Understanding aero vs throttle pos.

[HAHA]

Well - diesel and electric don't suffer as much from pumping losses so there is no such thing as a more restricted air intake when better aerodynamics results in less throttle.

However, if you p&g with the gasoline engine, you don't increase your suffering from pumping losses because you only use the engine with open throttle.

As been already said, you will always gain with improved aerodynamics, even in steady state driving. Engine efficiency probably drops a little bit more with a gasoline engine though.

[brucepick]

Sounds right to me.
Apparently there's plenty I could learn abut diesel - I never owned one, and drove one only briefly nearly 30 years ago (VW Rabbit).

[COcyclist]

In my diesel, coasting in neutral with the engine on, the ScanGuage reads 300 to 400 instantaneous mpg. There is no throttle plate creating vacuum and choking efficiency at low load conditions. The computer injects only enough fuel to keep the engine turning over. With better aero I can coast in gear more on long grades using no fuel at all. Fuel is cut off at anything over 900 rpm while engine braking (overrun) in the VW.

[JRMichler]

Originally Posted by HAHA

This should mean diesel or electric powered vehicles will respond better to aerodynamic improvements than gasoline counterparts....

- Unless we use pulse & glide

Sounds right to me. Gasoline engine efficiency drops off faster at part throttle than either diesel or electric. So those of us with gasoline engines get around that by increasing P&G as we decrease aero drag.

[Patrick]

Originally Posted by tjts1

No it doesn't. Taller gearing means lower RPM for a given speed therefore reduces manifold vacuum (higher absolute pressure) and reduced pumping losses. The amount of power to overcome rolling resistance and aerodynamic drag remains the same.

Power remains the same, but RPM is lower, therefore torque must be higher, requiring a larger throttle opening.

[tjts1]

Originally Posted by Patrick

Power remains the same, but RPM is lower, therefore torque must be higher, requiring a larger throttle opening.

No, with taller gearing for a given road speed, you still need the same amount of air to send the same amount of torque to the wheels. RPM decreases, absolute manifold pressure increases, throttle angle remains the same, torque remains the same. On a naturally aspirated engine you reach you peak torque for a given RPM when manifold pressure is at 100kpa regardless of the throttle angle. At low RPMs you can reach peak torque at 30-50% throttle.

[Patrick]

Originally Posted by tjts1

No, with taller gearing for a given road speed, you still need the same amount of air to send the same amount of torque to the wheels. RPM decreases, absolute manifold pressure increases, throttle angle remains the same, torque remains the same. On a naturally aspirated engine you reach you peak torque for a given RPM when manifold pressure is at 100kpa regardless of the throttle angle. At low RPMs you can reach peak torque at 30-50% throttle.

Power = torque X RPM. If you decrease RPM, you must increase torque to maintain a given power.

[tjts1]

Originally Posted by Patrick

Power = torque X RPM. If you decrease RPM, you must increase torque to maintain a given power.

You're thinking about peak torque at any given RPM. At part throttle cruise on flat ground you don't need that. You're just playing around inside the torque curve area. If you install taller gearing your airflow and torque remain the same even though you reduced the RPM for a given cruising speed. If you have a good OBD2 tool (scan gauge, ultra gauge, Torque app etc), get on the highway and cruise at a fixed speed in 4th gear on flat ground, note the throttle angle and MAF reading, then shift to 5th. You'll see the same angle and MAF reading despite the reduced RPM. Your injector pulse width will be different because fuel pressure is vacuum referenced. Thats why all OBD2 systems use MAF not injector PW to calculate fuel consumption.

[Patrick]

Originally Posted by tjts1

You're thinking about peak torque at any given RPM. At part throttle cruise on flat ground you don't need that. You're just playing around inside the torque curve area. If you install taller gearing your airflow and torque remain the same even though you reduced the RPM for a given cruising speed. If you have a good OBD2 tool (scan gauge, ultra gauge, Torque app etc), get on the highway and cruise at a fixed speed in 4th gear on flat ground, note the throttle angle and MAF reading, then shift to 5th. You'll see the same angle and MAF reading despite the reduced RPM. Your injector pulse width will be different because fuel pressure is vacuum referenced. Thats why all OBD2 systems use MAF not injector PW to calculate fuel consumption.

The equation holds at any RPM, torque, or power. Just like E = I X R, it's a mathematical and physical fact. If you decrease rpm for a given power, you must increase torque. If you lower RPM at the same torque, you have decreased power.

[tjts1]

Originally Posted by Patrick

The equation holds at any RPM, torque, or power. Just like E = I X R, it's a mathematical and physical fact. If you decrease rpm for a given power, you must increase torque. If you lower RPM at the same torque, you have decreased power.

Ok if you say so but this has nothing to do with throttle angle or the amount of air consumed. Throttle angle stays the same, MAF load stays the same given a fixed speed and taller gearing. If you don't believe me, try it on your own car. Its very simple test. Anybody with an OBD2 can do this.