Topic: CB750 K0 muffler design

[tlbranth]

Pretty soon I'll be having a go at building a new 4 into 4 exhaust system for my CB750 K0. This will include building the mufflers. I'd like to stick to the original sound as closely as possible. So If anyone (hondaman?) could fill me in on the original muffler design I'd be all ears. Thanks in advance for your help.

[PeWe]

Maybe a member has a rotten HM300 pipe that can be cut open?
The even earlier Lotus root pipes are more open with minimum of discs inside. Darker sound.

CB750 has its typical sound despite exhaust. I replaced my Lotus Root 4-4 pipes a few weeks ago, now Hindle 4-2-1, visited a friend on the countryside that heard that I was coming for a visit. He did not know that I have switched pipes.

[evinrude7]

https://www.ebay.com/itm/Honda-CB750-Exhaust-Baffles-Diffuser-CB-750-Baffle-18311-300-320-Set-of-4-HM300-/201687094880

[70CB750]

Cyclex has headers 4 to 4, just find mufflers. 

[JamesH_Sussex]

Pretty sure there is a thread on here somewhere?

[JamesH_Sussex]

Anatomy of a HM341
 https://r.tapatalk.com/shareLink?share_fid=13548&share_tid=131905&url=http%3A%2F%2Fforums%2Esohc4%2Enet%2Findex%2Ephp%3Ftopic%3D131905&share_type=t

Here’s one on the HM341...

[JDS]

I think the baffles has a large part to play in it. I've been told there is a difference in the sound of the 300 compared to a 341 and I believe the baffles were different. I just replaced a rather rusty set of 300s on my k1 with some much nicer 341s. The 300s are going into the scrap pile and if it would help you're more than welcome to the best one out of the bunch. Maybe a forensic autopsy would reveal something.

[754]

That would be where to start , he is in WA state.

[PeWe]

I think the baffles has a large part to play in it. I've been told there is a difference in the sound of the 300 compared to a 341 and I believe the baffles were different. I just replaced a rather rusty set of 300s on my k1 with some much nicer 341s. The 300s are going into the scrap pile and if it would help you're more than welcome to the best one out of the bunch. Maybe a forensic autopsy would reveal something.

Cut open autopsy of the HM300 and publish photos here would be perfect!

[kmb69]

I think the baffles has a large part to play in it. I've been told there is a difference in the sound of the 300 compared to a 341 and I believe the baffles were different. I just replaced a rather rusty set of 300s on my k1 with some much nicer 341s. The 300s are going into the scrap pile and if it would help you're more than welcome to the best one out of the bunch. Maybe a forensic autopsy would reveal something.

Baffles are way different. The baffle really doesn't do so much in the 341 pipes as they are heavily baffled internally.

[tlbranth]

That would be where to start , he is in WA state.

I'm not familiar with the nomenclature so I'm assuming 300's go on a K0. If so I would love to have one I could take apart. Would you please pm me with details? Thanks. I'd be happy to post pictures of the construction.

[ekpent]

300's were used on the 1971/K1 also. I think member Lordmorphy or something spelled like that was once making or having some luscious looking 4 to 4 CR style pipes made once.

[PeWe]

lordmoonpie    http://www.moonpie.co.uk/
Here an interesting thread about alternative 4-4
http://forums.sohc4.net/index.php/topic,61931.0.html

[ekpent]

 Lordy - Lordy -  lordy knew it was something like that PeWe    Good memory on you !

[PeWe]

That name is unusual..lordcreampie...hmmm lordmoonpie... I checked out his site about CR750 4-4 pipes a few years ago.

[tlbranth]

JDS was kind enough to send me a rusted HM300 muffler from his K1 CB750. I've cut it open to see its innards. I think input pipe had a small hole leading to it - seems like Hondaman said something about that. That part of this muffler is a bit too rusty to tell. Mark Paris....any specs on this?
I have a million other household projects to work on but building this exhaust is something I've wanted to do for a while so I'll putter at it. It'll take a while but I'll take pics along the way.

[MauiK3]

I bought “no number” K0 style re-pops from Yamiya. They are like early K0 pipes and only have a small removable baffle at the end. Great sound. I have not run them yet with the baffles out but I bet it would be a bit hooligan. ;-))

[PeWe]

I bought “no number” K0 style re-pops from Yamiya. They are like early K0 pipes and only have a small removable baffle at the end. Great sound. I have not run them yet with the baffles out but I bet it would be a bit hooligan. ;-))

Hi,
The baffles are not that small, almost like small beer bottles in size.
http://forums.sohc4.net/index.php?action=dlattach;topic=174454.0;attach=389179;image

The later has an end plug only, HM341 style from K2.

Interesting to see that pipe opened.
My Lotus root pipes have not the inner perforated tube, plus even fatter baffles, bomb shaped. That can be verified with baffles out, a big hole in the end that connects with the other pipe connected by a rubber.

My K2 stock build runs better with the stock wool around the Yamiya No Number (HM300) baffles! Surprise before vehicle inspection to reduce sound. They will stay. Bike sound good despite those that I was sure should choke the bike. It must be the backpressure that make the bike to run better from take off and throttle response at 3000-5000 rpm.

EDIT: If you think that the more open Lotus Root pipes will give more top HP than No numbers HM300 on a modified bike.....
Surprise!
My modified K6 1005 cc + more mods lost around 4-5 hp, got more torque and earlier. This despite better jetting according to dyno and A/F meter. Had way too rich needles. This with no baffles since the A/F meter had a tube to be inserted as long in as possible. Checked all 4 pipes. 2 runs of 2 pipes at each time. 93 vs 88 rwhp.

Lotus root with baffles in had almost same top hp as out , but earlier torque response at around 2500-4000, less from around 4000-6500 rpm. Both 4-4 setups did not want to scream above 8000-8500 rpm.  OK for engine durability but not for max Hp on the chart.
Saw same thing with different cams and 836cc with No numbers. Same 836 setup with 4-1 had no problem to enter 9500 rpm, over 10.000 with another cam.

Conclusion: Yamiya No number (HM300) work really fine on a stock bike and modified if 8000 rpm is enough on 5:th gear. I love the sound and look too.
Let's see if my K6 will get its 4-4 back on, now 4-2-1 to verify more power.

Maybe others have similar experiences.

[seanbarney41]

PeWe!  you are a champion for actually dyno testing real, available parts and ride testing...blowing away myths ands legends!  Keep it up!  I have always found that exhaust mods are a crapshoot.  There are just way too many factors involved in optimizing 4 stroke exhaust...but it looks like old time Honda engineers knew something that others don't

[PeWe]

Thanks, the dyno results are uploaded in the billet block sticky thread.

I recommend the HM300 style (Yamiya no numbers) for all that prefer the old nice CB750 look. I was really surprised about my stock K2 with airbox.

I'll add charts for the 4-2-1 when done at a chop that has functional Dyno and time. I have a call to reach over 100 at +9000 rpm ....