JamesMcD
JamesMcD HalfDork
10/22/13 10:31 p.m.

Has anyone else driven a car, which, if you floor it and let the car accelerate into the higher reaches of the tach, and then let off abruptly, will subtly and very briefly exhibit a surge of acceleration as the throttle closes?

What I think is happening is that as the throttle closes, there is a point at which the smaller throttle opening increases the intake air velocity, and this causes greater V.E., or better filling of the cylinders due to increased inertia of the intake charge. So for a split second you feel a surge of acceleration even though the throttle is snapping shut.

Does this make sense?

Do some drive-by-wire cars take this phenomenon into account? For them, is WOT sometimes achieved without literally going WOT?

Trans_Maro
Trans_Maro UltraDork
10/22/13 10:43 p.m.

My bike does this but I suspect it's because of a bypass valve that Keihin put in the carburetor to keep the bike form popping through the exhaust on decelleration.

I have the same carbs on another 'cycle and they don't do it but I have disabled the valves on that one.

djsilver
djsilver New Reader
10/23/13 12:09 a.m.

I know some cars open the idle valve or bypass valve on the way down when the throttle is closed at high rpm, to prevent it from dying on deceleration.

Kenny_McCormic
Kenny_McCormic SuperDork
10/23/13 12:18 a.m.

In reply to djsilver:

But the injectors don't come back on till about 1600rpm.

GameboyRMH
GameboyRMH GRM+ Memberand MegaDork
10/23/13 7:33 a.m.

Many E-throttle cars slow throttle closing if you let off the pedal suddenly for better emissions.

Also make sure you're not confusing it for the way slushboxes go into top gear if you let off the gas, under certain conditions it feels like they speed up a little when this happens.

kanaric
kanaric Reader
10/23/13 7:42 a.m.

My WRX does this, it's annoying but only does this when the clutch is engaged.

Josh
Josh SuperDork
10/23/13 7:45 a.m.
JamesMcD wrote: Has anyone else driven a car, which, if you floor it and let the car accelerate into the higher reaches of the tach, and then let off abruptly, will subtly and very briefly exhibit a surge of acceleration as the throttle closes? What I think is happening is that as the throttle closes, there is a point at which the smaller throttle opening increases the intake air velocity, and this causes greater V.E., or better filling of the cylinders due to increased inertia of the intake charge. So for a split second you feel a surge of acceleration even though the throttle is snapping shut. Does this make sense? Do some drive-by-wire cars take this phenomenon into account? For them, is WOT sometimes achieved without literally going WOT?

My Volvo does this. It's annoying. I think I've heard it attributed to the dual mass flywheel loading up the springs and then releasing when you let off the throttle, combined with the ECU holding the throttle partially open for emissions.

Knurled
Knurled GRM+ Memberand UberDork
10/23/13 12:36 p.m.
JamesMcD wrote: Has anyone else driven a car, which, if you floor it and let the car accelerate into the higher reaches of the tach, and then let off abruptly, will subtly and very briefly exhibit a surge of acceleration as the throttle closes?

It's probably the engine going into closed-loop as you shut the throttle. More power at a leaner mixture than "super rich protect the cat" WOT mixture.

m4ff3w
m4ff3w GRM+ Memberand UltraDork
10/23/13 1:53 p.m.
Knurled wrote:
JamesMcD wrote: Has anyone else driven a car, which, if you floor it and let the car accelerate into the higher reaches of the tach, and then let off abruptly, will subtly and very briefly exhibit a surge of acceleration as the throttle closes?
It's probably the engine going into closed-loop as you shut the throttle. More power at a leaner mixture than "super rich protect the cat" WOT mixture.

That was my thought.

stuart in mn
stuart in mn PowerDork
10/23/13 2:21 p.m.

I once had a car where the gas pedal would stay on the floor when I let off - that was fun.

You'll need to log in to post.

Our Preferred Partners
T49cfC6OvKbsAmwA1SWNOqU4ku9Irpn8OjHJiSt3Tbv09qKeakSU8Mps1lIxY1PC