Exhausts

[speedy king]

How vital is the condition of the exhaust to the power of the engine? I'm asking as i was about to falk out a large amount of money on either an engine re-build, new carb etc. as i'm losing some power on my rivals when i considered that last year i had a huge knock in the back that has left a MASSIVE dent in my exhaust underneath it (this is a 125cc Rotax exhaust) it had to be re-welded but i can not stress how big this dent is. I was told however that this would somehow help performance as it would act as compressing the gasses?

Would this hinder the performance of the engine as i read somewhere that the condition of the exhaust is vital for engine performance?

[1200Datto27]

Some back pressure is good, especially for making torque, but to much will strangle the engines power higher in the rev range, as it will not be able to breathe properly.

[speedy king]

about 50% of the underneath part of the exhaust is pressed in

[1200Datto27]

I would hazard a guess that it is to much damage done, you can usually cope with around 10% reduction in diameter, but anything more is going to cause an issue. See if you can borrow one from another driver, and you will be able to find out if it makes a difference.

[#15 tkm 4/]

do you have a picture of the damage?! just so we can see how big you are talking about

[speedy king]

I will try to get one. To try and put it in to words the big fat bit (ye, im good at this!) which i believe is the combustgion chamber where the exhaust gasses get turned around and go back to give more power, 50% of that is knocked in if not more than 50% massive knock in it. I will try and get a picture

[Superliner II]

A dent that big in the expansion chamber will cause a serious power loss, definitly not a gain, whoever said that was talking out their a***.

When the exhaust gasses get to the divergent cone part of the exhaust they expand which causes a negative pressure wave to travel back up the exhaust and pull more fresh charge into the cylinder while the transfer ports are open. If theres a dent in the exhaust gas can't expand as much so less negative pressure (vacuum) therefore less power.

When the exhaust gasses meet the convergent cone part a positive pressure wave travels back up the exhaust to stuff any fresh charge that has escaped out the exhaust port back into the cylinder. The dent in the exhaust will seriously reduce the effect of the returning pressure wave which should mean that you have more fresh fuel mixture that was meant to be pushed back into the combustion chamber escaping out the exhaust.

I had a small dent on my exhaust after it had come loose and fallen onto the brake disc. I felt the difference. I brought the exhaust to a local crash repair guy who had a gadget to pull the dent out without cutting it open. The motoX boys get their dents removed by plugging the ends of the exhaust and pressurising the inside while sometimes heating the area to be pushed out.

Whatever repair you make to it the exhaust will not be as new good ever again so a new one is your best bet if you want to be as fast as you were.

[speedy king]

The accident actaully happened May last year! I've done basically a season and a bit like that and i always thought i struggled on power but thought other people just had superior engines. However it occured to me yesturday that when i bought the kart from a team (i was a novice so wasn't even on the pace) i was leaving the others for dead along the straight until i had the crash. Definately will try to get the new exhaust before the race on saturday, i want to win this championship!

[hgmonaro]

I don't know anything specific about Rotax exhausts (they look a lot more complicated than our Yamahas) but for Yamahas there was huge differences in pipes. Some had different pipes for different tracks. So if the dint is still there.. get a new one quick! Same goes for the exhaust to header joiner (flex or solid) Make sure that's in tip-top condition too.

[Revracing]

Another point to think on is carbon build up, not only does this add weight but it also looses power, the only advantage is the engine will run quieter.

We used to periodically burnout our exhausts to keep them in as new condition...

[Alex Hodgkinson]

"I was told however that this would somehow help performance as it would act as compressing the gasses?"

Think about that scentence while you bear in mind that that part of your exhaust pipe is called the expansion chamber...

[1200Datto27]

Who told you? a fellow competitor? if so, maybe his thoughts where less than pure

[speedy king]

Originally posted by Alex Hodgkinson
"I was told however that this would somehow help performance as it would act as compressing the gasses?"

Think about that scentence while you bear in mind that that part of your exhaust pipe is called the expansion chamber...

I didn't actually know it was the "expansion chamber" when i posted this thread if you had noticed...

[Alex Hodgkinson]

lol yeah I kind of guessed Didnt mean to cause any offence if I did.

An expansion chamber works kind of like the inside of a wind tunnel, but in reverse.
It accelerates the burnt gasses out of the exhaust port (because the gasses have to suddenly fill a larger space, so they speed up) so that the next detination inside the engine can happen with as high a air/fuel content as possible (ie. with no carbon dioxide kicking around any more.)

So.. a smaller expansion chamber will result in more carbon dioxide being held inside the cylinder head each cycle, resulting in less efficient detinations, therefore less power.

[Alex Hodgkinson]

ooooh...Superliner II explained it better, listen to him!

[Superliner II]

Here is a kick a*se animation of the 2 stroke cycle that clearly shows the role the expansion chamber plays.

http://www.maximumsled.com/mountain/..._animation.gif

[speedy king]

Cheers for that.

Got a new exhaust, raced today HUGE difference! I wish i had it to start the season. I went from the back of the grid to 3rd and half a second off the winner over the finish line. Massive amount more power, couldn't believe it