question about aero

[gabi]

does a car feel slower or faster with better aero, given the same speed?

[Frank Lee]

Faster... why?

[Christ]

Perception can vary based on several factors, including how quickly the vehicle accelerated to the test speed.

If the individual were introduced to a vehicle which was already at speed, the perception shouldn't vary from test subject to test subject, regardless of aero.

[Frank Lee]

It could seem faster if it is more responsive to acceleration as basjoos claims.
But then it could seem slower because I've noticed that the quieter a vehicle is, the slower it seems to be going, and bellypans and such can quiet a vehicle down.

[Christ]

That's why I said that it could vary.

In a true test where one would hope to gather any useful information, most of the perceptive cues would have to be muted or averaged out over the sample because something as simple as a change in sound can change ones perception of speed, direction, proximity, etc.

[Hubert Farnsworth]

Quick question, if the aerodynamic drag component is reduced how does this change the amount of power the engine is producing at a given vehicle speed? For example lets say the car has a cd of 0.33 and a gear ratio spread such that the car is turning at 3000 rpm at 68 mph an N/V ratio of 44.12 rpm/mph. The engine produced maybe 50 horse power at this point, if the aerodynamic loading used to require 40 hp at this point and the aerodynamics is improved such that it now takes less power how can this effect the engine speed at a certain vehicle speed? I can see it allowing for increased coasting, however the gear ratio is fixed and therefore for a fixed vehicle speed the engine produces a certain power output at a given throttle input, say around 50% load (ie its a 105 hp engine)

[Frank Lee]

Come on Hubert, think about that one a little bit.

[Red_Liner740]

Originally Posted by Hubert Farnsworth

Quick question, if the aerodynamic drag component is reduced how does this change the amount of power the engine is producing at a given vehicle speed? For example lets say the car has a cd of 0.33 and a gear ratio spread such that the car is turning at 3000 rpm at 68 mph an N/V ratio of 44.12 rpm/mph. The engine produced maybe 50 horse power at this point, if the aerodynamic loading used to require 40 hp at this point and the aerodynamics is improved such that it now takes less power how can this effect the engine speed at a certain vehicle speed? I can see it allowing for increased coasting, however the gear ratio is fixed and therefore for a fixed vehicle speed the engine produces a certain power output at a given throttle input, say around 50% load (ie its a 105 hp engine)

Is this one of those "the answer is given in the question" type questions?

[dcb]

Originally Posted by Hubert Farnsworth

...however the gear ratio is fixed..

Depending on the car, it may be somewhat trivial to change the gear ratio. It is about $20 and a trip to the u-pull-it for my saturn to get a taller 5th gear.
Saturn S-Series 5th Gear Swap

My rabbit was pretty easy too, but needed more careful adjustment than the saturn when reinstalling the 5th shift fork (still could do it in the car).

*maybe* you have a C transmission, and can get a .725 5th gear out of a corolla or a tercel or a yaris? It might take some research to be certain: Toyota C transmission - Wikipedia, the free encyclopedia

Taller drive tires are another option, which may or may not affect drag, as well as swapping in a used tranny with better ratios (and a new clutch disk and throwout bearing while you are there)

There is no inherent scope to modding

P.S. if you need 50hp to cruise, you are driving a barn

[Hubert Farnsworth]

Originally Posted by dcb

Depending on the car, it may be somewhat trivial to change the gear ratio. It is about $20 and a trip to the u-pull-it for my saturn to get a taller 5th gear.
Saturn S-Series 5th Gear Swap

My rabbit was pretty easy too, but needed more careful adjustment than the saturn when reinstalling the 5th shift fork (still could do it in the car).

*maybe* you have a C transmission, and can get a .725 5th gear out of a corolla or a tercel or a yaris? It might take some research to be certain: Toyota C transmission - Wikipedia, the free encyclopedia

Taller drive tires are another option, which may or may not affect drag, as well as swapping in a used tranny with better ratios (and a new clutch disk and throwout bearing while you are there)

There is no inherent scope to modding

P.S. if you need 50hp to cruise, you are driving a barn

Ok the 50hp was pulled from thin air, even the Taurus I first learned to drive in only required 13-15 hp at around 50 mph or was it 60 I forget, but someplace I have the dyno slip that stated this. Also the 50 hp was derived from a 50% load based on the wot peak hp of 105 bhp and 100lb/ft or was that the other way around, I need to see which gear ratios/gearbox is in the car, I know its a 5 speed, but the overdrive isn't tall enough, I can engage 5th gear as low as 30 mph and then get it down to 25 around slowing down for corners, but it also turns 3000 somewhere near 70mph per the speedometer and the gps (closer to 68 at 3000 exactly by the tach). I'd like either a taller 6th gear on top of the five now as a way of reducing the revs further, but unless it can be done reasonably by some one with some mechanical inclination but no prior experience and for not too much money vs saving and replacing it when it did wear out.

I'm being serious, it doesn't seem logical that the engine would spin any slower even if the load is reduced due to the aerodynamics being improved, vehicle speed is a function of available tractive effort power - road load power- mass effective*acceleration (ie the inertial forces) reducing the aerodynamic component is incorporated inside the road load force, as an example I studied in a hybrid and electric vehicle course we took a theoretical extended range electric conversion of a Ford Fusion Hybrid and the peak power usage was 40kw or about 53hp for both the city and highway epa cycles, however with the net energy usage much more severe in the city cycle without regen, which was cut in half theoretically with regen, however due to the way the highway cycle works there is virtually no gain to regenerative breaking in the epa highway cycle test