Tuning and your exhaust system

[shockey1013]

So I was tuning my car on Thursday and dialed the AFR's in around 12-12.5. I took my exhaust off before the track on Saturday so I was just running open downpipe. My first run down the track the AFR's were wayyy lean, 13.5-14.0's. The only thing I did was take the exhaust off. Could the exhaust be that restrictive? I have a 3in all the way back with no cat. Blew my mind.

BTW I couldn't decide which thread this would be best for so I figured since it effected my tuning, it would go here.

 

[mchustle]

If your wb sensor is in the downpipe there is now fresh air mixing with the exhaust around the sensor, therefore if the af was right when you add more fresh air it will show leaner. I coul be wrong but I don't think it will mess with the tune other than your wb showing lean. If you want to run open dp I would move the sensor to the o2 housing and simulate narrow band, that should give you more accurate readings.

 

[rEclipserGST]

Where is your wideband sensor located if you took off the exhaust?

 

[xxNocturnalxx]

Can the W/B ever actually get fresh air? The turbine is always rotating, Is it possible for it to still pulse in like on a N/A car?

^ I'm guessing O2 housing.

 

[rEclipserGST]

You've never had an innaccurate wideband reading???

Yes of course it can. Mine is in the downpipe like normal. If you have exhaust leaks, it takes in fresh air.

 

[xxNocturnalxx]

If you have exhaust leaks yes, but they are talking about removing the exhaust..

 

[95talongirl]

^^.. whcih would be like a really large exhaust leak.

If the sensor is in the downpipe, then I could see it messing with the readings, as well.

 

[xxNocturnalxx]

So an exhaust leak down stream of the w/b sensor can cause false readings? That's probably your problem right there

 

[boost97gst]

Thats news to me. If they are saying an exhaust leak downstream can somehow cause a lean reading.... Learn somethin new everyday around here!

 

[viperlp01]

When I welded my wideband in my side exit exhaust it was about 4-6 inches away from the end of the exhaust and it screwed with my wideband readings. So I moved it much further upstream and I got clean readings.

 

[shockey1013]

My wideband is atleast 1 foot away from the end of the exhaust. I couldnt see fresh air getting all the way up there. Maybe at idle but not at WOT. Also I dont think it was a false reading since I was getting some knock as it leaned out.

 

[YZFR1]

Some information from the internet, might be of some interest to whom it might concern


Exhaust Pressure Wave Graph

The conditions that exist in the exhaust port during the exhaust stroke and when the intake valve opens can also have a significant effect on engine power. The goal is to minimize the pressure in the port during the exhaust stroke so that less power is spent pushing the exhaust out of the engine and to ensure that the pressure in the cylinder (as dictated by the exhaust port pressure) is low when the intake valve opens.

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Exhaust Port Pressure

The blue curve shows shows the pressure in the exhaust port (left side scale) relative to crank position in degrees (bottom scale). Of particular importance is what happens from the time the exhaust valve opens at EVO to the time the intake valve opens at IVO and finally when the piston reaches TDC at the end of the exhaust stroke.

In this example an unconventional exhaust header is used to generate a tremendous vacuum after the exhaust valve opens (EVO) and part way through the exhaust stroke (right quarter of graph). A conventional header would generate a high pressure in this region blocking the flow of exhaust gasess. This vacuum helps the exhaust gases escape and is maintained until a little before the intake valve opens. Unfortunately a high pressure is generated in the exhaust port leading up to when the intake valve opens. It is obvious that by further modifying the exhaust to extend the exhaust port vaccuum to include not only the exhaust stroke but the valve overlap period will result in significant power gains.

The theory of how an exhaust header works can be complex and the actual results are almost always different from the theory. There is no way to accurately predict how an exhaust header will affect the port pressures. The pressures must be measured and with a little experimentation, based on fact not theory, significant power gains can be realized.

 

[gprix1]

^^^^ That is a great explanation for an N/A car but not very useful if you run a turbo. I'm sure there is a little bit of the 'pulse' effect with a turbo car but with the turbine wheel always there in the exhaust stream, that effect is somewhat dampened and much less.
Although, it certainly appears to be enough that you need your wideband upstream from your exhaust outlet, this pulsing effect is not really enough that it can be exploited for making power on a turbo car the same way an N/A car does.

 

[YZFR1]

^Did you read the last paragraph the whole point is having to measure these pressures to know what is really going on, otherwise we are all just guesing

That was NOT a theory but just an EXAMPLE

 

[shockey1013]

Is this theory the same whether it's a turbo engine or a N/A engine?

NVM I didnt see the posts after the picture.