Oh, I get it.

Higher load = better fuel/work ratio. Drive up a hill and your engine will do more work per fuel consumed but will not consume less fuel per mile.

 

There are lots of things that have to do with good gas mileage

Gearing in the rear end
Gearing in transmission
how much throttlw you are on
engine size
vehicle weight
driving habits
etc

But also one thing you may have not known was that if you get gasoline from California, it is oxygenated, but your car realizes the oxygen content in the fuel mixture and runs the car richer anyways vs Arizona and I think the rest of the US, doesnt have oxygeated fuel so they do bout 10% better MPG than California does!!

Good job Sacramento for not doing your F-ing reserach and making us pay more for our gas and we still do worse fuel economy!!!

 

that's to lower HC though. They are more concerned about emissions/mile than mpg

 

Yes, it's no accident. There is no limit to their desire to make us poor in pursuit of their chicken-little fantasies. Glad I don't live there.

But anyway, does anyone know how significant cam overlap/idle setting is in losing MPG?

 

easy way to think about this is what motor goes in an 18 wheeler.

 

Big is more fuel efficient?? Yep, that's why the Honduhs run 400 CI engines...

 

Haha, quite so. Of course ultimately, all other things being equal, the big motor will eat more gas.

But, some things being unequal, specifically the kind of cam/tune/heads setup, that might be different? Comparing a "mild" bigger engine (402-427) to a "wild" 347.

 

I dont know how some of you guys are getting such bad gas mileage. I have a 99 TA A4 with 2.73 and get 27 mpg at 80 mph. My brother in-law has a 98 TA 6-speed with 3.42 and on the same 400 mile trip with the same exact mods as me got 28.5 mpg on the freeway. We followed eachother the whole way.

 

Is it that bad, do you think? I might should start a thread on that if so.

 

Valve overlap does play a part in gas milage. Think about it this way. You have your light dirty air that is being told to exit the motor. You have good air and fuel coming into the motor as well. When this air and fuel gets into the cumbustion chamber some of the air and fuel will leave and go out the exhaust.

 

Trey Z28 is right.

Everything being equal regarding weight, drag and speed: the hotter the combustion temp, the more efficient the burn of the fuel. More calories or BTU's of energy for the same quantity of identical fuel.

This is why little 4 cylinder engines screaming down the road get such good mileage. Others problems will crop up like shorter engine life for an engine that is putting out 75-80% of maximum power all the time, but it is more efficient.

So, if it takes 20 horsepower to cruise at highway speed you will do it more efficiently with a 25hp engine almost maxed out than with a 427 loafing along.

The most efficient 4 cycle engines hoped for are those made of ceramics that can operate at extremely high temps. As it is, only about 20% or less of the potential power from gasoline is converted to forward thrust. Not very good. If the engine could live at 800 degrees this efficiency will more than double, as I recall.

This said, my 402 with 481rwhp got 27mpg at 75mph cruise on Highway 5 between San Jose and LA. My 150hp (at the crank) Accord would get about 32mpg. I'll take the V8 please!

Chuck

 

The effect of throttling may have already been explained, but when you throttle a motor you decrease its efficiency. You will usually have a VE of about 75-80% with about 85% at peak torque which is your most efficient point in terms of cylinder filling. In terms of how much work each unit of air/fuel produces, your peak torque point is the most efficient. An unthrottled motor has fewer pumping losses and it manages to fill the cylinder with more air. As you fill the cylinder with more air, the dynamic compression ratio is raised and the burning is more complete yielding more pressure.

The ideal is a motor running at peak torque at WOT. This puts you at the engine's most efficient point, you are filling the cylinder to the maximum which makes your dynamic compression ratio higher, and your pumping losses are lowest.

In reality that's pretty impossible to run your motor at 4400 rpms on the highway with a motor this big at WOT, with a 25 cube motor that might work, to run at peak torque at WOT. As motors get more powerful, operating at WOT at peak torque becomes more improbable, so the peak torque efficiency part has to be tossed out the window. The only item of that equation in the previous paragraph (WOT & peak torque) that you are left with is WOT. It's best to run a higher throttle opening at lower rpms than a lower throttle opening at higher rpms. The higher throttle opening is going to fill the cylinder more on each stroke (getting you a higher VE and closer to peak torque efficiencies) and inrease that DC and your pumping losses are going to be lower. If you go high throttle + high rpm you are accelerating; if you go low throttle + high rpm you are lowering DC and raising pumping losses; high throttle + low rpm is what you want.

You already know the VE 85% etc. at peak torque. Included is a set of graphs from Taylor's book "The Internal Combustion Engine in Theory & Practice"; its a great book and you should buy it. What the graphs show is a line from right to left signifying the exhaust stroke, then left to right signifying the intake stroke, then right to upper left signifying compression stroke. It's in terms of pressure; obviously the full throttle & low vacuum modes create higher pressures because they are filling the cylinders more efficiently. This ups your DC and any pressure that's positive within the intake stroke will work with the piston. When you have negative pressures on the intake stroke, that works against the piston and ups your pumping losses.

Run a bigger motor at a high throttle opening in top gear on the highway for fuel efficiency; it's more efficient than a smaller motor running a lower gear and higher rpms at lower throttle. Then run a bigger motor at a higher throttle opening in low gear on the road for fun.
You will be within a few mpg on the highway of a smaller motor anyways, but that is the most efficient way to run it.

 

keep in mind- you might be getting into open loop at heavy throttle positions and running rich AF

 

Neat info.

Say, not to get too far OT but for you big cube guys with the good MPG's, what tires were you running at the time?

 

L78 15's @ 40psi


Chuck

 

Haha, whitewalls and everything?

 

Also forgot to mention a couple of pages later it has a graph of a motor running throttled. The frictional pressure loss is ~21 at 1/4 throttle, 19 at 1/2 throttle, 18 at 3/4 throttle, & 17 at full throttle. The graph is in terms of psi & that's at 1100 ft/min piston speed. At 2000 ft/min, the graph is higher, 1/4 throttle is about 28 psi and the others are the same distances lower (ie 2 less for 1/2, 3 for 3/4, etc). It just goes up & to the right linearly. It's from a 1948 thesis.

 

Thats amazing