determining best cruise RPM

[roadkilll]

is there a good method to determine the most economical cruise rpm for an engine?

i have a 78 f150 400ci engine a 1:1 high gear and 33" tires, i am prepareing to replace the rear axle gears and would like to swap in the best ratio for fuel economy. i know how to do all the math to figure the right gears to give me a certain rpm at a given speed but what i dont know is what rpm should i be targeting. the engine has been recently rebuilt going to higher compression (around high 9's maybe even 10:1) dual plane manifold and an RV/Towing type melling cam. the cam and manifold were picked to target the idle to 4500 rpm band.

the truck runs great and i am pleased with the performance even with the stock 3.5:1 rear gears i geat an average of 12 MPG highway/city mix. i just think the 3.50's might be to high and lugging the engineat cruise. most of my cruise is done at 55/65 but i do hit some 75mph streches. if i had to choose i would gear up for the best MPG at 55-65mph as i could add an overdrive for higher speeds.

i have found a digital dynometer (dynosim...i think) that lets you input all your engine specs and then gives you a torque/horse power curve. since my engine isnt stock i thought about spending the $200 for the program but i still wouldnt know how to pick the right rpm with out doing experimental runs with differnt gears

any help or direction would be great

[astroracer]

I think you are caught between a rock and a hard spot here. Your goal is to lower your gear ratio to improve highway mileage but, doing so, will also decrease your off the line performance with those tall tires. This will cause you to step farther into the gas on takeoff to get the same off the line feel. This will affectively decrease your total mileage.
If replacing the gears is only driven by a mileage concern I would re-think your direction and look into going to an A.O.D. trans or aftermarket O.D. unit. This would let you keep your current performance off the light and get you some mileage where it matters, at cruising speed...
Mark

[roadkilll]

i understand all those variables and am willing to deal with them but what i need to learn is how to figure the best rpm for cruise for a given motor

i have heard from some that you should cruise at the rpm where the torque and horse power curves cross but can not find any suporting data

obviously the ideal method would be to dertermine the horse power required to maintain cruise speed and then find the part throttle rpm that produces that horse power with the lowest bsfc

unfortunatly i dont have the resources to complete this method

again i am looking to determine the best cruise rpm for my engine?

[Reed]

if you were supposed to cruise at the point where your curves cross you would always be driving at 5252 rpm. that is way too high. cruise at the lowest rpm you can without it feeling like it is bogging down.

[CraigFL]

From an engineering standpoint, you would need to see the BSFC topographical map showing the efficiency for your engine. Brake Specific Fuel Consumption(BSFC) is the amount of fuel your engine uses per hour to generate a specific Horsepower. While as a general rule, it can be best to gear so that the engine RPM is lowest for a particular crusing speed, it is not always so. This is because the worst engine efficiency is when the throttle plate is barely open. Engine effiency is best with a wide open throttle. Efficiency can change a great deal between the throttle just cracked open and open a little wider so for this reason, it might be better to use a slightly higher(numerically) gear ratio.

[curtis73]

Adding to CraigFL's post, BSFC is only really applicable at WOT like he said. All bets are off with modest throttle openings. At small throttle openings BMEP and manifold vacuum are your best friends, but who has a graph of part throttle BMEP? In general, about 200-500 RPMs above the peak torque will save you a nice range. Anything below it will bog a little and anything above it will be neglegible to an extent. If you set it to 200 RPMs above your torque peak at 55 mph, that should maximize your mpgs from 50-65 or so. Above that, wind resistance and rolling friction will kill you anyway, so its no use tuning it for high speed in that truck.

If you're up for a test and tune, have several third members on hand in various ratios and install a vacuum gauge. Even a $10 parts store gauge with a long hose through the window will do well. Test them all and see which one gives you the most manifold vacuum at your target speed. Barring that, have a bunch of friends help you out and let them lend you their tires/wheels in smaller or larger (yikes) diameters. Then you can calculate the ratio you want from the size of the tire.

[roadkilll]

i would have to disagree with some things mentioned here .
1 BSFC is totaly relavent regardless of throttle setting it is the only real way to measure efficency.

2 wide open throttle on an automotive engines is far from the most efficent due to enrichment to make more power both carbs and injection do this. the most efficent would be in the cruise circuit or in closed loop and then still different rpms would provide different efficencys within that range

[curtis73]

Originally Posted by roadkilll

i would have to disagree with some things mentioned here .
1 BSFC is totaly relavent regardless of throttle setting it is the only real way to measure efficency.
2 wide open throttle on an automotive engines is far from the most efficent due to enrichment to make more power both carbs and injection do this. the most efficent would be in the cruise circuit or in closed loop and then still different rpms would provide different efficencys within that range

There has been quite a bit of disagreement on this topic recently. BSFC is a measure of how much power an engine can make from the BTUs given to it from the fuel. I hate to directly disagree, but it has almost nothing to do with MPGs. Especially at anywhere other than WOT since BSFC varies with dynamic compression. the actual compression at small throttle openings is quite different than at WOT and therefore the BSFC is almost unmeasurable due to its widely varying depending on load.

BSFC refers to combustion efficiency, NOT fuel efficiency. It has to do with how much power the engine extracts from the combustion, but has nothing to do with how much fuel the engine ingests. BSFC is probably the most misunderstood quality of engine operation. It is possible to increase BSFC by a large amount while reducing MPGs considerably and vice versa.

Your #2 is true of fuel efficiency, but not combustion efficiency. BSFC typically peaks at or about the torque peak at WOT, but drops off considerably as the throttle closes due to several reasons.

You'd do well, too to do some intense tuning on your ignition curve. You didn't mention your static compression, but my guess is that you can do MUCH better than either a stock or performance aftermarket distributor. You can probably get away with more initial and total timing with less advance from a ported source, but you sound like a very knowledgable guy and have probably already taken care of that

[roadkilll]

yes i have worked on the timing curve but i need more time to tune it i have a holley fuel injection system that lets me program the spark curve all i had to do was drop in a newer 460 distributor and plug it in to the computer

looks like im going to do some tire swaping to test for the best fuel economy at 60mph i have 28.5", 31" and 33" tire set to start with i will use gps for monitoring speed and will run a set course several times since with the different tire sizes the speedo and odometer will be off enough to really screw up the results