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Gary Kinghorn
31-08-2006, 03:44 PM
I know there is a relationship between exhausts, back pressure and power but how does it work?

If your bore is too large no pressure is there so power is lost

If your bore is too small you are restricting gasses and therefore power.

How do you know what the optimum is and how is the manifold involved?

Come on then clever trousers ;)

Floyd
31-08-2006, 08:12 PM
Back pressure is bad. Less resistance to the exiting gases means that there will be less 'pumping' losses. A small bore exaust will have more resistance than a large bore but it will have higher gas speed, which will aid low down torque.

The ideal exhaust would have a variable optimum length and a variable diameter, matched to revs but this isn't possible so the exhaust is optimised for the band that is most used and for quietness.

HTH

F

Dave G
31-08-2006, 10:56 PM
Can someone explain why any back pressure is good? How does it aid in increasing performance? Or how does it stop loss of performance is that a better way to put it? :)

Tony
31-08-2006, 11:37 PM
All I know is that one of the internal baffles inside my M3 exhaust has collapsed and it sounds awesome when 'pushing on' :D

Strangely I had a 205 diesel many years ago and the exhaust fell off that too from the manifold back. That sounded pretty damn good too :lol:

T

stevem
31-08-2006, 11:45 PM
'any' isn't neccesarily good, however there needs to be a balance as you'll get a higher overall throughput with a smaller pipe as the gas travels faster.

The faster you get the gas moving the better the scavenge effect when the valves are open - which means an optimal burn on the next cycle as the combusted fuel is cleaned out of the cylinder and 'new' mix pulled in.



... or something like that .... :) :unsure:

Gary Kinghorn
01-09-2006, 06:48 AM
There must be something in it somewhere else why would you need a complex manifold and not just have a pipe with a silencer on it.

See I do start interesting debates every now and then :lol:

Floyd
01-09-2006, 07:02 AM
You need a balanced manifold to create good scavenge effect as said before. Each time an exhaust valve is opened and the piston pumps the spent gas into the manifold, a ‘slug’ of exhaust gas travels down the pipe at a certain speed determined by temp and revs of the engine. The manifold will be tuned to be most efficient at a certain resonant frequency, usually in the band of most used operation. The resonance comes from the fact that the ‘slug’ will travel the fastest and with the least resistance and interference from other slugs from other cylinders in the system. If slugs hit each other they will slow up their speed, rather like traffic in a queue, which in effect causes back pressure.

You can tune a manifold for low down power, mid or high end etc but there is always a trade off compromise. Wrapping headers does 2 things; it stops radiated heat from entering the engine bay, which is good for induction temps and it keeps the exhaust gas hot, which is good for gas speed (the faster the better).

So a small exhaust will be good for low powered cars as gas speed will be high and also improving low down torque but it’ll impede high rev gas extraction. Horses for courses and all that.

F

pete_rallye
01-09-2006, 10:33 AM
So how does a turbo create masses of torque, compared to NA or even S/C? As that sits in the exhaust, it must be a restriction, or does it actually suck the exhaust gases out past certain revs?

Darren Langeveld
01-09-2006, 11:19 AM
Originally posted by Floyd@Sep 1 2006, 07:02 AM
You need a balanced manifold to create good scavenge effect as said before. Each time an exhaust valve is opened and the piston pumps the spent gas into the manifold, a ‘slug’ of exhaust gas travels down the pipe at a certain speed determined by temp and revs of the engine. The manifold will be tuned to be most efficient at a certain resonant frequency, usually in the band of most used operation. The resonance comes from the fact that the ‘slug’ will travel the fastest and with the least resistance and interference from other slugs from other cylinders in the system. If slugs hit each other they will slow up their speed, rather like traffic in a queue, which in effect causes back pressure.

You can tune a manifold for low down power, mid or high end etc but there is always a trade off compromise. Wrapping headers does 2 things; it stops radiated heat from entering the engine bay, which is good for induction temps and it keeps the exhaust gas hot, which is good for gas speed (the faster the better).

So a small exhaust will be good for low powered cars as gas speed will be high and also improving low down torque but it’ll impede high rev gas extraction. Horses for courses and all that.

F
Yeah - I knew that :lol:

Floyd
01-09-2006, 02:33 PM
The turbo is a restriction to the exhaust gas, it has to be. The energy of the exhaust gas turns the hot side blades, which turns the compressor blades. This subsequently increases volumetric efficiency beyond the capabilities of a N/A engine thus cancelling out the power loss used to turn the compressor. The engine will have far more torque due to better filling of the cylinders with air and fuel, which doesn’t just rely on the piston to ‘suck’ or create a vacuum to fill the cylinder with new ‘charge’ as it is forced in instead.

On a N/A engine, if you reduce ‘pumping losses’ you effectively release HP that’s already there in an engine but with a turbo, you add more power beyond the engine’s capabilities in N/A form. E.g knife edging a crank will enable the crank to cut through the oil easier and thus release another 5 or so HP from the engine. You can do the same on a turbo engine but the gain may be greater.


F

Pistonhead78
07-09-2006, 12:22 PM
I wouldnt say smaller bore = more power, cuz ur getting a smaller explosion in the bore, bigger exhausts mean u can get the exhaust gasses out faster and more petrole and oxygen in faster too, speciely if you have an injection and lambder sensor, not sure about non injection cars though :S lol

Stan
07-09-2006, 12:29 PM
So basically its all about sucking and blowing :lol:

Floyd
07-09-2006, 12:38 PM
I wouldn’t say small bore = more power either but you’ll need a small bore on a small engine for low down torque. The gas speed is the key and you need lots to spin the turbo early. When the turbo is on song, then you need a big bore and a large exhaust! You have to choose which ever is best for your driving style.

F

Pistonhead78
07-09-2006, 12:43 PM
Originally posted by scoobystan@Sep 7 2006, 12:29 PM
So basically its all about sucking and blowing :lol:
Yup "suck, squeez, bang, blow" and that is how it goes :D lol

mckeann
07-09-2006, 02:35 PM
Originally posted by Floyd@Sep 7 2006, 01:38 PM
The gas speed is the key and you need lots to spin the turbo early. exWhen the turbo is on song, then you need a big bore and a large exhaust! You have to choose which ever is best for your driving style.

F
exactly, gas speed is critical. thats what chavs with big exhausts dont get

Pistonhead78
07-09-2006, 03:20 PM
HAHA nice one chavs = :gayfight: