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Old 08-09-2005, 11:24 AM   #1
Konrad
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Lightbulb How do u know wat size the exhaust should be

Hey... im one of those guys that believe in the little things...

!!!

i think that a whole lot of little things make one big thing....

so a lot of small improvments on a car should make a big improvment...
and i seek improvment in performance...

anyway... how the hell do i know wat size exhaust a car should have?

for example... if i have a car with a 1L engine, i know thats it not a good idea to have a 3 Inch straight through big bore.... and with a v8.... youl probly lose some power if the exhaust is 1.5inch...

So is there a site with all this kinda faq?... cuzz i have a million of these Q`s...
Cuzz u know... theres about a million things to know about cars.
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Old 08-09-2005, 05:07 PM   #2
dunzo889
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The better the airflow, the more horsepower. Thats why people get big exhausts but dont go too big or else its just gay.
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Old 08-09-2005, 06:30 PM   #3
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alright, here is my thought, a big exaust is good for big engines. a small exaust is good for small engines. things to remember: you still need back pressure, you shouldnt run straight pipes unless you wide open all the time, and bigger is not always better. you can find information on this if you search for exaust related products, or you could call an exaust company (like hooker or flow master) for guidance.
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Old 08-09-2005, 10:52 PM   #4
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Quote:
Originally Posted by Konrad
Hey... im one of those guys that believe in the little things...

!!!

i think that a whole lot of little things make one big thing....

so a lot of small improvments on a car should make a big improvment...
and i seek improvment in performance...

anyway... how the hell do i know wat size exhaust a car should have?

for example... if i have a car with a 1L engine, i know thats it not a good idea to have a 3 Inch straight through big bore.... and with a v8.... youl probly lose some power if the exhaust is 1.5inch...

So is there a site with all this kinda faq?... cuzz i have a million of these Q`s...
Cuzz u know... theres about a million things to know about cars.

Suzzie or Charade? DOHC hairdryer, SOHC hairdryer or SOHC aesthmatic?

If you genuinely want to learn, there may be help at hand.
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Last edited by Wally : 08-09-2005 at 10:59 PM.
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Old 08-10-2005, 11:28 AM   #5
Konrad
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HEll YEAA i genuinly wanna learn...
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Old 08-11-2005, 03:55 AM   #6
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Ok let's take you 1 litre and I will assume it's an aesthmatic (no turbo). We might need to take this over several posts so you can absorb the process and give others a chance at rebuttal.

Remember we are doing this on paper which is no substitute for proper dyno testing and tuning.

What do we know about Japanese engines = they tend to fly in the face of convention, but we can still apply some general principles.

So the exhaust first:

there are several secret recipes on sizing exhausts and truth is the space constraints will necessitate some compromises. To give you time to find your cam and valve specs, lets go with the simplist = velocity/volume.

Time to get some basics out in the open first. This may all come as a shock to some, but perservere with me. I'll try to keep it simple.

In tuned systems the aim is to create a negative pressure(s) at the back of the valve as it's nearly closing to syphon out the last remaining gases, thus allowing more volume for fuel/air misture without exhaust gas diluting the burn. You have to pick an rpm you want this to occur at, but in the velocity/volume we ignore this and hope to jag some good syphoning at the right rpm in the process..

What we know is that if we just opened the ports up to atmosphere we would noticeably loose some low end power and fuel consumption would be increased. The reason for this is that the cam has been ground for certain angles, overlap, lift and duration based on, amongst other things, impedance from the exhaust (a lot of guys call this backpressure, but it's a combination of friction losses and dynamic losses ).

So in the four stroke cycle we have suck, compress, burn(power) and exhaust over two engine revolutions = 720. But the cam events are not neatly spaced over 180. The exhaust usually starts to open well before the piston has bottomed out on the power stroke. This is called blowdown and is designed to relieve/sacrifice a majority of the remaining gases after peak pressure in a controlled manner so that there isn't a whole load of pressure working against the piston when it starts it's upward movement. So you can imagine if you whack a big bore exhaust on your engine the decreased resistance to flow will cause the gases to exit faster than designed and you will lose some of the pressure that was supposed to keep the piston moving. Effectively you have unchoked exhausting gas at high rpm at the expense of usuable cylinder pressure (power) at low revs.

After blowdown the remaining exhaust gas must be expelled which is what the upstroke of the piston following the power stroke does. In fact the exhaust valve remains open for a short while into the suck (intake) stroke. It remains open so that more dense cool air/fuel mixture coming in will displace the residual hot gas and the negative pressure at the tail end of main exhaust stream has a chance to syphon as well. But now your big bore exhaust has caused a lower than expected residual exhaust pressure in the upstroke (because most of it dissapeared at blowdown) which means there is less escape velocity, so you can forget about any real syphoning and when the fuek/air mixture starts coming in there isn't going to be much to stop the piston pushing some of it out the open exhaust valve. The nett result is loss of power at low to midrange revs.

Step 1

Down to business. You can expect about 2.2 cfm/hp (1.39litres/sec/kW) of exhaust gas. So if you know your power you can simply say power/cylinders x 1.39 = exhaust flow l/s per port. Otherwise you will have to work out your actual exhaust flow which we'll rule of thumb again for a jap engine at cylinder volume (cc) x 300 / max power rpm = l/s. You can do your own displacement figures if you know your VE, clearance volumes, etc., but I'm casting a wide net here.

So lets say with your 1 litre is 333cc per cyclinder and peaks power at 5600rpm.


(333 x 300)/5600 = 17.8 l/s/port
17.8 x3 cylinders/1.39 = 38.5kW; which sounds about right.

Your port size is pretty fixed unless you decide to have it machined and Jap cars tend to have a slightly larger port area than the exhaust valve fully open area. But lets see what ii probably should be, once again not getting into too much maths, just another rule of thumb.

When the exhaust gas is travelling around 90m/s the sonic waves/pulses can cause up to 5 psi negative pressure on the back of the exhaust valve just prior to closing. Of course this means the average velocity of the gas during the stroke will be less and happens to be around 30 m/s.

17.8 l/s = 0.0178m^3/sec so if we divide by 30 m/s we should get the square area = 593mm^2.

The minimum diameter of the port is therefore sqrt (593 x 4/Pi) = 27.5mm.

So now we know the inside diameter of the exhaust pipe primary runner will be the larger of the port diameter or 27.5mm. I would suspect your port diameter will be larger than this.

Because you only have three cylinders you are stuck with a three into one system. So the tail pipe diameter needs to be sized next.

You could simply say "my engine is pumping out 53.4 l/s exhaust and at 30m/s my diameter should be 47.6 mm", which is close enough to 2" OD, but the gas is going to rapidly cool from the 900C at the collector to something in the order of 260C at the muffler thus slowing the velocity and increasing density.

We know that the pressure in the tailpipe isn't isobaric and it sure aint isochoric so we need to adjust to keep the velocity up to avoid cold plugging. So applying thermodynamics we need to rack the velocity in the equation up to 55m/s to compensate.

So the tailpipe diameter should be around 53.4l/s @ 55m/s = 35mm, which is around 1.5" OD.


Step 2

What length should my primary pipe be, you ask. Well in this method we use total volume of the displacement and clearance volume and try to make enough room in the pipe to accomodate it. So if your compression ratio is 9.5:1 and your displacement is 333cc your clearance volume is :

333/ (9.5 -1) = 39cc so total volume is 372cc.

Now what you have to remember here is your primary pipe is not the total runner length. You have a port runner between the back of the valve and the oulet port on the head. A lot is made of the port runner in shape, texture and volume. What we are going to have to do is take a guess of it's volume, unless you want to pull the head off your car and pour some measuring liquid down the port.

Lets say your port runner has a volume of 50cc, then the pipe runner has to hold 322cc. Simple maths :

take 593mm^2 pipe area from step 1 and apply

(322/593) x 1000 = 543mm long pipe required between port and collector.

I haven't checked my maths nor spelling.

And before all you guys with american pushrodasuaris' start up with me, yes I know your actual exhaust ports are a lot smaller than calculated in step 1, but that is why you have to go out and buy decent aftermarket ones to even approach the power per litre of Jap and Euro production cars.
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Last edited by Wally : 08-11-2005 at 04:02 AM.
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Old 08-14-2005, 05:24 AM   #7
Konrad
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thanks, ill try read it all again l8er, and tell u a few words i dont get...

but i might cross them over my dad first.
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Old 08-14-2005, 05:42 AM   #8
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2-3" is what you should aim for depending on how big the engine is or if its charged.
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Old 08-14-2005, 07:36 AM   #9
Wally
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Yeah right, 2"-3" on a one litre. I bet workshops see you coming from a mile away.
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Old 08-14-2005, 09:07 AM   #10
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1L? well, i'd recommend something between the 1.5" - 1.75" range, that's a mighty small engine.... i've never worked on an engine that small before, so i'd really suggest you listen to someone as smart as Wally or ChrisV.
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Old 08-14-2005, 08:41 PM   #11
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just curious but someone mentioned you need back pressure? for egr crap i can understand but if your modding for performance do you still need backpressure?
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