What is Over run
Discussion
As I have been led to believe, over run is unburnt fuel being burnt in the exhaust system, and hence the pops and bangs, once you lift off the accelerator.
Most noticable on cars with carbs - i.e. old school TVRs etc...
However someone will be along with a much more technical explanation
Most noticable on cars with carbs - i.e. old school TVRs etc...
However someone will be along with a much more technical explanation
as I understand it;
- on turbo cars the turbo spins to force fuel into the engine and therefore increase power (like fuel injection)
- the turbo is driven by exhaust gases.....the more exhaust gases the faster the turbo spins..the more fuel goes into the engine.
- if suddenly you don't need all of that fuel for power (ie lift the off the gas) the turbo can't stop spinning quick enough and puts some excess fuel into the engine which the engine doesn't need
It is this that is overrun...I think!
The fuel is then pushed through the engine and can ignite in the exhaust causing the turbo bang!!
- on turbo cars the turbo spins to force fuel into the engine and therefore increase power (like fuel injection)
- the turbo is driven by exhaust gases.....the more exhaust gases the faster the turbo spins..the more fuel goes into the engine.
- if suddenly you don't need all of that fuel for power (ie lift the off the gas) the turbo can't stop spinning quick enough and puts some excess fuel into the engine which the engine doesn't need
It is this that is overrun...I think!
The fuel is then pushed through the engine and can ignite in the exhaust causing the turbo bang!!
Sharief said:
Kozy said:
Sharief said:
Overrun? What does this mean?
Off throttle where the momentum of the car is effectively driving the engine.
Pops, bangs, flames or any other kind of sight or sound is something that happens on overrun, but the above description accurately describes what overrun is.
'Crackle' or flames on the overrun is where the engine is being driven by the car's momentum with a closed (or closing) throttle and the fuelling system is still injecting fuel.
The fuel then combusts in the exhaust, giving the popping and crackling sound
It doesn't really happen on modern cars as the more advanced fuelling systems stop injecting fuel. It happens on Carb fed cars as they are still chucking what is basically Idle speed fuel down the exhaust. It happens on some modern sports cars as the manufacturers engineer a few 'squirts' to give the effect!
The fuel then combusts in the exhaust, giving the popping and crackling sound
It doesn't really happen on modern cars as the more advanced fuelling systems stop injecting fuel. It happens on Carb fed cars as they are still chucking what is basically Idle speed fuel down the exhaust. It happens on some modern sports cars as the manufacturers engineer a few 'squirts' to give the effect!
neil_bolton said:
All interesting chaps, all interesting, I stand corrected 
Why is it more prominent on cars without so much silencing?
Because the silencers silence the pops and bangs from exhaust combustion as much (or nearly as much) as normal combustion. Cats seem to provide enough back pressure to stop the pops and bangs entirely.Why is it more prominent on cars without so much silencing?
neil_bolton said:
All interesting chaps, all interesting, I stand corrected
Why is it more prominent on cars without so much silencing?
Overrun is not more prominent on cars without as much silencing. Overrun is simply lifting off the throttle whilst in gear with the car moving. Everything else is a symptom of the way the engine is set up.Why is it more prominent on cars without so much silencing?
Marcellus said:
as I understand it;
- on turbo cars the turbo spins to force fuel into the engine and therefore increase power (like fuel injection)
- the turbo is driven by exhaust gases.....the more exhaust gases the faster the turbo spins..the more fuel goes into the engine.
- if suddenly you don't need all of that fuel for power (ie lift the off the gas) the turbo can't stop spinning quick enough and puts some excess fuel into the engine which the engine doesn't need
It is this that is overrun...I think!
The fuel is then pushed through the engine and can ignite in the exhaust causing the turbo bang!!
Thats just over fuelling which in a turbo charged car causes black smoke. Overrun is just s state of the engine (as said by otehrs) when you 'let off'. There is no real drive conditions or rpm conditions to overrun. it is just a term for when the engines rpm is higher than the throttle possition can sustain and thus the revs drop. This is used obvioulsy in engine breaking as it has the effect of the engine wanting to slow back down. - on turbo cars the turbo spins to force fuel into the engine and therefore increase power (like fuel injection)
- the turbo is driven by exhaust gases.....the more exhaust gases the faster the turbo spins..the more fuel goes into the engine.
- if suddenly you don't need all of that fuel for power (ie lift the off the gas) the turbo can't stop spinning quick enough and puts some excess fuel into the engine which the engine doesn't need
It is this that is overrun...I think!
The fuel is then pushed through the engine and can ignite in the exhaust causing the turbo bang!!
popping and banging from the exhaust isnt overrun as such. It is a result of overrun. More often than not in a tuned car or carburette vehicle (as agressive cams etc mean there is more likely to be an excess of fuel over normal running conditions that the engien is designed to cope with) where an abrupt change of throttle input (ie WOT or Wide Open Throttle to no throttle input) means that temporarily the car runs rich due to the metered air flow decreasing but there is a delay between metered decrease in air volume entering and fuel injected so as the revs die off quickly there is always a little too much fuel going in which does cause some excess fuel to either still be burning as it leaves the cylinder into the exhaust or be unburnt and ignite while inside the exhaust system causing the more noticable popping.
You may also hear this popping of unburnt fuel in modern cars more often now, for example new facelift mini cooper S models with a turbo charger actually induce this noise on purpose on overrun to give the car a sports sound when driven hard (sad I know but it does work, people love the noise!).
With regards to turbo charged vehicles in particular you will also notice some very tuned vehicles seem to make an excessive amount of noise during over run. This is a result of something half way inbetween what marcellus said and overrun... anti-lag. Where extra fuel is forced through the engine or even directly into the turbo impellor and ignited to create expansion forcing the turbo to keep spinning (with boost pressure being dumped via a waste gate so that the engine revs can fall) so that the boost pressure is available as soon as throttle input is present, thus avoiding lag and creating that symbolic rally car noise
:-D
Munter said:
neil_bolton said:
All interesting chaps, all interesting, I stand corrected
Why is it more prominent on cars without so much silencing?
Did you really just ask why cars with less silencing are noisier...? Why is it more prominent on cars without so much silencing?
I mean't is there something along the lines of less back pressure, or shorted exhaust run or something that helps promote it?
Obviously its going to be louder
kambites said:
If you want your car to pop and bang when you lift off, bypass the cat (or fit a low back-pressure sports cat).
Or have an air leak into the inlet manifold, which is the most common cause of popping and banging on the overrun!Edited by kambites on Thursday 24th July 12:45
Over run in it's purest form is literally when the engine is spinning faster than the throttle should keep it at. If you're spinning the engine at full throttle at 8000RPM and then you let off to 25% throttle, during the time the engine takes to drop down to 2000RPM (assuming it's linear) the engine is "on the overrun".
It is most used when the wheels are not engaged and there is a relatively quick drop in revs, such as changing gear with the clutch in, rather than letting a car gradually slow down like dorpping from 80 to 70 on the motorway. So when someone says "overrun" they generally men when the engine is being spun over by the inertia of the flywheel and crankshaft(Remember the lesson the other day about suck squeeze bang blow?) rather than the wheels.
The reason that you get a bang/pop sometimes, which is a SYMPTOM of overrun, is that for normal operation in a carbed car, the butterfly in the carb will allow the right amount of air in for the given RPM, this air is then sucked through the carb by the downward motion of the piston and mixed with the right amount of fuel.
On the overrun, the piston still suckes the mixture in, but the throttle, which is now partially closed, lets in less air than is needed, resulting in a "rich" mixture, the unburnt extra fuel then goes into the hot exhaust, where it ignites.
It is most used when the wheels are not engaged and there is a relatively quick drop in revs, such as changing gear with the clutch in, rather than letting a car gradually slow down like dorpping from 80 to 70 on the motorway. So when someone says "overrun" they generally men when the engine is being spun over by the inertia of the flywheel and crankshaft(Remember the lesson the other day about suck squeeze bang blow?) rather than the wheels.
The reason that you get a bang/pop sometimes, which is a SYMPTOM of overrun, is that for normal operation in a carbed car, the butterfly in the carb will allow the right amount of air in for the given RPM, this air is then sucked through the carb by the downward motion of the piston and mixed with the right amount of fuel.
On the overrun, the piston still suckes the mixture in, but the throttle, which is now partially closed, lets in less air than is needed, resulting in a "rich" mixture, the unburnt extra fuel then goes into the hot exhaust, where it ignites.
Gassing Station | General Gassing [Archive] | Top of Page | What's New | My Stuff