Effect of Track Day Noise Suppressor on Lambda Values

[aerobod - near CYYC]

With my new engine and the change in policy this year at my closest track to meet aggregate noise outputs during a given track day, as opposed to restricting noise levels of specific cars, I decided to remove my noise suppressor. Unfortunately on the last session there they determined I would be pulled off first in our track day club if they are getting close to exceeding the aggregate total, as I was the loudest car at 100dB at 15 metres from the noise measurement devices on the right (exhaust) side of the track.

With the old R400 spec engine I was 92dB at 15 metres with the noise suppressor fitted with 18 plates, about 6dB less than without the device. Here is that config:

I added another 6 plates (24 total) for use with the new R500 spec engine.

Today I decided to datalog driving the same route with full throttle acceleration in as close to the same places as possible, both with and without the suppressor fitted. Up to half throttle, there is no Lambda difference between with and without the device. In the critical areas from below max torque to the redline at full throttle it does affect the Lambda quite a bit. In the 6200-8200RPM range without the device I average 0.85 Lambda, but with the device fitted this drops to 0.79. Qualitatively there is little change in power output as Lambda 0.8 to 0.9 at full throttle versus power output should be a gentle curve peaking around 0.85-0.86 on a naturally aspirated engine, but with only a few horsepower variation over the range.

I think I will keep the mapping as-is instead of creating an additional map to swap between, just in case I forget to swap back to the normal map with the suppressor removed, which could give a catastrophically lean mixture. Also, the increased back pressure could increase exhaust and valve temperatures, so the extra cooling from the rich mixture will be beneficial in that case.

Likely I should be around 95dB at 15 metres with the device fitted. Most of the track days this year will be with it removed, though.

[ECR]

What’s the purpose of the alloy plates?

[aerobod - near CYYC]

2 minutes ago, ECR said:

What’s the purpose of the alloy plates?

The thin one deflects hot exhaust gas away from the edge of the fibreglass wing, the angled front piece is to deflect cones when hit at autocross, which is where the noise restrictions I had to comply with were brought in about 5 years ago.

[Colin_T]

My guess would be the exhaust restriction restricts the scavenging effect at higher rpm, so the richer mixture is from less air being drawn in to the cylinders. 

also possible there’s end gas remaining in the cylinder preventing room for fresh charge coming in. This will reduce resistance to knock too, so may actually require less ignition advance when using the trackday silencer too? 

[MattB]

Doesn’t adding plates make it louder?

MattB

[aerobod - near CYYC]

3 hours ago, MattB said:

Doesn’t adding plates make it louder?

MattB

It does, but I need to balance the restriction vs noise reduction. 18 plates gives me about 6dB noise reduction from previous measurements, I’m hoping for 4 to 5dB with 24 plates. The absolute noise reduction may not be as much, but the noise measurement devices on the exhaust side of the track should see a higher reduction due to the flow not pointing directly at them, with it being radial to the exhaust exit instead of axial to it.

[aerobod - near CYYC]

4 hours ago, Colin_T said:

My guess would be the exhaust restriction restricts the scavenging effect at higher rpm, so the richer mixture is from less air being drawn in to the cylinders. 

also possible there’s end gas remaining in the cylinder preventing room for fresh charge coming in. This will reduce resistance to knock too, so may actually require less ignition advance when using the trackday silencer too? 

I think they are likely scenarios, Colin. I don’t want to adjust the map due to it being optimised for use without the suppressor and switching maps is dangerous if I forget to switch back to the correct map when removing the device. I think the only significant detriment in staying with the same map is higher fuel consumption than optimum on track at WOT with the device fitted.