Marc Shaw

Okay, so after a recent conversation with someone regarding the use of either Cup (open) or drilled airbox versus a stock ("snorkel") airbox, I did an experiment today.

I went for a drive with the stock airbox with the Hammer measuring intake air temperatures ("IATS" on the Hammer menu). I then sat and let it idle for a bit to see the temperature changes. I then swapped out the stock airbox for a Fabspeed Cup airbox, did an "idle stabilization" with the Hammer, and did some more driving and idling.

I did this twice, driving the same road each time, all within the same 1 hour period in which the ambient temperature did not change. While driving, I maintained a steady 115 km/hr (about 70 mph) running at about 3500 rpm in 4th gear (with the spoiler up each time). I did not note the intake air temperature until I had been driving for about 15 minutes each time to ensure it had a chance to stabilize. I noted the temperatures while idling at the noted times from stopping (and the spoiler was down each time).

The results are below: Ambient air temperature: 26°C/79°F

Stock airbox
Driving:
30°C/86°F @ 15 minutes
Idling:
31°C/88°F @ 1 minute
34°C/93°F @ 4 minutes
35°C/95°F @ 5 minutes
36°C/97°F @ 6 minutes

Cup airbox
Driving:
34°C/93°F @ 15 minutes
Idling:
37°C/99°F @ 1 minute
41°C/106°F @ 4 minutes
42°C/108°F @ 5 minutes
44°C/111°F @ 6 minutes

I only reported one set of temperatures above because as soon as I put the stock airbox back on, the temperatures dropped back down and were exactly the same - replacing the airbox with the Cup one again did the same thing as the temperatures went right back up again.

Air intake temperatures matter, of course, as cold air is more dense and therefore give more power. The engine compartment of an automobile is a very hot place, with heat radiating off of the engine block and head, the exhaust pipe, and air passing through the engine cooling fan. It is very easy to see that any performance gain from reducing restrictions of air flow is quickly lost from increased intake air temperature.

So, I guess the question would be: is the improvement in induction noise at WOT worth the potential loss in power?

Marc
p.s. and I can only guess what a cone filter tucked under the rear window shelf in the engine bay would be like.

Heirsh

Nice Marc

Thank you for your efforts!

jeff522

That is really cool (I know bad pun).I have a cone filter with a MFS which sits almost at the firewall.Sounds like its time to move it into the rs 3.8 tail for some cooler air.Thanks Marc for your work and effort on the intake temp. This is what makes Rennlist so great

Chad T

Interesting data. Those temps are significantly higher. The snorkle on the stock box also doesn't appear very restrictive to me. Another thing with a cone filter is that they have rubber ends on them which would tend to hold heat longer. Might be interseting to see if the idle temps are any different with the wing up.

deep_uv

Great study Marc!

I too have pondered this subject and I have a few questions and observations to throw in to you and the other gurus of the group.

"Air intake temperatures matter, of course, as cold air is more dense and therefore give more power."

I noticed that the temperature difference when driving was only about 7 degrees F. Does the 7 degree increase in temperature outweigh the increased air flow in terms of horsepower/perfromance?

What's the ratio?

Is it linear?

Can you measure intake airflow CFM with the Hammer?

How do ambient temperatures affect the readings?

What happens to the idle temperatures with the spoiler up?

How quickly do the idle temperatures return to the driving temperatures once you go from idling to driving?

I wish I had a hammer so I could go find these answers........

Steve

pete000

More air flow VS cooler air. Not sure is better for power? Probably a wash... I use the stock air box with the snorkle, but cut a hole in the side to allow more flow. The Fabspeed lid has no snorkle and might not catch the cooler air coming in the lid opening. Nice study BTW...

Chuck Jones

Marc and Steve: GREAT study...equally great followup questions. Yes, this IS what makes this forum so relevant, informative, and helpful/useful. I will be interested in reading about replies to Steve's followup questions....I hope you indulge us Marc.

Regards, chuck

Chris M.

Great info Marc. This is exactly why I kept my snorkel like Pete above. Hopefully we're still catching some cool air.

c

Red rooster

Marc,
Excellent experiment. Good to see that stuff quantified. In terms of airflow capability the snorkel will comfotably flow all the air that a 3.6 motor requires.

In motor sport intake restricters are used to limit bhp. Numbers like 34mm - about 1 3/8 inches for our cousins down south - are used to limit larger motors.

The sealed airbox also acts as an air supply reservoir to smooth air demand pulses.

One possibility that I have never followed up is the use of the extended snorkel as used on the 993TT . On that car the air is pulled directly from the grill to avoid intercooler heated air .

All good stuff and well worth that 5 minute chat.

All the best

Geoff

Marc Shaw

I did a Google search and the ratio of air intake temperature to hp changes seems to vary from 1 hp for every 4°F to 1 hp for every 10°F. I don't know if it is linear or not - perhaps someone with more knowledge/experience can chime in.

No.

I don't know but, time permitting, I'll do the same experiment a few more times when the ambient temperature is different to see.

I thought of this but did not test it (I was already late for supper when my "5 minute" drive became over an hour long).

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.

I'll post pictures later but, if you look at the air inlet AFTER the airbox (ie. right before the AFM), it is smaller than the diameter of the snorkel. I doubt that there is more airflow with air holes of a Cup/drilled airbox as the snokel is not the limiting factor, the air intake tube exiting the airbox is.

That will be my next DIY project.

Yes, Geoff is the person that I had spoken to so gets full credit for the idea of the experiment and well as the rationale behind it - I just did the legwork.

Marc

agentpennypacker

Very interesting. How about using your cone filter and building a small heat shield (rounded?)going from the open part of the decklide, under the cone filter, and back to the fire wall?

Red rooster

Marc,
Dont belittle your tests !! You went out and attatched real numbers to something I and many others sort of knew to be true !

By my reckoning intake air temperature translates into density .So if the mass of air translates into bhp , then 1 degree C air temperature change equals about 0.3% bhp.

new temperature (abs )
----------------------------- - 1 = density ratio
old temperature (abs)

( -273 degree C = Absolute 0)
---------------------------------------------

For example 30 to 40 deg C

313/303 -1 = 0.03 = 3 %

So a 10 degree C rise in air temperature means a loss of 3 % power !

Maybe a physics guy out there can say thats crap , but it looks OK so far!

Makes the "extra " power from an open filter with its temperature problems look silly !!

Ah well . Enough of the summs but it is food for thought ?

All the best
Geoff

Geoffrey

Air density is a ratio and can be expressed by 2.7 * Aboslute pressure / Absolute Temperature. So, lets make some assumptions and say that we are at sea level which is 101.3kPa or 14.7psi absolute pressure. Lets deal with just the driving at 70 mph 86f vs 93f. So, the density at 86f is 2.7 * 14.7 / (460R + 86F) = .0727 lb/cubic ft of airflow. Density at 93F is .0718. Now, if we use a constant cfm we can find out the mass flow and determine the difference. Lets assume 100%ve, then a 3.6l engine operating at 3000rpm (cruise) requires 190.7cfm of air. the massflow is 190.7 * .0727 = 13.86lbs/cubic ft at 86F and 13.69lbs/cubic ft at 93F. So, the difference in temp and associated air density is fairly small and will result in a slight power difference, probably measureable on an engine dyno, but probably not the butt dyno. If we knew the size of the injector and the air fuel ratio, we could determine th injector pulsewidth in MS.

TroyN

Very cool stuff! Now to dig out my stock airbox cover and pack up my cup-style cover...

Robin 993DX

I did this same test several years ago on my 993, which was easier to do with the OBD-2 reader hooked up to a laptop.

Here is the link to the page
http://p-car.com/diy/intake/