JustinL

I've got lots of datalogs that show that revs stop climbing once fuel is cut.

Making something spin faster without adding energy or subtracting some moment of inertia defies physics. If you overshoot your limit the ecu has not realized that engine speed > rev limit, or the ecu has failed to cut fuel. These things don't happen instantaneously so like I was saying the feedback loop is one potential, or you are looking at the needle which is not a true representation of RPM and can overshoot.

Tom M'Guinn

Are you sure it's not the momentum of the tach needle that keeps moving up, rather than the actual engine speed? I've wondered about this before. Maybe it's time to set the rev limit to 4K or so and hook up a digital tach to find out...

Rogue_Ant

Then your rate of accel wasn't significant enough to make a good example. But if you have a single log showing more than your set rev-limit, your theory is busted.

Do you really believe that acceleration/momentum completely ceases at fuel-cut?

Chris White

acceleration/momentum are completely different functions.
Acceleration will stop the instant that the force is removed.

Chris White

The tach can be made to bounce around wildly…way off when the rpms are changing quickly.

If you blip the throttle repeatedly you can easily get the tach to bounce off the end of the gauge travel. It does help to have a light flywheel to really get it going!

951_RS

Lol just wait until you're done. It'll do it in third gear when the road is damp
I had some race gas in the other day and put it to 23 psi and it got crazy around 75-80 mph when boost hit

M758

Don't worry one bit.

M758

Once you pull the fuel the engine will not accelerate. In fact it will start slowing down right away given the compression ratio.

The only way to damage a motor by over rev when you have limiter is either by constant running or by a mechanical one. IE going from 4th to 3rd at 110 mph. That is bad. I have seen a number of 944 spec cars bend valves when missing shifts. At about 100 mph (top of 4th in a 225/50R15 NA) they get 3rd and the cars momentum runs the revs sky high as the clutch is released. This can cause valve float and contact. However if it did not do it you are safe. The bottom ends can take 8000 rpm for short durations. Valve train is the weak spot.

If you run for 3-4 hrs at 7500 RPM you may suffer wear related issues.

DanR

hit rev limiter a number of times in the heat of a race, that is what it is there for!!! (ie to save you from potential over rev damage)

Rogue_Ant

Man you guys are a tough sell...
Ok, this is fairly basic physics. Acceleration does not instantly stop when fuel is cut. It diminishes over time. Why? Because of mass, in the form of momentum.

Test it for yourself. Go out in 2nd gear and go WOT. Now, without taking the car out of gear, let off the throttle at, say 5500, and watch your speed increase for a short time after you let off the throttle...

Duke

De-acceleration fuel cut is delayed in the DME. With instant fuel cut at <1% TPS the car is becomes very jerky to drive. When I program stand alone system I usually set the delay to around 0.5 sec.
IIRC the stock DME puts the fuel back in at around 1500 rpm.

fortysixandtwo

You're right, it is basic physics.
No force = no increase in velocity. The instant (as in an infinitesimal period of time) force is removed, deceleration begins.
no: if, and, or but, do not pass "go", do not collect $200.

In your experiment above, what you're seeing is lag in the speedometer. I wouldn't exactly consider any of the instrument cluster gauges to be lab quality.

Another equation would be Ps+951=F
where:
Ps = Snowy parking lot
951 = 944 Turbo
F = Fun

Rogue_Ant

Ok, one last time.
If acceleration is, say 10mph/sec. That doesn't simply stop and immediately decelerate. You are not accounting for momentum. The time it takes for acceleration to diminish to zero and decel to begin. This all time that the car is still accelerating. (and 10mph/sec is nothing compared to the acceleration rate of free-spinning on snow/ice)

I can't tell you how many logs I've seen of exactly this on a 951, and on better equipment then our 951. But whatever, I'm not here to teach HS physics/algebra.

I'm done here.

fast951

Tom, with the PB set the rev limit to 4000rpm, try to hit it (you need to create a bit of load). You can monitor the tach and read the RPM in the PB. The PB is your digital tach.

951_RS

I see what rogue_ant is saying and I'm inclined to believe him.
The car doesn't go from 10mph/sec to 0mph/sec and then negative. That would be an instant loss in acceleration of 10mph/sec which would be pretty sudden.
Momentum isn't being accounted for. What causes deceleration is the forces working against inertia, such as air, compression, and internal friction. The forces of the engine are obviously much greater than these things or else cars would not move. It takes a bit of time for these forces to slow the momentum and cause the car to decel.
I see this everyday in my car when I get on it really hard, go halfway through a gear and immediately put the clutch in, the car keeps revving up with the throttle lifted. I do understand that's reducing the load, however. So not apples to apples there.