Honda-SOHC
SOHC.co.uk Forums => CB350/400 => Topic started by: Athame57 on March 28, 2025, 05:21:57 PM
-
I hope this isn't a Dumbo's question ;) but what does that collector box half way along our exhaust system do? Actually I'm asking because the web is producing different answers, also because I nearly developed a hernia today fitting a new one with downpipes.
-
Rust away.
-
Rust away.
That much I've noticed, I make efforts to control it though! ;D
-
They are akin to musical instruments, trumpet, trombone etc, tuned to make use of pressure changes.
The "headers" entering that collector section with it's change in volume send a negative pressure wave back up the pipe it's coming from (often, possibly wrongly, referred to as "back pressure " ) that helps to scavenge the combustion chamber by that negative effect just as the exhaust valve is coming to a close. This to improve spent combustion extraction and cylinder efficiency. Also, as timing overlaps on camshaft to have both valves open at that stage, it helps to pull in the next intake volume from carburettor.
That whole process, from the headers, is usually designed into system by altering header bore/volume/length to specifically "harmonise" at peak engine torque RPM (is that about 8,500 on 400 ?) This to bring peak efficiency in output and so maximum effort from the combustion.
The rest of the pipe, out through silencer to end exit, effectively does the same for much lower rpm. The overall pipe length being so much longer causes that negative "signal" to take longer in getting back to combustion chamber, usually arranged by the manufacturer to harmonise with low rpm running to help with engine response through that often used range.
The whole ensemble aimed at best combustion potential enhancing "tune" through the operating range.
As extreme example in contrast, a F1 naturally aspirated engine will have extremely short, wide bore, truncated headers, very short collector/outlet etc, all focused just on absolute peak very high rpm output with no other consideration.
-
Rust away.
After starting, Don't stop the engine until the exhaust is really hot, this will help to get rid of condensation and rust formation in the collector box. 8)
-
Thanks for this K2-K6 now I understand.
-
They are akin to musical instruments, trumpet, trombone etc, tuned to make use of pressure changes.
The "headers" entering that collector section with it's change in volume send a negative pressure wave back up the pipe it's coming from (often, possibly wrongly, referred to as "back pressure " ) that helps to scavenge the combustion chamber by that negative effect just as the exhaust valve is coming to a close. This to improve spent combustion extraction and cylinder efficiency. Also, as timing overlaps on camshaft to have both valves open at that stage, it helps to pull in the next intake volume from carburettor.
That whole process, from the headers, is usually designed into system by altering header bore/volume/length to specifically "harmonise" at peak engine torque RPM (is that about 8,500 on 400 ?) This to bring peak efficiency in output and so maximum effort from the combustion.
The rest of the pipe, out through silencer to end exit, effectively does the same for much lower rpm. The overall pipe length being so much longer causes that negative "signal" to take longer in getting back to combustion chamber, usually arranged by the manufacturer to harmonise with low rpm running to help with engine response through that often used range.
The whole ensemble aimed at best combustion potential enhancing "tune" through the operating range.
As extreme example in contrast, a F1 naturally aspirated engine will have extremely short, wide bore, truncated headers, very short collector/outlet etc, all focused just on absolute peak very high rpm output with no other consideration.
At Last!! I was beginning to think it was just me, but in the past I was regularly mocked when I criticised replacement systems that siamesed No. 1 and 2 cylinder, and then No. 3 and 4 cylinder. They never sounded as good because the exhaust pulses were not even. My "education" for this dates back to the late 60's at NSA sprint meetings. There was a guy called Mick Hands (I think). who sprinted a disgustingly fast supercharged 250 Honda, which I assumed was a CB72. When I spoke to him about it, he informed me that it was actually a C72, as the supercharger needed a 360 degree crank with even firing interval to work properly, pointing out that is why the CB 72 and 77 models had to have twin carbs.
-
Good memory of detail TM :) I recall that bike and Mick, but not knowing the details.
Some of my foundation of this comes from that era, in which Piper supported a CB 750 based drag bike with good record of tuning it. Just as important they published a very good manual of just what and how they researched and produced things, a very illuminating tome it is too.
How prescient/advanced of Mr Hands as that is an important element of current F1 engine that are Turbo/supercharged, in that the firing order of the V6 is arranged to deliver specific characteristics to turbine housing, the two banks having separate entrypoint to that cool side of turbo (they've electric drive too in helping fill in off boost dips, making it hybrid "super" orientation) all to drive the torque curve at maximum spread. This also facilitated by early closing of inlet cam timing as they produce so much intake pressure that closing before bottom dead centre artificially raises the final compression ratio available to them, over and above a pure geometric calculation in atmospheric engine. They use steel pistons to take the loading too.
In that contemporary era for CB 750 I designed and made an exhaust for my K2 that had very long narrow primary pipes, with short muffling end after collector, this looking for maximum torque. It felt good to me, but lent to a friend who was running 750 in casual form drag racing (he'd been running up to then "drag" pipes, that sounded odd and sort of flatulent) but with that experimental pipe his bike ran consistently 0.7 sec faster in 1/4 mile ;D it just came out of the start faster with that fatter torque curve, and not get caught again time wise before the finish. Interesting to get empirical data on something I made to good effect.
-
FWIW, in my old drag racing days (SS/F Dodge Challenger) we carried three different length collectors that we would bolt on to "adjust" engine power/traction/track/weather conditions.
-
FWIW, in my old drag racing days (SS/F Dodge Challenger) we carried three different length collectors that we would bolt on to "adjust" engine power/traction/track/weather conditions.
Sounds interesting, I like a potent V8 on "clattery" exhaust, even nice when running slow too.
It's a surprising effect just how much influence they can make in engine response isn't it.