Race Engine Forum UK

For some time I've known that turbo camshafts should avoid "symmetrical" timing characteristics - the reason being that a resonant frequency can be reached which causes pulsing and ultimately turbine failure. Having just accepted this I frequently now see that turbo camshafts run against the grain and indeed run symmetrical profiles.

Am I wrong or is there some secret to head/mainfold design that makes this old wisdom redundant?

Is it just that the "odds" of the resonant frequencies being reached are so incredibly small and minor in effect that it just isn't worth worrying about?

Dunno about this one Julian, but certainly the natural frequency of turbine blades is way above that of any engine order.

Pulse tuning - the only pressure wave/frequency related issue I know of with turbocharged units is about using the pressure waves to enhance the cylinder pressure around bdc in precisely the same way it is done on normally aspirated engines with interference exhaust headers.

GC

So why an asymetric profile in the first place - what does this gain you? Just to optimise the flow of gas to the turbocharger from the cylinders or ease the load on the turbo for filling the charge in the first place?

This asymetric concept, Julian, you'll need to explain in more detail what you mean please. Do you mean asymetric profile on opening and closing ramps on the cam, or asymetry between full lift ex cam timing one cylinder to another?

GC

Sorry - I refer to the asymetric timing of the camshaft

I'd need some persuading that it is beneficial in any way to have a variation in lobe centerline angle cylinder to cylinder on an engine in that way. Variation in event that is load dependent and done via variable cam timing or phase shift is another matter.

GC

With the asymmtric design you can have the typical "group N" sound :D

inlet ~270‚°, outlet ~250‚° -> have fun ;)

Pipercams have this! Hoermann also!
It's for a driveable torque curve because it shall be a turbo ;)

That is not what we are talking about Acki, and just different inlet and exhaust duration does not mean that the camshaft is asymetric.

GC

He could well be right though - I wasn't suggesting that each lobe would be operating at different timing relative to that cylinder's TDC. This is where my lack of formal education in this matter is likely to leave me unstuck.

the asymmetric cam it's the cam that the lobs have difrent flank and ramb profiles in each side (opening and closing the valve)
but the duration defferences between in. and ex. cam dosen't matter
i'm i right?



rgrds
dafnos

Asymmetric = the inlet and closing flanks are not the same profile. And yes I spelt it wrong, there are two m's..

Everything else is just different duration/timing.

GC