Engine rpm's increase under hard braking

[kabel]

I have experienced two instances where under hard braking (both feet in, full ABS engaged) I noticed the engine rpms increased, as if it was trying to anticipate a downshift. Is this something built into the computer? I suppose it is possible I did not have my foot fully planted on the brake and caught the accelerator as well… but I do not think that was the case. Anyone else have a similar experience or explanation?

[denverpete]

Originally Posted by kabel

I have experienced two instances where under hard braking (both feet in, full ABS engaged) I noticed the engine rpms increased...

No clue (is this a tip or manual). But, what the heck is "both feet in"? Are you braking using both feet or are you referring to some sort of Fred Flintstone method?

[kabel]

Originally Posted by denverpete

are you referring to some sort of Fred Flintstone method?

LOL! Funny. Sorry, yes we are talking about a manual tranny, "both feet in" referring to clutch and brake peddles "in" or fully depressed.

[Porschekid]

Originally Posted by denverpete

No clue (is this a tip or manual). But, what the heck is "both feet in"? Are you braking using both feet or are you referring to some sort of Fred Flintstone method?

I would assume he meant both feet (clutch and brake) fully engaged. And I haven't noticed this sensation... I would assume you bumped the gas if it isn't happening regularly.

[Porschekid]

Edit: He beat me to the punch.

[986Jim]

If your using both feet chances are your pushing on the gas at the same time and don't know it lol. If your car is tiptronic that explains how the rpm goes up with the brakes on, the torque converter will allow some revup with the brakes on, say 2500-3000 stall typically. Also if it is tiptronic it will downshift as the car slows down which you may notice on the tach as well, so there are a few possibilities here.

If your car is 5/6speed then you are probablly again pushing on the gas at the same time. When the clutch is fully engaged it's 100% connected to the engine, so if your braking and slowing the wheels your slowing the engine as well no matter what. It will slow right to a stall if you let it. If the RPM is going up then your braking and pushing the gas at the same time.

Either way, sounds like driving error to me.

kabel, why are you under such hard braking conditions? Did this happen at an autocross or DE event??

Did this happen during normal daily driving? If so, I suggest you increase your spacing between cars (and other stuff!).

Be safe.

[kabel]

Unfortunately both cases were on the street; I have had a curse upon me, every car I have purchased ends up in an accident with in the first month of ownership. So I have been extra cautious with my new toy as I get accustomed to it’s behavior. I am hoping the curse has been broken with the Boxster (been nearly two months now), but it sure seems like every other driver is out to get me! I am being a good boy on the street, I promise!

The pedals are nicely positioned for heel toe, much more so than in my other vehicles so even though I don’t think I am depressing both, I probably am. Just thought “hmmm. It is a Porsche, maybe it is smart enough to blips the throttle for me on a down shift?”

Ok, this is just a wild guess on my part, as I'm not yet fully up to speed on the workings of the Box yet. I know the power brake systems use engine vacuum.

Could you possibly have a vacuum issue/leak and the engine rpms are in response to this under hard braking?