What's causing slow dropping revs?
 

I have been driving my FRS for few months now and I noticed something that kind of bugs me.

When I rev up the rpms seem to drop much slower then in other cars. Anyone else noticed this?

It might sound weird but this bugs me a lot. I like when I can give a car quick blip between the shifts or to keep my foot on the gas a little longer after pressing the clutch to get the car to rev up. I enjoy the sound this makes but in the FRS the RPMs drop so slow it does not deliver the desired effect.

What is causing this behavior and can I change it somehow? Could this be fixed by a lighter flywheel or a tune?

Try a light weight crank pulley. If thats not enough, a lighter flywheel with the pulley should do the trick :)

I never had this problem, then again I have lowered my rotational mass to a maximum lol.

I've noticed the exact same thing. Seems like a software thing that you might be able to get rid of with a tune, but that's just a guess. I've been wondering what the cause is as well.

I'd be surprised if it were a rotational mass thing, since it seems like revs drop normally when shifting or rev matching.

I've noticed that mine does that upon occasion, when it's dropping to idle. It seemed to do it more after I put in a K&N drop in air filter.

Sometimes I cause it by having foot ever so slightly touching the gas pedal .. :bonk:


humfrz

This.
From dodgy memory having the throttle butterfly close results in a sudden drop in pressure leading to a spike change in afr. (I can't remember if it becomes richer or leaner.)

Richer. You're cutting off the air the ECU was expecting.

Thanks for the reply but I don't think that is quite right. Air sensor is upstream. I think it has more to do with the sudden drop in pressure causing boiling off of fuel on the side walls of the intake manifold.
I am going off distant memory and would be happy to be corrected but at this point in time I would be willing to bet 3 cents on this.

For a modern car I think the 86/frs is actually pretty good with this. I believe it is an emissions thing programmed into the ecu where the throttle is held open after the pedal is released to burn off any excess fuel in the manifold/head.

^ this for the most part. There is a whole set of overrun fueling tables and overrun requested load tables in the ecu that control how much throttle plate closure you get when off throttle at various engine and vehicle speeds. You can modify some of the tables to get more engine braking for example or to have the decel fuel cut end at a higher rpm when coasting down etc.

Software aside though, reducing the rotating mass will for sure speed up the rev-up and rev-down, though there are probably some improvements to be had with tuning.

There's decel fuel cut, throttle closing, etc. But let's go back to basics here--humor me for a second on my line of thinking. What's the difference between this, a covered wagon,

http://www.ft86club.com/forums/attac...1&d=1394112873

and this, Karl Benz's first mass production automobile with an internal combustion engine?

http://www.ft86club.com/forums/attac...1&d=1394112873

An explosion.

Explosions make your car go. The difference between a horse and buggy and a car is an explosion. The difference between a rolling chassis and a functioning car in the modern era is an explosion that generates torque/power (I'll use the the term torque from here on for simplicity). Torque makes your engine (and car) go, and in normal operation the only thing that makes torque is the explosion, called combustion. What slows the engine down? Lower rotating mass and more "negative" torque, torque that works against the acceleration of the engine. The first of the main two categories that create "negative" torque is pumping. This is work to used to get intake air and exhaust air moving through the engine.

http://www.ft86club.com/forums/attac...1&d=1394112873

In the Pressure-Volume diagram above, for discussion's sake the area under the green curve represents the combustion torque. The area under the red curve is the pumping torque--generally speaking, the more area under the red curve the more your engine wants to decelerate. The pumping work is caused by working against a closed throttle, backpressure, valve timing, etc. Pumping is the enemy of performance and efficiency.

The other thing that slows your engine down (creates negative torque) is friction. For our purposes, friction is a catch-all for everything else. It's the work/torque/energy used to drive the timing chain, to move the pistons against the bore walls, to actuate the valves. The basic idea then is:

brake torque = indicated [combustion] torque - pumping - friction .

The term "brake" comes from the brake on a dyno that resists the torque produced by the engine. So how do we get the engine to slow down quickly? We reduce rotating mass, and we increase negative torque. First that means shutting off combustion (making the green area in the above curve very small) by cutting fuel and spark. We close the throttle and if possible we could adjust other parameters like valve timing. Finally, we want to increase friction. What's that you say? Increase friction? Isn't friction bad? Isn't friction for an engine like weight for a vehicle--it basically hurts almost everything that we care about? How do we increase friction?

Whoops. You can't easily increase friction to slow down the engine (in our simplified story here). In fact, engines are getting lower and lower friction as time goes by due to new manufacturing methods, and pumping work is also decreasing. That makes decelerating the engine harder and harder on modern engines. All this is because of the push for more power per liter with better fuel economy and emissions.

TL;DR One of the reasons why engines seem to decelerate slowly nowadays is because it is a side effect of designing for more power/torque and efficiency

Interesting!

But, do you realize how much spare time you have on your hands .... ??

:D


humfrz

You do have 5x as many posts as me in like half the time...:D

I have noticed the same thing on my 6 speed Auto. The problem really presents itself when I have to stop quickly. The car seems to be fighting the brakes with the engine speed and no way to disconnect the power from the drive train with no clutch.