Exhaust Development

This page is going to be constantly updated over the next few weeks, in my quest to design and manufacture an exhaust system specifically for my engine tune.
As most people know, you can only go so far with porting then you come to big brick wall, where by you cannot extract any more power  from the engine.
The big power gains can be achieved by fine tuning the exhaust system. A couple of millimetres to long or to short can have detrimental effects to the power curve, as does only half a degree here or there on the expansion chambers.

When designing an exhaust lots of factors come into play, Port timing, Peak RPM, Exhaust port diameter, Horn coefficient and unsteady gas flow to name but a few.
First thing to do before we can go any further is to find out the exhaust gas temperature at the belly section of the exhaust, this is achieved by using a thermal probe, inserted into the belly of the exhaust to register exhaust gas temperature under normal riding conditions.
Why do we need to know the Exhaust gas temperature?
We need to know the exhaust gas temperature because the exhaust tuned length relies on the speed of sound for the scavenge pulse, where by the speed of sound changes accordingly with exhaust gas temperature.
For example:-

As can be seen from the table, for approximately every DegC it alters the tuned length by about 0.6mm, that may not sound like much but if you are 50 DegC wrong that makes a 30mm difference in tuned length.
The Exhaust gas temperature is taken mid-pipe to get an average temperature without interference from combustion temperature.
The probe was inserted in the top pipes as well as the bottom pipes incase of temperature variation between front and rear cylinders.

As you can see the probes have been lock-wired around the exhaust, as this is only a temporary arrangement. A 3mm diameter hole was drilled in the belly for the probe to be inserted.
Now the probes are fitted, I can take the bike for a run to record maximum exhaust gas temperature. ( Just have to wait for some dry weather )

Well I had a bit of a shock today I put the bike up on the dyno so as to record exhaust gas temperature.
On my pipe data I usually work on the figure of 600deg for EGT but to my amazement, the temperature in my pipes only reaches 440 deg on the rear cylinders and 430 degrees on the front.
This difference in EGT equates to a staggering 96.0mm difference in tuned length, to my previous pipes.

Now armed with this new information the manufacturing of the exhausts can begin. First thing to do is to gather all the information of my engine parameters, this can be seen below.

Then figures can be entered into the exhaust parameter calculator as shown below.

Hopefully this should give us a very good pipe specification (Notice I said hopefully)
Most parameters on the exhaust are hard and fast, the one grey area is to do with the horn coefficient, this is mainly an educated guess on what value to use.
Now that we have the dimensions for the exhaust, the exhaust then needs to be developed on the CAD system so that the piece parts can then be cut out on the laser machine.

As can be seen from above, the route that the pipe has to take is drawn on the computer to make sure that the pipe will fit the bike. The picture above shows the route for the top pipes, the problem with the top pipe run is the stinger part of the exhaust, there is not enough room on the bike to fit it in, to overcome this I have decided to go for an internal stinger, this is where the stinger protrudes into the divergent cone as well as protruding from the back.

Now the pipe run is correctly routed, the cones then have to be developed, this is done by means of 3D CAD software, which unfolds the cones to the correct developed shape. (Easy when you have the correct tools!!)
From this information a tool path is then created to be downloaded to the laser machine to be cut.

Once cut out you are left with the pieces as shown below.

These pieces then need to be rolled to form the cones as below.

As well as the sheet parts there are also a couple of parts that need to be machined

Once we have all the piece parts, the exhaust can then be welded together.

And finally after welding, the exhausts can then be fitted to the bike.

And there you have it, top pipes manufactured to fit my bike, now I just have to do the same for the lower pipes.

I'm going to be running the new pipes on the dyno this week so I shall post the dyno curves to this page in a week or so.
I've also made some adaptors to fit on the engine flange so I can alter the tuned length of the pipe, just so I can have a play on the dyno to try a few different things.

Results

The green curve (Run 052) are Swarbrick exhausts.
The red curve (Run 078) are my first prototype exhausts.
The blue curve (Run 092) are my set of exhausts as detailed above.

Well there you have it 123.9 BHP @ 10400 RPM
                                         64 FT/PNDS @10200 RPM

To be honest I didn't expect any more than 120 BHP due to the fact I'm only running 31.5mm carbs.
In the next few weeks I'm going to be fitting a set of TMX35's, but have got to heavily modify the inlet tracts to cope with the extra cross sectional area associated with the TMX's.
Then hopefully I should see figures around the mid 130's.