Maximum fuel economy can be achieved by keeping the RPM in the 'sweet spot' (maximum torque) of the cam. A well built motor will extend the RPM range of the usable torque curve. Increased compression will raise the whole torque curve and low valve overlap will help at low RPMs.

Danlee,
does this mean that if your motor makes peak torque at 3600 rpm and the driver cruised at 70 mph normally then it should be geared for 3600 rpm at 70 mph?

 

does this mean that if your motor makes peak torque at 3600 rpm and the driver cruised at 70 mph normally then it should be geared for 3600 rpm at 70 mph?

That is correct. Most RV type cams make peak torque below 2500 RPM, so at highway speeds they exceed the RPM where the best fuel economy is obtained. Overdrive units help keep the RPMs down to 2000 to 2500 RPM at highway speeds. All of today's newer cars and trucks operate like this.

Cams that produce peak torque at 3600 RPM typically have a lot of overlap, and lose much of their efficiency at low RPMs. You would have to operate at 3600 RPM nearly 100% of the time to take advantage of this.

A smaller cam and great intake and exhuast flow will produce a nearly flat torque curve. That will allow you to be near peak torque at any RPM.

The RV cam with the OD unit is probably the best choice for fuel economy, but a flat torque curve is next best.

 

Dan, you got any books or other recommendations for engine theory, as you continually describe? I'm always looking to expand my library.

 

Then why do many of these cars advertise torque at around 3 or 4k rpms? I understand your principle but it seems like some of theses new cars have high gears keeping the rpms below their torque that resides at higher rpms but I do not have any dyno charts to show so I am not very credible. I do not need to pirate this thread.

 

Ultimately, modern cars do not live up to their potential.

for instance, the 1992 honda civic dx get's better gas mileage than the modern hybrids.

ok, that's not really the answer, but it sounds good.

 

I think the torque curve would peak at a lower RPM at a part throttle condition? The peak torque that is shown on a dyno run would be at WOT. That could explain why numerically lower gears would achieve the maximum economy, assuming nobody is crusing at WOT!

 

I ran a simulation of a 400 motor with a 265DEH cam, a single plane manifold, and a 600 CFM 4 barrel carb. The peak torque occured about 4000 RPM, as did the maximum Volumetric Efficiency.

I then changed the carb to a 300 CFM 2 barrel carb. This is the smallest carb that I can simulate. The peak torque and maximum VE both dropped, and the RPM range dropped by about 500 RPM, but the torque curve and VE curve kept the same shape.

Changing the amount of air flow into the cylinder doesn't change the dynamics involved. The overlap is still the same as is the dynamic compression. These are strictly functions of RPM and valve timing.

 

OK so we cut the CFM by half and brought down the sweet spot by 500 rpm. Do you know what that 400 will do @ 50 CFM? I think that with small throttle and high vacuum you will find that CFM drop dramatically. I know that a tankless 50 CFM air compressor put out quite a wind from a 2" pipe. Seems considerably more that your typical idling Ford 400.

 

but this is exactly what makes a 4" stroke atractive besides getting a high torque number at low rpm it contributes to a wide flat torque curve, I just custom ordered an extreme energy camshaft with 270int and 280ext to get exactly 8:1 dcr, my torque in my simulator peaks at 3500 rpm with 460lbs (using a dual plane) but it starts at 437@2000 jumps to 458@2500 and goes really flat to 445@4500 , syms may not be exact and not even that accurate but shows you a realistic behavior of your engine stats when changing parts... mi opinion

 

blast from...

the 460 i was 'talking up' above is a 429....and i went 339 miles on 22 gallons...15 mpg.

I drive it watching an installe vacuum gauge. If you want to learn how to drive for mpg I bet just about anyone can get at least 3mpg more. You can learn how to get up to speed in the most efficient manner possible... amazing learning curve. (getting up to speed is where gas engines are least efficient. wifes 4 runner pos got up to 21 when she was getting 17-18....that 5 speed auto can be far more efficient when you use it as a manual. overdrive wastes gas when driving below 55 or so.

Some good, but old, testing liked engines in the low to mid 3's for displacement in the mid size cars of yester-year.. The 400 seems like a great match for a rig like ours and i plan on getting over 15 on mine...even on propane.

I still think the cleveland (or wheezer) stroked to 4" would be a great mileage build when matched with a xe256 cam, 9:1 Cr with roller cam and roller rockers....and a standard 4 speed with gearmaster over under...3.50-380's gears, final drive of about 2.5 with 235/85r16 10 ply tires filled to 50#. I bet i can get about 20 in my 77 1/2 ton.


also the 3" exhaust might be too much...flows higher than 2 - 2.25 which it needs and thus will lose torque.

 

This is rocket science! my question is how is weight calculated in all this?

 

weight needs energy to be moved and the more weight or quicker you get the weight up to speed the more energy used.

I am a firm believer that you save the most petrol by slowing the time of getting the weight up to speed and using gears to get it there more efficiently

I don't follow the need to use the rpm for max torque for mileage. The goal that i'm tryng to reach is to get the torque 1800-2500 rpm as high as possible and then to gear for that range when cruising. gears are just about everything as it allows you to get to speed using less energy.

i think every engine has a sweet spot (rpm) that is probably about 30% less than max torque where you can keep the engine balanced between work load and being usable...ie get up to speed in a reasonable fashion while saving fuel AND if you build the engine calculating this and get the extra 3-4 gears you need to stay there even if going 75 then you will get the best mileage possible. A flat curve = a bigger sweet spot.

I noticed that many overdrives keep an engine in too high a gear making the driver use the pedal to keep up to speed....shift down a gear keeping the pedal the same and you will go 3-5 mph faser indicating better use of energy.

again...get a vacuum gauge installed and learn how your engine works while getting up to speed then maintaining it.

 

well my setup turning 3100 at 70 gets 17mpg out of a 400 with a c-6 and 3.50 gears in a 71 f-100 short bed, i think the statement about how the cam's sweet spot is very true, because if your peak torque is at say 5500 on a small block with a huge cam, a cruise rpm of about 3600 would be great, but our motors with the said broad power and torque curves need not turn that hard going down the highway, but like said before it mainly depends on the cam and combination of parts and also carb jetting, i run a weiend action plus with a 750 double pump with 72 jets in front and 73 in rear with 6.5 power valves front and rear, it idles great and runs great, i think thats the reason most people get ****ty gas mileage because they just bolt on a carb and go instead of readin plugs and doing some tuning, also a blown power valve will cause it to run rich all the time too,

 

I used to have a pickup that got the same MPG empty as it got towing a 26' travel trailer 15-17 MPG............with or w/o the trailer attached kinda blows y'all's math, etc outta the water, don't ya' think?? ........the most important factor in keeping the MPG the same is 'driving style' ...............steady speed uphill & downhill