1974 Honda CB360 Garage Find Cafe Build

[Sonreir]

Do longer pipes give more back pressure ? Im just wondering !

Yes, longer pipes have more back pressure and back pressure is detrimental to an ideal exhaust system. The reason long pipes work does have to do with pressures, though.

The amount of exhaust gases that are removed from the cylinder is directly proportional to the pressure difference (called Delta P) between the gases inside the cylinder and the gases in the exhaust pipe. So basically, this means your cylinder will clear better with either higher cylinder pressures or lower exhaust pressures, or both.

So how does a long exhaust help with that? Well, it all has to do with inertia (like damn near everything in, on, and attached to an engine). When the exhaust valve in your engine opens a high pressure pulse of exhaust gases travel out of the cylinder and through the exhaust pipe. You can feel this effect just by putting your hand behind the pipe while your engine is running. What's important to us, however, is that because the gases travel in pulses, this means that a low pressure zone occurs behind each pulse. The relatively fast moving exhaust gas wants to remain moving and so it will pull along anything in its path. Because this path occurs within a tube, pretty much everything in the tube, including ambient air, is along for the ride.

Now earlier I mentioned that lowering the pressure within the exhaust will help clear out the cylinders of exhaust gases (this process is called scavenging). This is exactly what an exhaust pulse is already doing; it's lowering the pressures in its wake. However, if a pulse is given a chance to escape from the end of the exhaust pipe before the exhaust valve opens again, you're losing this low pressure wake because atmospheric pressure is now coming back in through the end of the exhaust pipe in order to help fill the low pressure zone. This also has the dual effect of causing the next exhaust pulse to slow down a bit as it encounters higher pressures within the pipe before fully exiting.

 

[crazypj]

There's also a bunch of acoustics involved.
Even with latest computer modeling, the process isn't really fully understood (just too many variables)
Empirical data collected over the last 100+? yrs has given some pretty realistic suppositions though
Pipe length and diameter can be worked out to reasonable accurate size although experimentation is still necessary for max performance.
We just need to be 'in the ballpark' for the operating rpm of engine

 

[BlueCB360]

There's also a bunch of acoustics involved.

Hell yes there is. Pipe diameter and overall length can be calculated so that harmonics will allow for resonance. Basically the sound wave achieved will help "push" out the exhaust gases. The trouble with calculating this resonant frequency is, exhaust gases cool as they flow out of the pipes. This screws up conventional calculations of which harmonics would be helpful.

 

[Sonreir]

And hence the inclusion of collectors into performance exhaust systems. Shorter sections of pipes means easier tuning and allows overlaps giving the pipes a broader "sweet spot".

 

[teazer]

OK, so the seat and pegs are coming and fork brace is somewhere waiting to be fitted. That's all good.

You might like to check out another DTT post http://www.dotheton.com/forum/index.php?topic=29489.190 for comments on frame and fork braces.

Front forks do still look slightly bent, but could be perspective. I'm not busting your chops, just making observations/comments. Try taking two straight rods and place one across the fork legs just below the top clamp and the other as close to the axle as possible and now look down on the two from above and any out of square is magnified by the cross rods.

BTW, mufflers are there to improve performance and avoid pissing off other road users and neighbors. Loud pipes lose ground as they used to say. They don't have to be stock (in my book at least) but they do need to take the bark out of the exhaust note. Noisy pipes are fine at the race track, but useless and annoying on the street.

 

[trek97]

What...Now i have to hire Stephen "friggin" Hawken, to interpret posts???? Why is it so difficult to just say "A little longer pipes may allow your bike to run better. Now Shut the F up and do it...or dont!

 

[Sonreir]

Well, I kind of view myself as a skeptic with most things and so some explanation of the whys and wherefores is required if you want me on board with any given concept. Anyone unwilling or unable to explain the theory and/or practice of an idea shouldn't really be passing it along. I expect that there are others like me around and so I do tend to go into detail on any given concept. I can include the Cliff Notes in the future if you'd like.

 

[crazypj]

What...Now i have to hire Stephen "friggin" Hawken, to interpret posts? ??? Why is it so difficult to just say "A little longer pipes may allow your bike to run better. Now Shut the F up and do it...or dont!

Ha Ha Ha Ha Ha
Dats funny dat is ;D

 

[ducatiboy]

Yes, longer pipes have more back pressure and back pressure is detrimental to an ideal exhaust system. The reason long pipes work does have to do with pressures, though.

The amount of exhaust gases that are removed from the cylinder is directly proportional to the pressure difference (called Delta P) between the gases inside the cylinder and the gases in the exhaust pipe. So basically, this means your cylinder will clear better with either higher cylinder pressures or lower exhaust pressures, or both.

So how does a long exhaust help with that? Well, it all has to do with inertia (like damn near everything in, on, and attached to an engine). When the exhaust valve in your engine opens a high pressure pulse of exhaust gases travel out of the cylinder and through the exhaust pipe. You can feel this effect just by putting your hand behind the pipe while your engine is running. What's important to us, however, is that because the gases travel in pulses, this means that a low pressure zone occurs behind each pulse. The relatively fast moving exhaust gas wants to remain moving and so it will pull along anything in its path. Because this path occurs within a tube, pretty much everything in the tube, including ambient air, is along for the ride.

Now earlier I mentioned that lowering the pressure within the exhaust will help clear out the cylinders of exhaust gases (this process is called scavenging). This is exactly what an exhaust pulse is already doing; it's lowering the pressures in its wake. However, if a pulse is given a chance to escape from the end of the exhaust pipe before the exhaust valve opens again, you're losing this low pressure wake because atmospheric pressure is now coming back in through the end of the exhaust pipe in order to help fill the low pressure zone. This also has the dual effect of causing the next exhaust pulse to slow down a bit as it encounters higher pressures within the pipe before fully exiting.

Isn't there also some theory that because of exhaust valve overlap on the compression stroke, in order to prevent fresh in-flowing gas from escaping out the exhaust valve there is a "reverse" pressure wave whereby the antegrade pulse of exhaust pressure bounces off the cone at the end of the muffler and travels in a retrograde direction back to the head and "pushes" fresh gas back into the combustion chamber? For this reason the "tuned" length of the header pipe, muffler and reverse cone are critical for optimal performance. Analogous to two-stroke exhaust pipes but to a lesser degree. Obviously a bike will run without a tuned length pipe. Just won't be optimal performance. In fact once many years ago after rebuilding my 600 pantah race motor I started the engine after 'forgetting' to put the exhaust pipe back on. No mufflers, no headers, no nothing. Started up fine but man!! The noise!!

 

[teazer]

Correct. That's the whole idea with exhaust design with a straight pipe. When a pressure wave reaches the end of an open pipe, the wave is reflected back in the opposite direction with the sign changed from positive to negative. That negative wave can get back to the head while the exhaust is still open, it will be reflected back down the pipe a second time and so on and so forth.

Exhaust lengths have more effect on power than one might expect and most engines work best with something in the 30-40 inch range for a straight pipe. Straight pipes also cause drag (resistance) so it's better to have a slight taper if possible.

 

[crazypj]

The other problem with a parallel pipe, even when correct length it only 'works' over a very narrow range, maybe 500 rpm at best.
I've always found Honda's work best with a shallow taper megaphone, around 3~5 degrees

 

[loganrm]

The headlight is pointing at the sky, and your key is missing too. Man this build went downhill fast. (I'm not sure witch smiley face is the one for being a smart ass)

I built in special refractive mirrors so that it can point up like that but project the light beam straight forward. I just thought it looked better like that. The ignition is like that on the newer cars. I just put it in my pocket and it fires the bike up by proximity.

 

[loganrm]

Wow, you go out of town for a few days and you come back to an all out war on your exhaust! In all honesty though, I really appreciate the comments made (including teazers!) on the exhaust and why/how it functions the way it does. As always you guys are super helpful and it means a lot for you all to take the time to post on the subject. Looks like I need to take a look at making some changes with my exhaust set up once I return home.

Teazer. Sorry for sounding like a dick. I had just gotten off one of many planes for the day and had been up for what felt like 48 hours. I might have been a bit blunt/bitchy. Probably had a little sand in my vagina.

Ok..That aside, I have been working with Xerox all week in New York and Arlen Ness has built several bikes for them. Check out this hideous monster! I'll say this about the bike. I don't like it but it is an absolute work of art and skill. It is extremely well built and a really off the wall design which I can appreciate.


Untitled by loganrm, on Flickr

Also, check out these two goobers in the "Jurassic Park" ride of the year.


Untitled by loganrm, on Flickr

While I'm at it...I picked up this book a couple days ago. It had this in it and I freakin' loved it.



Untitled by loganrm, on Flickr


Untitled by loganrm, on Flickr

 

[Ease]

Awesome! (The book)

 

[frogman]

MAN, that reminds me of the day MY dad told me he used to ride a stretched and chopped bike. Was right after I told him I bought my 360, He was like man MINE was a 360, 10" extended forks, hi-rise pull back bars, goofy ass tall seat, forward controls. Flames on the tank, chopped fenders, shorty shocks. AND I QUOTE, "After the 3rd time I laid that bitch over on that MUDDY ass road between the HOUSE and the HIGHWAY I said screw this shit, I would GIVE it away to anyone that wants it." It was 3 miles of muddy clay slip 'n 'slide, on a chop I bet it sucked.

Funny thing is, I BET I know that the guy he gave it too still has it tucked away in the back of a shed somewhere, 900 miles from here. ONE O' these days I will find out.

My dad's cool, He's one of the original Hooligans of the area, no hipster at all. ;D 8)

 

[loganrm]

Been a while on the posting here. Though I had the bike all wrapped up but BOY WAS I WRONG....

Took the bike to a mechanic to adjust the carbs and give it a once over. Test rode the bike at his shop and all was cool but when I left and got about a mile down the road the bike died. I took it back and he went over it again. calls me after the weekend and after a nice screaming match in the shop my bike now sounds like nickels are having a rave in te top end of my motor. Pulled the valve covers and I dont think there is any oil getting up there to the top end. I've got some nasty shavings starting to form. I'm not sure how bad it is yet. I'm gonna pull the motor and try to see what the damage is.

Any ideas as to whats going on in there? Another member told me that the 360 had top end oiling issues. Should I try to get a new oil pump assembly and install it or is it a lost cause at this point? The mechanic told me to find another motor and he thought his buddy had one...I told him to go get fucked.

 

[trek97]

Holy shit dude! Im sorry I cant help w the issue. But now you got me scared to death! I am halfway through the complete teardown and rebuild on my own engine!

 

[crazypj]

The pump is rarely a problem, usually someone has used silicon on gaskets and blocked oilway or left something out.
I've seen two clutch covers with the spring loaded piece that delivers oil into/out of filter missing (and heard of at least one on this site)
You don't need to pull motor to remove head or clutch cover

 

[frogman]

Yeah don't worry bud, we got plenty of experience around here to figure wtf went wrong. TAKE LOTS Of pictures as you tear things down, just incase you do find some evidence the jackass did something.

At least you have it back.

 

[loganrm]

The start was getting the bike back in my hands, which was freaking miracle actually. I did some research last night and I feel a little better about figuring it out but I am just pissed that this guy set me back so much. CrayPJ, i don't know if you want it to or not but every time I google searched my motor problems last night your name kept popping up! I will try to get that head off tonight and clutch cover to see what the damage is and if I went wrong somewhere. If you guys remember, I did have trouble getting that cover back on because of that gear not seating properly. I'm wondering if something went wrong there.