Throttle % while accelerating?
 

Hi I have just recently started on my quest for fuel economy in my 1995 saturn SL1 5spd. My best tank mpg to date is 38.4. Anyway to my question I was wondering what would be best for fuel economy - more throttle in a higher gear but less rpm or less throttle in a lower gear but higher rpm? Is it best to accelerate quickly say 75% throttle and shift at a higher rpm up to 40-50mph and then cruise or use about 25% throttle and shift at a lower rpm up 40-50mph then cruise? i know its based on the car and the engine. I know that the SOHC in my saturn makes peak torque at 2450rpm. Should i take the rpms higher than that while accelerating or shift around 2500rpm? Im just in search of the best way to get up to a certain speed for cruising. Thanks in advance

Between 70% to 100% throttle and shift between 1,800 - 2,500 rpm.

There is an article on www.autospeed.com that tests this I just can't place my finger on it right now. I'll edit it in if I can find it.


Edit: Found it!

Brake Specific Fuel Consumption
http://www.autospeed.com/cms/A_110216/article.html

About 2/3 to 3/4 throttle (not quite enough to throw it into WOT enrichment), and shift early (low RPM) is most efficient.

this has actually been a burning question of mine for a while now. make me wish I had a 5 speed again

Now, what about an auto trans?

For an auto, accelerate with as much throttle as you can while not driving the RPMs too high. After a while, you find the sweet spot, which is different on every car.

Best rate of acceleration is lowest manifold vacuum in highest gear that will produce the desired rate of acceleration. If acceleration is too slow use next lower gear, while maintaining low manifold vacuum.

Any throttle position below the point of full load enrichment is fine and it can range quite a bit from 20%to 80%. This is because throttle position does not determine load. Load is a function of manifold vacuum. Once manifold vacuum reaches 100%, additional throttle will not increase power unless you go to full load enrichment, or in the case of my VX you loose lean burn, which is another stage of enrichment.

In an auto transmission you want the lowest manifold vacuum you can maintain without causing the transmission to hold the lower gears any longer than its earliest shift point.

Every engine is more efficient when it has the maximum amount of air inducted into the cylinders at the bottom of the intake stroke. RPM too low you loose the harmonics of induction resonance and that benefit. RPM too high and you are loosing energy to friction and reciprocation losses, which are due to the accelerating and decelerating masses in all reciprocating engines. Good intake harmonics can actually increase the volume of air slightly on the intake stroke with the inertia of the incoming air contributing a sight "supercharging" effect.

I think it is not well understood generally, that very low throttle positions can virtually eliminate manifold vacuum in almost any throttled engine as long as the load is sufficient, which is accomplished by choosing higher gears than most drivers use for acceleration.

Think of it this way. I call it pulse accounting. Every combustion pulse is much more energy efficient when it uses all of the engines available compression, which requires the manifold restriction to be the lowest possible amount. When manifold vacuum is 0 the maximum amount of air is in the cylinder to be compressed and that will produce the maximum amount of power per unit of fuel consumed.

Without wide open throttle enrichment.

If you produce that amount of manifold vacuum at the RPM where your engines BSFC is highest, you have duplicated that BSFC, and that is as good as it can get.

regards
Mech

I'll try that. I figured out how to short-shift this auto under 3K RPM, but I can't push the pedal far for a little after the shift.

Though if I barely touch the throttle (1-2%), it'll shift even sooner, but accelerate like a turtle. It will act like it's in high idle at that throttle level.

Some times you can "force" a shift on an auto. Accelerate heavily 70-80% throttle when the rpm (the plural is on the r of revolutions) is about right for the next gear let up on the accelerator then push back down till just before it shifts down again.

On my Maxima it turns out there is an adjustment to the throttle positioning sensor. Basically I make the compuet think the throttle is closed more than it is making the tranny want to stay in a higher gear. It may be the same on other vehicles. On my Max it took loosening 2 phillips screws and twisting the senor in the same direction as the throttle plate twists, which made the tranny shift early.

I found this out my accident when I installesd a new TPS. I adjusted it reverse of how I described, and it downshifted sooner and refused to upshift without higer RPMs.

Hmmmm, wonder what happens if i go the other way....