Combution Chamber

Foekema-Kreidler / 50cc-Racing
Latest update Jan 4, 2009
Combution chamber imported?
The combution chamber is beside the exhaustpipe the most dicussed subject on forums about tuning on the web. Understandeble because with this, the gain of power is after a well designed exhaustpipe the most imported one. And no tuner will loose any power when maximum power is the goal. Beside the gain of power there are more advantages of a well designed chamber.

In the past and to day
In the 60th we did not more than the right compression. We only used standard Kreidler jockey heads. In january 2005 we start to make our first new calculated designed heads. And almost directly we had a gain in speed, even so that in Nieuwekerk, Jan de Vries came to have a look, what we had done with our aircooled 1968 racers, to be almost faster than his 1972 vanVeen GP racer. Jan and we (my brother Ricus and me) were with Aalt Toersen, the riders of the vanVeen team in the years 1967 and 1968, so we soon had a discussion about the subject.

Which details are most important?
For a well designed chamber the most important items are:
1. volume for the right compression
2. squishsurface to make the chamber as small as possible
3. squish clearance for the right speed (MSV) of gasses to the center.
4. Model of combution chamber also to shorten the burning period.

The right compression is still the most important issue for a combution chamber. This can be push up if the squishsurface and clearance are well designed. If compression is to high than the engine will not run smooth into high revs and will suddenly stop to a cerntain rev. At that monent the engine knocks and will soon get damaged.

Burning speed
A low squishclearance prevents the engine against knocking, while the gasses stay more cool in this area and also preserve the speed of gasses to the center of the chamber. This speed gave the gasses the last push to mix with air, which does make the flamefront faster and this shortens the burning speed. But the squish speed has also to be limited, because it may not exceed the speed of the flamefront, which is about 30m/s. Burning speed is after compression the most important issue of the combution chamber.

Effectivity of the chamber depends on the squishband and squish clearance. If more gasses are in the burning part of the chamber and not in the squish part, more gasses are effective in the burning period which makes the push on the piston more effective.

Model of the chamber
The model of the chamber is important for de burning period and also for the flow of the gasses. To shorten the burning period the ignition should take place in de center of the chamber, to make the distance to the edges as short as possible. There are many models from the past, but for speed engines we don't use them any more. The most used models today are:
1. conical chamber is easy to make
2. spherical chamber should be theoretical a better one
3. torroidale chamber should be the best of all, but is diffecult to make.
For 2 and 3 you need special equipement on the lathe, or a lathe connected to a computer for cnc purpose. Till sofar I only used the conical model and the results you can read everywhere on my website.

RPM is very important
The purpose of the engine and so the maximum rpm is most responsible for the design of the combution chamber. The chamber is even like the exhaustpipe dedicated to the same properties of the total set-up of the engine. It is even dangerous for the engine to use an exhaustpipe which push the engine to much higher revs, than for which it was designed, without a calculation of the chamber.

The right combinatiom
A right combination of all these subjects has an incredebly influance on the peformance of the engine. All details works together at the same time and pushed the power upwards. But the chamber design is also responsible for a very quit twiggeling of the engine when only a little power is needed.
It will be clear that calculation of combution chamber is almost that important as a well designed exhaustpipe. Without a calculation you never know how far you are from the limits and that might be dangerous for the engine.
Our practical experience is rising with almost 1000 calculation of total different types of engines, from 3,5cc glowplug to 500cc. But most from racing engines from 50cc to 125cc.

Each program a different MSV ?
Even like the calculation of the exhaust the MSV, which is calculated with several free programs on the web, is always different. And how does anone know what is the right calculation, if you don't know how it is calculated. Even when they are based on the formules of Gordon Blair, the differance is the same. That is why we did make our own program, that is the only way to understand exactly how the calculation is working. The MSV is a theoreticle number, because it is measured in a static situation. At high revs the situation is total different and may be this is the reason of different numbers of MSV.

Relation with exhaustdesign
As descrebed before the connection between the headdesign and the exhaust design is mostly the number of rpm. Other relations as temperature and pulsstrongness from compression are less important while these are also shifting a little with the rpm number.

Right references also important
Not only from own experience, but also from proven references, you know what the numbers are correspondending to a certain purpose of an engine. Because our active racing period starts in the 60th we had many friends in this sport, so there are many other experiences beside our own. One is a 125cc Honda engine, racing for the European championship which produce about 50hp. This engine is full electronic loaded to measure every thing we want to know. My son Raimond als has made some measurements possible, to know what the pressure is close to the inlet of the carburettor. For head controle each knock is recorded in the memory to read back after each run and so the right correction can be made in some way.
All products of big brands as Honda, Aprilia, Polini, Malossi, Piaggio etc. are collected for comparing if neccesary. The same we did for exhaustpipes.

Changeable chambers
Changeable chambers is a developpement for racing engines. It is easy to make and has many advantages over the standard head. For racing purpose you will have many spare ones with little changes in compression or msv. Not only watercooled engines can use it, also old twostroke engines can be updated with this product, so even provide these new technoly.

Combination request
Our advise should be, to send a combination request, also if you only need one of them. The one you don't need can be use to compare with the one in use. Besides the combination cost only a very little more.

What do you receive after a request
The head design will be delivered as you can see in the first pic of this page.
1. The left part are the properties of the engine and the chamber as compression and MSV
2. The right half starts with all the measurements which needed on the lathe, as outside diameters and the length of material
3. second part on the right half are the dimensions of a conical chamber.
4. third part on the right half are dimensions of a spherical chamber.
And a drawing of a conical chamber

With these measurements you are able to made the first chamber in one time in the right way. If the measurements are well used, than there is no need to controll the volume, but ofcourse you are free to do it. If you don't have right measurement it is hardly not done an takes much time to make one. With the calculation you get also an instruction.

Form for request
The form for request is a combination for exhaust and combution chamber, because many items has a relation with both. It is important to fill in the items as good as possible, if available with the works specificatons. Also the power and results of the current situation and if possible with exhaust measurements can be an advantage.
Download here the Fillin form send it by e-mail to: 50cc-racing e-mailbox, with the subject "Exhaust and/or Combution chamber".
Read the extra information and explenation of the fillin form

Air cooled heads with changeable chamber
Also for aircooled Kreidlers on the road, which are in many cases tuned for much higher speed, a well designed head and exhaust is very attractive to use. The included dedicated information is important to know what the borders are, between operation had to take place or is most adviceable.

More chapters about tuning
Explenation request form
Expansion exhaustpipe
Rotary Valve dimensions
Kreidler 6V racing engine

Calculation combution chamber(click the picture to read)

Example recalculation of Homoet6P (click picture to read)

Kreidler aircooled head with changeable chamber

Changeable chambers for aircooled head Kreidler racer

Kreidler fancooled head with changeable chamber

Cylinderhead for Kreidler vanVeen racer

Cylinderhead for Kreidler vanVeen racer

DRM cylinderhead for Kreidler racer

DRM cylinderhead new model for Kreidler racer

Halffabrics cylinderhead for Kreidler vanVeen racer

If the English language page is not ready, you may find a Dutch version.
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