Why does exhaust leak cause bad running?

[StuM]

Hi All,

As title really. I have read that blowing exhausts lead to bad running engines, but can anyone explain why please?

Thanks,
Stu.

[SILVERFOX2013]

hi m8 im no expert but i think it due to reduced back pressure due to hole or leaking exahust system afects combustion and exahust out cycle

[bigherb]

As silverfox says it's due to less back pressure, the system is designed for a small amount of back pressure, the exhaust and inlet valves are open at the same time for a short period at the end of the exhaust stroke so the incoming mixture purges the cylinder clear of exhaust gases, without the back pressure some of the incoming mixture goes straight through the cylinder into the exhaust system which can sometimes ignite causing backfiring.

[StuM]

Thanks both - popping on overrun too here so sounds like my clearly-blown exhaust is now primary suspect in my poor running.

I assume that the closer to the head, the worse the problem would be - ie. a hole in the silencer is less bad than one in the downpipe from the head?

Cheers,
Stu

[pionte]

Really good explanation of why a blow in the exhaust causes problems.

This is also why manufacturers spend a lot of time and money getting the exhaust bore size and length correct, and why when some pre pubescent oik decides to put a dustbin lid size silencer on his 1.1 Corse it probably goes even slower.

The average exhaust on your family runabout is a compromise. (as most things are) between cost/noise and performance.... That's why proper tuning companies will build an exhaust with exacting lengths and bore to optimise the back pressure to work in a narrow power band of revs... Coupled with camshaft and induction to suit.

This is one reason why variable valve timing works so well... It actually adjusts the timing of the exhaust valve ( usually, but inlet as well on cars like the M3) for pure HP less restriction is better but for Torque more is better, for years bikes had a valve in the exhaust that would open and close dependant on revs.... Cars have started to use them now too.

But yes... Holes are bad for performance!

[StuM]

Really good explanation of why a blow in the exhaust causes problems.

This is also why manufacturers spend a lot of time and money getting the exhaust bore size and length correct, and why when some pre pubescent oik decides to put a dustbin lid size silencer on his 1.1 Corse it probably goes even slower.

The average exhaust on your family runabout is a compromise. (as most things are) between cost/noise and performance.... That's why proper tuning companies will build an exhaust with exacting lengths and bore to optimise the back pressure to work in a narrow power band of revs... Coupled with camshaft and induction to suit.

This is one reason why variable valve timing works so well... It actually adjusts the timing of the exhaust valve ( usually, but inlet as well on cars like the M3) for pure HP less restriction is better but for Torque more is better, for years bikes had a valve in the exhaust that would open and close dependant on revs.... Cars have started to use them now too.

But yes... Holes are bad for performance!

Thanks for that.

Engine (and maybe box, not decided yet) out and into the garage over winter, so will be sorting exhaust and other bits then. One more trip to go before then, so might try a temp fix or will just put up with the bad running if not.

Cheers,
Stu