Peak power RPM change with air density

[Truckedup]

1. The loss in power and torque would be expected to be slightly greater than being absolutely proportional to air density, due to certain frictional and oil pumping losses being independent of air density. In engineering terms, the IMEP is substantially in proportion to air density, and the FMEP has a significant component which is independent of air density.

2. Little engines tend to have greater FMEP than big ones.

3. If you have a carbureted engine jetted for sea level ish and you take it to high altitude without rejetting, the loss will be greater than proportional to air density owing to it now being too rich.

The engines in question here have only two cylinders of 20 cubic inches each, OHV....I would think that fewer cylinders is less frictional loses ?

[pcnsd]

A 10 percent change in the RAD between the tuning day and race day will result in an appropriate 10 percent change in HP output.. Sometimes it is more. If you tune near the coast, mostly it is less.

[Brian P]

1. The loss in power and torque would be expected to be slightly greater than being absolutely proportional to air density, due to certain frictional and oil pumping losses being independent of air density. In engineering terms, the IMEP is substantially in proportion to air density, and the FMEP has a significant component which is independent of air density.

2. Little engines tend to have greater FMEP than big ones.

3. If you have a carbureted engine jetted for sea level ish and you take it to high altitude without rejetting, the loss will be greater than proportional to air density owing to it now being too rich.

The engines in question here have only two cylinders of 20 cubic inches each, OHV....I would think that fewer cylinders is less frictional loses ?

Relative to their power output, those engines will have higher frictional losses than a bigger engine of otherwise comparable design.

Torque output goes up in proportion to the cube of linear dimensions, friction tends to be in proportion to the square of linear dimensions, and the smaller engines tend to spin faster which increases hydrodynamic losses relative to the size of the engine.

The "absolute" amount of friction in the smaller engine may be less, but the torque output is reduced even further, so the proportion of friction ends up higher. If you cut the linear dimensions in half, you may end up with a quarter the friction but only an eighth the torque to begin with.

[MadBill]

...Torque output goes up in proportion to the cube of linear dimensions, friction tends to be in proportion to the square of linear dimensions, and the smaller engines tend to spin faster which increases hydrodynamic losses relative to the size of the engine.

The "absolute" amount of friction in the smaller engine may be less, but the torque output is reduced even further, so the proportion of friction ends up higher. If you cut the linear dimensions in half, you may end up with a quarter the friction but only an eighth the torque to begin with.

To stray a little further from air density effects: displacement increases cubically, but not torque, because V.E comes into the picture in that the valve area is only a squared function. Add in (all else equal) the possibly greater percentage of negative torque resulting from the increased spark lead required to usher the flame front across a bigger bore in a timely fashion and that due to the longer stroke and larger bearing journals the piston speed and linear bearing velocities are likely to be higher and I think the BMEP/IMEP balance of large vs. small engines is not immediately obvious.

[dorset]

The peaks will be around the same, just less. Possibly a gear change if you can't reach the RPM peak HP coincides with along with the usual tuning for that air density change.

^^^this is super interesting to me. i run an elderly triumph 650 twin at 800 feet elevation at loring, and i tune by RAD. i've never dyno'ed the motor, but my machine can pull 135 mph at 7200 rpm in a mile and a half there.

if i go to el mirage, peak horsepower will be at the same rpm at the higher elevation, just less, correct? so to do as well as i can at el mirage, why would i need to change the gearing, assuming i have it optimized right now for the air density at 800 feet? wouldn't my best times at a higher elevation occur at the same rpm, and just be slower?

[Truckedup]

The peaks will be around the same, just less. Possibly a gear change if you can't reach the RPM peak HP coincides with along with the usual tuning for that air density change.

^^^this is super interesting to me. i run an elderly triumph 650 twin at 800 feet elevation at loring, and i tune by RAD. i've never dyno'ed the motor, but my machine can pull 135 mph at 7200 rpm in a mile and a half there.

if i go to el mirage, peak horsepower will be at the same rpm at the higher elevation, just less, correct? so to do as well as i can at el mirage, why would i need to change the gearing, assuming i have it optimized right now for the air density at 800 feet? wouldn't my best times at a higher elevation occur at the same rpm, and just be slower?

So me and you are gonna have a shootout at El mirage ?

[MadBill]

The peaks will be around the same, just less. Possibly a gear change if you can't reach the RPM peak HP coincides with along with the usual tuning for that air density change.

^^^this is super interesting to me. i run an elderly triumph 650 twin at 800 feet elevation at loring, and i tune by RAD. i've never dyno'ed the motor, but my machine can pull 135 mph at 7200 rpm in a mile and a half there.

if i go to el mirage, peak horsepower will be at the same rpm at the higher elevation, just less, correct? so to do as well as i can at el mirage, why would i need to change the gearing, assuming i have it optimized right now for the air density at 800 feet? wouldn't my best times at a higher elevation occur at the same rpm, and just be slower?

Ignoring the reduced air drag at the higher altitude (and possible different rolling resistance) yes, same RPM for peak power; however running slower at the same RPM requires going up a tooth or so on the rear sprocket.

[dorset]

So me and you are gonna have a shootout at El mirage ?

you know we're going to have to go together eventually, because i can't afford it by myself.

[dorset]

yes, same RPM for peak power; however running slower at the same RPM requires going up a tooth or so on the rear sprocket.

yes.

my realistic choices are 19/43, 21/46, and 20/43. currently i'm fastest at 21/46, at about RAD 106. so far i've never been able to pull 20/43. but i can change stuff around in an couple of hours.

[MadBill]

I previously wrote: "ignoring air density..." but to expand on that reference, engine power varies directly with air density, but so does drag so depending on temperature and surface conditions, it's possible that the various factors will cancel out.

Just a SWAG, but I'd start with your 21/46 set but be prepared to switch to the 19/43 combo if necessary.

[dorset]

i have a dry primary, so i can change gearing pretty quickly.

19/43 gave me 134 mph at 7440 rpm. that's getting pretty close to blow-up on my old motor. i'd rather run more torque and less rpm, which is why i liked 21/46, once the motor would pull it. it gave me 135 at 7250 rpm. but either way it doesn't have to run up there for very long, except at bonneville.

haven't done that yet.

[Roundybout]

i have a dry primary, so i can change gearing pretty quickly.

19/43 gave me 134 mph at 7440 rpm. that's getting pretty close to blow-up on my old motor. i'd rather run more torque and less rpm, which is why i liked 21/46, once the motor would pull it. it gave me 135 at 7250 rpm. but either way it doesn't have to run up there for very long, except at bonneville.

haven't done that yet.

It takes say 100hp to run 100mph and you make that 100hp at 7500rpm. Now at altitude you're making 80hp at that 7500rpm. But you need 100hp to run that 100mph so you're only going as fast as 80hp will get you. You might not be able to reach the 7500rpm hp peak where that 80hp is so a slight gear change to get you back to the peak HP is in order. Hope that makes sense.

Question on tooth changes? Is a certain combo of counter gear and sprocket more efficient? I know certain combos produce a hunting tooth situation and can beat up a chain fairly quickly but not sure if efficiency is impacted.

[Brian P]

not quite; to use your numbers at altitude (80% of sea level density) if it took 100 hp to go a given speed at sea level, it will take 80 hp at that altitude.

To a first approximation, the reduced drag cancels the reduced engine power since they are both in proportion to air density.

The friction, the oil pumping, the water pumping, etc are not affected by altitude, and thus the FMEP will be a greater percentage of IMEP, and thus in reality, the engine will lose a slightly greater percentage of BMEP (i.e. torque i.e. power) than the straight percentage of air density.

[dorset]

To a first approximation, the reduced drag cancels the reduced engine power since they are both in proportion to air density.

^^^this suggests that gear ratio should stay the same even at reduced air density? gearing still puzzles me, i confess.

i know several people riding old british machines that have gone the same speed at bonneville as they do at lower elevations, with just jetting changes. one rides an old BSA 650, the other a 600cc BSA single. the latter embarrasses everybody else by going 150 mph on a thumper.

[David Redszus]

I previously wrote: "ignoring air density..." but to expand on that reference, engine power varies directly with air density, but so does drag so depending on temperature and surface conditions, it's possible that the various factors will cancel out.

Spot on Bill.

Given a vehicle with a Cd of .55 and FA of 9 sqft, at an altitude of 800 ft (28.9 inHg), would require 75 hp to
go 134 mph.

At El Mirage, at an altitude of 2840 ft (26.40 inHg) the same vehicle will make only 67.4 hp.

But aero drag would also be reduced. The same vehicle with 67.4 hp could attain a velocity of 133.25 mph.

Air temp and humidity have been held constant.

Can you use any fuel?