Pullback when you downshift

[Stallion]

I was just driving the Corvette today and I was thinking, you know how when you downshift you feel the car pull back, what exactly causes that? The compression of the engine?

 

[78SilvAnniv]

Pull back?

...do you mean by downshifting and the engine slows the car, appearing to pull the car backwards?
I don't know anything about compression...or what the 'cause' is for this feeling you describe...other than the engine slowing the forward momentum.
Perhaps someone else will chime in with a scientific explanation.
Heidi

 

[TomOB]

Here's my best try. If you are going a certain speed, in a certain gear, you're gonna have a certain RPM. downshift and now that mph translates to a higher rpm. the engine needs to speed up---quickly. In the most 'elementary' sense, the energy that is the forward motion of the car needs to be turned into various forms of energy in the engine. it must slow down the car (at first). Then there's the clutch and the shifting of gears....all these thigns coudl cause a sudden jerk or slowdown (shoudlnt be very significant), especially if you dont release the clutch slowly enough.
Tom

 

[Stallion]

Oh I see what you mean, Tom. It is the engine not at the high enough revs at that certain speed. That sounds logical. Thanks!!

 

[JohnZ]

Yup, engine compression is what slows it down, and if you feel a sort of a "jerk" when you downshift and let the clutch out, that's caused by the engine rpm being "driven up" to match the lower gear ratio you shifted into; with a little practice, you can "blip" the gas pedal before you let the clutch out to bring the engine rpm up to the higher level it needs for the downshift so it's a nice smooth transition without the "jerk".

 

[Stallion]

Okay, that's what I figured, John. Thanks!! And I pretty much have the idea that you need to gas it when you let out the clutch.

Thanks again!

 

[TomOB]

hey stallion,
did you find out that insurance info? thanks.


Just wondering how many vette heads know the theory behind their cars,. i am in no way trying to show off anythign or be condescending, but I coudn't help myself from learning all the science behind cars.
maybe thats just me.
tomas

 

[Stallion]

I haven't talked to my dad yet about the plan we have. When I do get all the info about everything, I'll e-mail you when I get all of the information on insurance. Shouldn't be but another day. Right now he's working but I'll talk to him in a little bit.

"maybe thats just me."

I'm just like you. I want to know what's going on in the Vette and why it happens. That's why I ask all of these questions. Knowing that the engine compression pulls back when you downshift isn't vital to car maintainance or even, I imagine, repair. But I'm just interested!!


John, speaking of which, what's the physics behind the compression pull back? What does the engine compression have to do with the power train in that idea? Thanks!!

 

[78SilvAnniv]

Me, too!

"maybe thats just me."
I want to know what's going on in the Vette and why it happens.

I also like to know "why".
As a girl, I feel I have an extraordinary amount of knowledge about the cars we drive, but I certainly don't know everything, and I like learning about new things.
I'm familiar with the basics, but it is always nice to know a little more and be able to hold a coherent conversation with my mechanic.
Impresses the mechanic, too!

I have learned A LOT by following Stallions threads. Thanks!
Heidi

 

[TomOB]

Heidi, i like to think the same thing..that I know a lot for a NYC kid. I went to school in manhattan (no im not a prissy private schooler with 4 bmws) and people don't even care what horsepower is.
When i fell for the vettes i had no one to reach out to for info (i didnt find this site until a year ago), so i justread websites and some engine builder magazines. I came up with my own definitions of torque and horsepower, i think it was "torque is the strength on the stroke and the horsepower must be how powerful it is" Then i took physics and learned the basics. That was junior year of high school. Inspired by the z06's efficiency i did an independent study in senior year on engines and efficiency. I was at too low a level to read the engineering texts so i did a lot of theorizing and calculations and simulations on my own. My physics teacher was impressed and i learned a whole lot. I came to Upenn and wanted to try some engineering but it interferes with my business curriculum too much.
Anyway, lets see if we can answer that. I guess my best explaination woudl be to try and turn the engine yourself. I've never done it but it cant be a walk in the park. Theres alot to do, you have to open and close valves, and then the cylinders have to intake air, compress it, then exhaust it (the power stroke would actually be easy to go through). Ok, now rotate the engine say 2000 times per minute. You can see it take a significant amount of power. Now imagine you're driving at 2000 rpm with say a final ratio (including wheels/tires) of some imaginary number 4. Downshift and lets say your ratio goes to 8. This would require an engine speed of 4000 to maintain the same road speed. Lets just assume that the energy to raise the engine to that speed is just double that of the engine at 2000. The energy has to come from somewhere. If you dont give it a little gas like John mentioned, it has to come from the kinetic energy of the car itself. If the car loses kinetic energy, it has to slow down somewhat. There's your jerk, if you do it quick enough.
Once again, the compression, as i interpret it, is just another reason why the engine is hard to turn and makes it hard to speed up. Theres also the flywheel and any other rotating mass that is before the clutch.
Tom

 

[Stallion]

No problem, Heidi!

I also like to hold educated conversations with people who are also into old muscle cars (dad, my uncle ). It's good stuff to know now to do.

Thanks for the explanation, Tom!!

 

[vettedoc]

Yup, engine compression is what slows it down, and if you feel a sort of a "jerk" when you downshift and let the clutch out, that's caused by the engine rpm being "driven up" to match the lower gear ratio you shifted into; with a little practice, you can "blip" the gas pedal before you let the clutch out to bring the engine rpm up to the higher level it needs for the downshift so it's a nice smooth transition without the "jerk".

On the racing circuit they use the heel and toe trick to blip up the rpm's prior to down shifting. It works well with a litte practice. I picked this up from my uncle who is a Ferrari person and former racer.

 

[Stallion]

On the racing circuit they use the heel and toe trick to blip up the rpm's prior to down shifting. It works well with a litte practice. I picked this up from my uncle who is a Ferrari person and former racer.

How would you do this? Wouldn't you just wait until you get to a certain speed to downshift and you shouldn't loose too much on the pull back, right?

 

[JohnZ]

Nope, when you need to downshift (under race conditions) you want to do it RIGHT NOW, so you "blip" the throttle to bring the rpm's up to where they need to be for the lower gear ratio before you let the clutch out; that makes for jerk-free engagement, minimizes wear on the clutch (because it doesn't have to carry the sudden load of "spinning up" the engine), and avoids possible unloading of the rear tires under heavy braking when the clutch engages. Under race conditions, this is done by using the ball of your foot on the brake pedal and your heel on the accelerator pedal so you can maintain braking effort while "blipping" the accelerator pedal to bring the rpm's up (some drivers prefer to do it the other way, using their heel on the brake pedal and their toe on the accelerator pedal - same result).

In normal street driving, you'd seldom be braking and downshifting at the same time (unless you're "playing racer"), so the heel-and-toe routine isn't necessary; just "blip" the throttle before you let the clutch out so the car doesn't "jerk".

 

[Stallion]

so you "blip" the throttle to bring the rpm's up to where they need to be for the lower gear ratio before you let the clutch out;

Wouldn't you want to do this everytime you down shift? Otherwise to just wait until you slow down, wouldn't that be not a good idea? Wouldn't you just rev the engine to get the high RPMs before you let off?

 

[JohnZ]

Let's say you're downshifting from 4th to 3rd (same procedure, regardless of which gear you're starting from); push the clutch in, shift from 4th to 3rd, "blip" the throttle to rev it up a bit, and let the clutch out. It's as simple as that - shift and "blip" at the same time, then let the clutch out - with a little practice, it'll be nice and smooth; saves wear and tear on the clutch.

 

[Stallion]

Okay, John, I understand now. Thanks!

 

[69blushrk]

Another good downshifting method

http://www.ferrariclub.com/faq/heeltoe.html

 

[Stallion]

Wow!! That's a good idea. But sounds like it needs lots of practice.