Fitting a performance 4 into 1 exhaust system to the SOHC 750-4:
1) What's presently available new and is there a favorite for looks and
performance?
2) Was there a 4 into 1 in the past (i.e no longer manufactured) which was a
favorite for performance?
Again, my thanks to all contributors on the list.
Roger
In general, the present 'crop' of 4-1 pipes does little or nothing for the performance in terms of header effects. Like Scottly said, the lost weight over some of the stock pipes (post-1975 models) added some pep to the bikes, though. In the "Thoughts of Hondaman" section, you'll find a post regarding performance of the 4-1 pipes of the early 1970s era, where only the old Hooker actually made any measurable difference, and it wasn't much.
Most of the vendors indicate that "the mainjet should be increased [some say 'how much', as well] when installing [the pipe]", which will always make the bike feel stronger, if it doesn't start fouling sparkplugs in the process. Today's fuels, with ethanol, usually run quite lean, so increasing the mainjet with the existing pipes has the same effect. I have noticed that the current vendors are careful to not mention that their pipe is the sole source of the power increase, though.
Some history: only the Yoshimura pipe(s) of the 1970s and the Dunstall 4-2 pipes make a significant HP difference FROM THE PIPE(s). The primary reason: these pipes are considerably longer than the bike, which is what it takes to make extraction suction viable with the high engine speeds and small valves involved. If you search the internet for "Mike Hailwood Honda Six" you will no doubt find the famous poster of him, riding away from the camera on his pipes, with all 6 megaphones pointed right at you: these indicate well just how long the pipes need to be in order to 'suck'. That particular bike is a 250cc 6 cylinder, DOHC GP bike from 1964-66 Honda racing days (they also made a 4-cylinder 250, 125, and even a 50cc twin GP bike in those days). The pipes themselves are nearly the length of the entire bike, on all of those bikes. The 750's situation is similar.
