Spirited driving fail? Over rev?

[Detroiter]

I would also like to add that in the original post OP said the RPMS were around 7200 while this happened. Wouldn't the tach still report RPM correctly even in a case of over rev? So it should have been showing ~9000 RPM rather than 7200. 7200 isn't even the rev limiter in our cars so I'm not sure how that could happen as well.

[Detroiter]

Originally Posted by stevesnj

If a dual clutch trans qualifies i'd say this is a drive by wire system with dry clutches, though there are wet clutch dual clutch systems also I believe.

I would say a dual clutch trans does not qualify as a manual While it does use a automated clutch setup I would not say it is a drive by wire system as there is no pedal to be depressed and have its position electronically sensed by the cars computer that tells a system to put *this* much force on the pressure plate.

[mav1178]

Originally Posted by Detroiter

I think you guys are misinterpreting what I'm saying. Because you are saying the same thing I am. It is mechanical and the electronics trying to deal with it is what CAN (not every time) cause the tires to give instead of the engine. The electronics could never "magically" stop the laws of physics. What I am saying is the ECU says "hey the engine is past 7600rpm that's not right" so it cuts fuel harshly resulting in a abrupt engine braking throwing a lot of resistance to the drivetrain. This has the potential to disrupt the rear tires enough to lock them up and drag the across the ground (similar to pulling the hand brake). To be fair it isn't actually locking them up but forcing them to spin at a slower rate than you are moving resulting in the loss of traction. If the harsh fuel cut engine braking is not a strong enough of a force to cause the tires to break traction then it will result in the engine over revving to like you said ~9k RPMS.

In your case with going from 6th to 2nd at 70 I'm sure the fuel cut was not enough to save the engine from over revving. If it was you would have heard a loud screech from the tires locking and the tail would have gotten lose and moved around a bit.

Sorry, the fuel cut is not enough to disrupt a mechanical overrev situation and "spin the rear tires"

It takes very little mechanical torque to spin our engines. Once wheel speed is connected to the engine via the wrong gear, the engine will overrev and you can damage the engine.

I suggest OP to take the car to have a leakdown test performed, to make sure the valvetrain is still okay.

-alex

[Detroiter]

Originally Posted by mav1178

Sorry, the fuel cut is not enough to disrupt a mechanical overrev situation and "spin the rear tires"

It takes very little mechanical torque to spin our engines. Once wheel speed is connected to the engine via the wrong gear, the engine will overrev and you can damage the engine.

I suggest OP to take the car to have a leakdown test performed, to make sure the valvetrain is still okay.

-alex

Do you have proof? Not talking shit or anything I'm just curious.

Having ridden in a 86 with someone who did not know how to drive manual the best the downshifted from 3rd to 2nd not as a over rev but when doing this they just dropped the clutch and did not rev match at all. The braking force made from this was pretty intense shoving me forward in my seat. I could completely see going from 3k RPM in 6th to letting off the clutch in 2nd would create a massive amount of braking force even before fuel cut.

[mav1178]

Originally Posted by Detroiter

Do you have proof? Not talking shit or anything I'm just curious.

My proof is all the guys that I've seen at the track with blown motors from a misshift.

I can probably count somewhere in the neighborhood of 30-40 cars in the past 15 years.

Almost all of them ended up with bent valves because the valves were floating at high RPMs. A couple threw rods out the side of the block when the piston was pushed.

But no, I don't have actual proof of exactly when and where every one of them happened, plenty of video on YouTube for research though, mostly from kids street racing.

[nico_rsx]

@Detroiter
Regarding rev-limiter or fuel-cut.
This doesn't have any impact here.
Any modern engine with injection, the fuel is already cut when you down-shift. There is already no fuel going to the engine. You can't cut something that's not there.


In a manual transmission engine, as soon as you completly lift the throttle, there is no fuel going to the engine. Not a drop.
The only exception is when the rpm is below 1200rpm (or little higher when cold).
Basically if your not idling or close to idling, and your foot is off the throttle, there is no fuel going to the engine, and the ECU is basically doing the same thing it does in rev-limiter (stop all fuel).
This is to save fuel, and provide some compression braking.
This doesnt apply to carburator engine.

[Icecreamtruk]

Stop throwing stop at each others guys. Detroiter is correct on saying an overrev from 6th to 2nd instead of 4th will lock the tires. Check this out

[ame="https://www.youtube.com/watch?v=TvDqFqvnxdk&t=7m8s"]R8 & GT86 Track Day - YouTube[/ame]

Going from 5th to 4th but instead getting second. Rear tires locked up, the tail stepped out and bam agaisnt the wall. Of course, sticky tires will make it harder to spin them and more likely to destroy the engine instead.

Nico is also correct in saying fuel is already cut while you are off throttle. And mav is also correct because with stickier tires, its the engine that spins more, not the tires lockout.

You are all right but saying different things in different ways.

By the way, I missed a few 4th to 5th upshits (2 to be precise) and gotten into 3rd. Both cases I felt something was not right so I realised the engine was revving up rather than down I clutched in. No damage was caused to the engine, but I did break the 8k rpm momentarly (saw it on video later, didnt have the time to check on the spot).

[Tcoat]

Originally Posted by Detroiter

I get what you are saying, but it limits RPM period do you have any proof it will not limit it during decel? I doubt it does anything but monitor the RPM and cut fuel if they hit the specified level. If you read my last post I said it will either be the engine or the tires giving way. If it's the engine yes it will FORCE it into over reving. What I'm saying is no matter what the rev limiter will still be in effect cutting trying to stop the over rev.

NO. It limits revs by cutting throttle. If you downshift too far cutting the throttle doesn't mean shit. It is not some magical device that will prevent an over rev for a downshift to the wrong gear. Yes the tires can give way or you can blow up the engine since it is trying to go well above the rev limit.

[nikitopo]

Originally Posted by Icecreamtruk

Going from 5th to 4th but instead getting second. Rear tires locked up, the tail stepped out and bam agaisnt the wall. Of course, sticky tires will make it harder to spin them and more likely to destroy the engine instead.

I can confirm I had the same issue when I accidentally shifted from 5th to 2nd. The rear tires locked up and the car started going sideways. It was possible to correct it and to not go against the wall. If the car was completely straight, then I would not have any issue.

[Detroiter]

Originally Posted by Tcoat

NO. It limits revs by cutting throttle. If you downshift too far cutting the throttle doesn't mean shit. It is not some magical device that will prevent an over rev for a downshift to the wrong gear. Yes the tires can give way or you can blow up the engine since it is trying to go well above the rev limit.

I could have sworn I read it was a fuel cut limiter and that it was potentially dangerous because of that(causing lean issues and such) but I'll admit I could be wrong.

But it is definitely not a magical device that will stop that from happening. All I was trying to say was that I have seen tires give way from a over rev mis shift before. I agree that in with the engine trying to go well above the rev limit it would most likely blow the engine.

[nikitopo]

Originally Posted by mav1178

My proof is all the guys that I've seen at the track with blown motors from a misshift.

Modern motors are not blow so easy by a miss-shift. I've seen sometimes professional racing drivers shifting deliberately from 5th to 2th to brake much harder the car. It just needs a lot of guts and to know how to do it. One of these drivers was for many years the WRC champion.


The aggressive rev limiter is there for a reason and I am laughing when I see tuners removing it

[churchx]

You brake not just by compression of fuel-cut engine but also by quickly wearing down synchros, usually meant to equalise differences of 500-1000rpm. Not advisable for anybody caring for long enough use. Though then again, what synchros in WRC cars transmissions, and if one overhauls coilovers on such in 1K miles or sooner, probably can rebuild engine & tranny more often too
But! If brakes are enough to lock wheels .. why use engine for that? Brakes are made for purpose, cheaper/simpler/quicker to replace wearables, quicker dissipate heat ..

[Shinchu]

you guys think the engine fuel cut is causing rear tires to lock during a missed downshift? (or upshift for that matter)


never heard of shift lock technique for drifting then huh?
watch the drift bible on youtube.


basically, as long as you aren't going faster than the max speed of the gear you are going down to (above would cause engine damage because your speed is higher than redline for that gear)
you dump the clutch without pressing the gas and the speed difference causes the rear tires to "lock" and then you hit the gas and its already in the powerband to hold the drift.
goes without saying don't go 100mph and drop to 2nd and think its going to be all dorikin supah dorifuto.


and no, fuel cut rev limiter isn't dangerous lol that's not what causes the rear wheels to lock. that's just plain operator error.
rev limiter will NOT prevent the engine from over-rev if the car is going too damn fast for that gear.


I love the "jumping out of the car" example haha pretty good description!

[nikitopo]

Originally Posted by churchx

But! If brakes are enough to lock wheels .. why use engine for that? Brakes are made for purpose, cheaper/simpler/quicker to replace wearables, quicker dissipate heat ..

This was not a WRC car, it was an ordinary car. Brakes are enough to lock the wheels in theory or in very old cars, in reality electronic systems nowadays will intervene to avoid such "dangerous" situations.