The Radium King

i think the turbos got vented fender liners as well, with the gt2 (introduced mid-way through the 996 model run). if you have a c4s you got turbo bodywork.

Macster

If the air got trapped the engine coolant temperature would rise even in less than 100F temps.

The air doesn't get trapped. It flows through radiator duct and condenser and radiator and exits where ever the exit is, under the just ahead of the front wheel well, or perhaps into the wheel well.

I've driven my 2002 Boxster with its less than optimum aerodynamics and with its radiator air exits just ahead of the front wheel wells and in 116F temps and in triple digit temps many times. While the coolant temperatue gets up to 226F it doesn't get any hotter than that.

texcwa

Thanks for the response Macster

rs10

I'm thinking yes, but ...

Wind tunnels can't be beat for design work. You can't hold smoke generators in front of a car going 150. And you couldn't see very well where the smoke was going over, under, around and through the car even if you could.

But no matter how much time you spend in a wind tunnel, you never know for sure how well the aero will work until you drive the car. For this and other reasons, final aero adjustments on racecars are made at the track.

On the other hand, the aero setup that minimizes laptimes might not be the best for street cars, where higher priority is usually given to fuel economy and top speed that to downforce. I certaintly don't want all the downforce I could get if I chose a GT2 bumper, wing, and lip spoiler, even if I do want more than I have.

However, while sending the Stig out on track to tweak the aero is not the right way for a street car, a good driver can confirm whather an aero mod produces significantly less lift in the front, rear, or both, as well as whether it leaves the car well balanced. Though regarding balance, what feels right on a typical track with speeds reached there might feel quite nervous at 177 mph.

So while I'd love to see wind tunnel data, I'm in principle willing to make aero mods without it. On the other hand, I'd be a lot more confortable if 10 drivers say "I've tried in on track, I've tried it on the autobahn, I'm a professional race driver, etc." than if just one driver whom I don't know says I tried it and it works. And so far, I don't think I've seen anyone say even that.

Surely someone on the forum has done this mod?

rs10

I'm a bit confused by the question marks at the end of the sentence. Does your 996 C4S definitely vent to the wheel wells?

If so, that seems like further reason to be comfortable with trying this on a Carrera. However, it could be that this works with the very similar C4S, Turbo, and GT2 front bumpers (where for some of the incoming air, the downward path would otherwise be even longer), but not on Carreras. Anyone know how the air is vented on a 40AE?

I can't remember, did someone say Porsche did this on one (or more) of the 996 GT3 variants? Or just that some owners of GT3 race cars did?

rs10

I know this is a bit off topic, but what are these about? Perhaps the first is an improvement which was introduced with the 996.2? Or ... ?

Slakker

You are missing the point. If you are able to take a high speed corner on a race track at 90 mph with 1.4g's and a 7 degree slip angle prior to the aero change and you are able to take it at 92 mph with 1.4g's and a 5 degree slip angle afterwards then you know it had a positive effect. This is easily calculated with most club level data systems. I have my doubts as to whether it would achieve a measurable difference but if it did you would be able to log it.

The other place you would see a difference is if you had a heat soak issue and it seemed to improve with this modification. But the stock brakes are amazingly good on the C2 and I have yet to see this as a problem that needs fixing.

wildbilly32

Yes it does definitely vent to the wheel wells.

TRK seemed to be indicating that 996's did not have radiators vented into the wheel wells. I was simply pointing out that some do indeed have that arrangement. He is correct that the C4S does have the turbo front bumper/ radiator air ducting. The front end on these cars are different and, therefore, I have no clue what impact a modification to a C2 would have on performance- cooling or downforce.

As has been said many times..."May the force be with you..."(my apologies!)

docmirror

As has been said, aero is tricky. Also, after testing in a wind tunnel where everything is controlled then the car is tested and tweaked on the track.

Cool real story from decades ago at Nascar: at one race, Petty lost his rear bumper during the race and noticed a significant speed improvement. Of course, Nascar wouldn't let him remove it from the car, so for the next couple races they would bolt the bumper on with a couple of soft brass bolts, and he would get accidentally 'bumped' so the bumper would fall off. Once off, they did not have to replace it, and he would get a nice speed bump. Tricky.....

Fairly low tech way to test a reduction in drag, but not downforce is to get a sensitive manifold pressure gauge like found in many aircraft. Do a test run and carefully note rpm, and manifold pressure(which can be easily translated into HP) during a baseline run. Make your mod, don't change anything else like add fuel, change tire pressure, etc and then make another test run matching rpm, and note any change in MP. A higher value in inches of Hg will indicate a less open throttle plate, and a reduction in torque and HP required under the same test conditions as the baseline test. The downside to this test is that many aero mods don't have a marked improvement until the car is moving fairly fast(since drag increases at the square of speed), so quite often the throttle plate will be nearly full open. Lower gear is best to reduce MP while keeping the rpm high as possible. So, if you can do the speed in fifth gear and also in sixth gear, it would be better in fifth to check MP delta.

This is not an exact method, but I've used it before on street stock cars when designing the cooling ducts and belly pans. You need a sensitive MP gauge, and a digital unit would be ideal.

rs10

Thanks for confirming that. Does your car also have the ducts that hang down in front of the wheel wells, where the air from the radiators is vented from a normal Carrera, or did Porsche close them up?

rs10

There are also ways to measure lift and downforce. Google will turn up a few. And I've been thinking about one which might work well with our cars.

996s with Xenon headlights have ride height sensors. Definitely at the front axle, probably at the back too. If one could tap into the data coming from these sensors, one could both determine how much lift one is getting front and rear, and also how different modifications affect that. And at least on some cars, it is possible to tap into the data. Here’s a link where someone did it with a Lexus: http://www.autospeed.com/cms/article.html?&A=113225

There is a simple way to translate changes in ride height into kg of lift or downforce. Put some weight between the wheels on one axle (e.g. in the frunk or behind the rear seats) and measure how much the height changes. This tells you how many kg are needed to move the height by X mm, so if you know the car rises Y mm at speed, you can calculate how much lift you are generating.

I believe there are also easier ways to test for an increase or decrease in drag from some aero change. The simplest, but not easiest to do, is seeing if it impacts vmax. One can also test changes in acceleration in 6th, e.g. from 90 to 110. Or changes in coast down time between similar speeds. (Though with the manifold pressure method, it may be easier to quantify the change in drag in terms of hp equivalent, or even the change in CdA.)

Anyway, it would be great if someone could figure out how to tap into, read, and save the ride height data. If so, I’d happily do this wheelwell mod, test it at Autobahn speeds, and report back in. I'd probably test another mod or two as well.

wildbilly32

Car not in air but there does appear to be an opening directly behind the rad fan in front of wheel wells to the bottom of the bumper cover. I do know when fans are running the best place to check for air flow is at the center of the leading edge of the front fender well coming from the louvers in the well.

Another thing of interest is on the parts list the 4S and the anniversary edition show four "air flaps" at the back of the fan air box. There are no similar listings for C2's. (?)

I think this discussion is above my pay grade!

The Radium King

hey rs10, you appear to be a technically-minded fellow, so here is a link ...

http://www.autoatlanta.com/porsche-p...section=801-75

this is the porsche online parts diagrams, provided by auto atlanta so that there is a price, and sometimes an image, tied to each item. i've no affiliation, but find it a readily-available source of parts info (and one of the places i stop when i am price-checking between sunset, suncoast, pelican, the local dealer, etc.). the particular page i am pointing you to is the 911-996-gt3-body-fender liners diagram. if you look at fender liners for the gt3 you'll see that they change mid-way through the model run. if you search the parts numbers, you will find images of each - the early ones are not vented, the later ones are. this was done with no significant change to the front bumper cover/aero (ie, the 996 gt3/aerokit unit). a similar search of other cars will show venting for the turbo, c4s, AE, and all 9x7 cars.

you asked about tensioners, tensioner pads, and oil pistons. you can find all these parts on the parts diagrams, and all have been updated. the m96 has an issue with chain slap on start-up. as an overhead cam engine, the crank is tied to the cams with three or five chains. these chains are held in place with metal tensioner ramps. the ramps are protected from the chains by plastic tensioner pads. the pads/ramps are pressed tight against the chains by chain tensioners and variocam actuators (depending on which chain we are talking about). when the m96 starts up, due to a number of reasons (light cold-weight oil drained down, no chain tension) the chains slap against the pads until oil pressure comes up (search 'start up rattle'). check your oil filter; there will most likely brown plastic bits - pieces of your tensioner pads bashed off by slapping chains. to fix this, porsche (a) made the plastic tensioner pads harder, (b) made the tensioners stiffer, and (c) updated a piston in the oil pump (the piston is a spring-loaded mechanism in the oil pump designed to bring oil pressure up quickly; the early piston was squared at the end and said to bind - the new unit is slightly beveled). the piston is a $5 piece that takes 5 minutes with your next oil change, so whether you believe this internet wisdom or not it is easy peace of mind. the three tensioners are $150 each but can be done without opening up or dropping the engine. and the pads, well, they are a big job that can be done with the engine in, but require opening up the heads. get a durametric, track you cam deviation, and change them when things get to +/- 9 (this is the deviation that triggers a cel). and start making your 'while you are in there' list - variocam actuators (any green bits in you oil?) chains, lifters, etc.

rs10

Thanks for more very helpful info! Shame I didn't know about the tensioners and oil pisten earlier, as some work was done in that area when replacing the IMS bearing! Perhaps we did replace a tentioner or two, I'll have to look.

Regarding the fenders, I googled the part number for the later GT3 part (99650412391), and while some vented images did turn up, when I was able to identify what car they are from, they turned out to be a different part, from the turbo. On the other hand, I found three photos that were apparently this later GT3 part, and none were vented. Did you find any images of a vented GT3 part?

If it's just the Turbo (and all Turbos, not just later ones), then the Turbo was the first 911 (or at least first 996) with the air vented into the wheel wells, and I'm wondering why - and also why they didn't repeat this on the 996.2 Carrera. We've already discussed the idea that it works well with the bumper. On the other hand, could it be to help warm up the PCCBs? This is inconsistent with the vents being on the C4S, but that might be because they fit other parts carried over from the Turbo (e.g. the bumper). (Though PCCBs were probably an option on the GT3 MkII, no?) Or it just could be that they had better things to do than to design a new wheel well part for the 996.2 and GT3 MkII.

The Radium King

here's a link to a late gt3 liner:

https://www.stoddard.com/99650412391.html

and here is the image:



you can see where the air normally vents downwards is blocked, replaced by a cutout (not vents) on the inside of the fender liner. trk out.