Popping Exhaust

[Guest scudoman]

Hi. I got a s7 with the usual 2 litre Pinto in it.

 

My question is whether popping on the over-run is normal, considering the length of the side exhaust and absence of a second silencer.......any ideas, I don't mind it as long as its not caused by valve problems etc...

 

 

Tony

[speedtripledan]

Mine does it even with a perfectly dyno tuned engine its unburnt fuel exploding in the exhaust, at night i see the odd flame shooting a foot out of the pipe

[Longboarder]

Very common. Mine does it even after rolling road and aftermarket injection. At least partly to do with the short exhaust design and worse if there is an air leak into the exhaust. Carbs do overfuel on overun and it doesn't mean the cam timing is necessarily wrong.

 

Nigel

[Guest Botbuilder]

Make sure there are no leaks on the exhaust. If the exhaust sucks air through a leak on one of the exhaust or manifold joints any unburnt fuel will pop. Without air/oxygen in the exhaust the fuel can't burn

 

[Guest chris brown]

Make sure there are no leaks on the exhaust. If the exhaust sucks air through a leak on one of the exhaust or manifold joints any unburnt fuel will pop. Without air/oxygen in the exhaust the fuel can't burn

 

Good theory Ali but I have not yet found someone with a hood that doesn’t pop away at some point on the overrun mine does it with just a little bit of throttle applied

[Guest peter2b2002]

my efi pops on over run , but if I give it a bit of throttle it won't

peter2b

[Guest Botbuilder]

Good theory Ali but I have not yet found someone with a hood that doesn’t pop away at some point on the overrun mine does it with just a little bit of throttle applied

My 2B used to pop and bang on the overun until I fixed the leak where the silencer joined the exhaust. I bit of Gun Gum corrected it. It's quite a common problem on old motorbikes too, simple physics really, no oxygen no combustion, no pop and bang

[Guest scudoman]

Thanks for the input lads.....feel better about it now but i will check for leaks.

 

Tony

[Grim]

First point.

If there is an air leak in the exhaust because of the pressure gasses come out, oxygen can't get in.

 

Air will get in, especially on over run. This is due to Bernoulli's principle. To paraphrase, the faster gasses are moving, the lower pressure they are.

In this instance the pressure will increase at higher rpm/heavier throttle, but remains low on over run.

Another example is an aeroplane wing - the air on the top surface of the wing has to travel further due to the shape of the wing, thus travels faster and creates a low pressure region and lift (as an aside, another component of lift is due to the angle of the wing).

 

 

Secondly, there are pressure waves in the exhaust, high pressure regions followed by low pressure. These may also reduce the pressure enough to draw small amount of air in.

 

The leak will be in the exhaust.

[speedtripledan]

Air will get in, especially on over run. This is due to Bernoulli's principle. To paraphrase, the faster gasses are moving, the lower pressure they are.

In this instance the pressure will increase at higher rpm/heavier throttle, but remains low on over run.

Another example is an aeroplane wing - the air on the top surface of the wing has to travel further due to the shape of the wing, thus travels faster and creates a low pressure region and lift (as an aside, another component of lift is due to the angle of the wing).

 

 

Secondly, there are pressure waves in the exhaust, high pressure regions followed by low pressure. These may also reduce the pressure enough to draw small amount of air in.

 

The leak will be in the exhaust.

 

[Guest mower man]

Air will get in, especially on over run. This is due to Bernoulli's principle. To paraphrase, the faster gasses are moving, the lower pressure they are.

In this instance the pressure will increase at higher rpm/heavier throttle, but remains low on over run.

Another example is an aeroplane wing - the air on the top surface of the wing has to travel further due to the shape of the wing, thus travels faster and creates a low pressure region and lift (as an aside, another component of lift is due to the angle of the wing).

 

 

Secondly, there are pressure waves in the exhaust, high pressure regions followed by low pressure. These may also reduce the pressure enough to draw small amount of air in.

 

The leak will be in the exhaust.

Thanks Grim after 45 years of knowing what it does I now know why and what its called regards mowerman

[Grim]

that equation is only valid for incompressible fluids.

[Guest woolly]

I use Mr Bernoulli's principle, I put holes in the bottom of my boat to drain the water out. Works great till you slow down, then you sink!

 

i also use Mr Bernoulli's principle, to keep me flying Works great till you slow down, then you don't fly

 

i also blame this bloke for making me change from clam shell wings to cycle wings

 

all in all he has a lot to answer too

[agent_zed]

p=rho does this bit stand for 'Robin Hood Owner'?

 

I thought it was soemthing like the venturi effect where a fluid passes an opening it draws air in?!

 

either way when i re-sealed my exhaust at the manifold to exhaust joint mine stopped popping as badly.

[Guest Botbuilder]

I don't know if it's Bernoullis Law or what but depending on your valve overlap etc. etc. The pressure in the exhaust system can become negative on the overrun and it will suck air allowing any unburnt fuel to ignite in the exhaust causing the popping and banging.