Yamaha TRX850 Enthusiasts Forum

Ok, so the question is, what am I better off doing? I already have a pair of staintune slip-ons mached with my stock headers. Am I better off keeping these, or will a full OVER system do a better job. I have heard that the OVER's a louder (is this true?), but are there any other advantages/disadvantages over/under the system I have already?

I have searched a little on here but cannot seem to find anything definitive

stock headers are very restrictive (mainly due to narrow bore and sharp bend at head)

big bore headers will improve gas flow

So are the OVER headers naturally a big-bore system?

Also is there a comparison in weight between them or are they similar?

Overs are a bigger bore system. 3mm in diameter larger at the head. Probably no weight advantage with all the springs used to hold it together. Ground clearance issues which require more finesse when hopping curbs and speed bumps. Also had to modify my belly pan to suit.

In the dim and distant past I dynoed my 98 green TRX with stock pipes, staintunes and staintunes without baffles. The staintunes were 2 hp up with baffles and 3 hp down without! However the sound and the 3kg plus weight reduction made them worthwhile. However for power delivery and top end the Overs will eat the staintunes for breakfast, lunch and dinner!

exhaust = max velocity
you want that air moving as fast as possible (450feet per sec is like, best).
stock headers move around 315ft/sec.

OVER actually DECREASE velocity, but increase cfm on a "stock" motor.. But if you have FCRs, holy crap, they are way better.

You will get power on a 100% stock motor with OVER full exhaust. But every time you do a mod, you get more power because the exhaust starts to work better.

Well FCR's are a plan in the future, but living in Australia they are going to cost a mint.

But if max velocity is the best and the OVER system will decrease this, will I gain power or lose when fitted to a stock motor?

you will gain power by having OVER, on a stock bike.

The more mods that you do, the more efficient the OVER exhaust becomes.

put a straw in your mouth. walk around the house using only the straw to breathe. Now run up and down the hall and try to only use the straw.
This is "running out of breathe" that a motor experience with a restrictive exhaust. You can only "unleash" a certain amount of power.

Now use a larger tube, like alfoil tube, dunny roll tube, etc (pref a clean one).
Repeat experiment. Notice you are able to run faster ?

Look at an olympic sprinters mouth. When they do the 100m dash. They dont hold their mouth open like a fish, they slightly restrict it to get max air speed over volume.
http://au.youtube.com/watch?v=4rBhV5RKV ... re=related - 1:50 has a reasonable view of this.

Its the same as the old "thumb over the garden hose" spray trick. The actual volume and pressure of the water coming out of the hose is a constant. If its closed over a bit, you get alot more velocity. But close it over too much, and you choke it off.

Hope this helps you a bit.