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Old 04-17-2020, 07:55 PM   #1
Bus Nut
 
Join Date: Apr 2018
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Year: 1995
Coachwork: Girardin
Chassis: E-350
Engine: 7.3L Powerstroke
How to use engine RPMs to our benefit

My expertise is with jet engines, and we generally use the throttle to set fuel flow for our desired outcome: max time airborne, max distance covered, min drag, etc...


I just got this tachometer installed on my 7.3L on my 4 window E-350 shortie, and am wondering if there's an equivalent with RPMs? I'm sure there's a lot more that goes into it, but is there a way to figure out what RPM setting would be best for fuel flow, or for climbing hills, or for accelerating onto the freeway? Most of my driving is on straight and flat freeways, or climbing up I-80 into the Sierra Nevada toward Tahoe (and back down), and I know speed isn't the best metric to use for max performance. Or am I totally misunderstanding this?



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Old 04-17-2020, 08:27 PM   #2
Bus Nut
 
Join Date: Apr 2018
Location: NorCal
Posts: 406
Year: 1995
Coachwork: Girardin
Chassis: E-350
Engine: 7.3L Powerstroke
I should add that I want to optimize everything in the following order:

1. Safety
2. Engine health
3. Fuel economy
4. Sick drag racing
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Old 04-17-2020, 10:56 PM   #3
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peddle to the floor ride the governor
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Old 04-18-2020, 12:07 AM   #4
FAC
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Find a dyno sheet for your engine and see at what rpm torque and horsepower crossover. That is generally a sweet spot to drive in. But an overpowered rig can lug better and an underpowered rig will need the extra downshift to take advantage of gearing to go up a steep hill. When you get familiar with your rig it will be easy to just feel the sweet spot, when more pedal starts to affect the speed less and less
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Old 04-18-2020, 01:26 AM   #5
Bus Crazy
 
Join Date: May 2018
Location: topeka kansas
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Year: 1954
Coachwork: wayne
Chassis: old f500- new 2005 f-450
Engine: cummins 12 valve
Rated Cap: 20? five rows of 4?
My thoughts....

Doing things by throttle..... Under acceleration, up those big hills, and towing. I think one item to keep an eye on is the Exhaust Gas Temperature. Just like in a piston engine aircraft. 1275 degrees is safe maximum. The higher you go, the shorter your time at the elevated temperature. I think many of the "diesel pick up truck" crowd find 1300 degrees to 1500 degrees acceptable depending on the time at those temperatures. I do not. I am expecting 200,000 miles to 300,000 miles from my newly built engine. I do not think the valves will put up with temperatures at or over 1275 degrees and return the number of miles I am expecting.

One method of keeping exhaust cooler is to pump in more air (bigger turbo) more volume of air ... The two down sides to this is too much boost, and poor low rpm throttle response, and black smoke.

so...... when going up the hills, accelerating, and towing, the object is to keep an eye on EGT. If you start running too hot and engine rpm conditions allow, downshift, bring up the turbo speed, meaning boost pressure, for more air.

My particular bus is set up for 1875 rpm at 65 mph. IF I need to, I can down shift to 5th gear and run 2525 rpm at 65 mph. The object is low rpm, low boost, low fuel flow at cruise speed level ground... up slight grades, at throttle but watch result of EGT. Start running too hot, I can downshift, bring up engine speed and keep same mile per hour because I can use the transmission gearing to increase torque at the rear wheels.

SPEED... well I should be able to go as far as 3400 rpm before I run against engine governor. If my math is correct that works out to 87 mph in 5th gear. and 117 mph in 6th gear. I dont think I will have enough power to pull top speed in top gear. I do think I will be able to pull 3400 rpm in 5th. I might be able to do 100 mph in 6th.... IF I can keep the temperatures in control.

Fuel economy... the lower possible engine rpm for the amount of road speed.... goes back to the 1875 rpm at 65mph... I figure peak torque is around 1600 to 1700 rpm.... I am running a different camshaft, valve springs, piston bowl design, and injector timing than stock, so I dont know exactly where my peak torque is now.

Drag race, pour as much fuel as you can... that unburnt fuel in the exhaust is going to bring up EGT fast, but a ten second run.... In my case, I also have aftermarket head studs and oringed engine block. I am supposed to be good for over 50 psi of boost or more.... but with my set up ... looking to keep that at 40psi to 45psi.

There is also the possible addition of water alcohol mix and or propane for more power and temperature control.

better fuel economy and power can be had by running the engine temperatures as hot as pratical. My engine when first put in use, was running 175/180 degree thermostats, I work at keeping the engine at 195/200 degrees.....

I have no electronic control over my engine at all... I have to watch EGT, boost pressure and engine temperature and make throttle adjustments as I go.

My engine is not set up any thing like most school bus engines, mine is re engineered -- hot rodded -- so.

For most busses, set the throttle on the floor and drive... most or governed so that the engine and transmission are safe. Unless you are going up hill or towing.... Then you can get some transmission oil temperatures that are too hot, some engine temperatures that are too hot, and I suppose some
EGT that are too hot, but I can not say I have seen school busses with EGT on the dash board. I do see some with transmission oil temp gages.

william
I hope I have answered a little bit of your question.
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Old 04-18-2020, 04:54 AM   #6
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Quote:
Originally Posted by magnakansas View Post
Doing things by throttle..... Under acceleration, up those big hills, and towing. I think one item to keep an eye on is the Exhaust Gas Temperature. Just like in a piston engine aircraft. 1275 degrees is safe maximum. The higher you go, the shorter your time at the elevated temperature. I think many of the "diesel pick up truck" crowd find 1300 degrees to 1500 degrees acceptable depending on the time at those temperatures. I do not. I am expecting 200,000 miles to 300,000 miles from my newly built engine. I do not think the valves will put up with temperatures at or over 1275 degrees and return the number of miles I am expecting.

One method of keeping exhaust cooler is to pump in more air (bigger turbo) more volume of air ... The two down sides to this is too much boost, and poor low rpm throttle response, and black smoke.

so...... when going up the hills, accelerating, and towing, the object is to keep an eye on EGT. If you start running too hot and engine rpm conditions allow, downshift, bring up the turbo speed, meaning boost pressure, for more air.

My particular bus is set up for 1875 rpm at 65 mph. IF I need to, I can down shift to 5th gear and run 2525 rpm at 65 mph. The object is low rpm, low boost, low fuel flow at cruise speed level ground... up slight grades, at throttle but watch result of EGT. Start running too hot, I can downshift, bring up engine speed and keep same mile per hour because I can use the transmission gearing to increase torque at the rear wheels.

SPEED... well I should be able to go as far as 3400 rpm before I run against engine governor. If my math is correct that works out to 87 mph in 5th gear. and 117 mph in 6th gear. I dont think I will have enough power to pull top speed in top gear. I do think I will be able to pull 3400 rpm in 5th. I might be able to do 100 mph in 6th.... IF I can keep the temperatures in control.

Fuel economy... the lower possible engine rpm for the amount of road speed.... goes back to the 1875 rpm at 65mph... I figure peak torque is around 1600 to 1700 rpm.... I am running a different camshaft, valve springs, piston bowl design, and injector timing than stock, so I dont know exactly where my peak torque is now.

Drag race, pour as much fuel as you can... that unburnt fuel in the exhaust is going to bring up EGT fast, but a ten second run.... In my case, I also have aftermarket head studs and oringed engine block. I am supposed to be good for over 50 psi of boost or more.... but with my set up ... looking to keep that at 40psi to 45psi.

There is also the possible addition of water alcohol mix and or propane for more power and temperature control.

better fuel economy and power can be had by running the engine temperatures as hot as pratical. My engine when first put in use, was running 175/180 degree thermostats, I work at keeping the engine at 195/200 degrees.....

I have no electronic control over my engine at all... I have to watch EGT, boost pressure and engine temperature and make throttle adjustments as I go.

My engine is not set up any thing like most school bus engines, mine is re engineered -- hot rodded -- so.

For most busses, set the throttle on the floor and drive... most or governed so that the engine and transmission are safe. Unless you are going up hill or towing.... Then you can get some transmission oil temperatures that are too hot, some engine temperatures that are too hot, and I suppose some
EGT that are too hot, but I can not say I have seen school busses with EGT on the dash board. I do see some with transmission oil temp gages.

william
I hope I have answered a little bit of your question.
Excellent info! Thanks. I'm wondering, though, how much of this applies to gas engines? How different are they?

Chris
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Old 04-18-2020, 12:11 PM   #7
Bus Crazy
 
Join Date: May 2018
Location: topeka kansas
Posts: 1,008
Year: 1954
Coachwork: wayne
Chassis: old f500- new 2005 f-450
Engine: cummins 12 valve
Rated Cap: 20? five rows of 4?
for gasoline engines...

Best mileage for gasoline engines.... once you are in top gear I think keeping the vacuum reading as high as possible would be a goal. So a vacuum gauge would be something to have on the dash board. When accelerating .. shift up to the next gear as soon as you can. The idea is keep the engine turning as slow as possible and get the job done. BMW did some research on this fuel economy thing.... there is a published paper on this research somewhere out there in the internet... anyways, the result of this work was the BMW 528e.

where do you run rpm for cruise speed....... that might take a little work and test drives.... I will give you an example. I have an old pickup truck, If I drive at 62 mph I get 16.5 miles to the gallon. It is a 1970 with a 350 v8 and three speed automatic. If I go 65mph i open the up the carburetor and vacuum drops and mileage goes to 15 mpg at 80mph i get only 10mpg.

generally higher speed, lower mileage due to aerodynamic drag. I picked 65 mph cruise speed for my bus as a compromise of getting there and fuel economy. I have one of those busses with a rounded bullett butt. I think a bus with a cut off square back end pays a higher price for speed that rounded back end bus. I would drop ten miles an hour for a short bus with a square back end... the longer bus has less drag penalty than a short bus.

The modern chevrolet v8 engines turn off cylinders when throttle position shows not so much power is needed. adds fuel economy. Porsche picked up 10percent better fuel economy for in town driving by turning off the engine at stops.


I'm rambling....

boils down to smallest throttle input as possible, for best fuel economy.
keep engine somewhere between peak torque and peak horsepower for best acceleration.

did this answer the question? I feel like I missed on this one.

william
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Old 04-18-2020, 12:54 PM   #8
Bus Nut
 
Join Date: Apr 2018
Location: NorCal
Posts: 406
Year: 1995
Coachwork: Girardin
Chassis: E-350
Engine: 7.3L Powerstroke
Thank you everyone for the info, this is super informative. It sounds like I'll need to install an EGT monitor next time I take it in. I looked in the "Caring and Feeding of the 7.3L" post and found this thread about installing an EGT monitor themselves... and I'm worried I'd dork it up and send metal shavings where they shouldn't go.


I spent some time this morning educating myself on automobile torque and horsepower, and my understanding is that best acceleration happens at peak horsepower. I found the following chart:





So the way I understand it is this: if I want to haul ass, set the throttle to hit 3000 rpm (peak horsepower); otherwise, an RPM between peak torque and horsepower is good (2000-3000 rpm). If I really want to prolong engine life, then I'll need an EGT gauge and basically set the throttle so as to not exceed a certain threshold.
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Old 04-18-2020, 01:02 PM   #9
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Year: 2003
Chassis: Chevy cut-away 6-window shortie
Engine: 6.0L Gasser
Quote:
Originally Posted by magnakansas View Post
Best mileage for gasoline engines.... once you are in top gear I think keeping the vacuum reading as high as possible would be a goal. So a vacuum gauge would be something to have on the dash board. When accelerating .. shift up to the next gear as soon as you can. The idea is keep the engine turning as slow as possible and get the job done. BMW did some research on this fuel economy thing.... there is a published paper on this research somewhere out there in the internet... anyways, the result of this work was the BMW 528e.

where do you run rpm for cruise speed....... that might take a little work and test drives.... I will give you an example. I have an old pickup truck, If I drive at 62 mph I get 16.5 miles to the gallon. It is a 1970 with a 350 v8 and three speed automatic. If I go 65mph i open the up the carburetor and vacuum drops and mileage goes to 15 mpg at 80mph i get only 10mpg.

generally higher speed, lower mileage due to aerodynamic drag. I picked 65 mph cruise speed for my bus as a compromise of getting there and fuel economy. I have one of those busses with a rounded bullett butt. I think a bus with a cut off square back end pays a higher price for speed that rounded back end bus. I would drop ten miles an hour for a short bus with a square back end... the longer bus has less drag penalty than a short bus.

The modern chevrolet v8 engines turn off cylinders when throttle position shows not so much power is needed. adds fuel economy. Porsche picked up 10percent better fuel economy for in town driving by turning off the engine at stops.


I'm rambling....

boils down to smallest throttle input as possible, for best fuel economy.
keep engine somewhere between peak torque and peak horsepower for best acceleration.

did this answer the question? I feel like I missed on this one.

william
Good enough for me! I've tended to keep the RPMs as low as possible, but wasn't sure what was best for cruising. I've only taken one long trip with our box, aka short bus, and fuel economy was averaging 9-9.5mpg, with us cruising 60-65mph, so I'll try dropping to 55 and see what happens then. I did notice driving into the wind, i.e. west, tended to get more poor fuel economy, but I didn't realize how bad it could get at higher speeds!

Anyway, thanks for the insights, both gas and diesel!

Chris
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Old 04-18-2020, 02:47 PM   #10
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Engine: T444E
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Mind you, the 7.3 Ford engine and the T444e are *NOT* the exact same engine! They are very similar, and use many common parts, but one thing I remember reading was the Ford version uses thicker head gaskets, to allow for rod stretch at the higher RPM's that the IH version does not run. As such, the IH version is typically limited to ~26-2700 RPM where the Ford version runs up to 32-3400 RPM.


Running against the limit isn't efficient. I feel like my IH version will cruise happily in the 1800 range, though the a 1:1 high gear (manual transmission) and 5.38 rear gears, I feel like traveling at speeds greater than 40 MPH (a gear swap is planned, probably in the 3.73 or 3.91 range). The Ford version will probably be happy in the 2200 range (someone correct me if they disagree) delivering the best power-to-fuel economy ratio around there.


Speed and wind drag will hurt fuel economy more than anything else, keeping engine RPM down will offset this but only so much. The fact is it takes a certain amount of energy to push air out of your way, and the faster you do this (and by extension, more air moved per minute, since you're covering more ground per minute), the more energy it takes. This is one very big reason many trucking companies limit the top speed of their trucks, it really does save fuel (yes, there's arguments for lower speeds being safer, but the fuel economy is much easier to track, document, and prove. Example, a Freightliner road tractor running 72 averages 6.5 MPG. Running the same truck at 62 brings the average over 8.2 MPG)
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Old 04-18-2020, 04:00 PM   #11
Bus Crazy
 
Join Date: May 2018
Location: topeka kansas
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Year: 1954
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Chassis: old f500- new 2005 f-450
Engine: cummins 12 valve
Rated Cap: 20? five rows of 4?
1995 7.3 diesel ford...

From a bit a look around to familiarize myself with the ford engine power/torque curves....

If I had to make a guess at and engine speed for best cruise, I would be aiming for 1600 rpm to 2000 rpm depending on the year of production. for the 1995 I would try to gear the bus as close to 2000 rpm for cruise speed as I could. If I could not do that... i would go below 2000 rpm, not above it.

power peaks at 3000 rpm.... shift at 3100-3200 for maximum poop. That is the kind of thing I am doing in an emergency........

If I could not get up a hill in top gear, I would downshift and work at keeping rpm as close to 2000 rpm as I could.. peak torque.... same for each gear up hills..... that 2000 rpm is going to be where you have the most grunt from the engine to move the weight/load.

I do not recommend hot rodding this engine at all... everything as a system is just too close to the edge of holding together. you add fuel or turbo, then you are going to have problems with the head bolts keeping it all together. then if you band aid that, and turn any more rpm, I think you are going to see cam/lifter problems. The rollers at the bottom of the lifters gives up and then the damaged lifter takes out the camshaft. If I owned a 7.3, I would drop in new lifters after 150,000 miles. replace headbolts with studs, I think that would go a long way of preventive maintenance with this particular engine.

william
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