Topic: Reducing back pressure in stock exhaust

[Redrider969]

I recently bought a 1974 CB750 with less than 7,000 miles on it (good news).  It was stored in a leaky shed for about 25 years and one of the pistons is frozen in the barrel (bad news).  Since I have to do some top end work anyway, I'm planning to bore the bike out to 836cc and install a new cam from Dynoman.  The folks there tell me these mods will not increase horsepower by much unless I add a less restrictive exhaust.  They recommend a Kerker 4 into 1 system. 

I really don't want to lose the look of the 4 pipes and would rather modify the stock pipes to reduce back pressure.  One suggestion was to gut the pipes and line the inside periphery with a fiberglass sleeve.  The question is how to do it.  Any insights on how to gut the stock pipes or other methods for reducing back pressure will be appreciated.

I should mention that the pipes on the bike are completely rusted out.  I am planning on buying a new set of stock pipes, and can get either the older "300" series or the newer "341s."  Since the older pipes have less back pressure, they might be a better place to start, but since I really don't know what the insides of either set of pipes look like, I have no idea if one would be easier to gut then the other  or if they can be gutted at all.

[ekpent]

The HM300's do not have nearly as many built in chambers etc as the 341. The 341 does not even really have a baffle pipe per say but just a diffuser on the end of the pipe as most of the baffling is built in. The 300's have less interior chambers and a real baffle pipe that could either be modified a little or just run it without. Still pretty pricey for new but you get the look. 4-1 may be an option with those other go fast mods.