ESE's works engine tuner

[F5 Dave]

Ahh, it was a long time ago sorry.

Although I remember when Wheels won it. He'd shown up with a shining red hub centre front kawi with a polished fabricated ally tank. We'd never seen anything like it.

But another year it had some farm bike tank perched at a jaunty position and rubbish lids for numberboards!
Again in my notasaccurate memory.

[wobbly]

HELP - I need a 951 SeaDoo cylinder to measure the transfer axial and radial angles.
Or can someone do this for me rather than sending the thing via courier.

[jonny quest]

Give me some reasons why a power valve 250cc motor would have a flat spot in mid range on dyno, and can be felt on the track. (Motocross)

Great bottom end power, goes flat right before PV opens, then hits way too hard when PV opens. I'm currently on a later PV opening which helped on the track, but it still has a flat spot before they open.

Good jetting, good fuel. No piston pin linking to ports, don't believe I have short circuit issue between transfers and ex port.

I haven't tried different pipes on track, but I did dyno a few different pipes, they all had this mid dip.

92-01 CR250R MX application.

[wobbly]

This can be seen in EngMod, and that is how I discovered the main cause, and the fix.
The PV is too low in its fully down position - like close to TPO.
Some PV can simply have the blades shortened, some can have the blade lifted with spacers.

The second issue is that in most cases the low PV position is combined with a mechanical or diaphragm servo, not an electronic actuator.
These system are very limited in the achievable rpm/span and speed, ie it opens way too quickly over a way too narrow rpm differential.
This is why you have a dip in power, as the PV is down too far, creating backflow into the transfer ducts, then it slams open too fast, giving a way too big of a delta in torque over a narrow rpm band.

Then of course there is an even bigger issue when only the main port has a PV, leaving the Aux ports fully open.
Not an issue in a T port, but a big issue in 3 port designs.
Lastly and completely incomprehensible, is KTM who open the Aux valves from the top downward - the reverse of the main blade.
Only because its mechanically easy to do with a gear drive setup between the two.

I was fully sponsored engines/parts for a MX Karting class with KTM125, as we had won 4 straight titles.
Spent 6 months trying to get it to work, never could even with an electronic servo.
Gave up and used a TM125MX as previously.

[jonny quest]

It is a triple port exhaust. Subs have rotary drums that open from bottom. Open area increases from bottom to top. I agree with you on the PV too low will cause this. Unfortunately this isn't the case. The main PV may be a little high. It's a ramp/ball bearing against a spring governor style. A different year governor opened later and slower, eased the power hit but dip was still there.

I looked at it again on a spare cylinder I have here. I can manipulate the stopper to lower the main PV about .5mm lower. It's the easiest thing to try quickly.

The sub exhaust ports are tear drop shaped. I may be able to get a little more area out of them without linking.

The YZ250, which is an excellent engine, opens the sub PV by sliding them open. (Think elevator door)

It's not ignition either, I have a completely different ignition on bike, didn't affect dip compared to stock ignition.

[wobbly]

I dont understand the logic, if the PV is too high in the down position it would not have a " dip " it should loose power everywhere down low - then hit hard as you describe.

[jonny quest]

The dip is still there with a earlier opening PV.

So logically, it would be a good test to try a lower PV to change the resonant tuning another way.

[Flettner]

A tandem twin with two x rear disc valve, how might that work. Using my twin exhaust port design = four separate exhaust outlets, yet Im only running two pipes. The cylinders will face each other with exhaust ports combining at the join. Both cylinders open exhaust ports at the exact same time, so no problem there. Exhaust ducts will curve a little to help the junction. Picture is not from the actual core I have in mind.
Gold card has arrived, last week of work next week, then free to fuck around with engine building.