Load vs. No Load

[daepp]

This question came up under another thread on about checking advance at 6000 rpm.

Does an engine suffer more being run at high rpms without a load vs with a load?

I know I cruise down the interstate in fifth gear with my little 2.4 running at 4000 + (-). Assuming you could keep it cool, would it be bad to run that same motor for extended periods while not in gear?

[Walter_Middie]

Why would you let the car sit at idle or at a high RPM if you aren’t going anywhere? Does it hurt it? Not in a big way – only microscopic wear and tear. If I am driving the car, I try not to hurt it by lugging it or over-revving it. But I also wouldn’t hesitate to run the car hard and use high RPM. That’s why I have the car – I’m enjoying it – and I’m shortening the life of the motor. It’s a trade-off.

When I start the car in the morning, I get it moving within a few seconds – I don’t let it idle to warm up. I keep the accelerating and RPMs in check until it is warmed up – then everything is fair game.

I sit and idle at stop lights, but I wouldn’t do the same thing in my driveway – I’m just not enjoying the car while it’s sitting in the driveway – why add wear and tear for no reason?

If I’m working on the car, then it idles for a while or it’s at high RPM for a while. Again increased wear and tear, but I’m doing something to it, so for me, the trade off is worth the small amount reduced engine longevity. But at the same time, I try to avoid running it in a waste-full manner (idling for no reason – or high RPM for no reason).

Bottom line – it won’t hurt the engine in a serious way, but why abuse it? I guess I have mechanical sympathy. I see the car as more than just an appliance.

[daepp]

Not planning on letting it rev hi for no reason.

They question came up about revving it to 6000 with no load while checking advance. I was just curious if that was worse than revving it to 6000 under load?

[psalt]

David,

Running an engine at high rpm with no load is worse than when it is under load. You should run away from anyone who tells you they hold the engine at 6000 rpm for more than a second to check the timing. Catastrophic engine failure usually involves failure of the connecting rod, primarily because of the way in which it accelerates, stops and accelerates in the opposite direction. Compression and expansion pressures partially offset the forces and failure is most probable at TDC on the exhaust stroke on the over run because there is little offsetting pressure. The pressure in the cylinder is directly related to the VE of the engine, which is very low when the throttle is partially closed, as with no load operation. At 6000 rpm under load, say at top speed of the car, the throttle is open much wider, the charge is much higher and the pressures are much higher.

[daepp]

psalt - thank you - that was exactly what I wanted to know. Somehow I suspected this was true but I did not know the technical reasons why.

[MichaelN]

I mean no disrespect, but this does not add up for me. I guess my objection begins with the idea that:
"Catastrophic engine failure usually involves failure of the connecting rod, primarily because of the way in which it accelerates, stops and accelerates in the opposite direction".

Actually, I think not. I am most familiar with catastrophic engine failure occurring because the engine is OVER-revved and the resulting valve float creates a collision between the still-open valve and the top of oncoming piston which, of course, creates mayhem. Perhaps the perception that an engine is more likely to fail when not under load comes from the fact that when engines are over-revved it is usually when they are not under load (such as when you miss a shift at red-line). There are other possible causes of catastrophic engine failure as well (I think Paul is referring to "throwing a rod"), but they too are likely caused by over-revving.

Modern properly maintained engines are very strong and run many, many thousands of miles. This is self evident. And they are quite capable of operating smoothly at any speed within their design limits for considerable lengths of time. I think that is especially true of the flat six motors in our cars. And as far as wear is concerned, every stress that contributes to engine wear (such as pressure on bearings, piston rings, and lubricants) increases enormously under load.

[Flieger]

1. Redline on a stock early S is 7300rpm. Since the force varies with the square of the speed, 6000 under load or no load should be no problem.

2. The rods are stronger in tension than in compression because buckling failure in most designs occurs before the material yield strength has been reached. Material yield strength is generally measured with a tensile test and the materials will react differently when subjected to the more complex loading in the actual part.

3. Valve float usually occurs before rods buckle. Rods usually only buckle on high power turbo motors or hydro-locked motors.

[daepp]

Wow - even more impressed with CP SLO!

For the record, when timing I have not held it at 6K for more than 5 to 10 seconds. But I have always wondered about it. It just seems harder on the engine than when I'm in the drivers seat - perhaps cuz you're practically in the damn engine when timing it!!!

[Jerome74911S]

All the technical explanations aside, I find it hard to believe that Porsche would specify checking advance using a method that would damage the motor. After all, none of these motors is at red-line when at 6,000 rpm. I think any difference in stress/wear is so minimal that it is not not worth considering.

I don't worry about it. It just sounds scary.

[daepp]

Originally Posted by Jerome74911S

It just sounds scary.

True dat!

[Flieger]

Originally Posted by Jerome74911S

All the technical explanations aside, I find it hard to believe that Porsche would specify checking advance using a method that would damage the motor. After all, none of these motors is at red-line when at 6,000 rpm. I think any difference in stress/wear is so minimal that it is not not worth considering.

I don't worry about it. It just sounds scary.

Agreed.

However, in the other thread someone mentioned a couple of minutes at 6000rpm. I think he was just throwing a colloquial term out there to mean a short while. I would not want to be in the engine bay of a 6000 rpm motor for that long, and what would he be doing for that long? Surely checking the timing cannot take that long. You just rev it a couple seconds to get a good reading then let it back down to idle.

I am not sure if oil temps could get out of hand after 6000rpm for a couple minutes sustained. If you did not have a radiator fan on the cooler and the engine was pretty 'hot' it might be bad.

A couple minutes of total time spread out over an hour would be fine though.

[Jerome74911S]

Yes, revving it to 6K for a few seconds to check is fine, but if it is wrong, do you go about adjusting it while it's going that fast? I haven't, but I'm just asking. . .

[RoninLB]

Originally Posted by Jerome74911S

adjusting it while it's going that fast? I haven't, but I'm just asking. . .

adjustine what?

The timing or air/fuel?

[Walter_Middie]

do you go about adjusting it while it's going that fast?

I don't see why you would try to adjust the timing while at 6,000 RPM. Just adjust it at idle, then it only takes a moment to open the throttle and see where the total timing is. No need to even go up to 6,000 RPM. Just rev it enough to see when the timing stops moving. It's very quick and easy.

[psalt]

Assuming you could keep it cool, would it be bad to run that same motor for extended periods while not in gear?


David,

Yes. In addition to the rod loads, there is apparently a torsional vibration issue with high rpm, no load running. BMW specifically warns against it in their M3 manual and some OEM's (MB is one) are now adding several thousand rpm lower rev limiters in neutral. I agree that blipping the throttle to check the max ignition advance is fine, but I don't agree with the poster who suggested several minutes of high rpm, no load running is fine. As a practical matter, the suggestion of holding the throttle open a fixed amount with a stop for minutes won't hold a certain speed, and will have the engine bouncing off the rev limiter .

[RWebb]

another way to look at this: when an engine dyno is used, is a load always applied?

[manbridge 74]

[QUOTE=Walter_Middie;6350939]I don't see why you would try to adjust the timing while at 6,000 RPM. Just adjust it at idle, then it only takes a moment to open the throttle and see where the total timing is. No need to even go up to 6,000 RPM. Just rev it enough to see when the timing stops moving. It's very quick and easy.[/QUOTE]


^^This: Different year models would have RPMs of varying amount where maximum timing advance comes in. I would well imagine it somewhere under 6K for most 911s?

Or did tests reveal that engine killing detonation was an issue when conditions were.... 100*F day, with high oil temps, with high 6000 rpm, with load of hill, etc? Load being a contributor to detonation as opposed to 6000rpm no-load windup NOT contibuting to det.

[Green 912]

Originally Posted by RWebb

another way to look at this: when an engine dyno is used, is a load always applied?

That has nothing to do with protecting the motor. How you going to measure work unless you make the motor do the work? Computer modeling?

IIRC the ajusted no load rev limits were there to help prevent momentum based overrevs as well.

[RWebb]

very little load has to be used to measure

actually, the engine is always doing work anyway

[Flieger]

But you need to have enough load so you can go full throttle without the engine accelerating if you want to do a brake dyno. Or you can do inertial dyno and measure the angular jerk along with instantaneous angular velocity but I think brake measurement would be easier and more reliable. Although, one would only really need accurate angular position and time measurements then differentiate it 3 times and know the inertia of the dyno. Then you would also have to account for the engine's own inertia, though.