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Why does blipping the throttle work with clutch disengaged?


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Kmac5130
New User

Oct 28, 2021, 4:53 PM

Post #1 of 3 (143 views)
Why does blipping the throttle work with clutch disengaged? Sign In

I drive a 2004 Jeep Wrangler 4.0L with a 5-speed transmission. I have a question on rev-matching when downshifting, specifically about blipping the throttle. When downshifting, I understand how double clutching while blipping the throttle can allow for a quicker downshift, because the clutch is fully engaged when the throttle blip occurs, so the input shaft and counter shaft speed up and allow me to easily shift the gear lever into a lower gear when I press in the clutch the second time.

What I am struggling to understand is how blipping the throttle still works even without double clutching. If I am in a high gear, press in the clutch, and blip the throttle while the clutch is still pressed in, I can just as easily shift into a lower gear as if I were using the double clutch method. I understand that in both scenarios the engine’s RPM is increasing, but when I blip the throttle while the clutch is still pressed in while using the second method I described, why does the transmission allow me to shift into a lower gear? From my understanding, when the clutch is pressed in fully, the engine is not applying rotational force on the input shaft or counter shaft, so a blip of the throttle should not be speeding either of them up. And if the counter shaft is not speeding up to match the output shaft’s speed, then why can I downshift so easily when I do this? My vehicle does have synchros, but that does not explain why blipping the throttle with the clutch pressed in seems to speed up the counter shaft.

If, for example, I want to downshift without blipping the throttle and without slowing down, my gear lever does not easily let me shift to a lower gear, unless I tried to force it in. I also want to be clear that my vehicle is not electronically preventing me from downshifting in any case. It seems clear that the counter shaft is sped up in both cases either when I blip the throttle with the clutch pressed in, or when I shift to neutral and blip the throttle while I have the clutch completely engaged (not pressed). My only thought is that there is still a small amount of friction between the flywheel of the engine and the clutch, even when the clutch pedal is completely pressed in, which would speed up the input shaft and counter shaft when I blip the throttle, allowing for me to downshift smoothly without slowing down.
I’m sure everyone knows, but when I say “blip the throttle”, I mean a quick tapping of the gas pedal to raise the engine’s RPM’s.

Any info on this would be appreciated. Thanks


(This post was edited by Kmac5130 on Oct 28, 2021, 4:58 PM)


Hammer Time
Ultimate Carjunky / Moderator
Hammer Time profile image

Oct 28, 2021, 5:38 PM

Post #2 of 3 (131 views)
Re: Why does blipping the throttle work with clutch disengaged? Sign In


Quote
My only thought is that there is still a small amount of friction between the flywheel of the engine and the clutch, even when the clutch pedal is completely pressed in, which would speed up the input shaft and counter shaft when I blip the throttle, allowing for me to downshift smoothly without slowing down.


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Kmac5130
New User

Oct 28, 2021, 7:19 PM

Post #3 of 3 (113 views)
Re: Why does blipping the throttle work with clutch disengaged? Sign In

Thanks for that. I couldn't find that anywhere else, and I don't have mechanical experience to have figured that out.






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