Steezy

So when I first started driving my friend would always downshift to slow down before a turn. I don't mean downshifting while turning, I mean downshifting a while before turning. I ask him "Doesn't that waste gas?" he told me that it doesn't because the car is using natural combustion to slow down.. So I started doing it everywhere because I figured at least it was saving some break pad. Well my dad was in the car with me one day and I did it and he said "You know you're wasting gas by doing that". I explained what my friend said and my dad disagreed and said "Any time the car is at higher RPM's, it's using more gas." I don't know who to believe... Input?

TibFreak

I would be siding with you good ol' dad on this one.

Steezy

What I don't get is why... You're not giving it any gas and isn't the car just using the idle to slow down?

radu_rd2

If you are not giving it any gas you probably don't waste gas; he is right that higher rpm wastes more energy but in your case that's what's slowing down the car. It might consume a bit more gas than idling tho, not sure. If you want to know real bad, I'll check next time I drive, the AEM FIC program shows the injector duty cycle

It's stupid however because you are saving your brake pads which are cheap and easy to replace but instead are using up your clutch every time you downshift (unless you rev-match first which would waste a bit of gas heh). You are also using up your engine more.

Use brakes to slow down, that's what they're for

TibFreak

I can't break it down like most members on here can, but it seems like common sense that higher RPM's would use more gas. And it seems like it would be harder on the engine.

Steezy

i never really thought about the clutch, I do still have the stock clutch and I've got 120,000 miles so I probably should be thinking about the clutch more than anything, and I learned how to drive a stick in this car so it's been stalled a few time lol

CudaJoe

well, actually it's an interesting concept you bring up. You ask why does it burn more gas whne your not introducing more gas to it correct? But if you think about it more closely, you are actually only able to open the throttle using the "gas Pedal" so what are actually introducing is more air flow into the motor. The motor has a programmed fuel rate for the amount of air your allowing to enter the motor at a given time.

Two things to look at when controlling the amount of air into the motor is...

1.) How open the throttle is (how hard are you pushing on the gas pedal)

2.) What RPM are you riding at (the higher the RPM, the faster the fuel/air mixture is being consumed.)

What you can get out of this is that when you down shift, you are burning a faster rate of fuel/air mixture even though your not opening the butterfly. The butterfly will in fact, will open itself to allow the needed air in for proper mixture.

It's all relative really, you have two options when it comes to slowing down, one way saves you on gas but causes your breaks to wear quicker. The other way allows your breaks to last longer but eats your gas(not by much though).


I guess this is too much. The answer to your question is, your not giving the car gas... you only do that at the gas station... the engine controls that

njsharkracer

I'll make this real simple. If your driving a manual tib and when you downshift and the engine is over 2k rpms with your foot off of the gas pedal, the engine will shut off the injectors. Therefore your not using any gas. Once you either give the gas pedal a push again or the engine drops below roughly 1800 rpms, the ecu will turn the injectors back on. If you drive a manual tib you'll feel the injectors turn off when you let off the gas over 2k rpms and feel them kick back in below 1800 rpms. I have a scangauge that reads this info and I can confirm it.

DTN

The car makes an effort to maintain stoichometeric ratios. The faster your engine is going with the TB closed, the more vacuum it will pull. The more vacuum it pulls means the more air that can come into the manifold. The air in the manifold will go into the engine and the engine will try it's hardest to keep that stoich so that it does not detonate.

Your car draws in 1L at various pressures per revoloution. If there is less of an opening, then less air will come through but it attemts to get as much as possible by increasing the vacuum. The higher the vacuum, the more air will go through the little opening at the throttlebody or IAC.

The higher the RPM, the more air will be pulled into the intake manifold. The more air, the more fuel is used.

radu_rd2

A vacuum gauge shows more vacuum when you're engine braking than at idle. Meaning there is less air going in per RPM.
But all in all, I guess more air could be going in overall.

Whether the ECU actually tries to keep stoich ratios at these times, that I do not know. There is no issue of detonation as there is barely any air going in compared to WOT.

I will check with the FIC