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Would anyone know at what RPM the turbo kicks in?

It is impossible to sense any sound variation while cruising and I cannot find this piece of info anywhere...
Thanks.

Yannis,
Turbos are always spinning if the engine is running. Exhaust is always flowing when running which is what spins the turbo.
If your talking about max boost pressure, that can vary some what by load conditions(props, hull design, boat weight, gear weight, wind, tides, etc.) but generally in marine applications on a planning hull, once on top in cruise mode is when you'll see max boost.

A good way to tell is when the exhaust goes from blackish (more fuel than air vs rpm) to pretty much clear exhaust by sight.

Thank you, Bruce.

I was thinking that, like in sports cars or heavy trucks, where you suddenly hear a whoosh sound, as they accelerate, it is when the turbo kicks in.
And, as the rpms drop to allow for a gear shift the sound of turbo stops, only to reappear when the rpms go back high again.
Perhaps cars are different?

Yannis - not the same motor, but we have a pair of 4LH-STE 230hp - smoke enough to clear mosquitoes for miles up to about 2,100rpms. They clean up as turbos start to work slightly above that, then north of 2,700rpms you hear the distinct whistling and the response of additional throttle increase is significant as they are into their prime power area. Hope this helps for comparison. Best of luck.

Thanks, Bob, it helps indeed!

Also, why different motors? I thought that the 10hp diff. is a US thing for pollution or something.

Yannis,
In a gas application the other other noise you may be referring to is the blow off valve. In between shifts when the throttle plate closes the boost pressure has to be released from the system prior to the throttle plate. Diesel engine does not have a throttle plate which is why you don't hear the release of pressurized air.
Other noises you may hear on sports cars with turbo can be the waste gate opening which realeases ecxess exhaust gas from the system when full boost is acheived. You will notice it far more if the system is open to atmosphere rather than routed back to the exhaust down stream of the turbo.

Yannis,

I don’t know the rpm, but I do know for sure you know when it kicks in. Its not secretive at all.

Dug

Kevin, thank you. It makes sense.

Dug, thanks, but why do I not understand when it kicks in?
Not only do I not hear any turbo whistling after a given rpm, I also do not feel any special acceleration. It all progresses linearly all the way from zero to 2900-3000 rpms that I cruise at. Funny...
Now that you told me, I will try to notice if any such thing will happen to me too, I will carefully observe the progression of rpms until I notice something. But not until next May or so!

Yannis wrote:Kevin, thank you. It makes sense.

Dug, thanks, but why do I not understand when it kicks in?
Not only do I not hear any turbo whistling after a given rpm, I also do not feel any special acceleration. It all progresses linearly all the way from zero to 2900-3000 rpms that I cruise at. Funny...
Now that you told me, I will try to notice if any such thing will happen to me too, I will carefully observe the progression of rpms until I notice something. But not until next May or so!

Yannis

The problem is you are on the bridge. Let someone run it and you will hear the difference. Once I go over 1900 RPMS the noise in the cockpit increases substantially. Big difference from 1900 to 2000 RPMs. That is with my 270 Cummins.

Tony,

Each very first trip of the season I cruise with the engine hatches wide open, so as to see the bilge and listen to the engines.
I strech back and look down as I accelerate.
I havent ever noticed a whistling sound...I may have to go check my ears.