How do I reduce horsepower?

[ViviTheMage]

get some horses to pull it...

 

[VinylxScratches]

1. Sell the truck
2. Buy a car with a 4 cylinder engine.
3. Profit!

 

[LTC8K6]

ZV, I understand the explanation.

So what is going on with the race cars? They don't seem to get worse, but better mileage with the airflow restrictor. Not speaking of Nascar, of course, but the more modern race cars. The airbox restrictor increases the number of laps they can run on a tank of fuel, it does not decrease it.

 

[RGUN]

Originally posted by: LTC8K6
ZV, I understand the explanation.

So what is going on with the race cars? They don't seem to get worse, but better mileage with the airflow restrictor. Not speaking of Nascar, of course, but the more modern race cars. The airbox restrictor increases the number of laps they can run on a tank of fuel, it does not decrease it.

Because the flow restriction will reduce horsepower and as before, horsepower is simply a result of combining a specific amount of fuel with a specific amount of air. If you have less air in the cylinder, it will require less fuel. This only works in this case because a race car will be spending most of its time accelerating at WOT, so this is a situation where the vehicle is always at maximum horsepower, and would therefore benefit from an air restriction (in terms of fuel economy). If they raced at a constant speed, they would not save any fuel because of what I mentioned earler, and ZV pointed out again, the intake restriction will result in reduced efficiency.

 

[LTC8K6]

Thanks, ZV...

 

[ayabe]

Strip the interior, remove the spare, take the bed off.

 

[Zenmervolt]

Originally posted by: RGUN

Originally posted by: LTC8K6
ZV, I understand the explanation.

So what is going on with the race cars? They don't seem to get worse, but better mileage with the airflow restrictor. Not speaking of Nascar, of course, but the more modern race cars. The airbox restrictor increases the number of laps they can run on a tank of fuel, it does not decrease it.

Because the flow restriction will reduce horsepower and as before, horsepower is simply a result of combining a specific amount of fuel with a specific amount of air. If you have less air in the cylinder, it will require less fuel. This only works in this case because a race car will be spending most of its time accelerating at WOT, so this is a situation where the vehicle is always at maximum horsepower, and would therefore benefit from an air restriction (in terms of fuel economy). If they raced at a constant speed, they would not save any fuel because of what I mentioned earler, and ZV pointed out again, the intake restriction will result in reduced efficiency.

To say that horsepower is simply the result of burning a specific amount of fuel is to oversimplify and to assume a constant efficiency value.

An airbox restriction reduces efficiency which not only reduces horsepower, but also increases the amount of fuel required to produce a given quantity of horsepower. Remember that the amount of horsepower an engine provides at the crankshaft is whatever energy is left over after internal inefficiencies have been accounted for. This includes things like heat generation, internal friction, and the applicable item here, the creation of a vacuum in the intake manifold.

Intake manifold vacuum is a manifestation of pumping losses and is an inefficiency (in fact, the lack of a throttle butterfly in diesel engines is one of the three main reason that diesel is so much more efficient than gasoline, the other two being high compression and a higher specific energy content, by volume, in diesel fuel). Also note that the inefficiency I'm talking about here is not in terms of miles per gallon (or, for an engine alone, gallons per hour) but horsepower per gallon.

For example:

My 951 will average 30 mpg at a steady 60 mph. That's 2 gallons per hour of fuel consumption for the engine and, at a steady 60 mph on level ground the aerodynamics dictate that the car should require no more than 15 hp (being pessimistic and back calculating based on a measured top-speed of just over 170 and a horsepower figure of 300 at the crank).

If I floor the throttle at 60 mph in 3rd gear, the engine will be near the power peak of 300 hp, and fuel consumption will drop to around 5 mpg. That's 12 gallons per hour.

Horsepower has increased from 15 to 300, gallons per hour have increased from 2 to 12. Horsepower has gone up by a factor of 20, fuel consumption has gone up by a factor of only 6. Clearly, the engine is vastly more efficient at turning fuel into power when the restriction in the intake is removed even though the overall vehicle efficiency (in mpg) has declined.

Adding an additional restriction in the intake system makes the engine less efficient at both ends of this process, not just the top end, so to make that same 15 hp at 60 mph, the engine would need to be working harder and specific fuel consumption might be 2.1 gallons/hour instead of 2.0 gallons/hour.

As for why the race cars get better mileage with the restrictors than without them, notice that I have already demonstrated that engine efficiency (in terms of fuel consumed per horsepower produced) is not necessarily the same as vehicle efficiency (in terms of fuel consumed per mile traveled). Overall vehicle efficiency has a lot to do with aerodynamics, gearing, and driving habits among other factors. Note that restrictors are used on tracks that are high-speed (as a means of keeping overall vehicle speeds down).

This plays into vehicle fuel efficiency in several ways:

- Low downforce wing setups. High-speed tracks generally have high banking and sweeping corners, which allow cars to have less downforce and, accordingly, less drag, which yields better efficiency.
- More constant speeds. Another result of the higher banking and more sweeping corners is a reduced variation in speed. It is more efficient to stay at a steady, albeit high, speed continuously than it is to always be braking and accelerating. This too helps vehicle fuel efficiency.
- Gearing. Taller "high-speed" gearing setups allow higher speeds for a given RPM, which increases vehicle fuel economy.

Those three variables greatly overcome the inefficiencies introduced by the airbox restrictions, but, aside from changing driving style, they are not feasible in the everyday world in which the OP is driving.

ZV

 

[LTC8K6]

Note that restrictors are used on tracks that are high-speed (as a means of keeping overall vehicle speeds down).

I specifically excluded Nascar...restrictor plates are fundamentally different than what I am referring to because they are under the carburetor.

Restrictors are put on the air intakes of ALMS cars, and LeMans cars, etc., to reduce power, and they decrease fuel consumption as far as I have ever heard. They are used at all tracks, which are all road courses. They are not used to reduce speeds, but to try to equalize different engine/vehicle packages.

Intake restrictors visible here on an Acura:

http://www.blogcdn.com/www.aut.../007_acuralmp1_opt.jpg

 

[RGUN]

Originally posted by: Zenmervolt

Originally posted by: RGUN

Originally posted by: LTC8K6
ZV, I understand the explanation.

So what is going on with the race cars? They don't seem to get worse, but better mileage with the airflow restrictor. Not speaking of Nascar, of course, but the more modern race cars. The airbox restrictor increases the number of laps they can run on a tank of fuel, it does not decrease it.

Because the flow restriction will reduce horsepower and as before, horsepower is simply a result of combining a specific amount of fuel with a specific amount of air. If you have less air in the cylinder, it will require less fuel. This only works in this case because a race car will be spending most of its time accelerating at WOT, so this is a situation where the vehicle is always at maximum horsepower, and would therefore benefit from an air restriction (in terms of fuel economy). If they raced at a constant speed, they would not save any fuel because of what I mentioned earler, and ZV pointed out again, the intake restriction will result in reduced efficiency.

To say that horsepower is simply the result of burning a specific amount of fuel is to oversimplify and to assume a constant efficiency value.

I was intentionally not bringing the efficiency of the drivetrain/moving components into the description. Burning a specific amount of fuel will infact produce a specific amount of energy. Once this translates through to the driving wheels, it will be subject to whatever efficiencies exist.

 

[Zenmervolt]

Originally posted by: LTC8K6

Note that restrictors are used on tracks that are high-speed (as a means of keeping overall vehicle speeds down).

I specifically excluded Nascar...restrictor plates are fundamentally different than what I am referring to because they are under the carburetor.

Restrictors are put on the air intakes of ALMS cars, and LeMans cars, etc., to reduce power, and they decrease fuel consumption as far as I have ever heard. They are used at all tracks, which are all road courses. They are not used to reduce speeds, but to try to equalize different engine/vehicle packages.

Intake restrictors visible here on an Acura:

http://www.blogcdn.com/www.aut.../007_acuralmp1_opt.jpg

Ahhh. OK.

Still a different scenario, but simpler to explain.

Race cars spend most of their time at full throttle, intake restrictors effectively reduce the maximum throttle opening, which reduces full throttle power (and full throttle fuel consumption). They develop less horsepower (to reduce speeds of certain cars, that's what "equalizing" means in racing), and while fuel consumption per horsepower goes up, fuel consumption per mile goes down because horsepower drops much more than per-horsepower consumption increases.

Example: Let's say that an unrestricted engine burns 30 gph to produce 300 hp. That's 0.1 gph per horsepower. Now, add a restrictor. Let's say we get 28 gph and 250 hp. That's 0.112 gph per horsepower. Better fuel consumption over time, but worse per hp developed.

At cruise on the street, gph/hp is more important since at legal speeds it's HP that is the (relatively) fixed need. And since most people don't use full-throttle acceleration, chances are that a street driver would never enter the realm where a restricted intake would start limiting power (the limitations really only start to come up near full throttle).

Does that make any more sense? It's clearer in my head but as I type it seems to be more confusing than I think it is.

ZV

 

[Zenmervolt]

Originally posted by: RGUN
I was intentionally not bringing the efficiency of the drivetrain/moving components into the description. Burning a specific amount of fuel will infact produce a specific amount of energy. Once this translates through to the driving wheels, it will be subject to whatever efficiencies exist.

True, but even within the engine it's not the same amount of useful energy. Some (most) is wasted in creating heat. Some is wasted to create manifold vacuum. Some is wasted to overcome internal engine friction. All of that occurs before anything is available at the crankshaft to power things outside of the engine and has nothing to do with drivetrain efficiency. That's what I was getting at.

It is entirely possible for engine A to burn gasoline at a rate of 20 gph and produce 300 hp at the crankshaft, while engine B burns 20 gph and only produces 280 hp at the crankshaft. The same total energy is produced by the gasoline going through both engines, but engine A is more efficient even before one considers the drivetrain.

BTUs are, "simply a result of combining a specific amount of fuel with a specific amount of air." Horsepower, however, is not.

ZV

 

[LTC8K6]

I think we need to call Mythbusters. Can we work a conveyor in somewhere?

 

[Zenmervolt]

Originally posted by: LTC8K6
I think we need to call Mythbusters. Can we work a conveyor in somewhere?

You know, I just thought of the simplest illustration:

Do you get better or worse mileage when your air filter is dirty and clogged? A dirty air filter is equivalent to adding an intake restriction.

I'm up for the mythbusters though, that would be kind of fun.

ZV

 

[KillaKilla]

A more open exhaust, either through a larger pipe or more open muffler, will help fuel economy and improve mileage. I'm assuming it's naturally aspirated, so a turbocharger would also increase fuel economy by increasing the air pressure on the intake side. A more open intake can also help, though this can result in more rapid fouling, because barring a much larger filter, less restrictive means less filtration of the incoming air. Taller final-drive ratios will also improve mileage, to a point. An engine has an ideal speed for generating a given amount of power, and you'd want you gearing to put you engine speed at just above what it needs to be to maintain speed at just about full throttle, like in semis.

None of these, except possibly the exhaust or an ECU change are cost-effective though, as the mileage gains are somewhat minimal compared to the cost to purchase and install them. If you need a high-efficiency truck that will still be able to haul everything you need, Chrysler still sources the top-of-the-line engines for their Ram pickups from Cummins. Having driver one with the old 5.9, I'd say they are hilariously (and often dangerously) powerful, rugged as hell, and get about 27 MPG on the highway with cruise at 65 MPH(flat ground). Mileage on diesel pickups will drop off very quickly as you increase speed because they are geared far lower than they should be, to improve acceleration, at the expense of fuel economy.

Oh, scratch that, they've moved to a 6.7 liter. I have no idea what the fuel economy will be on those, but torque is up from the (stupidly high) 610 ft-lbs to 650. I... I don't know where you would put that much torque. That's well above 2 mustang V8s, in a 6-cylinder.

 

[MovingTarget]

5.7l v8? Perhaps you should consider a conversion to propane/cng... Depending on how/where you drive the truck, it could save you a lot of $$ over the long run. (The tax credits help too...)

 

[Zenmervolt]

Originally posted by: KillaKilla
A more open exhaust, either through a larger pipe or more open muffler, will help fuel economy and improve mileage.

Only if the stock exhaust is terribly restrictive. Headers would help far more than a larger pipe or a different muffler.

Originally posted by: KillaKilla
I'm assuming it's naturally aspirated, so a turbocharger would also increase fuel economy by increasing the air pressure on the intake side.

Flat out wrong. More air means more fuel. Always and forever. If the turbo is producing boost, fuel economy will go down. Turbocharged cars tend to get good mileage because they are off boost at cruising speed. Adding a turbo will not increase mileage.

Originally posted by: KillaKilla
A more open intake can also help, though this can result in more rapid fouling, because barring a much larger filter, less restrictive means less filtration of the incoming air.

Intake debris won't cause fouling unless you're pulling in oil. Pulling in more debris would allow contaminants into the engine and could potentially accelerate wear of the cylinder walls and piston rings, but it is very unlikely to cause fouling.

Originally posted by: KillaKilla
Taller final-drive ratios will also improve mileage, to a point. An engine has an ideal speed for generating a given amount of power, and you'd want you gearing to put you engine speed at just above what it needs to be to maintain speed at just about full throttle, like in semis.

Semis, and other diesel engines, don't have a throttle. Engine speed is controlled simply by varying the amount of diesel oil that is injected into the cylinders.

Furthermore, you do not want the engine to be gear to maintain speed at "just about full throttle". For fuel economy you want to gear the car such that the engine is turning as few RPM as is possible without lugging the engine.

Originally posted by: KillaKilla
None of these, except possibly the exhaust or an ECU change are cost-effective though, as the mileage gains are somewhat minimal compared to the cost to purchase and install them.

Well, this, at least, is true and accurate.

Originally posted by: KillaKilla
If you need a high-efficiency truck that will still be able to haul everything you need, Chrysler still sources the top-of-the-line engines for their Ram pickups from Cummins. Having driver one with the old 5.9, I'd say they are hilariously (and often dangerously) powerful, rugged as hell, and get about 27 MPG on the highway with cruise at 65 MPH(flat ground). Mileage on diesel pickups will drop off very quickly as you increase speed because they are geared far lower than they should be, to improve acceleration, at the expense of fuel economy.

Oh, scratch that, they've moved to a 6.7 liter. I have no idea what the fuel economy will be on those, but torque is up from the (stupidly high) 610 ft-lbs to 650. I... I don't know where you would put that much torque. That's well above 2 mustang V8s, in a 6-cylinder.

Actually, diesels are geared taller than normal vehicles because they produce enough torque to deal with the taller gearing. This is also done to alleviate the fact that diesels typically have a smaller usable RPM range between idle and redline.

For example, the Cummins diesel in Dodge trucks redlines at 3,500 RPM while the gasoline V8 redlines at 5,800 RPM. While diesels typically come with shorter (numerically higher) final drive ratios, the transmission ratios tend to be taller to compensate. For example, the overall gear ratio (automatic transmission) with a 4.10 rear end is 2.75:1 for the gasoline V8, but 2.56:1 for the Cummins; the diesel has taller gearing.

Mileage for diesels falls of with higher speeds because the engines aren't designed for high-RPM efficiency. Diesels are designed for low-RPM stump-pulling and they tend to lose efficiency at high RPM.

ZV

 

[LTC8K6]

My 5.7L Hemi does very well in the city and on the highway. Particularly once you learn how to keep it in 4 banger mode as much as possible.

 

[LTC8K6]

ZV, the explanation for needing to change a dirty air filter always seems to be that the engine will run rich as the filter gets clogged.

But that really should not be true with a modern car, should it? A modern engine measures the air flow and keeps the mixture correct anyway, doesn't it?

Within it's limits, of course.

The actual reason behind needing a clean air filter relates to the loss of power and efficiency on a modern vehicle, I suspect.

 

[Zenmervolt]

Originally posted by: LTC8K6
ZV, the explanation for needing to change a dirty air filter always seems to be that the engine will run rich as the filter gets clogged.

But that really should not be true with a modern car, should it? A modern engine measures the air flow and keeps the mixture correct anyway, doesn't it?

Within it's limits, of course.

The actual reason behind needing a clean air filter relates to the loss of power and efficiency on a modern vehicle, I suspect.

Yes, on old non-feedback carbs a clogged/dirty air filter increased manifold vacuum, which caused the carb to meter a bit rich. Modern EFI (and even feedback carbs) would use the information from the O2 sensors to keep the mixture close to stoichiometric though. An air filter would need to get pretty bad on a modern car before the effects became noticeable to the driver.

ZV