A while back there was a discussion regarding efficiency and larger tires/gears. I had asked a question about engine efficiency and rpm about something I had read a few years back pertaining to rpm range, torque and HP. Regretfully, no one had seen anything like that.

Well, in the Oct 08 issue of 4WP Off-Road Adventures magazine, I found on pg 42 what I had been looking for. It states:

"The general rule for gas engines says they are most mpg efficient at 30-35 percent of maximun rated rpm and at 75-80 percent of rated torque."

Okay, so that's the formula I was looking for. So then, our wimpy 3.8 creates (in stock form) max HP at 5150 rpm and max torque at 4150rpm. According to those figures and affter doing the math, that means our engine is most efficient at 1545-1802 according to the HP and 3112-3320 by going off the torque figures.



Alright, that didn't clear anything up for me.

Any thoughts?

 

I'm not going to solve the problem, but here's a couple thoughts about the provided information.

You wrote, "gas engines says they are most mpg efficient at 30-35 percent of maximum rated rpm", but your math uses 30-35 percent of Peak HP RPM.

Are you using a torque / rpm graph to do the torque calculation? Taking 75-80 percent of the peak torque RPM will not give the proper RPM range.

 

Woods, yes I used the HP value for peak hp at 5150 rpm. I guess if you do go off peak rated hp it would be 5600 which would mean 1680-1960 rpm.

Yes I was using a hp/torque rpm graph off the hytertech site.


I am curious about your last statement: "Taking 75-80 percent of the peak torque RPM will not give the proper RPM range."

What is the rational behind this? I know that going off the torque numbers the reading seems awefully fast, but I'm also wondering if I am interpreting something wrong or what. Or do the numbers really not match up well because our motors suck?

Edit: Sorry mods, I see you moved this out of modified but I had posted it there since the original post I was referring to had to do with gear/tire combos and efficiency. Also, if you are going to make any kind of change to gears/tires, it is a mod.

 

Ok, what u said made sense until i looked at the chart. Max torque is 197 lbs. So 75-80% of that would be a torque value of 147-157 lbs. So then by looking at the chart as to where our 3.8 makes that torque, it looks like the engine would have to be turning 4k rpm? Like I said, am cornfused.

 

Factory says peak torque is at 4200 RPM on '07's. It is always more effecient to run on torque curve. 75% is 3150. I doubt anyone but speeders like me would be running that high.

Not that might be the most efficient shift point, but the cruising speed should be down around 2k unless the enging is lugging like it can when going uphill in 6th gear at that rpm.

When ever going uphill it is best to take it out of overdrive whether it be stick or auto unless you are driving high enough on the torque band that it really does not matter.

 

Those numbers don't equate to mileage. They only refer to HP and torque. I always follow my KISS method. Just keep the RPM as low as possible. Also too, that formula cannot in no way whatsoever in the world or universe hold true for all engines. Is that for inline or V engines? If it's for V engines, is it 60 degree or 90 degree? Is it for a flat or pancake engine? Is it for a radial aircraft engine? Is it for a two stroke or four stroke? There's no way possible that one formula can establish specific parameters such as that on every single type gas engine made. It just ain't possible. The only formula that I know of that fits anything close to ALL type engines is that HP and torque are always the same at 5250 RPM. I've been at it a long time, and I just don't think one formula for efficiency would work across the board.

 

Don't be confused, just because the stock torque curve does not fit a standardized model. What I'm learning from this, is that there are some big exceptions to this rule.

I don't know anything about torque curves. I wonder if the "hypertech" torque and HP curves don't fit the model much more accurately. They may be indicative of more typical curves. Also, that graph is marketing material. I've not read up much, on the hypertech programmer, but if that graph is accurate, I'd think a lot more of my friends would have them. 160 lbs. compared to 50 lbs. at 1500 rpm

 

?? HP and torque are always the same at 5250 RPM ??

Hate to say it, but that does not hold true, especially across all types of engines. Far a simple example I had a V6 engine that had peak HP/redlined at 5200rpm, that particular engine would not go above that and there was no limiter.

 

My take on your statement is that the best efficiency is 1545-1802 RPM & 180-192 ft-lbs. If you have ever seen a BSFC (brake specific fuel consumption) map for an engine, think of the first number as on the x-axis (speed), and the second on the y-axis (torque). Ideally, you get the best efficiency on the "island" of the BSFC map. This rule of thumb that you quote may be true for older "mechanical" engines, but it probably goes out the window when computers started controlling the engine (allowing the engineers much more flexibility for optimization).
Take a look here for a generic picture:
http://www.eng-tips.com/viewthread.c...=144109&page=9

 

Sorry, but I am not incorrect. Anyone in the know in the automotive world will tell you this phenomena is true. There are a bigillion articles about this, but here is a very good one from Peterson Publishing. Probably the best in the automotive publishing business. If you don't believe them, well, what can I say?
http://www.carcraft.com/techarticles...que/index.html

From the article: "Because torque and rpm are divided by 5252, torque and horsepower are always equal at 5252 rpm. If you solve the equation at 5252 rpm, the rpm value cancels out, leaving horsepower equal to torque. If you plot torque and horsepower curves on a graph, the lines will always cross at 5250 rpm (rounded off). If they don't, the curve is undoubtedly bogus."

This has been a well known fact ever since the introduction of the firsy engine dynomometers. It has been covered in depth in every automotive magazine more than enough. Evidently, you don't get out much.