Expansion chamber

Special Parts

Foekema-Kreidler 50cc-Racing
Latest update Aug 18, 2008
Exhaust expansion chamber
To make an expansion chamber you first have to calculate the dimensions. This is a very complicated subject. Every item of the twostroke proces has an influence to the dimensions. It is so complicated, that you will never finnish this job. Also the big GP racing factory brands are continue changing their designs a little. Every new engine concept has it own exhaust design. There is nobody who can tell he knows all about it. In the most positive way you might say it is a very well compromise. The tuning lenght of the exhaust is based on physics, the temperature, the speed of sound are the calculated elements. Why are so many calcultion programs and also the tuning information on the market so different? I don't know. But most tuners use a different tuninglength, or does apply it different. Many tuners now a days still don't calculate anything at all, they are used to their lay-out and know the results of it some way. Some tuning literature don't show, beside the tuninglength no calculations at all. They only advises some specs of the exhaust parts. For a young new tuner not an easy subject to become familiar with.

How in the sixty's and how we do it today
Our methode in the sixty's was to use reliable references of the Yamaha racing engines, and experiment with small changes.
To day we calculate and see on screen, what will be the effect of a little change in detail. Also the differences of two or more existing pipes is very clear in this program.
We collect since 1964 a lot of wel working and reliable references which are comparised in our calculations.
In 1968 our private Kreider runs a topspeed of 168km/h in the Assen GP and was at that time one the fastest in the world. Now we know that the exhaust desine was most responseble for this result.
To day our sprinter runs 100km/h in less than 7 sec, which is enough to set new WR on the 1/4 mile, 1 km and 1 mile with standing start. And still we are very sure that this it is not the end. The current calculation program which we apply is built by Delft University and correct with our references as described above. At this moment we are very sure about a good results, but we also knew that we are never ready and still trying to make it better.

Standard expansion exhaustpipes
On the motormarket you can buy several standard expansion racing exhausts. These are cheaper than a handmade dedicated one. Standard pipes can work well, but not always. It is important to know to what purpose the exhaust has been designed for. So don't buy a pipe designed for 70cc to use it on a 50cc engine, or use a pipe designed for offroad racing on a roadracer. If you do so the result could never be optimal and sometimes even dangerous for the engine. In some cases it is very atractive to buy a standard exhaust and made some changes as has been calculated. One example is the Homoet 4P, in paticulairy when it is used on a 60cc engine like many Kreidlers. If you buy a standard exhaust, always ask the following questions:
1. for which purpose and what kind of engine, displacement ccm3, the exhaust has been designed for.(racing/sprinting/offroadracing/enduro).
2. which cylindertuning is dedecated for the exhaust, for the right rpm.
3. for the right powerband it is necessary to know how many and how close the gears are.
Don't think that a little porthigh (timing) difference will be a directly a disaster, but sure, the result will be different a little anyway.

Exhaust dimensions service
We started the exhaust dimension service end 2003. Within a short time we had so many, that we have to decide to ask a little money. Still many enthusiast even far over the Dutch borders, already use there own handmade pipes with our dimensions. Here are some responses:

Exhaust design for an Yamaha in Portugal dec 7, 2006
Hello Luc,
It is 00.37AM, and I just arrive home because I was ajusting my son`s Yamaha DTLC, It is very nice, I don�t know the max speed but it is fantastic, it is one diference with the exhaust incredeble.
Thank you for the explanation about the ultra SAP CDI, for street road it is not recomended. Yesterday I ask you if it is possible assemble only the your CDI, because the ignition of my son`s bike it is with lights, and he need lights.
If I will problems with the ignition of the TZ125 engine I will ask you one ignition, because it will be one bike for race, and the price for this it is razonable.
I am very happy with the exhaust, fantastic.
Best regards.

Exhaust design for a Kreidler 60cc
I received your dimensions for my 60cc. I weld the exhaust and tried it directly and became very enthusiasme. A high rpm with much power, I can drive to 85 and easy shift to the 5th gear and than pull trough to 110 km/h, right up and against the wind. It is far the best exhaust I ever tried... from 2 famous tuners in Holland. I had my doubts when I saw the dimension, but it works... I will put the result on the Kreidler.nl forum as well. I'm very greatfull and a happy new year.

Exhaust design for a Honda MT80
I espected that a calculated exhaust was good. But that good,... that was far above my espectations. The engine setup was: 32 mm carburettor and 6 valve reedvalve, special ignition and a Proma80 exhaust. This was already very fast, but when I went testing the new one and it came within its powerband, it was far more powerfull then before. So a very well exhaust!

Exhaust design for a Honda 50cc Dutch competion crosser
Hi Luc,
I just like to respond on the dimensions I got from you a little time ago. I had an exhaust which look like that of yours. But only for the belly and the baffle cone. So I decided to make new header and defuser parts. This was not without succes. The engine does made the rpm's you had written and also is strong in the lower rpm's. So strong that even in very bad and soft offroad parts we were very fast. In last offroad event we reached for the first time the second place so also for the first time on the podium. So are very glad but also interested how you calculated that dimensions. Anyway thanks...

In what way and how do you get the Exhaust dimensions?
With the exhaust dimensions you get also the next information:
1. Dimension of all parts in length and diameters, incl. for hydroform.
2. Dimensions how to draw it on paper including an example.
3. A freeware program to print the template parts 1:1 on paper
4. Information about the powerband, the engine will be operating.
5. Advise about your setup and timing, when the current is not quiet right.
6. Instructions how to made the exhaustpipe in the easiest way.
7. Tuning advise if needed.
Some examples you see pictures on this page.
Download the form "request for exhaust dimensions"
Fill in form
Send it to e-mail: 50cc-racing, with the subject "Exhaust".
Within a few days you will get all information.

Tools to make an exhaustpipe on your own
To make an exhaust by your own, you need the following tools:
1. steelsiccors,
2. gloves for protection (very important)
3. a drillingmachine with a grinding disc (thx Guus), 4. welding tools, best is Tig welding, but normal or with copper will work too.
5. bankscrew and masive metal staf in a few diameters.
6. enough time, some experience, precision and no hurry.

Chambers for cylinderheads are also very important
For maximum performance it is adviseable to use a well calculated chamber. Hereby it is important to have a correct squish band in the width, the clearance en velocity and that in combination with the exact compression ratio, for the right rpm. With the calulations you also get all the information for the late as the degrees for a conical chamber and squishband or the radius for a hemispherical chamber and squishband. Without a right calculation it is a hell of job to get the righ combination of all data. If you cannot make it by youself, you can let it made by us.
Download here the Fillin form
Send it to: 50cc-racing e-mailbox, with the subject "Calculation Chamber" and within a few days you get all the properties.

Read about the special: Ftt Homoet 4P

Exhaust calculation (click the picture to read)

Example Hydroform exhaust calculation (click the picture to read)

Exhaust cones for a SpartaMet racer

SpartaMet racer

Scooterexhaust in 2 parts

Combution chamber calculation (click the picture to read)

If the English version is not ready, you will automaticle be connected to the Dutch version
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