How to dyno a 996?

Over the weekend I had my 996 dynoed, but had a few problems. It was dynoed on an above ground Dynojet. When we started, we had the engine lid open to connect the tack signal. In the shop, the dyno is back against an overhead door. The door was closed. On the first and second runs, the car onlyu dynoed in the 230's with hitting peak and stopping to pull anymore power at 5500 rpms. As a test on the 3rd run, I re-routed the tach signal cable and closed the engine lid. Immediately, it jumped up 10 hp on the next run; but still killing the power about 5000 rpms. For the 4th run, they opened the back door. The car jumped up in power again to 270 at 6200 rpms. Above 6200 rpms it was dropping off again and not pulling higher as it should. I believe it is an airflow issue in that it is not getting the additional airflow it woudl be getting if driven on the street. I have figured out a couple of things. First, the airbox needs the rear lid closed to direct and funnel air. Second, the rear spoiler is designed to pull air via a ram air effect from air traveling over the car and along the rear window. From this I am curious as to how other people dyno these cars? I have 2 ideas for going back to the dyno: placing a fan directed toward the back of the car on the roof or opening the airbox, propping it up and directing a fan into it.

First of all, if they don't know how to dyno the car, I'd be VERY cautious. As I'm sure that you know, dyno figures change based on many factors such as temperature, humidity, etc. Also, it must be calibrated to the car and establish maximum engine revs. Then, there is a 15% conversion factor to get from wheels to the crank. As for the results, when my car was dyno'd, the engine lid was lifted and a fan was directed to the engine. This helps are flow to keep the engine cooler (simulate driving). In the dyno pull, the car is in 3rd gear to the preset engine revs. To get the "factory specs" the dyno numbers should be around 280 hp. Steve 2003 996

Check out these links from Dinan and from Car and Driver . Changing air flow, temperature, and hood-up/down configurations can make a big difference.

What gear did you ran your car in for the max. horse power run? You should be in the 4th gear as that is the most linear gear ratio. Using any other gear will have a lower horse power reading. The suggestion of the fan on the engine bay area is a definitely a requirement, as our cars is rear engine. Opening or closing the decklid shouldn't make a difference, as I have always ran them with the decklid open and didn't have this issue with the 993s, wouldn't think the 996 would be any different. The Dynojet uses a CF (correction factor) formulation to calculate the output power, the CF is determined by altitude, temperature and etc. It is a factor that converts the actual result to a standard factor across the board. In other words the horse power output you saw on the computer is manipulated to reflect difference in environment so that you can make sense of the power output test from one run to another regardless what temp the car was Dynoed in and even on a different day (But only on the same machine, assuming they keep their Dyno machine calibrated and maintained correctly). Every brand of chassis Dyno machine uses a different CF factor and formulation to get their "justified" horse power reading. For example, "Mustang" Dynos always reads more horse power than "Dynojet" brand chassis machines, but it really doesn't mean that the car is really making that much power, just their version of the result. The nature of the chassis Dyno is for you to determine baseline vs. changes to help tunners tune a car for maximum output. What was the CF factor? Normally it should be in 1.02 range, was it a Dynojet 246C machine?

We ran all the runs in 4th. I believe it was a Dynojet 248C. The correction factor was 1.04 based on the heat and humidity. My results were before correction. As an FYI, I have dynoed many cars over time. This is the first that I've had problems with. I have been talking to a couple others who have dynoed, including Ruf Auto Centre. It has to be dynoed indeed with the lid closed and a fan directed into the rear lid to get proper results otherwise the engine won't get the same airflow it would get on the street.

I had some issues with dyno'ing my C4 as well, but I assumed they were awd drive issues. Perhaps it did have something to do with the rear lid being up....I had an odd dip in all of my graphs around 4000 rpm. There will be a nice steady (although lower than expected) curve up to the that point, then a hesistation and big dip in the chart, and then a fast pull to redline. On the street this hesistation doesn't exist... Shawn

Here is a 996 C4S Dyno on the MAHA running on Californiatastic 91 Octane. P-Wheel: Power at the wheels. P-Drag: Power loss due to drag P-Norm: Adjust Power at the Crank M-Norm: Torque in NM Ign: Ignition Timing Vair: Grams of air/sec Redline was almost 7.1k

Ditto. I also had some unusual dips. During the dyno runs, you could hear the car miss at high rpms on the dyno. On the stree no issues. I have a printout copy, but I am getting the files for the runs. I'll post the dyno after I convert the files.

Here are 2 of the runs. The red line is the first run with the lid open and the back overhead door behind the dyno closed. The blue is the last run with the engine lid closed and the back overhead door open. The temperature was 97 degrees and the humidity was 6%.

My perference and the only dyno I put any credence in is "The Great White Dyno". I and several others I have known have used it with varying success. It requires a lot of preparation, expense, dedication and patience to get your official results, but they are ones that no one will challenge. A horsepower or torque number means absolutely nothing here, only a two way average speed thru the "flyin' mile" measured within a thousanth of a second. Many have come here to test their best only to become very disappointed with the reality of what they have. This dyno does not lie! This is the place where you get sunburned on the underside of your chin, this is the Bonneville Salt Flats.

Nick, This is an interesting test, but in no way more accurate in measuring HP. This test only gives you average speed at a mile and is subject to many factors that don't exist on a dyno. At a mile, it is an abreviated top speed test. Aerodynamic forces increase at a cubed rate as speed increases. While the 2-way average cancels out head/tail winds, it does not compensate for the effects of sideways airflow on aerdynamic drag. Anyone who has experience as a pilot will quickly understand this. The greater the sidewind, the more they dynamic drag. There are other variances including lack of correction for humidity and temperature, air contamination from sand and dust effecting engine breathing, surface friction/irregularities and driver skill. Driver and surface are completely eliminated as factors on a chassis dyno. There are SAE standards to compensate for humidity and temperature. Salt flats is a great place for measuring top speed, but hardly any value at measuring HP. Mile acceleration runs are no more accurate that using the SAE power conversion formula for 1/4 acceleration runs; which is only 3% accurate. If you really want to isolate it, use an engine dyno (as governments and manufacturers for measuring rated engine power). Beyond that, a chassis dyno is next accurate (based on SAE and JIS standards). The fuzzy part of a chassis dyno is guessing the conversion from at the wheels to at the flywheel. Low powered FWD cars are around 1.2. Rear wheel drive mid-power cars are around 1.15. High torque cars like Vipers and Supercharged Mercedes V8s are around 1.10, but to measure it that accurately you need an engine dyno.

What is the so-called manufacturer HP rating right off the boat of an 01 C2? Can't find any reliable info yet. thanks!

According to my manual, net-horsepower: 296 hp/221 kw at 6800 rpm

I don't believe that is correct for an 01'. The 99'-00' were rated at 296 HP. The 01' was increased to 300 HP at the same rpms. I am not sure of the changes, but the rating was the same for US as ROW.

02.....you are incorrect. The 99 was rated at 296 and the 00 and 01 were rated at 300.

Is anyone in the DC area interested in doing some dyno runs?

Wash DC Porsche- I'm interested in dynoing my 951, it would be cool to get a bunch of people in the area and doing a dyno day.

Well, I'm very interested.