Liquid cooled

[Brian993]

I have a non 911 question, maybe some one has the answer as the Boxster bbs is dead. Do liquid cooled (boxer?) engines like the high revs as do the air cooled?

[masraum]

I think the Boxster engines are supposed to really scream. 7000 redline or something like that.

[911jon]

BigB, the characteristics of an engine in terms of maximum attainable rpm are not determined by the cooling method. These are determined fundamentally by the laws of physics limiting the piston velocity. In practice by the configuration of the bore/stroke ratio and the quality and design of the components.

In general, short stroke/large bore (over-square) permits higher rpm (and therefore power output per unit volume) but limits torque at lower engine rpm. And vice-versa. This is because piston velocity per revolution deceases with increasing bore/stroke ratio.

Component design is also critical; for high rpm engines, reciprocating mass (pistons, con-rods, valves) should be minimised. As a good 911 example, that is why the '88 Carrera Club Sport was fitted with hollow-stem valves to permit the higher rpm limit of that car over the standard 3.2 Carrera.

Engine design is a complex subject and there are other issues which all have a bearing (no pun intended!) on the case, but consider crankshaft design, engine casing flex etc.

The remarkable and still exemplary packaging of the 911 is in part due to the compact simplicity of the air-cooled dry-sump power unit. Water cooling would not have fitted in there!

I am not an engine designer and I am sure others can explain better than I; but I hope this is of some assistance.

[Joeaksa]

Considering that the latest group of Formula One motors goes almost to 20,000 rpms, is liquid cooled and usually survives for the race, the key getting a motor to rev high is to throw lots of money at it and pray.

It is usually a factor of bore versus stroke, overhead cams and extremely well balanced parts. The new things on the horizion that I heard is a electric actuated valve that has no spring to bounce and can be "adjusted" while running. Pushrods for valve actuation really limits a motor, thus overhead cams are a must.

Air or liquid cooling has no real factor in max. rpm. The rotary (wankel) motor of years past was redlined at 8-9000 rpm but would rev to 14,000 easily. The limits in the motor were the cooling system which would not keep the motor cool at this speed. Course no valves, pistons and rods helped as well..

Joe

[speeder]

Assuming that a motor is in good working order and has oil in it, all engines, (car,truck,motorcycle,lawnmower,etc.), can be taken to their "red line", ie. maximum recomended speed, which is always on the conservative side.

With some motors, like most traditional American iron, there is no real desire to run at red line because they make most of their power down low,(torque), but there is no harm in red-lining them- they usually have a fairly low red line anyways. It is just not desirable. All real sportscars, (and motorcycles), are designed to be revved,(sp?), because that is where the fun is.

This is partly a late answer to your "safe to redline?" question, but they are related. Bottom line is to maintain vehicles well, and drive the daylites out of them, when conditions are safe of course. All Porsches are designed for this. Enjoy.

[Superman]

My old SOHC Honda 750 m/c used to redline at around 11000 rpm, if memory serves me correctly. 12,000 rpm could be achieved. 750 divided by four is 188cc per cylinder. My SC is 3000 divided by six which is 500cc per cylinder. So you see, the much higher redline was possibly because the pistons and rods (reciprocating mass) were so much smaller. At the time, Honda was just coming out with the transverse-mounted six-cylinder 1300cc m/c that would do about 60 mph in first gear! The designer of that engine had previously built a 2-stroke 50cc twin-cylinder engine that could achieve 25,000rpm!

So, it's all about reciprocating mass. This is interesting to the engineers, but all you and I need to know is keep clean oil in it and don't exceed maximum rpm as stated in the owners' manual.

[Bob Chapman]

BigBPainta
Yes! I can testify on the basis of having some seat time in my son's ex '99 2.5-l Boxster. I take the '89 Carrera's engine to redline at least twice on every drive. I do it because it's fun, and it seems that it's the way it should be run. The Boxster was the same, also '89 Honda CRX si. But not the '94 300ZX.

But you already know all that, don't you?

[Brian993]

Thanks for your input, I was mainly asking because the power is definetly up at the top. It really starts to fly around 5000 rpms and keeps going until around 6500 - 6800. You can feel it come on, I like it so I do it alot. But I would really like to keep it fun, and high repair bills would definetly change all that. So I guess its safe to say the liquid cooled can be treated the same as the air cooled if maintained properly.