ESE's works engine tuner

[WilDun]

that were so successful in the Thiel inspired Aprilia cylinders.

WOBBLY,
I'm assuming that the man you are talking about here is Jan Thiel - Is he the same guy from Holland who built the successful Jamathi 50cc racers in the late sixties and maybe early seventies?

Will.

[wobbly]

Yep, he is the MAN.

[F5 Dave]

Last night I encountered the most inconsistent bike I've run on the dyno; my GasGas EC300 (with road tyre fitted of course).

What I thought was going to be a simple find the right mainjet ballpark 1/2 dozen runs turned into a fiasco of chasing down why it wasn't revving out (which I never really found a smoking gun but it does now).

During all this I found that I could greatly influence the run by where it started. There was a dirty great hole between 5-6000. If I started the run say at 4000 it would have to pull through this & would have a suppressed run until it almost cleared - but still with a lower peak figure and a steeper curve than if I run it from 6000.

Not sure if the dip is due to PV opening too early, or if that's a mechanical governor type thing that wouldn't be such a problem on the dirt where it may spin up quicker than on a 600lb roller.

Ever run stuff this inconsistent?

I usually run 3 runs with every change & on most bikes find I need to disregard the first pass for it to clear out. This became all over the place sometimes. Don't think it was entirely a temp related thing. Tried a few mainjets but didn't affect things too much.

[speedpro]

Trade it in on a CRF Dave, Honda, 4 stroke, nothing but goodness.

[F5 Dave]

Nah man. No point in riding a fart fart dirt bike. This has low end stomp and rips when you ask it to + sounds gorgeous. And it's lighter and starts on the side of a slippery hill with a simple prod on the lever. Love this bike.

[wobbly]

Sounds more like the PV isnt opening at all untill its too late.
If the blade is jamming, the force from the mechanical ball/ramp type governor may eventually be able overcome the problem at high rpm.

[TZ350]

During all this I found that I could greatly influence the run by where it started. There was a dirty great hole between 5-6000. If I started the run say at 4000 it would have to pull through this & would have a suppressed run until it almost cleared - but still with a lower peak figure and a steeper curve than if I run it from 6000.

Ever run stuff this inconsistent?

Yes, and I feel your pain.

My experience with our GP125 bikes (single exhaust port, no PV) I found it depended on how cleanly they pulled through the power dip before coming onto the pipe. Anything that struggled there then had a bad run compared to starting the run cleanly above the dip. I put it down to wet fuel loading up the engine and it taking time to clear.

Both carburation settings and/or chamber design seemed to do it. An aggressive pipe seemed to be worse and required more care with getting the fueling curve right. Which makes sense when you see the power dip as the negative effect of the pipe resonance working against the engine before rpm increases to where the pipe resonance starts to help the engine make power.

Maybe its that three times past the main jet thing again. As we often had to look at the complete fueling curve, the bad engines often ran rich in the dip. Some engines liked a power jet others did not and changing the main jet plus air correction and sometimes the needle jet nozzle height to get the curve right as it passed through the dip on WOT was required.

Wob makes a good point about the PV possibly being stuck.

[richban]

Hi Guys

I have a question. What is the best position for the plug in these Heads? I know that Wob mentioned 6mm from piston with a Toroidal style but what about these jobs. tip of electrode protruding a couple of mm? Is there are hard and fast rule.

Cheers Much RB

[F5 Dave]

I took the top cover off to observe the PC. It seemed to operate pretty easily and pretty much full as soon as I blipped the throttle with any zing. Might be worth pulling it just to be sure.

[wobbly]

A toroidal shape or the flat roof bathtub as used by Aprilia both have very steep sides up and away from the chamber/squish band sharp edge.
This is in part because they have to, to get the cc volume correct,but also to create more turbulence from the squish action.
Pulling the end face of the earth electrode down into the middle of the burn space, close to the piston, also means that the initial ionised kernel from the spark is directly in line
with that part of the gas being consumed that has the greatest turbulence, thus it will burn the fastest - increasing deto resistance.
The chamber shape you have there does none of those things - ie old fashioned rubbish, start again.

[richban]

A toroidal shape or the flat roof bathtub as used by Aprilia both have very steep sides up and away from the chamber/squish band sharp edge.
This is in part because they have to, to get the cc volume correct,but also to create more turbulence from the squish action.
Pulling the end face of the earth electrode down into the middle of the burn space, close to the piston, also means that the initial ionised kernel from the spark is directly in line
with that part of the gas being consumed that has the greatest turbulence, thus it will burn the fastest - increasing deto resistance.
The chamber shape you have there does none of those things - ie old fashioned rubbish, start again.

Stink one. The sides are way steaper than the original VHM design. The corner radious will need to be like 10mm. I will try it and make more.

[husaberg]

Frits posted a pic of the Aprilia Cylinder head. A detailed drawing if memory serves me correct

This kept me thinking for a while. Maybe it is a misunderstanding stemming from the way you define a toroid. The decisive factor is that the radius of the combustion dome originates off-centre; its center is offset from the cylinder center line. That may or may not lead to the spark plug intruding into the combustion chamber, depending on the radius, the offset and the plug thread diameter.

The first two pictures show the Aprilia combustion chamber with a flush plug. The third picture shows a Wobbly-head with a deeply-intruding plug. The last picture shows the turbulence in a toroid head, initially caused by the transfer streams and enhanced by the squish action.
The intruding plug seemed a logical step but tests at Aprilia have shown that an intruding plug costs power, probably because it causes a scavenging shadow where spent gases are not fully removed by the incoming fresh mixture, so Aprilia-chambers have a flat area around the plug hole.

http://www.kiwibiker.co.nz/forums/at...8&d=1329656811

http://www.kiwibiker.co.nz/forums/at...9&d=1329656824

wobbly head
http://www.kiwibiker.co.nz/forums/at...1&d=1329656583

http://www.kiwibiker.co.nz/forums/at...3&d=1329658303

[richban]

http://www.kiwibiker.co.nz/forums/at...3&d=1329658303

The actual shape is very close to this. The pic's are not doing it justice me thinks. It sure is a balance act to try create a head that gives you what you want.

[richban]

How I have been working this out.

[wobbly]

The important things to see from the above is that the Aprilia drawing shows a VERY flat bathtub and was found to be the best design for a domed piston.
The Toroid became into being as an adaption of the bathtub and proved to be the best design for a flat top piston from a huge amount of testing I did for
a Belgian company making Honda after-market cylinders.
Both are trying to get the spark gap down very close to the piston.