garrett376

The fan AND the intake would pull from the spoiler grille per what colin says... the intake is sucking so much air that it's doubtful it ever gets a chance to heat up - at least enough to make a difference.

But these hoses, etc., are probably in an area where there is less flow (less than the many hundred cfm that an intake + fan moves), so they get hot. Right at the intake air is getting sucked in at such a high rate that it's unlikely to make any difference. I'm with Geoffrey (and Jack) on this one - increased power shown on a dyno is enough for me to keep my cut open airbox! Well, I was going to keep it even without that fact! I think the intake air temp findings and conclusion is misleading for those determining whether to keep the stock box or not. Isn't a dyno result conclusive enough?

PS 951North - that C4 Widebody for sale in LA is a beauty... and a good price, too!

951North

Trust me, if I had the room... best looking 911 ever IMO.

Sorry if the NACA thing is old or previously debated, it was the first thing that I thought of to get the best of all worlds. I'm admittly new to 911 air cooled but I want to learn as it WILL be my next p-car.

JackOlsen

It's good to think outside the box. But in this case, there really isn't a problem to fix.

And NACA ducts like that were fitted to the rear fenders of early versions of Ruf's Yellowbird cars to feed air to the intercoolers... and it worked out that the 911's aero qualities pulled air out through the holes instead of letting it in. Ruf dropped the ducts as a result.

Red rooster

Garrett,

I dont want to get heavy on this stuff but it is a basic fact of engine operation that power is directly related to intake air temperature.
I havent seen anyone claim that this is not true !

Geoffrey says that he has seen a cut air box give more power- on an engine dyno ? I dont remember any figures for intake temperature.

Others, myself included, have seen intake temperatures rise in hot, normal driving conditions and be slow going back to normal.

What this is saying to us all, is that there is an issue here that needs to be understood so that the right modifications for the cars use can be made.

If you are happy that the cut box is giving more power then maybe a thorogh check on intake temperature may lead to further work to give even more !

Maybe a piece of air duct hose could be worth +10 bhp ? I guess I have spent too many years struggling for the last little bit of bhp to close my mind to potential gains !

Just my $0.02 !!

All the best

Geoff

JackOlsen

I don't want to belabor this either, but... I tried it.

The temperature difference was as great as 25 degrees in steady-RPM use. There was no noticeable power increase at all.

On the track, the temperature difference disappeared. I suspect that creating a longer 'straw' for the engine to suck air through might have been hurting output, ultimately. But we're talking about probably insignificant amounts any way you slice it.

Geoffrey

Intake air temp changes will change the air density due to pure physics and formulas posted above. The dyno information I posted above was on a 964 Cup car on a chassis dyno which has very little air moving across the engine as it is stationary with only a fan blowing in the general vicinity of the fan. I did not have a Hammer to test IATs so I can't say on that point of data.

951North

Yup I'm learning...

Red rooster

Jack,
Thats an unusual set up ! Most cone filters I have seen are on an adaptor plate
right back in the hottest area of the engine bay !! as a performance enhancer !!!

From what has been said it looks like hard track use will even out the engine bay temperatures and not be much of a problem ? I am more interested in road use where this temperature situation can be a PIA.

Guess we all have a view but at least this has been "aired " .

All the best

Geoff

JackOlsen

The K&N filter was attached to an extension piece to get it right under the grille.

I've since gone back to a more traditional setup.

NineMeister

It never ceases to amaze me how many times we end up deliberating over answers to the wrong questions......

To return to the original poser from Marc:

...so I read the actual question as "what happens to the air intake at Wide Open Throttle and does it affect power?" which I believe Geoffrey, Jack and myself have covered with the answer that under sustained full throttle intake air temperature into the engine itself is unaffected by the design of the airbox.

However, that said, I do think that anyone disagrees with Marcs findings that at part throttle and in traffic the intake air temperature increases substancially with the use of a non-shielded air intake.

Perhaps more interesting is that if you are using the standard air temperature probe in the air flow meter to measure the temperature you have a problem regarding the response speed of the sensor to air temperature fluctuations, and I have witnessed this on my dyno installation many times whilst playing around on 964's with Motec. Most sensors have the same problem because the measurement element (usually a thyristor) is normally fully shielded from the airflow within a plastic housing. I assume that Marc's measurements have been done through the "actual values" on the Bosch hammer via the Motronic ecu, so even though the numbers given will reflect the measured temperature of the sensor element in steady state, once the throttle is nailed the actual intake air temp will drop almost immediately, whereas the measured sensor temperature will invariably lag behind. I believe it is this lag that has confused the issue somewhat.


So lets get down to numbers. A few years ago whilst mapping a Motec equipped 964RS on a third party dyno installation I recall measuring a clear 10bhp power difference by moving the location of the one cooling fan that was available at the time. In the end I found that blowing air directly into the engine bay lost power because it also recirculated exhaust gas back into the intake; the best location was to put the fan at the side of the car to blow the exhaust gas away from the back end and also spill fresh air over the offside quarter panel and into the air intake.

These findings were the driving force behind the subsequent 9m dyno installation, where we replicate how the cars operate on the road by drawing hot air out from under the engine through a grille in the floor whilst also blowing fresh air down on top of the engine lid for the intercooler and air intake - i'll see if I can post a suitable photo later.

Marc Shaw

I should have said:
"So, I guess the question would be: is the improvement in induction noise at WOT worth the potential loss in power at part throttle?

I deliberately kept a steady state rpm during the testing to try to minimize other variables and only alter the airbox configuration.
Obviously, doing this type of testing at WOT would be difficult to do safely off a track-setting.

I do not have the standard MAF, but rather have the ProMox unit installed that uses a Bosche HFM5 AFM - in case that makes a difference as to how quickly the air intake temperature probe can respond to temperature changes.

From post #10 in this thread:
deep_uv, "How quickly do the idle temperatures return to the driving temperatures once you go from idling to driving?"
my reply, I did not time this exactly but it seems that air temperatures would drop back to a stable 30°C/86°F with the snorkel within about 3-4 minutes, while it took upwards of 10 minutes for them to drop back down to 34°C/93°F with the Cup airbox.

This would suggest that the air drawn into the air intake of the snorkel is preferentially drawn through the grill while the open airbox is sucking hot air for quite a while and that the blower fan does not change the air in the engine compartment every few seconds.

Marc

Geoffrey

Colin,

I can second exactly what you have seen across the pond. The 3000cfm carpet blower fans do recirculate exhaust back into the intake and you will lose power. Running the engine at 185F oil temps compared to 210F degrees will gain you power just as running cooler cylinder head temps will will gain you power. It is all about density and intake air temps are just one factor. Regarding the intake air temp sensors, there is quite a difference between normal sensors and "fast response" sensors and can be seen very clearly in a turbocharged engine. Below are two log files from similar 911 Turbo engines on different tracks, but you can get the idea. You can see the rate of change on the air temp as the engine is on boost between a normal AT sensor and a fast response AT sensor.

Red rooster

Colin,
Temperature is measured by a thermistor. A thyristor is a gate controlled power switch.
Interesting point about the thermistor time constant but Mark has a HFM5 Mass flow sensor with a fast temperature response time.
What was interesting was that Mark did his measurements on the road under real life conditions. I had seen similar/worse scenarios on chassis dynos but never bothered to follow it up with a proper road test.

Following Marks tests I am sure that road use of the 964 means that this temperature rise/power loss effect is real . I think that we have all noticed a flatness in response after a summertime traffic jam. A good solid blast is the fix, maybe in every sense !

If I could find somewhere to live where continuous full throttle was acceptable road driving maybe this whole subject wouldnt matter ! but then I dont think I would be on my own for long !!

In the end, basic physics says that the cooler the intake air , the more power.

All the best

Geoff

Steven C.

This is a very interesting question being discussed and one that whoever designed this cone air filter was thinking about. I got this with my car when I bought it in a box of spare parts. Have no idea why the guy took it off or where he even got it but it is an attempt to solve this problem. My car now has a regular K&N stuffed up by the firewall.

Chuck Jones

"It never ceases to amaze me how many times we end up deliberating over answers to the wrong questions......"

God I love it when you guys get relevant!! As a casual reader, and non-technical Porsche owner...this thread has been a great read. The subject matter, although a little more technical than the average discussions I normally see here...has been a VERY informative and interesting thread. I thank all of you who have particpated in this with your in-depth assessment of the air temp issues. You also put it in terms that we...who have slightly less than a PhD in auto mechanics....can actually understand what it is you're saying. Thanks. Chuck