Cool Air and HP Increase

[DANNOV]

My personal expertise is finace and investments: but, I admit to being an amateur engineer. I am trying to get as HP out of my 1985 3.2 Carrera as possible in DE and autocross events. There is a classic text, "The Sports Car Engine" written by Colin Campbell in 1964. He states that: "since motor racing is a warm weather sport there is everything to be gained by trying to draw carburetted air from as cool a source as possible" He goes on to calculate something called "Charles' Law" to illustrate that you can gain 1% in brake HP for every 4 degree centigrade drop in aspirated air temperature. See page 244 for those few who still have this book. He also speaks of the boost to the induction system from the forward velocity of the car, and if this air is directed into the induction system it will also lead to a further increase in HP. I am probably one of the few people who still has this book - I owned a TR-4 many years ago and also tried to get as much HP out of that beast as possible.
My question is: if I rigged a piece of flex pipe that was sealed at one end to the intake of the air filter box and the other end was placed in the air intake area of a Carrera tail, would I expect to see a HP increase. Some may say: why bother it would be small. I say it is a 20 minute $4.95 modification. Could it be that cool air ramming on a fuel injected engine has less of an effect than it has on a carburetted engine? What do you engineers say? It seems to me that some of the big old V-8 muscle cars boosted of cool air ramming. Why wouldn't this work with a 911 with the cool air pickup located in the air intake of the wing, in that location it would certainly pickup cool air and the ram effect could be significant. Some of you will note that I am the same guy who asked the question about the chin spoiler a few days ago, after reading everyone's comments I have decided to go with both the spoiler and the wing, it seems to be the safest way.
DANNOV

[Bill Verburg]

Look at the Le Mans GT1s from a few years back. The factory placed the scoop atop the green house for the ram air effect.

On a 911 there is a high pressure region around the air intake in the lid. There is also a virtual hurricane of air moving through the engine compartment, ~1500 liters/sec at 6000 rpm. As such, cool air can be found just about anywhere in the rear portion of the engine bay. This is not the case where water cooling is used. In a water cooled car there is relatively little air moving though the compartment and what air there is has been heated by its passage through the radiator. Ducting cool air in this situation is much more profitable than in the air cooled situation.
In a front engine car there is a convienient relatively high pressure area at the base of the windshield.

Ruf placed NACA ducts in the quarter panels to collect cool pressurized air.
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Bill Verburg
My Home Page My Pelican Gallery page My Porsche Owners Gallery Page


[This message has been edited by Bill Verburg (edited 10-25-2001).]

[ChrisBennet]

Originally posted by Bill Verburg:

...
Ruf placed NACA ducts in the quarter panels to collect cool pressurized air.

An interestng side note: Alois Ruf was a guest speaker at last year's Zone 1 Tech Tactics and he spoke about these ducts on the Yellow Bird. He said that where they had placed them turned out to be a low pressure area and as a result the air wound up going backwards through the intercooler and out the duct! For this reason only the original car had them.

[This message has been edited by ChrisBennet (edited 10-25-2001).]

[This message has been edited by ChrisBennet (edited 10-25-2001).]

[Superman]

I have no doubt that cooler air gives more power. But moving the intake mouth outside the engine compartment won't change that. If I start a cold engine at zero degrees, and drive it two hours in zero degrees the entire intake plenum will be zero degrees when I stop. Even the oil filler cap is cool. At 6000 rpm, the fan moves 1500 liters PER SECOND of air. Fifteen minutes after shutoff, everything's warm.

There is another reason why you might move that mouth outside the engine compartment. With the fan moving all that air into the engine, there must be negative air pressure in there.

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'83 SC

[DANNOV]

It is a good point about all of that air being ducted into the engine compartment; but, isn't all of that air ducted over the cylinders? Perhaps some gets to the air intake but the intent of the engine designer is to cool the engine and it looks like that is where most if not all the air is going. I must admit that my idea seems so simple that if it worked, every 911 would have a cool air ram installed by the factory many years ago. But, as an after thought, if the 911 engine gets so much cool air why do we have so many posts about hot running 911's.
DANNOV

[Bill Verburg]

I wondered why they stoped doing the NACA ducts, I thought it might have been money.

A lot isn't always enough! I remember some stories about IMSA racers trying to get cool air to their 911s. Bottom line was they were never able to get enough gain to make it worth while, in fact I believe that they lost power in general through those efforts.
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Bill Verburg
My Home Page My Pelican Gallery page My Porsche Owners Gallery Page




[This message has been edited by Bill Verburg (edited 10-25-2001).]

[rscupper]

There must be some advantage to inducting air - I believe most club racing rules disallow such a set-up in all but the most unlimited classes. Anybody know for sure?

[Bill Verburg]

Mike Bavaro ran the roof top ducts in his PCA GT3, ala GT1, on his old car. His new one, again PCA GT3, does not have it.
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Bill Verburg
My Home Page My Pelican Gallery page My Porsche Owners Gallery Page

[APKhaos]

Charles Law is right. As my old engineering prof sagely said "For every complex design problem there will be several answers which are simple, elegant, and wrong."
The Ram air induction method is generally considered to be incrementally valuable above around 80MPH, provided you can get the intake up past the boundary layer. Hard to do on a 911 without looking absurd.

I'm keen for improvements on my 85, as well. Does anyone knows of a good source of tuning/setup info for the Motronic DME?? I'm thinking that 15 years of wear and accumulated gunk around the inlet airflow meter be degrading performance. Someone recently posted about having detected a severe over-rich condition due to loss of airflow meter spring tension. He claimed a really significant improvement by re-tensioning the spring [there is a ratchet adjucter in the air meter], leaning out the [sealed] mixture preset from three turns factory setting to eleven turns, and adjusting the idle to suit the new settting. Sounded like trial and error.

I'd really like to get the induction side of the engine restored to ex-factory-like condition, adn would not be surprised to pick up some of the HP lost to wear and gunk over the years in the process.
Any one BTDT???
Tony K
85 Carrera

[cim]

gain 1% in brake HP for every 4 degree centigrade drop in aspirated air
temperature.....yes....but -7% for every 100ft above sea level?!......

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www.angelfire.com/va3/vw

[firecall999]

Originally posted by DANNOV:
But, as an after thought, if the 911 engine gets so much cool air why do we have so many posts about hot running 911's.
DANNOV

The oil also carries a lot of heat away. Which is why people mount oil coolers in the front. I guess the air alone isnt enough to cool the engine.
Even big micro chips require liquid cooling. a big heat sync and a fan to push or suck air over it just doesnt carry away enough heat.

Are there diminishing returns on the cooling provided by air?
i.e. the faster you move the air and the more of it you move the more cooling you get. But I assume only up to a point.


A lot of Ricers try to get a cooler air in take. A popular mod for Hyundai Coupes (Tiburon) is to mount the air filter on a long tube at the bottom of the engine bay closer to the road, where it gets no obstruction from a cooler air source. Or at least thats what the people that sell them claim.

Alex

[911pcars]

I think Campbell and Smith in their competition engine reference manuals also suggested an exit for air as it passes over the air intake. A sole entry point showed that air turbulence increases in the ram tunneling and disturbs smooth (laminar) air flow. However, I've seen modern race cars with what looks like a ram air system (and a single entry point for air), so I can't say for certain who's right as there are variables that may work on a track car, but not be so effective for the street.

I also think any ram air effect is lost as soon as you put an air filter in the induction air stream. I do agree that cooler than ambient air temperature is better for power. Notice how much better the engine runs when the weather is cool and damp? Ask someone with a turbo how much their engine loves lower charge temperatures.

Regards,
Sherwood Lee
http://www.seinesystems.com

[R22tech]

I tried it last night on my 3.2 without the airbox cover and a very low restriction filter secured in the box. I found NO difference in performance or drivability and the only change in sound that I could tell was a hissing/whistle type of sound. Seems you would have to modify the "throat" of the airbox to change the sound.

[emcon5]

Originally posted by cim:
temperature.....yes....but -7% for every 100ft above sea level?!......

Are those numbers right? If an engine loses 7%HP/100 feet, it would produce zero HP a little over 1400 feet.

Even if the 100 is supposed to be 1000 feet it seems a little suspect, as piston powered aircraft can operate above 14,000 feet.

Huh?

Tom



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82 911SC Coupe

I'm not sure of the HP losses due to reduced air density (altitude) but I am sure that any piston engined aircraft operating at those altitudes (10,000 feet and higher) would have forced induction.

[Bill Verburg]

Don't confuse theoretical benefits, which every one can agree on(they're in the book!) with practical benefits(I hooked up xyz air intake and reduced my lap times at Watkins Glen by .45sec)
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Bill Verburg
My Home Page My Pelican Gallery page My Porsche Owners Gallery Page

[5axis]

Take a look at a GT2 wing. Big air inlets on each side. Maybe Rarly has done some internal ducting to get the cold air to the intake??

[RarlyL8]

Yes, the ducts are functional. I didn't chime in earlier because this is a major modification for very little return.
The ram air effect is real and it can be fealt in the seat-of-the-pants dyno, especially on a cool day when the engine is hot.
Hot oil + cool inlet air = HP
Obviously the increase in power isn't much, but enough to make the engine feel more "alive".
When the 3.3 turbo is installed the ductwork will go to the intercooler instead of the air intake.

[911pcars]

Cool air is a relative term. If the ambient air temp. is 100ºF +, there isn't any cool air; maybe just cooler air. Excuse me for the shameless commerce, but this guarantees "cooler" than ambient air for whatever purpose:



Sherwood Lee
www.seinesystems.com
http://members.rennlist.org/911pcars

[masraum]

Hmm, my thoughts
1 I think the engine compartment of the 911 is already full of cool near ambient temperature air due to the fan sucking the air into the compartment.
1a You probably don't want rain/water getting forced into the engine due to your cold air mod. I would avoid this unless you never drive in the rain and you always remove/block it off when you wash the car.
2 Forced air induction is something different altogether than cold air induction.
3 Actually loosing 7% per 100ft (which sounds loff to me, but what do I know) wouldnt' result in 0 power at 1400ft. It would actually never go to 0, because the figure for the 7% would decrease as the hp decreased, and forced air induction would counteract that phenomenon.
4 911 engines are called air cooled, but in reality they are oil cooled. Maybe the earliest models were air cooled, but for a long time they have had oil coolers of some sort. Most of the heat from the oil will be lost outside of the engine compartment.

Just my thoughts, definitely no expert

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Steve
Rennlist Member
PCA Member
'88 targa