New Stainless Steel version. (Please note the one shown in the photo is one of our earlier mild steel ones)
Designed from experience and a bit of theory (see below) and tested and developed on the Dyno.
We built a number of different exhausts of different diameters, lengths and designs and this version has so far bested all alternative designs we have back to back tested on the dyno on Liam's very quick FXR 150 bucket.
Typically 20% power gain over stock FXR 150 exhaust when used with a muffler that matches our recommendations. Dyno charts available on request.
Please note we can weld muffler retention springs on the end of the header pipe on request to suit your chosen muffler. (We prefer spring retention to clamps).
Your exhaust performance considerations do not stop at the entry point to your muffler. The muffler is part of the exhaust system and your choice of muffler will effect performance. We recommend a muffler with a straight through perforated baffle pipe with an internal diameter of 32mm. Using a muffler with a larger or smaller internal diameter baffle pipe will either constrict gas flow or slow down the gas flow. A muffler with a larger outer diameter body or a longer or shorter muffler with this size baffle pipe will change the sound but will have an almost neglible effect on power. A larger diameter muffler will produce a deeper softer sound. Mufflers typically either have fibreglass wool packing or steel wool packing. A tightly packed muffler will be quieter and produce more power with our header. We prefer fiberglass-wool packing as it is easier to tightly pack but will require repacking more often.
Back Pressure Vs gas speed:
Back pressure will restrict power on a four stroke engine. A four stroke engine unlike a 2 stroke engeine does not need back pressure to help control inlet and exhaust port timing. A too small diameter pipe will cause back pressure and a loss of power.
Slowing gas flow will restrict power on a four stroke engine. Too large a pipe will slow gas flow and reduce power.
Consequently an exhaust should use the minium diameter tubing that does not introduce back pressure.
We have chosen our manifold diameter after testing different diameter headers on the dyno. We also tested a stepped exhaust system but discovered that this caused a loss in power on the FXR 150 engine we used in our tests. It seemed the one we tested combined the worst design elements, and initial pipe size that caused back pressure combined with a too large a stepped pipe that slowed the gas flow.
The timing of gas flow and sound pulses (which travel through the exhaust at two different speeds) exiting into an expansion chamber or the end of the exhaust effects the effieciency of extraction of gasses from the engine and insertrion of inlet charge when the valves are open. In general a shorter exhaust will give more top end power and longer exhaust may produce more power low down.
We have tested a couple of different length exhaust systems and found that when used with a muffler that matches our recommendations that a pipe that ends a few inches in front of the rear wheel produces more horsepower both down low and at the top end than the other systems we tested. The perfect length will depend on what modifications have been done to your engine. However relatively small variances in length of about 50mm will only make marginal differences to the overall power across the revs used when racing an FXR 150 bucket.
An added bonus of a shorter system is that the muffler can be mounted under the engine out of harms way and it also puts helps centralise the mass of the bike which aids handling...and of course it is also quite a bit lighter than a full length system.
We do not claim these are the ultimate design headers for all FXR bikes. We can only state these are the best ones we have tested on our 3 modified reference engines to date. We believe that this header when paired with a muffler meeting our recommendation will be a very good match for most FXR 150 race bikes.
Please email us if you would like to see comparitive dyno tests of these headers with all the different systems we have back to back tested on the dyno.