Question re: how engine revolutions "decay"

Interesting topic.

I can only help with a small portion of knowledge: The rate at which an engine revs down is part of the engineering process. When it revs down too fast use a heavier fly wheel or anything with a similar effect, vice versa. I have this from a former colleague who studied automotive engineering.

Now the guesswork: The engine components in a sports car are much lighter (because of material and design, just compare the pistons of a Ferrari to those of a "normal" car....). So this is maybe why they store less kinetic energy which leads to a faster drop of rpm when releasing the throttle?

 

Not a dumb question at all. I'm no expert, but a highway cruiser would have a heavy flywheel for efficiency, while a sports car that changes speeds rapidly would want a very light flywheel for quicker acceleration. I would think all sports cars and luxury GTs have this engineered in.

Then there are issues of a small 12 cylinder car's piston/drivetrain inertia vs. a larger V8 or less. More moving parts in the 12 but smaller, lighter parts. Ferrari went to 10 cylinders for the F1 cars yet has 8 and 12's for the road cars. Not all decisions are done by engineers obviously; the 8's were seen as an "American" cylinder count...

Ken

 

It takes Work to keep the rpms up. When then energy being applied to do the work is stopped, then the car slowly returns to its rest state. How fast it returns relates to how easily it gets up there. Lighter parts, faster it gets up there, faster it falls down.

See this thread:

http://ferrarichat.com/forum/showthread.php?t=21735

*At least thats how I fig it*

 

Personally, I think of it as a quality of the car. Makes it harder to drive well and thus harder for joe schomo to drive well. Then again it is my favorite car, so if it was just thought out poorly, I don't care...its still a damn awesome car.

 

Alot of it also has to do with parasitic loads on an engine. You have alternators, a/c, pwr steering, water pump, timing belts, transmissin input shafts and clutch assemblies etc, all being turned by the engine. Which will affect the rate of speed and engine can rev up and down. If you have ever worked on an old chevy 350, you will notice a huge difference when you run the engine with no belts attached to the accessories in they way it rev's up and down. But as soon as all the stuff is attached again it really slows it down.

Many car makes compensate for this load by going to light weight internals inside the engine...crank, pistons, valves, hollow camshafts etc. And like others have said they will also use lighter clutch assemblies and flywheels.

 

Doody hi,we know we are in atmo 14.somthing PSI depending on where you are on the planet,ok youve opened the throttle and there is still a vac down there so atmo is still runing in, then you snap it off ,the 14 somthing atmo lost it down the plenum tract but still active gas but soon put to sleep,
so we know the flywheel is holding the energy just look at f1 engines kill the spark it stops dead same as a bike motor no flywheel,
i belive there as to be an issue with vac in the cylinders during throttle off,
think of it as when you throttle stright back on the plenum tracts should charge up with good atmo pressure as long as you dont let it get close to idel rpm,
good post Doody young up and coming techs should be taught this,not to indepth its very complex,

cheers
Dale.

 

Most sports cars these days have a computer between the accelerator and the mechanical throttle control. So pressing the gas pedal does not directly activate a butterfly valve as in the old days. Instead, it tells the computer what you want to do and the computer then decides how best to accomplish it, even imposing some nanny overrides if necessary. So the computer ultimately controls the throttle mechanism.

So you're right, the throttle response has been engineered via software in the typical modern sports car. fwiw, the 996 has terrible throttle over-run compared to a 360 (ie, the revs decay slowly when you take your foot off the gas).

 

This is an interesting subject. I had always learned when up shifting into the gear and while clutch is being released to feather the throttle for a smooth transition (works well in my 996, and my BMW). On the other hand, depending on the driving style (i.e. going through the gears medium to moderately fast) this technique doesn't work as well for 360M because the throttle response is so sudden getting the right amount at the release of the clutch is just too cumbersome and result in jerky and excessive clutch wear. I do instead blip the throttle on the upshift and they are smooth as butter. On the other hand if going through the gears fast RPM doesn't have time to drop too much the blipping is not necessary. Good topic indeed.

 

The first serious race car i ever drove was a pumped up striped out cherry speedster with an 8 pound flywheel. very quick engine revs and very quick engine decay.
Since that time long ago, I have built a few hot rod motors, toyota, porsche, volkswagen and on each of them, I have lightened the flywheel or bought a light weight unit, even once going to an aluminum pressure plate, carillo rods and forged pistons with thin taper wall wrist pins all to reduce the MOMENT OF INERTIA to get it to rev up and decay Quickly. Fast throttle response is a joy to drive, even though i still stall the race truck with the 11 pound steel billet flywheel frequently, it is still a lot of fun to drive.
Some emission equipment causes slow decay but the biggie by a BIG margine is the flywheel and clutch combo. Makes it easy to drive, makes it smooth, dampens out those vicious little power pulses too.
My guess is that parasitic drag is roughly the same, alternator, waterpump ps pump.

FWIW when i have to do a clutch on the 308, i will do a light weight flywheel and a borg & Beck multiplate, (read low inertia and strong) clutch, assuming that the cost isn't too outrageous.

happy motoring,
chris