ESE's works engine tuner

[wax]

A squish band area ratio of 50% is a good general value. Wobbly may be kind enough to explain in more detail why that percentage is smaller on engines with a fixed ignition timing.
Forget about your 'small angle towards the combustion chamber'. The angle of the squish area should exactly match the piston crown, so use a domed squish band on a domed piston.

Thanks very much
I did chop a piston in half then scanned it and fired it into cad so i could get the measurement perfectly
From here I will 3d print and then it will show me how good i was.
Im reading that a bath tub is a good shape on a dome piston so I will go with that,
These are to go into and existing head and so its becoming hard to get enough length into the spark plug to get the distance I need off the top of the piston.
The old dome was a hemi style with the dome 15mm off the top of the piston with a straight cut squish that really wasn't one

[wobbly]

The basics you have the good info so far.
50% SAR with closest safe squish depth is generally the best place to start.
Conventional wisdom is that a bathtub works on a dome , this was initiated into the common knowledge base by Jan's experiments at Aprilia and I do know he tried a flat top
without any gain.
But this is not the case with a conical dome , and the narrow squish as is used in KZ kart engines.

I did some serious tests in the early 2000's with the conical shaped domed pistons .
At the time a 7* conic was the std shape and the tech rules were ( are ) a strict 13cc to the top of the plug hole.
Several GP teams were using toroidal shapes , especially after unleaded was mandated ( that word again ) to help prevent deto that was all but impossible with Elf Mitso 130 octane rocket fuel.

But A work around the CIK rules , that would normally prevent using a toroid in KZ , was to drill a tiny hole at an angle from the toroids highest point into the plug threads to allow trapped air to escape.
This proved to tech that the chamber volume was legal , when using the simple method of filling to the top of the threads with the piston at TDC.
With the toroid , and the plug ground electrode 4 mm from the top of the dome , this was worth on average 1.5 Hp with a huge gain in deto resistance - allowing much leaner mixtures to be used.
Sadly not long after I started doing this , the CIK instituted a new screw in plug gauge that had exactly 2cc volume , thus preventing the " work around " of a drilling.

The need for a very narrow , and slightly diverging , squish when using a straight line ignition , is that the actual effect of proper squish geometry is to eject high speed turbulent eddies into the
smoothly advancing combustion flame front - and this dramatically increases the speed of the expanding " bubble " of burning A/F mixture.
Thus the time needed around TDC for the combustion to be completed , is reduced - and this effect is exactly analogous to advancing the ignition timing.

In a KZ the static spark advance is a serious compromise between generating sufficient mid power Vs preventing deto in the area between peak torque and peak Hp.
A side effect is that retarding the static advance allows greater overev as well.
So , without the luxury of a proper digital , retarding ignition system , it becomes impossible to generate the power needed at low rpm , as well as achieving overev power when an efficient squish is used.

It appears that Honda and Yamaha were convinced a toroid is the best for a flat top , as all of their "fast " engines used this, and I have proven it works best on a conic.
And based on that , I believe a toroid , in most 2T tuned engines is the best choice .

This was reinforced by another test done for a customer who revived , then remarketed the air cooled Webco head design used on old Yamaha singles and twins.
I back to back tested a normal stock configuration hemispherical , vs a bathtub , vs a toroid . These were all CNC machined with the exactly same squish and cc on a domed piston.
The result was a clear win to the toroid with an average of + 4 RWHp ( in a 250 twin ) , and the bathtub was exactly in the middle with +2 Hp average.
But the real kicker was that the toroid then allowed an extra 2* of static advance to be safely wound in , gaining another + 2Hp with no deto issues at all.
The result was much , much more dramatic when running higher compression for the common 110 octane race fuels available, and allowed in USA.

[wax]

The basics you have the good info so far.
50% SAR with closest safe squish depth is generally the best place to start.
Conventional wisdom is that a bathtub works on a dome , this was initiated into the common knowledge base by Jan's experiments at Aprilia and I do know he tried a flat top
without any gain.
But this is not the case with a conical dome , and the narrow squish as is used in KZ kart engines.

I did some serious tests in the early 2000's with the conical shaped domed pistons .
At the time a 7* conic was the std shape and the tech rules were ( are ) a strict 13cc to the top of the plug hole.
Several GP teams were using toroidal shapes , especially after unleaded was mandated ( that word again ) to help prevent deto that was all but impossible with Elf Mitso 130 octane rocket fuel.

But A work around the CIK rules , that would normally prevent using a toroid in KZ , was to drill a tiny hole at an angle from the toroids highest point into the plug threads to allow trapped air to escape.
This proved to tech that the chamber volume was legal , when using the simple method of filling to the top of the threads with the piston at TDC.
With the toroid , and the plug ground electrode 4 mm from the top of the dome , this was worth on average 1.5 Hp with a huge gain in deto resistance - allowing much leaner mixtures to be used.
Sadly not long after I started doing this , the CIK instituted a new screw in plug gauge that had exactly 2cc volume , thus preventing the " work around " of a drilling.

The need for a very narrow , and slightly diverging , squish when using a straight line ignition , is that the actual effect of proper squish geometry is to eject high speed turbulent eddies into the
smoothly advancing combustion flame front - and this dramatically increases the speed of the expanding " bubble " of burning A/F mixture.
Thus the time needed around TDC for the combustion to be completed , is reduced - and this effect is exactly analogous to advancing the ignition timing.

In a KZ the static spark advance is a serious compromise between generating sufficient mid power Vs preventing deto in the area between peak torque and peak Hp.
A side effect is that retarding the static advance allows greater overev as well.
So , without the luxury of a proper digital , retarding ignition system , it becomes impossible to generate the power needed at low rpm , as well as achieving overev power when an efficient squish is used.

It appears that Honda and Yamaha were convinced a toroid is the best for a flat top , as all of their "fast " engines used this, and I have proven it works best on a conic.
And based on that , I believe a toroid , in most 2T tuned engines is the best choice .

This was reinforced by another test done for a customer who revived , then remarketed the air cooled Webco head design used on old Yamaha singles and twins.
I back to back tested a normal stock configuration hemispherical , vs a bathtub , vs a toroid . These were all CNC machined with the exactly same squish and cc on a domed piston.
The result was a clear win to the toroid with an average of + 4 RWHp ( in a 250 twin ) , and the bathtub was exactly in the middle with +2 Hp average.
But the real kicker was that the toroid then allowed an extra 2* of static advance to be safely wound in , gaining another + 2Hp with no deto issues at all.
The result was much , much more dramatic when running higher compression for the common 110 octane race fuels available, and allowed in USA.

Thanks again Wobbly.
This is where i am at the moment. The squish has the perfect curve to the dome of the piston. The design of the head the dome is going into make it’s hard to get the plug down far enough. I’m thinking I will have to go to long reach plugs im at 19mm on this plug but it’s not enough. This is a dome I trying to produce for the masses and so I can’t be using an expensive race plug.

[wobbly]

Move the top o ring down onto the horizontal face - means the spigot can be shortened to nothing if need be.
Also add a thread cooling slot , you can make it say 6 radial pockets with support bridges if you are concerned with strength , but as the cover supports the plug boss its probably not needed.

[husaberg]

Move the top o ring down onto the horizontal face - means the spigot can be shortened to nothing if need be.
Also add a thread cooling slot , you can make it say 6 radial pockets with support bridges if you are concerned with strength , but as the cover supports the plug boss its probably not needed.

As an aside, i would go with the one piece and make it out of Aluminum.

[F5 Dave]

Are you feckin kidding? Blue is Cool. Might want to make it fully round though. I can see you losing a lot of compression as is.

[koenich]

Move the top o ring down onto the horizontal face - means the spigot can be shortened to nothing if need be.
Also add a thread cooling slot , you can make it say 6 radial pockets with support bridges if you are concerned with strength , but as the cover supports the plug boss its probably not needed.

As always, thanks for sharing the knowledge!

Any rule of thumb to design the toroid? There are quite a few ways to achieve compression xy...
- squish area is clear
- h1 depth of spark plug
- h2 height of bowl
- r1 radius starting at squish area
- r2 radius transferring from r1 to spark plug
If r1 starts normal to end of squish area the radius is given by the height of the bowl...plus is r1 = r2?

[wax]

Are you feckin kidding? Blue is Cool. Might want to make it fully round though. I can see you losing a lot of compression as is.

Its a 125 so only half a real engine, This is why i only went for half a dome
the blue is because its a yamaha

[wobbly]

General design comments are that R1 isnt. Its straight @ 60* for about 1/2 H2.
H1 can be varied but I make it such that the ground electrode is around 4mm- 6mm from the dome ( compression dependent ), with the plug body flush.
The flat diameter around the thread end should be a minimum of 17mm or the alloy overheats locally due to no threads on the plug end.

Originally NGK's spec for the old B10EGV type plug was that the unthreaded end should protrude 1.5mm into the chamber.
But im not sure if this is the case now with the latest plugs , and anyway this is specifically not allowed in KZ tech rules , so I havnt done any testing there.
Except I milled the end off a R7232A on a rotary table , the longer plug used in Honda's with the under plug deto sensor , such that the body was flush per the rules , but the ground electrode
was sticking well into the chamber on a small unmachined upstand - made no more power , bugger it.

[SwePatrick]

It is not the mechanical efficiency I worry about but the thermal one. The manufacturer only characterize it to 1.8bar while Alex wants to run much higher. Just extrapolating the curves to 3bar will already heat the air to well over 300degC if it can reach that and absorb lots of power. Hopefully I am wrong...

The thermal rises also if the mechanical seal get better.
Less 'slip n slide' for the air around the lobes makes for colder air.
But in alex case, the high boostnumbers we saw is a fruit of having the piston make a secondary compression of the air in the crankcase.

And i saw some other guy talked about coated rotors, coating will most certanly be destroyed by the fuel.
Autorotor(lysholm) said long ago that you should run no fuel into their supercharger due to coating couldn´t take it.

Teflonstriped as in big fuel classes(and more) in dragracing is the way to go, but needs service, and should never be ran dry.

[lodgernz]

Many times, Wobbly has talked about the dangers of using old fuel that has lost significant amounts of aromatics, and the risks of associated engine damage.

This is a video I took at a recent Ruapuna meeting.
A 4T bucket that is fast enough to beat the best 150 2T, with a matt black tank, sitting in full sun.

Perhaps the lack of aromatics isn't such a huge risk for a 4T, I don't know, but as 2T people, we need to know about this.

[wobbly]

Yes , fuel conservation is a pet fetish of mine.
I cant imagine the number of destroyed pistons/cylinders I have seen that simply would not have happened with fresh fuel.

Anyone I work with in karts or bikes knows rule 1A , all fuel must be stored in 5L containers , with no air space, then mixed and used one at a time.
A 20L pail or a fuel tank left 1/2 empty overnight will degrade from 98 pump unleaded to 91, if you are lucky.

The instant any container is opened the Reid Vapour Pressure that has flashed off the light aromatics into the empty space , will blow out and disappear to atmosphere.
Then the lack of finely atomized fuel droplets that burn quickly in the A/F mix , makes the egt drop ( falsely ) as the big blobs of low octane fuel don't burn ,and as soon as you lean down you deto
if lucky , or seize if not.

When tuned to the edge of deto at all times this is super important for jetting repeatability, where 1 jet too conservative means you may miss pole by 2/1000 of a sec.

Maybe most 4T engines are running such low bmep they are nowhere near the edge of destruction , and this is not such of an issue.

[F5 Dave]

One of my dirtbikes has been sitting almost a year. I might need to drain the gas into the van. Good upper cylinder lubricant. Who know what I'll do with it when I upgrade to fuel injection carrier.

Wonder what people think when they used to follow me smelling of Av Gas and synthetic.

[PVO]

Hi everyone, I'm seeking clarification on a few aspects of crankshaft design and hoping for some input. Another member graciously shared some empirical numbers with me from his twin-cylinder engine and I wanted to get a feel for the analytical side too.

1. Crank Web Material Required to Support Press Fit Pin - Many pages back Frits noted a good rule of thumb to use for supporting the press-fit of the big end pin is around 1/2 the diameter of the crankpin. A press-fit assembly is relatively simple to set up in an FEA. Is it accurate to believe that if the press fit is FEA'd and there is no yielding of the crank web material around the pin, then there is sufficient material around the pin or are there other considerations I haven't accounted for?
   
2. Big End Pin Engagement - Is there a similar rule of thumb for determining how much big end pin engagement is required?
   
3. Crankshaft Stiffness and Deflection - If the crankshaft could be loaded and simulated in FEA is there a threshold for the amount of permissible deflection? Presumably, this number would be minimal, 0.025mm (0.001")?
4. Target Interference Fit - Does the amount of interference fit scale with output power or is it relatively constant across engine size/power output?

The engine I'm working on is an 850cc 90-degree twin (targeting 160hp NA and more later on with boost). Of the many design considerations two of the most critical will be sizing the big end pin and deciding on the crank architecture - no center bearing (like as drawn) or integrating a center bearing.

Husaberg - thank you for your input and ideas on converting the rotational axis. I think I've found a layout that won't require too much complication.

Thanks all for reading!

[Niels Abildgaard]

The engine I'm working on is an 850cc 90-degree twin (targeting 160hp NA and more later on with boost). Of the many design considerations two of the most critical will be sizing the big end pin and deciding on the crank architecture - no center bearing (like as drawn) or integrating a center bearing.



Thanks all for reading!

What engine type will be donor for cylinders,pistons and conrods?