Broad band power for the Kawasaki A7 350 rotary valve twin
The Kawasaki A7 Avenger 350 cc rotary valve 2-stroke twin is a fun little bike.
I was surprised to find how quick these things are, and do hear they were very progressive for their time!
A friend of mine was in need of a set of chambers for his red A7….
There was a set of pipes on his bike that he wanted me to copy in stainless steel. However after measuring the length of those chambers I was suspicious that they would run well on the stock A7 motor, as the tuned length calculated to be for about 9000 RPM’s! This is beyond the red-line of the stock motor. Someone suggested that these pipes may have been for the A1 250 or for the A7R racer. The later may have been true because the port timing of the A7R was designed to produce peak power around 8000-9000 RPM, the stock street A7 motor is ported for around 6000-7000 RPM peak power.
After some thought it was time for measurement, math, and mock up. Below (center) you can see the original to-be-copied vintage pipes. To the right were the first two sets of test pipes (mild steel) next to the originals in my truck.
I made a slightly innovated copy of the originals keeping the 9000 RPM ish tuned length, and a longer set, tuned to about 7000 RPM with fat bellies and a more torque oriented enduro taper. The test pipes were made from mild steel with loud as heck stingers!
Here is the test bike at the test strip. After tons of seat of pants testing it was clear that the larger, longer, fatter pipes were the winner. They made great power from low revs right up to the stock red line of about 8000 RPM.
Here is a little bit of video of testing. The owner of the bike seems happy in the videos. Even with these low quality videos you can hear the WICKED sound of the stinger pipes!!!
Knowing that the larger pipes were the clear winners I set out to make a small run of them in stainless steel with silencers. I call these pipes “A7 Bombers.” Here you see laser cut flat parts, some cone rolling, and a few rolled cones in 20 gauge 304 stainless.
And more rolled cones…
Some fit up and welding. Stainless pipes must be purged with argon on the inside to TIG weld or else the inner surface of the weld turns to slag and become brittle. Baffles for the glass pack silencers are on the right.
Here is my first lot of pipes for the Kawasaki A7. Top: Silenced Bombers in polished stainless, Next: mild steel test pipes with stingers, Next: Silenced Bombers in polished stainless, Next: Silenced Bombers in mild steel, Next: silenced one off test pipes in polished stainless, Bottom: Silenced Bombers in stainless steel. Note: Only the bombers run well on a stock motor!
Also a later couple sets of A7 Bombers sporting polished stingers, adjustable mounting tabs (as on my Denco replicas), header clamps, and full custom mandrel bent stainless headers.
Silenced Bombers in stainless steel, on an A7R replica.
A handsome A7 with Bomber Pipes in mild steel with stingers.
Kawasaki A7 awaiting restoration with Silenced Bombers in polished stainless.