modified stock exhaust question

[whiterabbitt]

has anybody cut their stock megs down? I am thinking of cutting them off just behind the rear hanger, as that is where the last rust hole is, and something inside is making a lot of noise.

I will keep the airbox sooo....

 

[whiterabbitt]

well I did it, and I now know what was making the racket, the baffle tube in the small end of the r/h meg was rotted free and rattling around. when I cut the pipe 1in from the rear hanger this tube just fell out, so I cut the left baffle out with a hacksaw blade and about 8 strokes. I think it looks hot, like those rockets with the little pipes sticking out from under the swinger. I hope to post some pics soon. I may be adding aftermarket or homemade baffles, we will see how it goes

I was wondering what the experts recomend for a jet size to start with, keeping the airbox.

 

[Nessism]

For the sake of motorcyclists everywhere, please put some mufflers on that beast of yours. It's your right to ride a motorcycle, but not to annoy everyone within earshot of your open exhaust.

Harley mufflers are typically very cheap to purchase and readily available. Any muffler guy should be able to weld them on for you for next to nothing.

 

[whiterabbitt]

I did not ask for your opinion of my choice, and even as it is it is quieter than every harley in my neighborhood and there is one at every other house it seems.

I would still like to know what a good starting point for tuning would be

thanks for the advice in advance

 

[Nessism]

I did not ask for your opinion of my choice, and even as it is it is quieter than every harley in my neighborhood and there is one at every other house it seems.

I would still like to know what a good starting point for tuning would be

thanks for the advice in advance

Sorry but I refuse to help people with open exhausts tune their bikes. Some of the more open minded folks around here might though.:-\\\

 

[7981GS]

As you are keeping the stock air box, I seriously doubt that it would need much if any jet changes.
Perhaps a little richer on the air screws?
Read the plugs after a ride or two.

Daniel

 

[8barreldart]

I cut down my stock mufflers about where you did and then ordered some 12" cheapo mufflers from JCwhitney. used the stock heat shield to cover up my ugly welds, Sounds good, had my buddy ride it up and down my street and it is much quieter than 3/4 of the bikes around. Not a expert on jetting but maybe go up a size on the mains, little out on the fuel screw . heres a pic of the mufflers if your interested

 

[7981GS]

I have friend that bought a "take off" HD Sportster muffler for $20.00 off of Ebay
and installed it on his Suzuki 650 Savage.
It sounds better than stock!

Daniel

 

[whiterabbitt]

I cut down my stock mufflers about where you did and then ordered some 12" cheapo mufflers from JCwhitney. used the stock heat shield to cover up my ugly welds, Sounds good, had my buddy ride it up and down my street and it is much quieter than 3/4 of the bikes around. Not a expert on jetting but maybe go up a size on the mains, little out on the fuel screw . heres a pic of the mufflers if your interested

thats about what I have left. good looking bike btw, love it!

I stuck some 107.5 mains in and still lean, has no topend power, tops at a ton like it is out of breath, thinking 115 mains, helpful input welcome.

to Nessism and others,
I understand that a lot of people have issues with loud bikes. I dont live in california, there are no noise laws in the cornhusker state regarding motorcycles, and you cant hear my bike from 6 blocks away idling at a stoplight like a ton of the h-d's in my area. After riding around a bit yesterday I am aware of the fact that in order to be annoying I have to be speeding too or spending entirely too much time in 1st gear. oh yeah I just cant leave out all the rockets that run around without silencers at 10k+ rpms, you can hear some of them a mile off, IN TOWN.

 

[Don-lo]

You need to realize that in high performance motors (Suzuki vs HD), cylinder filling and evacuation are very dependant on exhaust back-pressure, which you have effectively eliminated. I doubt the motor will ever run very well without some back pressure restored.

 

[Guest]

You need to realize that in high performance motors (Suzuki vs HD), cylinder filling and evacuation are very dependant on exhaust back-pressure, which you have effectively eliminated. I doubt the motor will ever run very well without some back pressure restored.

agreed in order to make that motor run anywhere near correctly you are going to have to bring that exhaust system together some how to scavenge some back pressure, you can use some shorty mufflers , 0r even if you want it that loud see if you can find a four into one header and run it through an open meg or some thing the 4into 1 will work as a collector and create some back pressure.

 

[GS750]

You need to realize that in high performance motors (Suzuki vs HD), cylinder filling and evacuation are very dependant on exhaust back-pressure, which you have effectively eliminated. I doubt the motor will ever run very well without some back pressure restored.

agreed in order to make that motor run anywhere near correctly you are going to have to bring that exhaust system together some how to scavenge some back pressure, you can use some shorty mufflers , 0r even if you want it that loud see if you can find a four into one header and run it through an open meg or some thing the 4into 1 will work as a collector and create some back pressure.

I totally agree!!!

My GS750B had a 4 into 1 exhaust with a large aftermarket can style muff of unknown brand, I fitted the wiseco 850bb kit to it approximately 2 years ago and at the same time I overhauled the head, valves, ignition was replaced with new coils (which were matched to the boyer brandsen electronic ignition that I had on it already).

I have K&N pods as well, and always found that it was a real PRICK to get it running right, even with all the "good tools", vacume synchroniser, colourtune plugs etc etc......

It was easy to tune but it was such a major pain in the ar5e to actually get it running right over all ranges. I could tune for low speed and it would be a dog at high speed, could tune for high speed and it was a dog down low, could tune for mid range and it was a total bastard at both high and low speed....... I was pulling my hair out and after about 15 jet changes, carby stripping, changing this, that and the other, I simply decided to just put up with it.

I recently got myself a parts bike which had a Tranzac system on it, and since the muff was much smaller than the one my GS750B had on it, I decided to "try it" on my bike, after all, what did I have to loose????



Well, the difference was UNBELIEVABLE!!!! I thought that my bike ran like an "older 850cc" after I did the motor to 850bb, but once the Tranzac was installed.... HOLEY CRAP!!!!

It goes SO NICE NOW!!!!

And it SOUNDS GOOD TOO!!!

My $0.02 (including G.S.T.)

 

[whiterabbitt]

Ok thanks for the info, I am not a performance tuner, just learning. Makes those baffles seem a bit more attractive...... Now to make or buy em? after seeing what was inside the originals, and a few cheapo's, I think I can make something like it.

I would like to note that I really like the way this looks, and just can not argue with how solidly the stock pipes are mounted, so the idea is to run with it, headaches and all.

heres to a new season of fun!

 

[Tom R]

Backpressure bad! Velocity Good.

 

[whiterabbitt]

Backpressure bad! Velocity Good.

I dont understand, which is it. backpressure to scavenge or velocity, I thought backpressure helps with low end, velocity with high end.
the stockers have a tube about the diameter of the header tubes that runs all the way through from collector to last baffle, with a few holes in it, (part of the reason I didnt think it mattered that much, I could see all the way through the pipes) nothing but air pressure to slow the exhaust gasses, unlike harley pipes that have the tube blocked off to force the exhaust through the packing, or some cheapo's that use a screen rolled into a tube and attached to a washer or plate of sorts.

 

[Don-lo]

Backpressure bad! Velocity Good.

I think this post is being sarcastic, since the format is Neanderthal-speak.

Things are happening fast as your valves open and close (50 times a second @ 6000 RPM). Your engine was DESIGNED for back pressure, and that slows the exhaust speed a little. If it comes out too fast, it creates too much of a vacuum in the exhaust port. This is a problem because of your valve overlap. Just before the exhaust valve closes, the intake opens so it can push all the burned gases out the exhaust. It is an exactly timed process. If there is too much vacuum at the exhaust, good air and fuel will get sucked out also, shorting the mixture in the cylinder and weakening combustion. The extra fuel and air in the exhaust also can ignite, causing backfiring.

Make sense?

 

[whiterabbitt]

Ah Ha! soo thats why I lose power at high speeds, vacuum! the air rushing past the exhaust is sucking my mix right out the tailpipes, or something like that, right? I havent experienced any backfiring except when I close throttle real fast at medium speed but it is more like the popping sound sportbikes have at trackday than a big bang like a car makes.
If I stick something like "torque cones" in my pipes it will help all that?

thanks Don-lo you are a great help in my quest for knowledge.

 

[tkent02]

That's a really sweet 550.
My 650 has the harely mufflers and it sounds great, maybe a bit louder than stock but not much.

 

[Tom R]

It seems to me that backpressure is a bit of a misused term. I can't see the advantage of having your cylinder push against additional force to clear the exhaust gasses. In order for scavenging to take place the pressure on the exhaust side has to be lower than that on the intake side. The faster a volume of gas moves, the lower the pressure (this is why carburetors work), which is why the exhaust velocity is important to create a lower pressure in the header and increase scavenging.
Like everything else there must be a compromise. What rpms, timing and displacement your running determines the amount of exhaust your trying to ram through your pipes. As a rule of thumb, smaller pipes are better for low rpm because this is where they produce the most scavenging effect, but result in backpressure at higher rpms. Bigger pipes produce less velocity at lower rpms, and therefore, little or no scavenging - but at higher rpms (if your cam allows) they come into their own.
I'm sure I missed something somewhere - but it's late and my bed's calling.

 

[Don-lo]

The air column that is passing through the head has momentum, so the pressure outside the intake valve will always be high, and the pressure outside the exhaust valve (after the bulk of gases have rushed out) will be low. At TDC after exhaust, there is a small slug of gases on top of the piston that cannot be pushed out, and "scavenging" does not pull them out because it is a dead end cavity. At this point, the intake valve opens just before the exhaust closes (valve overlap), and the slug of gases is pushed out the exhaust. This is a very precise process that is determined by the cams and aligned with the intake and exhaust pressures. You want the exhaust valve to close when the slug has been pushed out, before the fresh air/fuel can get sucked out. Backpressure is a very simple concept: it is the resistance of the air going out the exhaust valve. If there is less resistance to air escaping, then more air will go out during valve overlap. This means fresh air/fuel will be going out too, decreasing the air/fuel for the next combustion.

Scavenging is a complex term that is more often misunderstood and misused. It is NOT the process that removes the last exhaust pocket from the combustion chamber (see above), but rather it promotes an advantageous back pressure wave that cuts off the end of that slug. Using back pressure waves has dramatic effect in 2 stroke expansion pipes: a reflected sound wave in the pipe cuts off the changeover at the transfer port. 4 strokes use precisely timed valves, so there will be no dramatic change. In the 90s, Yamaha put EXUP valves in the exhausts of their FZRs to decrease back pressure at high RPM, but now engines rev higher with more power without such aids. Also, look at a turbo collector pipe sometime: extremely short pipes running immediately into a blockage (turbine). Do you think there is any "scavenging" going on there?

Suzuki exhausts are a system coordinated with engine setup to give optimal performance over a wide range. Changing exhaust sizes or removing mufflers will change back pressure, and the motor setup will no longer be optimal. It can be readjusted, with slotted sprockets, precision tools and accurate knowledge (not just buddy's hunch), but it can be a lot of work. The science for this has been around a long time. In fact, Suzuki got it very, very right a long time ago.