Aggro_Al

Yes, in general, if you gave the engine more throttle it will increase in RPMs and if you stopped giving it throttle it will eventually just stall. This should happen whether your car is a 6MT or a 5AT. This should happen whether you have the clutch disengaged or engaged unless you reached the absolute limit of the engine. But, it takes time for the engine to accelerate and de-accelerate. It's not instantaneous and this throttle action didn't have anything to do with the blip that happens when you step on the clutch.

On the 6MT, the ECU did not blip your throttle. That short blip was caused by you disengaging the clutch. When you disengage the clutch, you've seperated the engine shaft from the rest of the drivetrain at the flywheel. The engine is free from the load, after the flywheel, that it was once spinning. You've decreased the torque load.

If everything else stayed constant then it's simple math.

RPM=(HP*5252)/torque

Just by disengaging the clutch you will unload the engine, your RPMs will go up because of less torque load. This will continue until resistance and friction from the engine brings your RPMs back down. You can counter this loss by adding more throttle to bring your RPMs back up.

When you re-engage the clutch, you replace the load that was once disconnected causing a drop in RPM from the neutral position unless you are accelerating from the throttle to overcome this loss. This is why engine braking works. You are now using engine compression to decrease your RPMs which in turn lowers your velocity.

On the 6MT, unless you are manually rev-matching and double clutching for better performance, the synchromeshers will rev-match for you in most circumstances. The synchro is a lightweight ring with spines that allows the engine shaft and gear shaft to match speeds and couple (hopefully, when the clutch is disengaged). There is no ECU that controls the clutch or shifting process on the 6MT. If the 6MT has electronic controls for shifting, I would like to know where it is?

The 5AT can do automatic downshift rev-matching in manual mode because it has the software to do it and it uses a computer controlled clutch. In auto-mode, the 5AT's computer fully snychronizes and torque manages the first four gears. No rev-matcing necessary.

You would need the software and controls found in one of the automatic transmission (F1 type or Tiptronic type) to do automatically controlled blips and rev-matching. The only way that I'm aware of rev-matching on the 6MT is by manually double clutching or heel-toe.

If you need proof, find a straight level road with no traffic. Accelerate to about 25- 30 mph in 3rd gear. While doing your best to maintain a level throttle, step on the clutch but don't change gears. Note which way your tach went? Now, re-engage the clutch in the same gear. Note which way tach went now?

Aggro_Al

You lost RPMs because you took your foot off the throttle, you gained back some RPMs because you disengaged the clutch and you lost RPMs again because you re-engaged the clutch.

up2nogood

Gotcha. Nice job Aggro!

spacemn_spiff

iTrader: (3)

This is called inertia and also friction is involved.


I dont know what you are trying to do by using the Power equation. If you want to compute the change in momentum then you need to use momentum energy equation and conservation of momentum principle. So there is something wrong with the fundamentals in your usage of just power equation there.

Power = Torque x Angular speed = and also is equal to Force x Linear speed

If you think your equation is different than above then your equation is derived from this equation because you are using different units on both sides of the equation, thats why you need those constants.

Load doesnt factor into the equation for increasing RPM until you keep the power same, in this case since you are not pressing the throttle, no power is provided. So rpm should not increase.

If you are assuming that the torque in the energy equation is the load torque, then you are wrong my friend. Its torque produced by the engine. In my case, there is no torque produced becaused I am not pressing the throttle.


Synchros are nothing but clutches, a special kind - conical friction clutches and final engagement to the gear after synchronising is done by dog clutch.

So either ECU is blipping or delaying the throttle closing after pedal is released.

350Zenophile

iTrader: (20)

I can't believe the ECU on a MT vehicle would blip the throttle in anticipation of an up shift. The later is the only reasonable explanation I've heard if you guys experiencing this are 100% sure you aren't pushing the clutch in before completely letting of the gas.

We do have drive-by-wire throttle and I'd bet good money it was engineered to roll off the throttle closing to smooth out the on-off transition. I remember reading reviews of some early DBW systems acting more like On-Off switches compared to traditional throttle cables.

I wonder if this might not be an early Model Year issue that was refined in later ECUs. Quick poll: What MY do you have if you are experiencing this?
( )2003
( )2004
( )2004.5
( )2005

Aggro_Al

The flywheel can store energy and maintain momentum. The flywheel is there to smooth out the power pulses of the engine and to provide smoother shifting while the clutch is disengaged. The heavier your flywheel is the more energy it can store, the downside is that it takes longer to accelerate. A lighter flywheel can accelerate faster but won't store as much energy. Check with those that have replaced their stock flywheels for lighter one. They can accelerate faster but they also have bigger swings in RPMs while shifting.

The bottom line is the 6MT will not automatically rev-match or blip for you. You have to rev-match or blip manually.

That short RPM spike is caused by clutch disengagement not the ECU. Try the test I mentioned above and then tell us that it doesn't happen. The reason I said to keep a level throttle is so there is no up or down effect from the throttle.

King Tut

iTrader: (1)

Just another thing to toss in is that I have found this is more common on cars with an electronic throttle instead of a throttle cable.

spacemn_spiff

iTrader: (3)

I am going to test out with the foot completely off the throttle and depress the clutch. Mind you, this is not my first manual car, I have driven manuals all my life, so I know how to synch the gear shift without revving the engine.

Lets not talk about function of flywheels please because I have designed flywheels and momentum compensation systems for my job. The explainations that have been given so far as non-technical as can be.

Also doesnt the cruise control take an input or have a sensor that detects if the clutch is depressed or not. May in combination with that ECU knows what the clutch pedal position is. I havent tested cruise control my Z.

350Zenophile

iTrader: (20)

You're really reaching here man. Yes it does, but how would your ecu then know if you were going to shift up or down based on that? I'm sure if Nissan had designed some proprietary rev blip/match system from MT's we would have heard about it through normal PR channels.

Aggro_Al

Yes, cruise control does have a sensor. On the 6MT, cruise control momentarily disengages while you are shifting gears and then readjusts after you finish shifting. Stepping on the brake or on the clutch will turn cruise control off.

spacemn_spiff

iTrader: (3)

So there you go, so there is a possibility that ECU can blip. And I am trying to find out if someone knows the officially.

It just doesnt know up/down, just probably knows that you are going to shift, its easier to rev the engine down than rev the engine up when you shifting because you are working with engine friction.

If we dont know for sure, then we dont know, I just noticed the blip and am trying to find out whats going on. What I am damn sure of is that its not because of momentum and there is something to it other than that.

Aggro_Al

Umm, I never said that. I said that cruise control is off while you are shifting or braking.

Cruise control is off while you are shifting on the 6MT. On the 6MT, once you re-engage the clutch to get cruise control to work again, you can no longer blip or rev-match the transmission because the gear is already connected. Even if you have an F1 type transmission you couldn't do this. Once the clutch plates are connected it is bad to have them slip. This doesn't apply to the 5AT, the 5AT has a torque converter and it has the ability to uncouple or unlock and slip without disengaging the gear.

Again, just stepping on the throttle or letting off the throttle will, in general, increase RPMs or decrease RPMs whether the clutch is engaged or disengaged or whether it is the 6MT or the 5AT. I believe just about every car operates on this principle. Depress the throttle to go faster and release the throttle to slow down.

The 6MT will not automatically blip or rev-match.

spacemn_spiff

iTrader: (3)

I know you didnt or neither did I intend to say that you did. I just simply meant to imply that if ECU knows the clutch is disengaged, it is as simple as a software or logic change to blip the throttle.

If you realize, ECU is aware of all the change in inputs, such as throttle, brake, clutch, so to do something as simple as a blip, like I said its just a software change, may be a couple of lines of code, if even that.

Cruise control also is a software in the ECU, that analyzes all the inputs and decides what needs to be done when it gets the right inputs. I agree that ECU may be pause the cruise control when clutch is pressed and cancels it when brake is pressed.

Also, BTW AT does not engage and disengage gears as you indicated, it has clutch or bands for each gear ratio that lets power flow through a particular gear ratio and not others.
So it doesnt need synchros or dog clutches, it has planetary or epicyclic gear train to minimize size/space or for compactness.

Aggro_Al

I think you misunderstood what I meant. I never said that torque converter based ATs engage and disengage gears like an MT because they don't. You are absolutely right that TC based trannies always have power flowing through them unless they are in neutral or park and that they use planetary gearsets, several clutches and bands. What I was trying to illustrate was that you couldn't blip or rev-match an MT while the clutch was engaged because an MT is not suppose to slip while in gear. I guess I didn't make it clear that I was making a comparison that for a plate clutch to slip normally that it had to be disengaged from the gear where as an AT just has to unlock or uncouple to allow slippage without disengaging from the gear.

You are right that also that the 5AT doesn't use synchros or dog clutches. I never said that the 5AT used them. What I did say is that the 5AT is able to do automatic downshift rev-matching in manual mode because it had the software and hardware to be able to do it. And I also said that rev-matching wasn't necessary on the 5AT in auto mode because the computer controls the synchronization and the torque management of the first four gears.

Some of the F1 type auto transmissions, like SMG and DSG, do have automatic rev-matching built into the transmission. This type of transmission uses most of the same parts as a standard MT but also uses computers and electronic controls like the Tiptronic type auto transmissions to automate both clutch and the shifting. The transmission in the Z 6MT is not this type of transmission. The Z has a standard manual transmission. The driver controls the clutch and the shift process manually. In the cars, with automatic rev-matching the driver only get to tell the tranny when to shift and the computer does the whole shift process automatically including the rev-matching.

The 6MT does not have automatic rev-matching or blipping.

Kolia

iTrader: (2)

Guys,

Drive by wire means we are not opening the throttle ourselves. The ECU does that for us. He also recognize how fast we hit the throttle and will smooth the signal for drivability purpose. The ECU will "retard" throttle release to minimise rough transitions (a problem of early EFIs).

If the engine revs up when the clutch is depressed, it only because power (ie fuel) is still injected at that time. Otherwise, deceleration is instantaneous, you both know that. "Unloading" the engine by separating it from the tranny will not accelerate it...

spacemn_spiff

iTrader: (3)

Thanks Kolia for the information. Engine is getting fuel for a split second after the clutch is depressed, just what I was saying.

Kolia

iTrader: (2)

All evidences are pointing that way.

But I don't think it's because the clutch is depressed. Rather, just a "lag" when comming off the throttle. If you time yourself right, the engine revs do not go up when clutching.

spacemn_spiff

iTrader: (3)

Agreed.

Aggro_Al

In a perfect world, we would hope to get 100% torque transmission from the flywheel to the drivewheels while the clutch is engaged but, unfortunately we don't. We have parasitic drivetrain loss which causes a difference. Also, while the clutch is disengaged the engine isn't moving the car.

Look at anybody who has dynoed their cars and you will see the numbers at the drivewheels is always less than numbers at the flywheel. After you adjust for gearing, wheels and tires, all that extra loss from the flywheel to the drivewheels is attributed to parasitic drivetrain loss. When you step on the clutch you seperate the engine shaft from a lot of that parasitic drivetrain loss and a portion of that loss is returned to the engine. You also have to consider that the engine isn't being applied to moving the car while the clutch is disengaged. This is the spike you see in the RPMs.

If you do the test I described before and keep the throttle as level as possible, we will have minimized any effect the throttle has in the change of RPM while disengaging and engaging the clutch. Leave the car in the same gear so that we are disengaging and engaging the same gear and when you step on the clutch your RPMs will shoot up without help from the throttle. When you re-engage the clutch your RPMs will go down without any change in throttle position.

Again, the Z 6MT does not have automatic rev-matching or blipping. On the 6MT you have to do it manually.

Kolia

iTrader: (2)

I’m not sure I understand correctly what you’re trying to explain Aggro Al.

You’re basically saying that an engine kept at 40% throttle will spin faster with zero load than one fighting mechanical and aero drag? I think it’s pretty obvious to everybody.

The only way to minimize the effect of the throttle in that kind of experiment is to be completely OFF the throttle. In which case, when we depress the clutch, the rpm drops immediately because the inertia of the car was what was keeping the engine spinning. No fuel is injected until the engine drops to 900 rpm or so. It then switch to idle mode and prevents the engine from stalling.

One more thing the ECU is measuring is the actual load of the engine. Driving at a steady 50 mph on a flat road or going up a hill at 50 mph does not require the same amount of fuel and ignition timing, even though rpm will stay constant. It measures engine load by reading the throttle position versus engine speed/acceleration and compare it to what these should be if the car has not loaded. It adjust to compensate.

This is why maintaining constant throttle position is not a valid test. The ECU will immediately detect the off loading of the drive train and cut back on fuel trim. The short time it takes to adjust is probably what we feel as rpm stepping up.

Again we see that the increase in rpm is due to fuel injection, not any flywheel inertia energy storage. The flywheel will never accelerate on it’s own when you unload it.