ESE's works engine tuner

[Peljhan]

As we are speaking about heads and squish etc. I have a riddle here and maybe you guys have some ideas before I get into it.

In 2021 I made a new chamber for my Tomos D6 [50cc watercooled, bridged exhaust 195°, two stransfers 128°, piston port, PVL kart ignition]

New head has better cooling and more structural rigidity but it lost power drastically. Dimensions should be same as with old head, except squish went from old 0,55mm to new 0,48mm (will measure both heads again in future).
You can see black graph was best power a year earlier and there was no driving between tests. Red graph is new head and I never saw so much power lost and I made lot of modifications over years.
Another mod made in same time was repositioning of EGT probe (5mm thick stack gauge probe) from about 250mm exhaust length measured from piston to about 120mm length. I tought this was the case for low power but with removing a probe there was not much difference - blue graph.

[wobbly]

Without making a comment on the actual effect of changes , what's going on with the weather correction for this software.
You have the difference between 19 to 25*C air temp and 0 to 60% humidity.
0 cant be correct , but is the air being corrected properly ?

[Peljhan]

Uh, I think I forgot to input weather data (correction factor) for red curve
Can't do it now, only when measuring. It is cheap software.

[wobbly]

Do you have a choice of Correction.

[41juergen]

@Wob: you mentioned some when that the distance of the spark plug tip to the piston shall be approx. 6 mm as shown in my sketch... is that valid for flat pistons as well for domed once or a setup with a straight angle/ flat dome?

[wobbly]

For a flat top I have been as close as 3.5mm , but only because the chamber was very small for Methanol.

[lodgernz]

For a flat top I have been as close as 3.5mm , but only because the chamber was very small for Methanol.

I always try to position the plug gap at the point where straight lines from the squish band meet.
Is that wrong?

[wobbly]

That works just fine for a dome , but of course cant be done with a flat top.
In that case its just a matter of juggling the toroid chamber shape and the spark gap position.
I dont know how much is too much as Ive never had a chance to test it properly , but I believe you can get the toroid roof too high , ie the plug boss down very low, and that this geometry may
affect the scavenging loop arriving from near the rear wall and travelling across the chamber.

[Peljhan]

Do you have a choice of Correction.

No, I can't even export it in excel or something else.
If we use only corrected blue and black curve, there is still missing complete power top on blue curve with new insert.

[dtenney]

Looking for a bit of help, please. Is there a standard or a formula for calculating the needed distance from the piston crown to the spark plug? This is for a square bore/stroke 85cc motor.

[Peljhan]

Looking for a bit of help, please. Is there a standard or a formula for calculating the needed distance from the piston crown to the spark plug? This is for a square bore/stroke 85cc motor.

I doubt you will find a formula for that. I had a gap lower than 1,5mm on 90cc (18hp) "square" aircooled engine. I was experimenting with head inserts and needed to put a 1,5mm shim under sparkplug in one instance to get more clearance. When I forgot to install a shim once, piston tuched the plug and engine didn't start. It was working fine with about 1,5mm gap, the only difference I saw was clear spot at the center of piston dome, but no detonations visible.

[jonny quest]

Thanks Frits



https://www.kiwibiker.co.nz/forums/s...post1130892885

Good catch Husa! No one had any information back then either!

[wobbly]

Peljhan , the factors in play to kill top end are MSV , that obviously has changed dramatically with the closing up of the clearance, compression , that should be the same cc
or finally maybe the head was deforming previously - as you said the new one was alot stronger.
In any case the instant result of any factor like this is a big drop in EGT - so you need to start logging this , as to get a real correlation between any two setups you should
be adjusting the jetting to hit a target egt number..
The fix is a reduction in ignition timing , past the point where the power graphs cross.

Re the plug position , the pointers for this exist in Jan finding that a flat roof " bathtub " chamber gave better power over a simple part spherical shape, that pulls the plug gap down closer to the piston.
Then we have the toroidal shape , that pulls the plug down even further , and testing I did showed this was absolutely the best for a flat top piston.
The only other factor as mentioned before was my feeling that having a very deep toroid roof , with a large plug boss intruding into the combustion space , may ideologically seem good for flame front positioning
but could in fact start to disrupt the scavenging loop flow.

[41juergen]

Thank's a lot for all the feedback!

[Wos]

Very very interesting Diskussion... answers about head, flat top, domed, distance plug...squish and the special influences😎 wow...😲

My next question

the angle of squish band to the head shape ending to ...for example " bathtube"...


Which is optimum " crossing" angle of both geometric " flow directions"?

Crossing 90 degree?... thats what i do🤔

or with flow from head in direction of squish?...angainst it🤔✋

Thanks for continuing exiting answers✌🥰

Wolfgang