Fuelly Forums - Full Throttle at low RPM, or High RPM and low throttle

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This gets into the big engine small engine problem. I have a friend with a 2.5L v6 in his Sennica and see really poor MPG readings until he gets into a higher gear with his automatic at about 40mph where as in my 1.5L xB I get really high MPG readings lugging it at idle in 5th gear at 20mph like 70mpg on the level road.

Quote:

Originally Posted by theholycow (Post 150040)

If you're going faster, you're doing it wrong. You may not gain a fuel economy improvement; you may in fact lose fuel economy.

I often shift when I've only reached 1200RPM. You may need to go lower than that. The strategy may not be compatible with your car. Experimentation will answer all of these questions for sure.

what is 1200 to holy cow is 1600 or so for me

Quote:

Originally Posted by JanGeo (Post 149613)

At low rpm it doesn't take much throttle to drop vacuum to near 0 on the intake since even a small throttle opening allows more air to flow into the engine than it can pump so you really are not doing much by pushing the pedal further other than feeding the ECU with throttle position signals that may richen the mixture even further and push yourself into open loop.

This is correct. I have a vacuum/boost gauge in my car and I can see the vacuum per throttle angle. u can get to 0 vac. without opening the throttle all the way.

on a map of maf the throttle angle isn't used for fuel enrichment unless the throttle body is open x amount in x amount of time. quick acceleration needs more fuel (in theory). if your moving steady its not even going to do anything. rpm's increasing is going to add more fuel. but like said, at 0 vacuum your at a higher efficiency.

I wanted to add you can get to 0 without going into open loop as I've seen it on my vacuum gauge. also how the tps.

a vacuum gauge is great for exactly this cause I know exactly how much throttle to use to get to 0. you can also see lugging, the needle shakes.

Quote:

Originally Posted by theholycow (Post 150040)

By faster, I meant accelleration. What you described is how I drive normally because of how the car is set up. It's very hard to drive it normally. It's lloking promising. I'll see after this tank is done.

Yup, that's what I was talking about too. I don't accelerate any faster than the traffic around me. If you can't control your acceleration enough by shifting because you have too much idle-RPM torque, it won't work for you.

Even if you can, it still may not be the most efficient way to drive your car...it sure is for mine, though.

Isn't lugging an engine damaging to the rod and main bearings?

Lugging will destroy the engine quickly. Modern engines are equipped with knock sensors and can't lug. They run or they stall. At very low RPM with wide throttle openings they may rumble or growl; the extra vibrations may prematurely wear stuff. I avoid letting that happen.

I would think 75% throttle at such a low rpm would just be hammering the bearings constantly. The engine is attempting to spin faster but cannot due to the resistance of moving the car forward so as the combustion happens it's forcing the pistons/rods down but they cannot move so the force would be put on the rod and main bearings.

That's my thoughts on it but I have nothing to back it up.

At such a low RPM, there just isn't much power being made. There's so much more torque at higher RPM that the lower RPM can't even begin to stress that stuff.

Basically, the difference between 50% throttle and 100% throttle at, say, 1000RPM, is merely the energy wasted sucking air through the throttle...it's not making much more power, pulling in much more air, or using much more fuel.

Unfortunately I can't even use that technique....oh well. Thanks for the explanation.