80 kz650 exhaust

kzbobber2 wrote:

it would run fine even without the mufflers,ive seen it done,with a rejet. motors a motor right,just got headers on my race car

I'm sorry. You are right - a motor is a motor and a KZ650 will run great with no exhaust back pressure - you just need to jet it correctly. Enjoy

its all good,i wouldnta said that but you caught me in a funk. cheers man

i have sporty mufflers now,to chase the deer away,dont sound bad

I'm curious to learn more about this topic. I want to change the can on my 650 as well to a straight pipe. Would I have to rejet? What size?

How does "back pressure" play into all of this? Will it decrease performance?

Gorham wrote:

I'm curious to learn more about this topic. I want to change the can on my 650 as well to a straight pipe. Would I have to rejet? What size?

How does "back pressure" play into all of this? Will it decrease performance?

The effects of undesirable and desirable backpressure is a science unto itself. Engineers have spent tons of $$ and time researching and experimenting in this area tuning exhaust systems to extract maximum power and efficiency from engines. There is no one "universal" exhaust that works best with all engines; for each different engine there is an exhaust system that works best. To give you an idea of the thought that goes into designing an exhaust system and tuning back pressure to provide the optimum pressure wave and scavenging effect, here's a paragraph extracted from WIKI:

"The exhaust ultimate power valve (EXUP) is a device fitted to selected Yamaha motorcycles (FZR, YZF, XV1900, R series,MT-01) and snowmobiles (Apex) that constantly adjusts the back pressure within the collector of the exhaust system to enhance pressure wave formation as a function of engine speed. This ensures good low to mid-range performance.

At low engine speeds the wave pressure within the pipe network is low. A full oscillation of the Helmholtz resonance occurs before the exhaust valve is closed, and to increase low-speed torque, large amplitude exhaust pressure waves are artificially induced. This is achieved by partial closing of an internal valve within the exhaust — the EXUP valve — at the point where the four primary pipes from the cylinders join. This junction point essentially behaves as an artificial atmosphere. The alteration of the pressure at this point controls the behavior of reflected waves at this sudden increase in area discontinuity. Closing the valve increases the local pressure, inducing the formation of larger amplitude negative reflected expansion waves. A servo motor controlled by the ECU opens and shuts the valve. The EXUP valve operation goes from being almost fully closed at idle speed, through to being fully open at 9000 to 11000 RPM (2500 RPM for XV1900).

The Triumph Daytona 675 motorcycle also has the EXUP valve."

Here's what an EXUP looks like:

EXUP

Now I'm quite sure some Yamaha owner somewhere will say "I just cut the exhaust system off near the bottom of the frame in front of the EXUP and now the bike performs better" but I am skeptical of such statements to say the least. Ed

Thanks Ed.

Given that, I agree in general to an extent that when our bikes were designed they weren't just slappin' parts together and it's best to use the stock setup when it's available. But I also think restoring the bikes I have back to stock is boring...

Also my 650 didn't come with the stock exhaust. I have a 4-1. It sounds good, but I want something louder that's simple, cheap, and looks good. Like a straight pipe or turn out.

I understand stock pipes are very hard to come by, plus many folks prefer the 4 -> 1 pipe. No problem, some 4 -> 1 pipes look and perform great. The point I was making was that simply cutting off the pipes will be detrimental to performance. Best test to prove this theory is to remove the pipes totally (now we're really talking zero back pressure) and see how the bike runs. :laugh: Years ago my friend tried this with a small Honda and started it up in his basement. His mother, who rightfully never fully trusted him or me, started screaming down the basement steps to leave her washing machine alone! :laugh: Ed

would removing the baffles from the exhaust of my 1980 kz650 effect its performance?th

Of course; it will make it slow and loud.

Yes. (see 650ed's reply)

PLUS!
Ride by a cop here and you will get
a $250 first time fine
a $500 second fine
a $1000 third fine
the next time you will appear on a milk carton

Ride by my house once and you will most likely go free
Ride by twice and you might get an ice cube in the head
fired by a surgical tubed wrist rocket slingshot. (my uncles favorite)
Ride by repeatedly and you will appear on a milk carton.

Most people who would cut off mufflers are not going to go through the time or expense of getting it to run right again and will either junk it, part it out or sell it off at a loss.
Most of them are still young and stupid and karma will make sure when they get some girl preggers and have a kid and then there will be some new kid who decides it is HIS right to ride a horrible sounding/running noise maker and make this new baby wake up every night at 3am screaming/pooping and peeing. OR even better he wakes up some neanderthalic ape who hunts down the smart ass punk and slaps some teeth out of his head.
And do not go with the loud pipes saves lives argument. It has been disproved time and time again. Loud pipes causes fatigue, hearing loss and startles drivers causing dangerous situations. You are the one riding on an open hunk of metal and guess who is more vulnerable?
Oh yeah and loud pipes do not make your junk any bigger. Just your brain smaller.
For the record I am not talking about a aftermarket tuned exhaust I am talking about open pipes with no baffling. Such as those who merely cut off their mufflers or run straights.