View Full Version : Noisecam - B&S Cylinder head noise

Keith Oakley
10-Nov-10, 11:28 AM
Having considerably reduced the fan and exhaust noise on my B&S powered F35 test craft it's becoming apparent that cylinder head noise is one of the loudest remaining noise sources. The noisecam system can identify where individual noise sources are located but one of its limitations has been that it couldn't accurately locate sources when there are 2 or more sources on the same frequency that are physically close to each other - typically less than half the wavelength of that frequency.

I'm now working on a software upgrade to hopefully overcome that. The picture below shows the craft under the 2D noisecam array on the lawn last march. There are also two noisepics from that test, both at 967.7hz ie 14E (14 times engine rotation frequency). One noisepic shows a noise (red splodge) on the left hand cylinder head and the other (taken a fraction later) shows the right hand cylinder firing.

I suspect cylinder head noise on an air cooled B&S 20bhp (upgraded to 28) 2 cylinder 4 stroke is something we are stuck with. Incidentally that funny wooden box on the top of the engine is an experimental airbox to reduce intake noise. It works but other noises are louder so no overall benefit.

Keith Oakley


10-Nov-10, 08:50 PM

Was the noise at the cylinder head or the exhaust? the blades on our old 35 used to get bad soot marks due to the venting of the silencers onto the blades. That was at 3 times engine revs as the we had a 36 to 72 ratio and the engine ran at 3600 rpm with the 6 blades. I think it make 3 times or is it 12 times not sure......
Interesting tho if it is the heads. St John at Hertford Uni has looked at diesel heads actually lifting off the barrels under high revs. Could we be looking at this? Did you take the heat shield off? Ours used to resonate so we took it off.

Interested in how you can solve this

K@ bearwood

Keith Oakley
11-Nov-10, 08:12 AM
Its a 2D noisepic (I haven't got the new software on 3D yet) so it could be on the inlet manifold, cylinder head, exhaust manifold, or even the rockers/valves. It isn't the exhaust outlet, thats way up on the side of the duct, and I know the exhaust manifold isn't leaking. On your craft noisecam showed the noise was coming from the silencer exhaust outlet which was pointing directly at the fan. With an exact 3:1 ratio and 6 blades your engine and fan were in sync so that a fan blade was always lined up with the exhaust when it went bang, hence the soot marks on the blades.

The heat shield is off, but as a thin piece of tin only partially covering the aircooled head I doubt that stops much noise from emerging. I think this noise is perfectly normal, it's just that its normally drowned out on most F35s by exhaust and fan noises up to 15dba louder. I suspect the noise might be less on a better engineered and watercooled engine, but head noise is still there. Below are some professional noisepics from BMW of their mini engine with a similar pulse of noise from each of the 4 cylinders in turn. You can see the 1 3 4 2 firing order and that the no1 cylinder is loudest (its orange!)

Keith Oakley


12-Nov-10, 08:39 AM
Ive been thinking about putting noise proofing under my cowlings to try and soak up some noise, have you thought about doing that?

Keith Oakley
12-Nov-10, 08:53 AM
Yes but my experiments with engine cowlings so far have shown little effect because with an aircooled engine you need so many big holes to let the airflow through. I think the message is that head noise is around 10-15dba below the exhaust and fan noise on most craft, so sorting exhaust and fans is first priority.