Quote:
Originally Posted by comptiger5000
The smaller engines don't get better mileage in normal driving patters due to being underpowered. I've compared my Jeep with a 5.9L V8 to another with a 4.0L I6. I drove the 4.0 for about a month in my normal driving pattern. Since getting mine with the 5.9, and adjusting my driving habits to the higher powered, but very similar vehicle, I am getting within 0.2 - 0.3 mpg of the 4.0 in my normal pattern of short trips and city driving. My engine has 2 more cylinders and almost 50% more displacement. The Jeeps are very similar in weight. My tires are about an inch shorter (225/70/16 vs 245/70/16), axle gearing is the same (3.73). Trans gearing is slightly different. Mine is (1st - 2.45, 2nd - 1.45, 3rd - 1.00, 4th - 0.69). The 4.0 is (1st - 2.74, 2nd - 1.54, 3rd - 1.00, 4th - 0.69).
The bigger engine allows me to gain speed MUCH faster while keeping the engine in its efficiency band. The only problem is, in heavy downtown traffic, everyone else picks up too slow in their little 4 cylinder cars. Optimal MPG for me comes with letting the auto shift at 2100 - 2200, which allows moderate throttle. When stuck in heavy city traffic, I've seen it shift as low as 1400 rpm picking up from a stop on flat ground. At that point, I'm on the gas pedal so lightly, that I can't press it any less without coasting.
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I do understand what you're saying here, and I'll to it with this...
I think there is a fairly specific power to weight ratio that car's should be designed after. Most of you guys in these V8's are getting pretty gas mileage, due to the power to weight ratio causing the engine to not be under heavy loads, because these engines have low end torque.
Now, for a 4 cylinder powered car to theoretically get twice the gas mileage, the power to weight ratio's will need to be similar. The very second the engine is struggling, FE goes out the window. However, even in a large V8, most V8's these days are reaching very high effeciency levels.... I mean, I can go get a 360 Cube motor that makes over it's displacement now, stock?? Wow, what an efficient engine, or is it over efficient??
Also, there's a certain point where an engine will be over efficient and you've gone past perfect efficiency. And it takes more fuel than it did before when it had "less" power.