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914World.com _ 914World Garage _ Rear air
Posted by: worn Oct 6 2014, 08:55 PM
Looking at the muffler I have to admit there is a challenge here, but it would be nice to pull air out from beneath the car and to generate downforce without resorting to an actual spoiler. I don't see them,so am wondering if it is a cSe of beating your head against the wall. I followed after a nice example on a 944 last weekend. Engine is in the front and I don't know if it helps. Not interested in posing, I want to suck air and shoot it upwards.
Pointers would be welcome.
Posted by: Elliot Cannon Oct 7 2014, 01:18 AM
If you want true down force at the rear, you need a wing. A spoiler decays lift and doesn't necessarily provide down force. It's been my experience that the 914 (at least mine) at speed needs more down force in the front. Diffusers I know absolutely nothing about. 
Posted by: maf914 Oct 7 2014, 06:02 AM
There is an article in the October 2014 issue of Excellence magazine with a nice orange 1977 air cooled 911 set up for track days. It has a full rear under floor with diffuser. It looks good, but the article doesn't really go into detail about the performance of the diffuser or cooling issues with the engine.
I remember reading that the Porsche factory and Porsche customer teams had to make compromises with the 956/962 cars under floors and diffusers. Completely enclosed for max down force during qualifying or open louvers to allow airflow for engine cooling during the race. The air cooled engine configuration, as well as the flat six arrangement, was a liability. With water cooled engines the heat could be managed in other ways without compromising the diffuser and under floor.
Posted by: worn Oct 7 2014, 08:10 AM
There is an article in the October 2014 issue of Excellence magazine with a nice orange 1977 air cooled 911 set up for track days. It has a full rear under floor with diffuser. It looks good, but the article doesn't really go into detail about the performance of the diffuser or cooling issues with the engine.
I remember reading that the Porsche factory and Porsche customer teams had to make compromises with the 956/962 cars under floors and diffusers. Completely enclosed for max down force during qualifying or open louvers to allow airflow for engine cooling during the race. The air cooled engine configuration, as well as the flat six arrangement, was a liability. With water cooled engines the heat could be managed in other ways without compromising the diffuser and under floor.
Yeah, that was sort of my thinking, the air has to flow from top down in the engine and that is that. So I am guessing a good front air dam would be more effective overall. Except I keep hitting them on driveways and practically anything around me.
Posted by: mgp4591 Oct 7 2014, 09:53 AM
If you want true down force at the rear, you need a wing. A spoiler decays lift and doesn't necessarily provide down force. It's been my experience that the 914 (at least mine) at speed needs more down force in the front. Diffusers I know absolutely nothing about.
I also had someone on this site claim that our cars get light in the rear over 100 mph. Mine hasn't been over 20 (until it's built) so I have no idea... any experience with this? How about ways to increase downforce in front without creating too much drag?
Posted by: stownsen914 Oct 7 2014, 11:07 AM
I put a diffuser on the back of my 914 racecar. It required a large amount of work to fit, including custom headers and exhaust, redirecting exhausted cooling air from the cylinders/heads out the side of the car instead of out the bottom (which is now covered by flat bottom and the diffuser), redirecting the shift linkage, lowering the car to a silly low ride height to take advantage of underbody aero, etc., etc.
It was fun engineering it. I still need to put some data acquisition on the car to illustrate what if any benefit I'm actually getting out of all this.
Scott
Posted by: Elliot Cannon Oct 7 2014, 12:07 PM
If you want true down force at the rear, you need a wing. A spoiler decays lift and doesn't necessarily provide down force. It's been my experience that the 914 (at least mine) at speed needs more down force in the front. Diffusers I know absolutely nothing about.
I also had someone on this site claim that our cars get light in the rear over 100 mph. Mine hasn't been over 20 (until it's built) so I have no idea... any experience with this? How about ways to increase downforce in front without creating too much drag?
I have absolutely no experience as a race car driver so I have very little time above 100mph. I just remember the Yellow Zonker at 125 seemed to me to be very loose in the front.
The looseness of the front end seemed to be inversely proportional to the tightness of my sphincter. I was gripping the seat cushion so tight it kind of made the five point harness redundant. 
Posted by: effutuo101 Oct 7 2014, 12:26 PM
I think Dana did one.
If memory serves, a front air dam and rear spoiler are the way to go. For the rear, you are simply pushing the cortex further back so the rear doesn't lift.
My wife maxed out my old 2.0 and had no issues with stability over the century mark.
Posted by: effutuo101 Oct 7 2014, 12:27 PM
I think max speed is what, 130 in stock config?
Posted by: mgp4591 Oct 7 2014, 12:44 PM
How about a rear underspoiler, kinda like the 944 Turbo? 
And a splitter for the front to keep that end stable...
Posted by: whatabout1 Oct 7 2014, 12:49 PM
I remember a story of a Porsche team that tried reversing the cooling fan to create down force.
Must not have worked....
Posted by: worn Oct 7 2014, 01:11 PM
If you want true down force at the rear, you need a wing. A spoiler decays lift and doesn't necessarily provide down force. It's been my experience that the 914 (at least mine) at speed needs more down force in the front. Diffusers I know absolutely nothing about.
I also had someone on this site claim that our cars get light in the rear over 100 mph. Mine hasn't been over 20 (until it's built) so I have no idea... any experience with this? How about ways to increase downforce in front without creating too much drag?
I have absolutely no experience as a race car driver so I have very little time above 100mph. I just remember the Yellow Zonker at 125 seemed to me to be very loose in the front.
The looseness of the front end seemed to be inversely proportional to the tightness of my sphincter. I was gripping the seat cushion so tight it kind of made the five point harness redundant. Well that is the kind of detailed technical data I was...well, ...Thanks Elliot!
Posted by: worn Oct 7 2014, 01:13 PM
How about a rear underspoiler, kinda like the 944 Turbo?
And a splitter for the front to keep that end stable...That was the example that had me started. Thanks.
Posted by: worn Oct 7 2014, 01:14 PM
I remember a story of a Porsche team that tried reversing the cooling fan to create down force.
Must not have worked....
Hmmm. At some point on this board there was talk of 917 fans shooting into the sky due to hub failure. So I am thinking those at least were (when working) thrusting air down.
Posted by: veekry9 Oct 7 2014, 01:24 PM
https://www.youtube.com/watch?v=9y0SMqACTyM
In a 914 the greatest drag is the house.
After this and improving induced drag,an attempt at evacuating the high pressure beneath the car is proven to be effective.
An observation of the most recent LMP cars will show the methods without the fans.
A incredibly effective method,channeling the air to the low pressure area.
Posted by: damesandhotrods Oct 7 2014, 01:41 PM
If you want downforce I’d say buy a wing and either book some wind tunnel time or buy some data acquisition equipment. Because there will be some tuning involved. Considering all of the science involved in making ground effects actually work I have a feeling any diffuser and skirts you add will only be cosmetic.
Posted by: sixnotfour Oct 7 2014, 02:04 PM
all this should help you
http://www.cassidy-online.com/porsche914/aerodynamic_aids/
anklebiters diffuser;
Attached image(s)
Posted by: veekry9 Oct 7 2014, 02:47 PM
http://www.962and904.com/
Or build a replica to get that downforce feeling.
The idea of a car with low drag and sufficient downforce at public road speed is good one.
Gravity will provide the downforce and a teardrop shape will reduce drag.
Study aerodynamics.
Posted by: worn Oct 7 2014, 04:37 PM
all this should help you
http://www.cassidy-online.com/porsche914/aerodynamic_aids/
anklebiters diffuser;
I read that page many times. What I do not get though is why the big axx spoiler produces less downforce than stock. Willing to believe but surprised.
Posted by: damesandhotrods Oct 7 2014, 05:03 PM
all this should help you
http://www.cassidy-online.com/porsche914/aerodynamic_aids/
anklebiters diffuser;
I read that page many times. What I do not get though is why the big axx spoiler produces less downforce than stock. Willing to believe but surprised.
It is very simple. The wing is in dirty airflow, and the angle of attack is probably less than optimal. A wing needs to be set up and tuned. And you can’t just stick it anywhere. Learn from teenage tuner crowd…
Posted by: mgp4591 Oct 7 2014, 09:00 PM
My understanding of a diffuser is to reduce drag by increasing the percentage of laminar airflow from under the rear of the car, effectively (after tuning and research) helping the air pull the rear end down by keeping the air attached to the surface. You also see alot of cars that have upward facing vents behind the rear wheels to help that turbulent air exit out and up, hopefully pushing the rear end down. At what speed all this work is effective I have no idea so I'd guess you'd have to look at successful example (supercar or racer) and find out from that source what works. Of course the rest of their design has to be taken into consideration also but I'd think that some copying wouldn't hurt... Any and all comments are welcome as I well may be up in the night on this. ![popcorn[1].gif](style_emoticons/default/popcorn[1].gif)
Posted by: mgp4591 Oct 7 2014, 09:13 PM
Or we can google images and results of wind tunnel testing on a brick with vortex generators installed... 
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