I borrowed SPL meter from work and did some measurments, all "C" weighted("A" weighted will be 3-4 db less, but I wanted to capture entire spectrum)
Inside the car:
Idle 68.5
Driving steady 2K rpm 79-80
Driving steady 3K rpm 85-86
Driving steady 5K rpm 89-90

Accelerating to redline-peak 109.5db

Outside, car parked in garage, door open
Engine 0.5 meters above the hood 74db
Back, idle, 0.5 meter from exhaust tip at muffler level 87db
Rev up to 3.2K in neutral peak 108.4(0.5 meter)
Rev up to 5.2K in neutral peak 135.9(0.5 meter) Seems awfully high, I think that engine resonated off garage walls and it added up.

SPL meter is ANSI and IEC 2 compliant, whatever that is.

 

Manufacturers use acoustic dummies to measure............simulate human ear directivity and response. A spectrum analyser or software can visualize the output jack of SPL meter. This FREE software will blow you away just use SPL meter as microphone on a good laptop:
http://www.qsl.net/dl4yhf/spectra1.html

http://www.binaural.com/NeumannPhoto.html
http://www.neumann.com/?lang=en&id=c...00_description

Q45tech stands for technical not technican: BSEE, MSEE, MBA

 

Now measure it with your stereo on as loud as you normally have it to hear over the car noise. This is why I added B-Quiet all over the spaces I could get to and insulation behind the front wheels.

 

Meter's range is only up to 8000 Hz, so stereo measurments is not accurate. At full blast with no distortion it was 100 db. I have lots of Bquiet in doors.

 

Thanks, I will definitely play around with that.