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This " free" horsepower sounds too easy?


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#1 mariner

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Posted 29 November 2019 - 13:42

According to this , drag racing orientated, article a big enough forward facing scoop can add up to 20% power at very high speeds.

 

https://www.enginela...cing-air-inlet/

 

It seems to go against all the "use a naca duct" advice in road racing and does'nt take account of the large scoop drag. 

 

However , ever since the Drag racing world got hold of real time propshaft torque measurement and proved a top fuel engine does give 10,000 hp pus I have been wary of decrying drag racing stuff.

 

Any thoughts?



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#2 Charlieman

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Posted 29 November 2019 - 15:30

The claim is about 30% on 3,000 hp.

 

"To do that, the math says they need close to 4,000 horsepower. Yet, stationary dynamometer tests of these engines routinely only register 3,000 horsepower or less."

 

Think about a ram duct, if such a thing exists. You are a globby thing of air in free flow. You are heading towards something like a snorkel. Do you dodge the snorkel or do you dive down the snorkel?

 

Because you are a globby thing of air, you dive for the point of lowest pressure with all of your mates ahead of you. One mate goes to the left of the snorkel, one to the right and one straight down. You and your mates behave like that because you (globby things) try to do the easiest thing. You do the easy thing -- and there is no ram effect because free air will always move away from positive pressure.

 

"With a forward-facing air scoop at lower vehicle speeds, the engine is pulling air into the scoop."

 

Aah, you have a negative pressure area.



#3 FirstnameLastname

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Posted 29 November 2019 - 22:51

The power of horses seems such an outdated way of measuring engine performance.

#4 Fat Boy

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Posted 30 November 2019 - 00:27

The power of horses seems such an outdated way of measuring engine performance.

 

There's a formal definition. If you want to work in SI units, fine. If you don't like the name, no one cares.



#5 gruntguru

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Posted 01 December 2019 - 08:03

The science of ram air has been well understood for at least 100 years. There is a fair bit of unnecessary waffle in that story.

 

Essentially the maximum possible gain in charge density is governed by Bernoulli (converting 1/2x(rho)xv^2 dynamic pressure to static pressure) and the most efficient way to achieve that (from a drag perspective) is a NACA style duct or an F1 style air horn.



#6 Allan Lupton

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Posted 01 December 2019 - 10:41

The science of ram air has been well understood for at least 100 years. There is a fair bit of unnecessary waffle in that story.

 

Essentially the maximum possible gain in charge density is governed by Bernoulli (converting 1/2x(rho)xv^2 dynamic pressure to static pressure) and the most efficient way to achieve that (from a drag perspective) is a NACA style duct or an F1 style air horn.

Unnecessary waffle is pretty usual when the non-technical try to understand an established principle.

As a former half-rho-v-sqared merchant (as we were often called) it is sometimes my lot to try to explain that it's charge density that's wanted rather than charge pressure, although, as it's only the pressure that you can measure, that's what gets recorded - and it is relevant as the gas laws show.

With supercharged engines the other difficulty is that we should be interested in the charge density (evidenced by pressure) in the cylinder as the inlet valve closes, rather than that in the inlet manifold. Realistically, we cannot measure the pressure in the cylinder, but we can and do measure the manifold pressure and whereas most folk would regard a high pressure as desirable, some would say it shows that the engine is not using the air (or air/fuel mix) at the rate it is being supplied by the supercharger and any ram recovery system.

Boring-on about other subjects available :lol:



#7 Greg Locock

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Posted 01 December 2019 - 11:05

300 mph (say) is 134 m/s, which would give a 10% boost. At best.



#8 mariner

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Posted 01 December 2019 - 14:52

Thank you, I knew it sounded too easy.

 

Mind you at practical level the huge dragster scoops seem small vs the DFV ones of the 1970's on a relative speed and BHP basis.

 

A DFV struggled to get to 500 bhp, a top fuel/funnycar engine has 10,000 bhp so 20:1. Obviously the top speed of the dragster is 50% higher and a lot of the bhp comes from oxygen released from the nitro. but  from pics the opening ratio looks more like 5-6:1.



#9 Greg Locock

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Posted 01 December 2019 - 20:04

That may be too simple a calculation, but it is in the ballpark



#10 gruntguru

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Posted 01 December 2019 - 23:34

Thank you, I knew it sounded too easy.

 

Mind you at practical level the huge dragster scoops seem small vs the DFV ones of the 1970's on a relative speed and BHP basis.

 

A DFV struggled to get to 500 bhp, a top fuel/funnycar engine has 10,000 bhp so 20:1. Obviously the top speed of the dragster is 50% higher and a lot of the bhp comes from oxygen released from the nitro. but  from pics the opening ratio looks more like 5-6:1.

Makes sense. The CSA of the opening should reflect the power of the engine. It has no effect on pressure in the intake - unless of course it is too small.



#11 Fat Boy

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Posted 03 December 2019 - 17:52

300 mph (say) is 134 m/s, which would give a 10% boost. At best.

 

5-10% is not an inconsequential gain. If that's coupled with the fact that the flow path is nice and the air it's drawing is as cool as possible (as opposed to that from near the tarmac), you're looking at a substantial HP difference. The scoop itself is generally located in a place which would be disturbed by the tail of the car anyway and it often smooths the airflow from around the cockpit.



#12 Greg Locock

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Posted 03 December 2019 - 21:27

Sure, but it isn't 20%



#13 Fat Boy

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Posted 03 December 2019 - 23:38

Sure, but it isn't 20%

 

Fair enough. It's not like a hot rod magazine to overstate things, is it? They didn't talk about the baseline much, did they?