With the help of my lovely wife, I have made the air pressure measurements on my 914 at speed. The reference point was the ashtray in the dash. The ends of both hoses were buried in the center of 2" blocks of open cell foam to prevent air movement from affecting the pressure readings. I realize the reference point is not going to be at exactly zero, but as a reference it will show pressure diffrences between different parts of the car. The windows were rolled up, and the targa top was on. Here are the numbers, in kilopascals:

at 45 MPH

front center of front bumper: 0.21
middle top of front trunk lid: 0.03
inside front of front wheel well: 0.00
middle top of roll bar: -0.03
middle of rear window: -0.03
inside engine compartment (coasting at idle): -0.01
inside rear wheel well: 0.02

at 65 MPH

front center of front bumper: 0.44
middle top of front trunk lid: 0.02
inside front of front wheel well: 0.01
middle top of roll bar: -0.05
middle of rear window: -0.04
inside engine compartment (coasting at idle): -0.03
inside rear wheel well: 0.03

After our last measurement, we rolled down the driver's side window and drove at 45 MPH to see what difference that made on the reference point. The rear wheel well measurement went up from 0.02 to 0.07, which means that at 45 MPH the pressure at the ashtray is 0.05 kPa lower with the driver's side window down.

Whelp, there's my little numerical contribution to the 914 community.

--Royce

so, if i read your numbers right, there's air being sucked OUT of the engine compartement at speed!

interesting

can you please pinpoint the exact spot you used on the front hood.
Andy

Open up the lights!

I think you will be suprised.



M

Isn't that what the engine cooling fan is supposed to do?

What is the conversion/relation between kilopascals and mmHg or PSI or Atms.?
The impeller can generate a neg. pressure, but that also limits the flow. Where in the engine compartment was the sensor?
The rear roll bar and window has a neg. pressure enviroment as well. Thus, opening the engine lid to the GT style would open the impeller to a greater neg. pressure enviroment. That would suggest the need for a positive pressure cold air intake.
Deserves a classic post status.
L. McC

Bondo, great data. Thanks for doing this.

I misread those numbers at first. I'm used to seeing absolute numbers in kPa from MAP sensors, but those are obviously from the reference.

The converstion is 100kPa = 1bar = 14.5psi. In the data above, 0 would be "equal to atmospheric". -0.05 would mean 0.7psi below atmospheric. 0.21 is 3psi above.

On the wheel well measurements, was the sensor in front of, above, or behind the tire? It's interesting that the pressure is positive there.

I'm not surprised the engine compartment is negative. The negative area above it from the back window won't help airflow in, the fan will be working hard to blow air into the space below the tin, and the engine is consuming a goodly amount of air, too. Ducting air from one of the rear wheel wells to the engine intake should help power some.

It would be interesting to know the pressure under the car at these speeds. The relative pressure between the engine bay and the underside of the car affects the cooling fan volume.

Smoothing the bottom to reduce drag also increases air velocity and reduces pressure. Modern performance cars attempt to do this. And of course racing cars dsigners go to great lengths to tune the underbody for maximum downforce from either flat bottom or venturi designs. The bottom of a 914 is fairly rough so I would guess flow is probably turbulent and pressureis higher relative to the engine bay. Just a guess!

Ducting air from the wheel well also depends on temperature. Temp need to be taken inthe wheel well and in any ducting. Also that ducting will impact the intake tuning (if that exists). The possibility of ducting cool air at ambient pressure is certainly attractive.

Dave

I just realized I made a mistake with the conversion numbers above. The conversion factor of 100kPa = 14.5psi is correct, my later numbers, however, were not. 0.21kPa is not 3psi, it's 0.03psi. This looks suspicious. That would mean the pressure rise at the front bumper at 65mph is only 0.6psi, which seems very low.

If the numbers are not in kPa, but in % of 1 bar (so the 0.44 figure, but rather 0.44bar), then 0.44bar = 6psi. If the numbers are in mPa (mega pascals), then it would be 60psi at the front bumper. This would also imply at that engine bay is at 0.4psi below atmospheric (which sounds about right) if in %/1bar, or 4psi if mPa (which sounds high and, if true, then ducting the intake would make a BIG difference).

We sure of those units?

I'm sure of the units, but the reference point is probably not at atmospheric pressure. The numbers can't tell you what pressure is at a certain place, only the difference in pressure between measured points.

As for air being sucked out of the engine bay, that can't be known without measuring the pressure at the underside of the car, which I didn't do. I would think the pressure must be lower down there or the porsche designers would have made the cooling fan suck air from underneath and blow it out the top. The engine compartment measurement was taken next to the air cleaner, on the driver's side. There was no rain tray.

One of the possible explanations for the front wheel well measurement being higher than expected is I have no front valence. (it got caught on a curb and pretzelized a couple months ago) Sorry my 914 is such a beater I would imagine that you could lower those numbers even further with a an LE spoiler or something like that.


--Royce

Oh, and the point on the front hood is the approxamate center of the hood, as eyeballed by me.

--Royce

i'm interested in the device - it should also be possible to use a sealed chamber as a reference and get absolute numbers relative to 1atm.

Anyone got a wind tunnel?

I'm getting really interested in this stuff... :-)

Mueller: do you reckon we can build one ;-) lol.

Fiid.

But the numbers seem at least 10x too low. I just can't swallow 65mph against the front bumper only generating a 0.06psi increase in pressure. Either the pressure in the ashtray was changing a lot more with speed than I'd think it would, or something else is wrong. I wonder if the foam threw off the results?

I think I'll build something using a MAP sensor that reads above 1 bar. I'm pretty sure I have an MPX250 in the Megasquirt kit I've not assembled, which should be 2.5bar absolute, or 1.5bar above atmospheric.

fiid:

Go to the airport and see if they will let you park your teener behind a jet waiting to take off - you'll get a 300 MPH wind tunnel effect (if your teener stays glued to the ground).

I see it now, aerodymanic kits for the teener that make it look like a rice rocket. Also, you are not allowed to paint them and the BURSCH must be chrome and round.

It is interesting data though.

Ok, I'll

I'm not entirely sure that most of the stuff you see on rice rockets has anything to do with real aerodynamics. I'm most interested in building a venturi tray for the underside of the rear than can actually suck air through the engine, plus increase downforce.

My favorite rice-aero trick is putting a f-off great wing on the back of a front wheel drive car. Great. At speed the rear wheels are welded to the floor and the driven and steering wheels are floating. Seriously smart.

If you had the top off the cockpit the reference was definately not zero. If you ever ride without the top with the heater on you can easily notice the increased warm air flow at speed as the high velocity air over the cockpit creates a low pressure and pulls the heated air through. If you had the top on and windows open, it probably pressurized the cockpit. I think top on, windows up, vents closed would get you closest to 1 atmosphere or 1 bar.

I like the idea of some extensive wind tunnel time. Full size with a rolling road.

Was just having a little fun.

1st - it's been around a long time and that is a full spoiler on the front of the car, riding about 1/4" off the madcam. This will create negative pressure under the car, especially if combined with side skirts. It should create a situation where air is drawn through the engine intake to underneath the car. Yeh, I know it's considered ricer but we used it on the race car (without the sideskirts)

2nd - the effectiveness of this was shown by Jim Hall on the 2J Sucker car, alabeit he had a snowmobile engine drawing air too!

3rd - For the venturi effect your talking about, I would think that a square duct from the front spoiler to a splitter underneath each engine bank with the top not sealed would create a high velocity air and at discharge create a negative pressure area below the engine. Hey, they tried it with little flaps stuck to the body.

Oh well, it is obvious I have no idea of what I am talking about - so I'll just shut up (yeh, right).

wow! great finds...

can someone send me a pic pf a stock impeller... i have an idea!

Why were the engine bay measurements taken while coasting (at idle)?
What were they at cruising speed?
What year is the 914? So we know what bumpers it has, lower engine bay flaps...

For those interested about aerodynamics (and pressure zones about the tires ) see below:
http://www.icbm.org/erkson/ttt/aerodynamics/

Okay, info is now there

My Porsche raceshop guy (they race 914s) has always told me that air flows out of the engine grill at speed vs. flowing in.

http://www.zotz.biz/_wsn/page4.html

I did them at idle to minimize the effects of the engine fan because I was purely interested in aerodynamic data.

I didn't measure the pressure with the engine at RPM.

It's a 1973 914 (both halves) and has stock bumpers and no front valence whatsoever. It does have the cooling flaps at the bottom of the firewall (minus 30 years of scraping things)

From 914TImo's website...