Keep her above 4000rpm and she'll thank you for it every day.

[blue2000s]

Originally Posted by MNBoxster

Hi,

There are lots of other factors. An engine never achieves complete combustion, if that were true, we'd never need emissions gear.

But, there is an RPM range in each engine where the combustion is most complete, and from the sound of it, it's in the 4k range for the M96 engine.

But other factors contributing to long life, maybe even the dominant factors, are that the Crankspeed dependent Oil and Coolant pumps (run off the crank-driven serpentine belt) may be in their most efficient range here as well.

All the engineer was speaking about was Carbon build-up, but a steady supply of Oil and Coolant will mean more to longevity than the buildup of a little Carbon. Carbon is softer than steel or aluminum and actually acts as a lubricant and sealant. You need some, but too much is a problem because it causes hotspots (retains heat) in the valvetrain and interferes with heat transfer to the head where the coolant can carry it away.

And, you need to put this into perspective. Incomplete combustion takes places at either end of the Tach. At some place north of 4k rpm, the piston doesn't remain at the top of the Compression Stroke long enough to combust all the fuel, even with an altered timing curve. This is why you often hear an exhaust popping on deceleration from a hard run, because unburnt fuel has accumulated in the exhaust and ignites there instead of in the cylinder where it's supposed to.

The interesting thing to me is that Porsche doesn't gear the Tiptronic or the Control Unit to shift at 4k rpm, I wonder why they wouldn't do that? And, the Tip cars generally have a more trouble-free life than the Manuals...

Happy Motoring!... Jim'99

Carbon can also build up on the intake side. As fuel is exposed to a hot intake valve, it can solidify and build up. The build up within the ring seals is also partially irrespective of combustion. The faster engine speed gives the fuel less time to sit on hot surfaces as it is raced into the combustion chamber.

Also, don't forget about fuel quality and the presence of oil, which are also contributors.

[Brucelee]

"The interesting thing to me is that Porsche doesn't gear the Tiptronic or the Control Unit to shift at 4k rpm, I wonder why they wouldn't do that? And, the Tip cars generally have a more trouble-free life than the Manuals"

That IS actually pretty interesting.

And BTW- if very high RPM were good for engine life, imagine how long a NASCAR engine should last! Driven JUST BELOW REDLINE for hours on end and with fresh Mobil 1 to boot!

Think of it, a million mile NASCAR engine!!!!!!!!!!!!!!!!!!!!

[MNBoxster]

Originally Posted by blue2000s

Carbon can also build up on the intake side. As fuel is exposed to a hot intake valve, it can solidify and build up. The build up within the ring seals is also partially irrespective of combustion. The faster engine speed gives the fuel less time to sit on hot surfaces as it is raced into the combustion chamber.

Also, don't forget about fuel quality and the presence of oil, which are also contributors.

Hi,

Well, definitely too much Carbon buidup isn't good, but the fuel is meant to actually bathe the Intake Valves as part of the process of cooling them.

The only 2 ways a Valve can shed it's heat is to either transfer that heat to the fuel which flows past it, and/or for that fraction of a second in each cycle where the Valve is closed and actually in contact with the Valve Seat, thereby transferring it's heat to the Head where the Coolant carries it away.

Again, too much Carbon, especially on the Valve Seat, and you interfere with this heat transfer by essentially creating an insulating layer.

I also agree that the quality of the Fuel, and the general health of the engine (where Oil isn't leaking into the Comcustion Chamber) will play a large part in preventing excessive Carbon buildup...

Happy Motoring!... Jim'99

[blue2000s]

Originally Posted by MNBoxster

Hi,

Well, definitely too much Carbon buidup isn't good, but the fuel is meant to actually bathe the Intake Valves as part of the process of cooling them.

The only 2 ways a Valve can shed it's heat is to either transfer that heat to the fuel which flows past it, and/or for that fraction of a second in each cycle where the Valve is closed and actually in contact with the Valve Seat, thereby transferring it's heat to the Head where the Coolant carries it away.

Again, too much Carbon, especially on the Valve Seat, and you interfere with this heat transfer by essentially creating an insulating layer.

I also agree that the quality of the Fuel, and the general health of the engine (where Oil isn't leaking into the Comcustion Chamber) will play a large part in preventing excessive Carbon buildup...

Happy Motoring!... Jim'99

The fuel is primarily sprayed onto the intake valve not to cool it, but to enhance the atomization of the fuel. It is a side benefit that the heat from the valve is carried away by the air/fuel. The valve is also cooled by the air/fuel rushing past it on the intake stroke. FSI engines show that the intake valve can opperate just fine without spraying fuel on them.

Typically, it's the exhaust valve that has the cooling problems as the hot gasses of combustion run past it and the valve's size is somewhat smaller than the intake. This is why you'll find sodium filling in exhaust valves and not on the intake side.

[Rail26]

Cuts your gas mileage by about third though...I have been trying it lately.

[victoreric]

Very interesting & thanks for sharing that. Now I don't have to feel guilty about pushing mine to 5000. That's when the engine let's out that nice growl.

[MNBoxster]

Originally Posted by blue2000s

The fuel is primarily sprayed onto the intake valve not to cool it, but to enhance the atomization of the fuel. It is a side benefit that the heat from the valve is carried away by the air/fuel. The valve is also cooled by the air/fuel rushing past it on the intake stroke. FSI engines show that the intake valve can opperate just fine without spraying fuel on them.

Typically, it's the exhaust valve that has the cooling problems as the hot gasses of combustion run past it and the valve's size is somewhat smaller than the intake. This is why you'll find sodium filling in exhaust valves and not on the intake side.

Hi,

I actually meant the AF mix, guess I oversimplified. I know the Boxster has Sodium-filled Exhaust Valves, so does my Esprit...

Happy Motoring!... Jim'99

[z12358]

On the highway, the straights, and mostly when my fiance is with me I keep it between 3k-5k. On the twisties, and especially when riding solo, the 5k+ growl informs me about the area to shift. Few times during a run, I'll take it to 6.5k. On the track, the needle rarely goes below 4.5k. There's something about this car that makes high rpms so inviting and right.
Z.

[djomlas]

yea,..not every car likes to be in high rpms but wow do these cars like it

[Paul]

It's not mine, but here's a happy Porsche:



In Germany they are driven this way for hours with no ill effects!

[Back2DTM]

i just redline a couple times a day to keep the engine clean