More power at higher speed?

laissez-faire

laissez-faire

laissez-faire

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Do cars have more power at higher speed?

Travelling at say, 120mph, must force a lot of air into the engine.

Is this a contributory factor to most cars exceeding the manufacturer top speed, because that's based on theoretical top speed using power at stationary and a drag coefficient calculated in a wind tunnel?
 
all speedos over read, so when you're doing 120, you're probably doing around 110mph, maybe even less..

i got my leon to an indicated 230km/h (143mph) down a long hill in germany, even though the top speed is supposed to be 211 (132)..

having more power at higher speed depends on where the power is in the rev range. mine was at around 4000 rpm (diesel), so there really wasn't a lot of power there ;)

attaining such a speed on a flat would have taken a week..
 
Since the engine is already sucking in the air at high pressure I think the speed of the car moving through the air will make a negligible difference to the amount of air going into the engine.
 
i know sports bikes do as most use ramair (great hole in front of fairing) to ram cold air straight into airbox. Don't think there are many stock cars that have a airbox setup like this though ?

manufacturers of big bikes claim >10bhp at high speed with this.
 
laissez-faire said:
Do cars have more power at higher speed?

Travelling at say, 120mph, must force a lot of air into the engine.

Is this a contributory factor to most cars exceeding the manufacturer top speed, because that's based on theoretical top speed using power at stationary and a drag coefficient calculated in a wind tunnel?
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Yes probably more power, if the intake has free flow into it.

I'd say top speed is more likely to be tested via driving a real car on a top speed circuit at say like mira.

But yes you can work out the theoretical top speed, using this method can often be used to gain initial gear ratios for vehicles.
 
Adz said:
Since the engine is already sucking in the air at high pressure I think the speed of the car moving through the air will make a negligible difference to the amount of air going into the engine.
Click to expand...

Even the best air intake systems can only go up to 20% higher than atmospheric pressure.
 
saitrix said:
Even the best air intake systems can only go up to 20% higher than atmospheric pressure.
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Only 20%?! Thats a massive amount of ram air effect and would result in 20% more power.

To the OP, it all depends on the air intake design, obviously it varies massively too. More pressure = more fuel = more power though
 
Simon said:
Only 20%?! Thats a massive amount of ram air effect and would result in 20% more power.

To the OP, it all depends on the air intake design, obviously it varies massively too. More pressure = more fuel = more power though
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Yeah I was meaning in the best possible case scenario you can get 20%, which a road car is going to get no where near that.
 
I notice this effect in my golf mk1 amazingly!

If I neutral it at 70, it idles at about 1100, if I then come off at sit stationary, It drops to 900ish!

Always thought it must be colder air or something like that to cause it, weird really :)
 
I remember doing quite a lot of reading on the ram-air effect for a big thread when I used to use the Peugeot GTi boards when I had my 106, even down to reading bit of a few papers on aerodynamics.

Ram air is all but useless on a road car - it has no effect at legal road speeds, and only really a negligible effect at 150+mph. Potentially then, that's why road bikes have factory fitted "ram air" intakes. But they're not big wide scoops as everyone sems to think.

The traditional view of a ram-air scoop is a fallacy. A big wide mouth, tapering down into a narrow intake is all but useless. It doesn't force more air into the intake at all. So, big scoops like this intake, extremely common in the peugeot scene are not really worth it:

peugeotramair.jpg


What is better is a narrow air intake, with a slightly wider pipe opening out behind it. The air entering the narrow intake rapidly expands to fill the wider pipe, creating lower pressure and sucking air in at a much higher pressure than any extra air can be "rammed" into a wider scoop.

When air-box regulations were free in F1, you can see reasonably clearly on the Ferrari that there's a tall but narrow air intake. Look at the shine on the green and red stripes around the side of the airbox, and you can see the bulge in the airbox:

Ferrari312T.jpg


(can open, worms everywhere!)
 
Flukester said:
i know sports bikes do as most use ramair (great hole in front of fairing) to ram cold air straight into airbox. Don't think there are many stock cars that have a airbox setup like this though ?

manufacturers of big bikes claim >10bhp at high speed with this.
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what he said. ram air is very effective on bikes as airbox size is limited. on a car this is increased in size hence the ramair effect having little effect however you do see people with quad lights removing 1 for this very purpose.
 
I'm sure there was an article in MCN about the RamAir effect.
It did give more power at top speed, but they couldn't go very big on the air duct because the extra drag it causes would negate the extra power it would give.
 
mjt said:
all speedos over read, so when you're doing 120, you're probably doing around 110mph, maybe even less..

i got my leon to an indicated 230km/h (143mph) down a long hill in germany, even though the top speed is supposed to be 211 (132)..

having more power at higher speed depends on where the power is in the rev range. mine was at around 4000 rpm (diesel), so there really wasn't a lot of power there ;)

attaining such a speed on a flat would have taken a week..
Click to expand...

For some reason my current TT seems to be absolutely spot on, ive tested it at various speeds with a tomtom and also a garman device and it matches exactly, even with those safety signs that have the speed that flashes up, every other car ive driven and still drive show about 32-33 on speedo for the sign to flash up 30, my tt shows 30, bit bizzare why its the only car ive driven that shows it spot on?
 
Bradmax57 said:
For some reason my current TT seems to be absolutely spot on, ive tested it at various speeds with a tomtom and also a garman device and it matches exactly, even with those safety signs that have the speed that flashes up, every other car ive driven and still drive show about 32-33 on speedo for the sign to flash up 30, my tt shows 30, bit bizzare why its the only car ive driven that shows it spot on?
Click to expand...

i also seem to remember ducati and aprillias as being spot on. my honda on the other hand. 140mph+ and i get done for 128mph?? bugger.
 
RAM AIR

Operating Conditions
Temperature = Tair = 20C = 293K
Atm. pressure = Pair = 14.7PSIA
Cpair = 1005J/KgK
K = 1.4
The Cpair and K are constants for air.

Case 1 @ 100Kmh (62Mph) = 27.78m/s

Calculating temperature of the ram air

Tramair = ((Vcar^2/2gc)/cp) + Tair
Tramair = (((27.78m/s)^2/2(1kgm/Ns^2)/1005J/kg) + 293K
Tramair = 293.4K

The temperature increased by 0.4K or 0.4C.

Pram = Pair (Tram/Tair)^(k/(k-1))
Pram = 14.7PSIA (293.4K/293K)^(1.4/(1.4-1))
Pram = 14.75PSIA - 14.7PSIA
Pram = 0.05PSIG (gauge pressure)

So as you can see driving 100kmh will only have a gain of 0.05 psi! now lets try for 200kmh.

Case 2 @ 200Kmh (124Mph) = 55.5m/s

Calculating temperature of the ram air

Tramair = ((Vcar^2/2gc)/cp) + Tair
Tramair = (((55.5m/s)^2/2(1kgm/Ns^2)/1005J/kg) + 293K
Tramair = 294.5K

The temperature increased by 1.5K or 1.5C.

Pram = Pair (Tram/Tair)^(k/(k-1))
Pram = 14.7PSIA (294.5K/293K)^(1.4/(1.4-1))
Pram = 14.97PSIA - 14.7PSIA
Pram = 0.27PSIG (gauge pressure)

By seeing how the velocity of the car increases the ram air effect...it is barely anything! I'll conclude by saying that in racing circles where a 1/100th of a second counts, it's worth it, but don't let Pontiac fool you into thinking it works on the street.

(stolen from http://hondaswap.com/forced-induction/myths-electric-turbos-ram-air-41316/)
 
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