Undertray ?
01-13-2011, 05:11 PM
#17
Nordschleife Master
("Security: unlicensed videographer, display 27 - Swarm, Swarm!")
Hmmmm.... Interesting how the factory's reverted to the ol' exposed engine in this, their latest rendition of "from all we've learned"...
Reinforces my prejudice that the 964 engineers were given a mile-long leash toward indulging their efficiency & design whims. Not so much adult supervision.
01-13-2011, 05:26 PM
#18
just thought about this some more,
I have watched that video several times and having known that the watercooled cars run no engine tray, it has always made me think about a reason for the tray on the 964 and 993.
Do you think the reason its off on later models is due to the lack of air forced downwards? I mean the aircooled cars do have a massive fan forcing air down to the floor at the rear, through the engine and into the very same air that they trying to remain constant to help reduce lift. Could this have contributed to turbulent air and rear lift if not employed? Hence one of the reasons for them using something that they also knew may increase temps to some degree when driving slow?
Watercooled cars are sealed and have no negative effect to air under the rear of the car. The watercooled cars are far better for many efficiency reasons and this could be one of them.
If any of this is right, its possible the tray negates the excessive airflow force down by the fan which may contribute to lift, and it was only when no air was forced down on the watercooled cars that it could be remove, would that seem reasonable?
I have watched that video several times and having known that the watercooled cars run no engine tray, it has always made me think about a reason for the tray on the 964 and 993.
Do you think the reason its off on later models is due to the lack of air forced downwards? I mean the aircooled cars do have a massive fan forcing air down to the floor at the rear, through the engine and into the very same air that they trying to remain constant to help reduce lift. Could this have contributed to turbulent air and rear lift if not employed? Hence one of the reasons for them using something that they also knew may increase temps to some degree when driving slow?
Watercooled cars are sealed and have no negative effect to air under the rear of the car. The watercooled cars are far better for many efficiency reasons and this could be one of them.
If any of this is right, its possible the tray negates the excessive airflow force down by the fan which may contribute to lift, and it was only when no air was forced down on the watercooled cars that it could be remove, would that seem reasonable?
Last edited by Unkle; 01-14-2011 at 04:01 AM. Reason: grammar/spelling
01-13-2011, 05:41 PM
#19
Nordschleife Master
(With that, they'd have solved lift AND recaptured cooling energy as... Thrust! Hey - has anyone looked into that?... ala, P-51 radiator recapture efficiencies)
oh - nevermind.
01-13-2011, 07:06 PM
#20
Nordschleife Master
Some interesting stuff here about what the undertray is designed to do but for me the fact that removing it was an easy weight loss of 6.36 Kg sealed the deal 
01-14-2011, 03:35 PM
#21
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In my opinion, I think the undertrays primary purpose was to reduce 'Drive By Noise' a big thing back then. There was more and more legislation to comply with DBN being a new one if I remember.
Looking at all the NVH material on the side shields shows it must have been an issue.
Aren't water cooled engines by their nature quieter, which is maybe why the water cooled don't carry one.
Like Steve and a lot of others, I'm happier to run without one for weight saving as well as the fact that you can see all the oily sorry I meen shiney bits!
Looking at all the NVH material on the side shields shows it must have been an issue.
Aren't water cooled engines by their nature quieter, which is maybe why the water cooled don't carry one.
Like Steve and a lot of others, I'm happier to run without one for weight saving as well as the fact that you can see all the oily sorry I meen shiney bits!
01-14-2011, 03:53 PM
#22
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I believe bobster964 is right on. I read somewhere (I think in Adrian Streather's book) that it was specifically noise regulations in Switzerland that drove the installation of the tray. If I find it again, I'll post the page number.
01-15-2011, 04:51 AM
#24
All good reasons for leaving it on and removing it.
However does anyone know the angle of the undertray under the engine? From the floor, running front to back?
If its 7 degrees approx it would be interesting...
However does anyone know the angle of the undertray under the engine? From the floor, running front to back?
If its 7 degrees approx it would be interesting...
01-16-2011, 05:13 AM
#25
The underside of all the 964s was enclosed with floor pans and sealed. These panel run the entire length pf the chassis from the front bumper all the way to the rear.
The rear panel was designed not just to cover noise requirements but also to improve high-speed aerodynamics and to better control the directing of engine cooling air in from an air duct in the floor pan horizontally and out the rear. There is absolutely no evidence as is discussed elsewhere in this book, that the rear engine cover damages the engine in anyway or form. Unless the 964 live in a very warm and dry environment, it is recommended that this rear cover stay installed. For all the European reading this book leave it on.
The Carrera RS was also fitted with these underside panels front to rear, the difference being that the panels were not sealed. This reduced the corrosion or long life guard to three years. The Carrera RS was a road-registered race car. If there were negative issues related to the installation of the rear cover, they would have been discovered by 1992 and the rear cover would have been left off.
quoted in full and worth adding to this thread.
01-17-2011, 04:53 AM
#27
lol
Yeah i have thought the same, although it has been in use on my car for 18 years, so probably not going to be an issue now.
Thankfully we don't have to put anything in that area like a diffuser to reduce lift/wake etc like new production cars or performance manufacturers 430/360 as Porsche did it for us in a basic form. Perhaps its just pure chance that while trying to reduce noise they stumbled on a diffuser design (albeit without vertical strakes) now copied on production cars, but I'm not so sure it was by complete chance...
Interesting topic: this isn't directly related but found it on a motorsport forum and worth a read. Not sure i fully understand it all, but this general information below, the first page info and Adrians book resulted in the tray staying on for me. I just think we may have more going on under the rear of the car than a simple temp issue.
Diffuser:
it's purpose is to slow air back down to near-freestream speeds before exiting the underside of the car. If there is a big speed differential you effectively have a jet mixing with the car wake and the energy loss is huge, resulting in a much reduced mass flow in the channel formed by the underbody and the ground.
The downforce comes from two aspects.
Reducing the resistance of the underbody increases mass flow under the car. Given that the size of the channel is effectively fixed and we're well away from Mach number effects, the more air flows through the faster it has to go and, by Bernoulli's principle, the static pressure has to drop. Lower pressure under the car gives you the downforce.
Secondly, the upsweep of the diffuser means you're changing the momentum of the flow and giving it a vertical component. Action & reaction means air goes up = car goes down. More downforce.
With respect to the original question, 7-10 degrees is about the limit, depending on how smooth you can keep the floor all the way along the car, where your cooling air exits (hopefully not underneath Oops), whether you have deepend/sharpened/extended sills, what rear wing angle you're running, how close it is to the back of the car and whether you run proper endplates or strakes in the diffuser.
If you optimise all of these then 15-20 degrees may be possible. If you have a close-mounted rear wing (like on a single seater) then 30degrees may be possible (like F1).
Critical things to sort are no upstream disturbances, having a smooth transition in the floor surface between flat and inclined (no panel seams at the apex of the diffuser) and shielding it from the rear wheel wakes with deep endplates (F430 Ferrari) which come as far forward along the inside of the rear wheelarch as possible. Then it should give you a healthy of downforce (both at the rear AND at the front) but for minimal drag.
To summarise, a diffuser stuck under a car with no other aero devices will work without creating turbulence at 7 to 7.5 degrees angle. More angle MIGHT be possible, but only time in a windtunnel will allow that judgement to be made, so 7 degrees is 'safe' for everyone.
If you have a rear wing, then all that is still true, but the likelyhood is, if the package was designed together and a wind tunnel and/or accurate modeling used to come up with the design, then the greater angle you have may work without separation and going to a lower angle may reduce your downforce from the underbody, and may even upset the downforce generated by the rear wing.
(Bernoulli's Principle)
The diffuser increases the area of floor that 'sees' this low pressure by extending the low pressure area further rearward. As a result the area of the floor subjected to the lower pressure is greater so total downforce is increased.
993 GT2 Evo panels and diffuser
Yeah i have thought the same, although it has been in use on my car for 18 years, so probably not going to be an issue now.
Thankfully we don't have to put anything in that area like a diffuser to reduce lift/wake etc like new production cars or performance manufacturers 430/360 as Porsche did it for us in a basic form. Perhaps its just pure chance that while trying to reduce noise they stumbled on a diffuser design (albeit without vertical strakes) now copied on production cars, but I'm not so sure it was by complete chance...
Interesting topic: this isn't directly related but found it on a motorsport forum and worth a read. Not sure i fully understand it all, but this general information below, the first page info and Adrians book resulted in the tray staying on for me. I just think we may have more going on under the rear of the car than a simple temp issue.
Diffuser:
it's purpose is to slow air back down to near-freestream speeds before exiting the underside of the car. If there is a big speed differential you effectively have a jet mixing with the car wake and the energy loss is huge, resulting in a much reduced mass flow in the channel formed by the underbody and the ground.
The downforce comes from two aspects.
Reducing the resistance of the underbody increases mass flow under the car. Given that the size of the channel is effectively fixed and we're well away from Mach number effects, the more air flows through the faster it has to go and, by Bernoulli's principle, the static pressure has to drop. Lower pressure under the car gives you the downforce.
Secondly, the upsweep of the diffuser means you're changing the momentum of the flow and giving it a vertical component. Action & reaction means air goes up = car goes down. More downforce.
With respect to the original question, 7-10 degrees is about the limit, depending on how smooth you can keep the floor all the way along the car, where your cooling air exits (hopefully not underneath Oops), whether you have deepend/sharpened/extended sills, what rear wing angle you're running, how close it is to the back of the car and whether you run proper endplates or strakes in the diffuser.
If you optimise all of these then 15-20 degrees may be possible. If you have a close-mounted rear wing (like on a single seater) then 30degrees may be possible (like F1).
Critical things to sort are no upstream disturbances, having a smooth transition in the floor surface between flat and inclined (no panel seams at the apex of the diffuser) and shielding it from the rear wheel wakes with deep endplates (F430 Ferrari) which come as far forward along the inside of the rear wheelarch as possible. Then it should give you a healthy of downforce (both at the rear AND at the front) but for minimal drag.
To summarise, a diffuser stuck under a car with no other aero devices will work without creating turbulence at 7 to 7.5 degrees angle. More angle MIGHT be possible, but only time in a windtunnel will allow that judgement to be made, so 7 degrees is 'safe' for everyone.
If you have a rear wing, then all that is still true, but the likelyhood is, if the package was designed together and a wind tunnel and/or accurate modeling used to come up with the design, then the greater angle you have may work without separation and going to a lower angle may reduce your downforce from the underbody, and may even upset the downforce generated by the rear wing.
(Bernoulli's Principle)
The diffuser increases the area of floor that 'sees' this low pressure by extending the low pressure area further rearward. As a result the area of the floor subjected to the lower pressure is greater so total downforce is increased.
993 GT2 Evo panels and diffuser
Last edited by Unkle; 05-21-2011 at 04:20 PM.
01-17-2011, 12:39 PM
#28
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Mine is off at the recommendation of my mechanic. Acording to him, it "cooks" the engine.
In my opinion it doesn't matter one way or another. I'll never see speeds high enough for the undertray to provide any sort of downforce.
In my opinion it doesn't matter one way or another. I'll never see speeds high enough for the undertray to provide any sort of downforce.
01-27-2011, 05:23 AM
#29
I think i drive above 80mph each time i take it out (on a private road) so i think its comes into play near this speed.
Saw this in a excellent CGT thread, its interesting to see the extent of the undertrays and diffuser taken off for engine removal and cleaning, and brings to mind whether the 964 is the only sports car where owners actively remove underfloor panels? Also included a Ferrari 360 which has extended vertical strakes and also covers the engine, i wonder if the 360 owners also remove for reducing heat as this also covers the engine and if that is a concern that has been adressed. The 964 (Red circles) is that for air flow or strengthening?
Anyone got a spare wind tunnel to test the 964 with and without?
Saw this in a excellent CGT thread, its interesting to see the extent of the undertrays and diffuser taken off for engine removal and cleaning, and brings to mind whether the 964 is the only sports car where owners actively remove underfloor panels? Also included a Ferrari 360 which has extended vertical strakes and also covers the engine, i wonder if the 360 owners also remove for reducing heat as this also covers the engine and if that is a concern that has been adressed. The 964 (Red circles) is that for air flow or strengthening?
Anyone got a spare wind tunnel to test the 964 with and without?
Last edited by Unkle; 05-21-2011 at 04:20 PM.
