Question about torque

[gungadin]

I was told that when it's necessary to accelerate that I should do it at the rpm of maximum torque for my engine.
Can this be true?
I'm getting great fuel economy results driving my old 911 porsche but this idea goes against my driving techniques totally.
Maximum torque for my engine is at 4000rpm
Maybe I'm missing the point.
But I guess my question is...What rpm range should I be using when I need to accelerate?

[Jay2TheRescue]

Originally Posted by gungadin

I was told that when it's necessary to accelerate that I should do it at the rpm of maximum torque for my engine.
Can this be true?
I'm getting great fuel economy results driving my old 911 porsche but this idea goes against my driving techniques totally.
Maximum torque for my engine is at 4000rpm
Maybe I'm missing the point.
But I guess my question is...What rpm range should I be using when I need to accelerate?

I don't know about Porsches, but let me tell you what I have learned by running a ScanGauge on my full size pickup truck. I imagine your results would be similar as both vehicles have powerful engines with low gears.

I've noticed that if I touch the pedal at all in lower gears, even the slightest "feather touch" my instant MPG falls to 1 or 2. What I do is accelerate briskly (not slow, but not quickly either) until I'm in 3rd gear, then I start backing off the accelerator. My logic is that since touching the pedal at all in the lower gears makes me go down to 1 or 2 MPG I have nothing to loose, just get out of the lower gears and into a more MPG friendly gear quicker. I do this at about 25% throttle. This may or may not be the "peak torque" but on my engine its probably close as I believe a 350 hits peak torque somewhere around 2,000 to 2,500 RPM. Anyway, the goal is to neither accelerate like you're running from the cops, nor like you're grandma driving to church on Sunday. You want to be in the middle.

-Jay

[GasSavers_BEEF]

I have been playing around with the idea of getting up to speed faster for better FE.

I usually give it half to three qtrs throttle until I am at 40 mph at which time my car goes into 4th (my top gear) then I baby it from there. this has done me pretty good. my last tank was over 38mpg which is awesome for me.

I have also heard that engines have less pumping losses with open throttle. as long as you don't hit open loop at which time it just starts dumping gas in the engine to get you going faster. the pumping losses would not change in close vs open loop but the gas consumption would.

[gungadin]

Yes this is what I am instinctively doing and succeeding with. My 20 mile drive to work is mostly 99% rural or on a freeway with few stops. Additionally I'm using downhill grades as opportunities to accelerate. (coasting up the other side). My car is very low to the ground and it helps me to see even the slightest hill or dip in the road.
I've gone from 18mpg for what used to be normal driving to consistantly over 24mpg. I can't quite break the 25mpg barrier but that's my goal for now.
I don't really see me revving to 4000rpms ever in my particular technique. The engine is a performance engine and (I think) the max torque is situated at that rpm for when one would be spending alot of time "performing". There's plenty of power at 2-3000 rpm to get me up to speed and up hills as well.
Otherwise,,I guess I'm lucky that the car is pretty light (2500#) and it will coast for a long way since the shape doesn't offer much wind resistance and if I want to take a sharp corner I can carry lots of speed without touching the brakes.
HA! funny how my mind used to associate my foot on the accelerater pedal with poor fuel economy and now I'm equally concsious about touching the brake pedal.

[Jay2TheRescue]

Originally Posted by gungadin

Yes this is what I am instinctively doing and succeeding with. My 20 mile drive to work is mostly 99% rural or on a freeway with few stops. Additionally I'm using downhill grades as opportunities to accelerate. (coasting up the other side). My car is very low to the ground and it helps me to see even the slightest hill or dip in the road.
I've gone from 18mpg for what used to be normal driving to consistantly over 24mpg. I can't quite break the 25mpg barrier but that's my goal for now.
I don't really see me revving to 4000rpms ever in my particular technique. The engine is a performance engine and (I think) the max torque is situated at that rpm for when one would be spending alot of time "performing". There's plenty of power at 2-3000 rpm to get me up to speed and up hills as well.
Otherwise,,I guess I'm lucky that the car is pretty light (2500#) and it will coast for a long way since the shape doesn't offer much wind resistance and if I want to take a sharp corner I can carry lots of speed without touching the brakes.
HA! funny how my mind used to associate my foot on the accelerater pedal with poor fuel economy and now I'm equally concsious about touching the brake pedal.

I have found that the opposite of your hill theory works best on my truck. I coast down the hills, and accelerate going up. You may want to try this.

[gungadin]

OK, I hear that. I do coast down many hills but when I need to gather speed and I have a convenient downgrade to do it on, I just figure I can get that speed easier with gravity on my side.

[Jay2TheRescue]

Originally Posted by gungadin

OK, I hear that. I do coast down many hills but when I need to gather speed and I have a convenient downgrade to do it on, I just figure I can get that speed easier with gravity on my side.

When I do that in my truck my instant MPG drops from as high as 80 mpg to 20-30 MPG. For me its better to take the high MPG's when I can get them going downhill, and take the 15-20 MPG going up the next hill. The goal is to get the highest average mileage. Taking the big numbers when you can get them is a part of that. Chances are that unless these are small hills you will need to accelerate before you get to the top anyway.

[gungadin]

I'd love to have a scan gage but I just don't think there's a place to plug one in on my '76. I'm just trying to craft an artful and instinctive technique.
The wifes Sonata has an "avg mpg" display that I can scroll to on the instrument panel. I'm driving her crazy....I reset it when I drive it and drive it frugally and register something like 28mpg. Then she gets in and reads that and when she's done it's down to 19mpg. hehe....."Do the math darling...I can go 150 miles further on a tank than you can!...Whatever,,you're buying the gas...

[Loserkidwac]

With my Mazdaspeed I've been getting 25-27mpg normal mixed commute and 30-33 on highway compared to the EPA 21city and 27highway...in my case the care runs very rich and is boggy at WOT or 1/4 throttle so I usually accelerate fairly briskly and the coast along and keep some light pressure on the throttle or if its highway I might step on it a little and coast again...this seems to work well but I imagine it is fairly specific to my car...I also run 2 step hotter spark plugs to try and alleviate the some of the rich condition, I noticed a decent jump in my mpg when I installed them...much cheaper then buying a fuel management system for a DD car...

My thoughs are..in theory...if you started to accelerate at the point of max tq, wouldn't the torque curve start to drop off and you'd actually be using the engine out of efficiency, at least torque wise? Someone care to enlighten me? Any thoughs one accelerate at the point where HP and TQ cross over? Correct me if I'm wrong don't TQ and Hp always cross over at the same point on a dyno graph around 5200rpms? Wouldn't this be the more efficient even though HP is falling the TQ is still climbing...just a thought on my part...please discuss I'd love to here from some knowledgeably people!

[Jay2TheRescue]

I think you will never know these figures exactly unless you dyno'ed your particular vehicle. I still don't see the merits of running the RPM's that high. Just hurry up and get into a higher gear. Don't race to get there though. I rarely exceed 2,000 RPM in my truck. I can achieve brisk acceleration without a double digit fuel rate.

-Jay

EDIT: I looked up a dyno from a vehicle with a similar drivetrain and fuel system as mine.

http://www.hypertech.com/media/dynoc...%20-%2057l.pdf

The torque hits a plateau at about 2,000 RPM, but you don't reach peak HP until 4,500 RPM. I'm happy with getting the most torque with the lowest fuel rate.