And that while The yamaha ignition curve is very conservative...only 23 degree to 3500...then falling quickly

Exhaust timing was low too...if i remeber correct about 184

Easy to make a monster out of th 250 crossers 🤪

Grüße Wolfgang

[Frits Overmars]

Why did the oems use cl8se to 2mm of squish on YZ250 and CR250?

reliability. accepts any fuel. even expired

I beg to differ. Less squish makes the engine less forgiving, not more.

[Wos]

Why did the oems use cl8se to 2mm of squish on YZ250 and CR250?

I beg to differ. Less squish makes the engine less forgiving, not more.

Frits,

Explain less forgiving thanks


Do you mean the tolerances and reliability? Think with 1mm gap should be ok !?

Too my fellow, the engine was brutal...shot him directly on a overseas container nearby the cross track😯🙉

We continued with a fantic 125... yz motor😉✋

[Wos]

Frits,

Explain less forgiving thanks


Do you mean the tolerances and reliability? Think with 1mm gap should be ok !?

Too my fellow, the engine was brutal...shot him directly on a overseas container nearby the cross track😯🙉

We continued with a fantic 125... yz motor😉✋

Frits!

I am thankful for all your given advice here...our teacher

Question

How would you solve the beastyness of an 250 market given crosser?

Enlarge gap? Guess no!?

Smoothen with ignition curve and reducing compression...
Another pipe?

And how would you create/ optimice a market given 250cross single ...as used by factory teams ....for a real pro Champion?😯🤪👍

Exiting Questions to me!

Thank you very very much!

Grüße Wolfgang

[wobbly]

I think we have a mix up in terminology.
First there is how forgiving the engine is mechanically , that speaks to how sensitive it is to the fuel/tune etc.
Having a squishband that effectively " doesn't work " by being too big , ie low MSV ,makes it more prone to detonation.
But on the other side of the coin , having aggressive squish velocity via high MSV makes it very sensitive on the throttle response ie harder to ride for a novice.

[jonny quest]

The YZ is a full on race bike. The CR is a full on race bike. The KTM SX is a full on race bike.

Makes no sense to me why the squish is so large. It's not manufacturing tolerance either. There is a reason they do it.... I haven't heard a good reason why.

[Frits Overmars]

Frits, Explain less forgiving thanks. Do you mean the tolerances and reliability? Think with 1mm gap should be ok !?

With 'less forgiving' I meant: less tolerant of sub-optimal fuels, for example fuels that have been exposed to the outside air for a long time and have lost a large part of their volatile components. And also less tolerant of mixtures with sub-optimal fuel-air ratios, of sub-optimal ignition timings and of sub-optimal (i.e. high) engine temperatures.
1 mm squish gap should be OK for a 250cc MX single, as most of them have a 72 mm stroke. If it were my own bike, I would narrow the gap even further, to 1% of the stroke.

How would you solve the beastyness of an 250 market given crosser? Enlarge gap? Smoothen with ignition curve and reducing compression... Another pipe?

I would not dream of enlarging the squish gap. I want the fastest combustion possible, which will permit me to ignite it late and so minimize any pressure rise before TDC.

Smoothening the ignition goes without saying: find the ignition timing curve that gives the best power curve, simple as that.

Reducing the compression ratio will make the bike more rider-friendly in most circumstances, and that includes road racing at Grand Prix level, as we learned when the factory Aprilias were compelled to switch from heavy-leaded fuel to no-lead fuel.

A different pipe? Absolutely. We (i.e. my friend Jan Schäffer and yours truly) performed a lot of tests and the best results came from pipes that practically 'castrated' the engine: several horses were gone missing. But throttle response became so much smoother that lap times improved, and the rider's condition was less strained during the course of the race. It proved a 'golden Griff ins Klo' as you Germans would call it, not just for kiddy bikes like the KTM 65SX, but also for top level riders.

EDIT: I see that Wobbly outbraked me. In my defense I can argue that it's two hours past midnight where I live while he's already had his first saturday morning coffee. Cheers Wob.

[husaberg]

Why did the oems use cl8se to 2mm of squish on YZ250 and CR250?

Parts variation. So if the worst combination of parts is used it will not hit anything.
i.e longest conrod rod with the shortest head and cylinder the lowest deck height crankcase and the tallest piston with the thinnest base and head gasket. You will not end up hitting the head when it's revved in anger.
That's why instead of the intended factory intended .9mm or whatever, you end up at 2mm or worse.
If you are racing a class where you have to use std parts or measurements. You use these tolerances to gain the best possible advantages or combinations.
Or you can normally modify them to gain the best of these acceptable tolerances.

example
https://www.kartengineshop.com/user/...cc%20Mini2.pdf
https://www.kartengineshop.com/user/...c%20KZ10-C.pdf

These would be a good source of info if you say wanted to map and engine for one of the tunning programs.

[Wos]

With 'less forgiving' I meant: less tolerant of sub-optimal fuels, for example fuels that have been exposed to the outside air for a long time and have lost a large part of their volatile components. And also less tolerant of mixtures with sub-optimal fuel-air ratios, of sub-optimal ignition timings and of sub-optimal (i.e. high) engine temperatures.
1 mm squish gap should be OK for a 250cc MX single, as most of them have a 76 mm stroke. If it were my own bike, I would narrow the gap even further, to 1% of the stroke.

I would not dream of enlarging the squish gap. I want the fastest combustion possible, which will permit me to ignite it late and so minimize any pressure rise before TDC.

Smoothening the ignition goes without saying: find the ignition timing curve that gives the best power curve, simple as that.

Reducing the compression ratio will make the bike more rider-friendly in most circumstances, and that includes road racing at Grand Prix level, as we learned when the factory Aprilias were compelled to switch from heavy-leaded fuel to no-lead fuel.

A different pipe? Absolutely. We (i.e. my friend Jan Schäffer and yours truly) performed a lot of tests and the best results came from pipes that practically 'castrated' the engine: several horses were gone missing. But throttle response became so much smoother that lap times improved, and the rider's condition was less strained during the course of the race. It proved a 'golden Griff ins Klo' as you Germans would call it, not just for kiddy bikes like the KTM 65SX, but also for top level riders.

EDIT: I see that Wobbly outbraked me. In my defense I can argue that it's two hours past midnight where I live while he's already had his first saturday morning coffee. Cheers Wob.

Frits🙃

We dont outbreak us here on that superior forum... NO NO NO👍😂

many thanks to wobbly too...many thanks to you Frits for very detailed answers 👍👍😎

[Wos]

Parts variation. So if the worst combination of parts is used it will not hit anything.
i.e longest conrod rod with the shortest head and cylinder the lowest deck height crankcase and the tallest piston with the thinnest base and head gasket. You will not end up hitting the head when it's revved in anger.
That's why instead of the intended factory intended .9mm or whatever, you end up at 2mm or worse.
If you are racing a class where you have to use std parts or measurements. You use these tolerances to gain the best possible advantages or combinations.
Or you can normally modify them to gain the best of these acceptable tolerances.

example
https://www.kartengineshop.com/user/...cc%20Mini2.pdf
https://www.kartengineshop.com/user/...c%20KZ10-C.pdf

These would be a good source of info if you say wanted to map and engine for one of the tunning programs.

I do not think this is the reason today, even when many parts and tolerances are involved.

For older days you may be right 😉

Diffrent bikes differ todoy only about 0,1 to 0,2 mm in serial gap these days...

Specially with undomed piston so so easy to measurue in production..to be save...

Grüße Wolfgang