slider

I know in Nascar, if a car spins, there is a good chance the motor will re-wind if you don't keep the clutch in. Completely different beast, clearly. But I think you certainly can get these motors to run backwards with enough force.

simsgw

Okay, quick version since I have an appointment. You're right that we can do some amazing things if we apply enough force to a device. If your avatar is related to your work, Joe, then we both know that's true.

However...

The assertion was that we can create those overrev's reported in the DME listing by spinning the car. That isn't the same thing as saying the engine was turned backward for some number of revolutions. No, that assertion includes two specific assumptions -- which some may have called facts in earlier threads -- that I cannot accept at all and one that sounds... possible, but with serious caveats. I will explain, but quickly first in case I have to leave: The assumptions are that the engine can be driven to a backward rpm higher than it was turning forward, and second, that the ignitions will continue in that state and be recorded in the DME log.

Discussion. As usual with such a hypothesis, we'll start by assuming it is true and see where that leads us. Thus: "Wow! Look at that DME reading. Must have happened when he spun in turn nine."

The car is traveling at any speed you like, and what we care about is the rpm, so let's say it is near the red line to give this hypothesis a fighting chance. Since I need something specific to avoid injecting equations into this, let's suppose it is in third gear when it spins and go from there.

1. Initial speed, 105 mph, rpm 7300. (I'm guessing here. Haven't checked the exact mph per 1000 rpm for a 997.2S. Work with me.)

2. Spin is initiated when the rears go beyond the sweet spot, the point of maximum adhesion. Car rotates because at that point, the tire patch has no significant directionality to it. That is, a tire pushed into that regime does not 'care' whether it is traveling sideways, forward or backward. But as the speed drops the tire hooks up again, as it always does. I managed this once on turn... eight I think it is at Sears Point in heavy rain. But the tires hooked up after I rotated 180 degrees, so I had to steer in reverse until the car slowed enough to snap it around forwards. (Fortunately, it was a test day so I didn't have cars going over the top of mine.) That is the condition we need to deliver that force to spin the engine backward. The car must be traveling backward for some measurable period of time. Otherwise the tires won't hook up and we can't deliver a sustained torque high enough to slow the engine to zero rpm and accelerate it back up to -8000 rpm.

3. The car was going at 105 mph, which required 7300 rpm. It must now be going ten percent faster to reach 8000 rpm in reverse. That is, 115 mph backward. Either that, or the damn fool driver downshifted to second in the middle of a spin. I have seen people befuddled enough to leave the clutch engaged in a spin. I will grant that. I have never seen anyone downshift once a spin was established and then re-engage the clutch. Could the downshift have caused the spin? Certainly, but then we have to assume he shifted into second while traveling at 7300 rpm in third. Yes, I've seen that too, and it is entirely plausible that such a person so incompetent might also not disengage the clutch. But I submit we have violated the premise. The overrev (whether forward or backward for the crankshaft) occurred because of a downshift at too high a speed, not the spin. Moreover, I seriously doubt you could begin spinning up the engine from 7300 to the point where the wheels lose adhesion, say 8000 and then manage to cancel that kinetic energy and inject enough energy for the change of crankshaft speed required. We're talking about going from 8000 rpm to minus 8000, which is 16,000 rpm of change. Even a racing engine with near-as-snot no flywheel at all takes a noticeable length of time to change rpm by 16,000. With such a car, you might make this happen. With a 911 street engine, I don't believe it. The spin would complete before the engine could be induced to change speed by that much. First objection This is the mild one that I might accept with evidence. It is a bloody lot of kinetic energy in the rotating masses of the engine that we are trying to reverse in the time it takes to come to a stop as the car spins. The engine will overrev forward. We know that, but will it overrev backward as well while it spins? Possibly, but I would need serious persuading, as noted.

3. Next, suppose it spun without a missed shift. Just too much speed and not enough pavement. As the car rotates, it slows. So we have the second objection which is a tough one: Insufficient speed to over rev the engine. We were at 7300 rpm going forward. Ignore all the gyrations. Pick up the car and turn it backward at that same speed. The engine -- if it can -- will be going 7300 backward. Not more. No overrev. I seriously doubt an engine will tolerate that by that way, but I can't testify to it personally, so we'll suppose it can. What it cannot do is somehow accelerate the car to a higher speed so the wheels can in turn shove the engine over 7500 rpm in reverse. No energy is available externally to accomplish that change of speed, and it sure ain't coming from combustion of fuel once you do that to the engine.

4. Finally, go back to that initial stage of the spin. What happens if you add power at that point? Well, you pinwheel of course. The rear tires are already past their limit of adhesion, so if you add torque they will just spin faster. They can't accelerate the car until they hook up and they won't hook up as long as you keep your foot on the accelerator pedal. If the driver does not remove power, the spin will be quite dramatic, but there is no chance of "rewinding" the engine. We have enough torque to keep a spinning wheel from hooking up at... well, any speed I suppose, but certainly at the mid-range speeds we're discussing. We might well over-rev the engine applying torque to a spinning wheel, but we preclude any chance the engine will be slowed to a standstill and then forced into reverse motion. Overrev? Yes. But caused by the fool keeping his foot in it. Not by rewinding the engine.

5. Okay, he didn't disengage the clutch, but our driver did take his foot off the gas pedal. (Huzzah! He's not a complete idiot.) The car rotates to something like a full 180 degrees, and then the tires hook up, as happened to me that day, but this guy leaves the clutch engaged. The torque from the rear wheels begins slowing the engine (along with the car of course). Not difficult with clutch engaged. Now what happens when the engine reaches idle rpm and continues to slow? Well, try it sometime at a stoplight. The fire goes out. An engine will not continue the four-cycle combustion process while stopped. These are not motors, though we say that sometimes. An electric motor can be stopped and will produce its highest torque at the point. An engine cannot be stopped. Or rather, it can of course, but not so that it continues operating. Alright, that just means we have no resistive torque except the pumping losses as we continue to reverse the rpm, as we head into negative rpm ranges. Of course, the car has slowed drastically by now, so we can't possibly reach the over-rev condition we hypothesized, but I already complained of that as objection two above. My final objection is that the fire went out. The engine management computer records ignitions in the report we call the DME. No ignitions will occur once the engine passes zero rpm heading toward those reverse revs.

Is it certain the fire will go out? Well, not strictly. I could code up an engine computer to time the ignitions in reverse. But why the hell would I? And if I don't it's just like propping one of the old aircraft engines. With magneto ignition, they can backfire and reverse direction long enough to break your arm. Maybe even take off a forearm, but nothing worse. Engines do not run in reverse.

Sorry this is so rambling. I have to run, as I said. I don't object to the idea that we can force an engine to turn backward if we don't disengage the clutch in a spin. You have to be fuddled to do that, but loss of control can do that to people. What I object to, and will not accept without direct evidence from my own instrumentation, is the idea that first, the engine can be driven into a reverse over-rev condition, to minus 8000 rpm or beyond, since you weren't going that fast forward; and second, that the engine management computer will permit, let alone record, ignitions while the engine is spinning backward.

Call me stubborn if you like.

Incidentally, not an objection but an observation before I leave. We are taught to disengage the clutch to avoid flat spotting the tires, not to avoid rewinding the engine, but I can see how that might arise in higher formulas than I drove, with the ultralight rotating mass of those engines. What I believe is that having done that to an engine, you won't be studying DME reports for evidence. You'll be walking the track trying to find the odd pieces that flew off. Not from the rpm intrinsically but because the bits aren't designed to transfer that much torque backward and I expect something will seize before you get past -2000 rpm. That's just a hunch and not evidential, but I'd sure want a salvage car to test, not my own.

Gary

slider

great response, Gary. I agree DME readings will not tell you if an engine has been spun backwards. However, given the DME-noted abuse, I would fully expect under that kind of hard track driving, the likelihood of spinning the car is high and therefore getting the engine to rotate in an opposite direction is also higher.

simsgw

Boy, I'll agree with that. With as little as a donut for incentive, I would believe this guy used his Porsche to pull a race trailer to track days and ran the Baja off road race each season in it.

Gary