Project RX-7

So, time to get cracking with the data I've gathered so far.

This is Excel Dyno's input screen. I've boxed in red the required input data:

The top box "Vehicle" requires all the vehicle specific information. Note that % weight on the rear wheels is not equal to the weight distribution of the car. Since this program simulates the car accelerating in a straight line, the actual % weight should also take weight transfer to the rear wheels into account. I've calculated this based on the assumption that the static weight distribution of the RX-7 is 50/50, the CG is located at 1/3 of the total height of the car and a longitudinal acceleration of 0.5G.


I couldn't find any information regarding the FD3S's frontal area. be it the 93-99 or the 99+ version. All i could find out was that it was "revised" for the face-lift. Danke very much. Since it is a vital part of the simulation (the program neglects air resistance if this data isn't provided) I've had to guesstimate it based on whatever information i could find about other cars. I am aware that this will generate a significant amount of error compared to real world results. However, since this value is kept constant throughout the comparison, it does not affect the outcome of the test.


Note: While looking for data regarding frontal area, i came across various forum threads which suggested this could be measured. Either by working on the car with a tape measure, or by heating the car's body in the oven and throwing it against a huge block of cheese to measure the imprint. While i can understand that most people don't care about how big the surface that moves the air is on their car, it's a pity the information isn't readily available to enthusiasts.


On to the engine

Excel Dyno requires the construction of power and torque curves through a table as you can see on the left. There is an option to specify whether the input data is power to the wheels or flywheel, further adding to the accuracy of the test and saving the user precious time. The problem is unless you dyno the car yourself and input the data acquired (which is probably the way the program is intended to be used) it is extremely rare to find horsepower and torque curves with values in table format on the net. Fortunately however I don't actually need a torque curve, since the program calculates torque automatically (yes, in case you didn't know horsepower is a function of torque) There are plenty of dyno charts to choose from online and one website seems to have collected a fair few of them: http://www.catenet.net/dyno.php
There is an embarrassing level of choice, from 20B to 13B engines, with varying level of modifications (all declared) and age. I tried to choose a relatively stock 13B with a little over 10psi of boost and a smooth curve, and ended up selecting this:

http://www.catenet.net/graph.php?car177=1&SUBMIT=GRAPH

which belongs to Bill Setter. Congrats Bill on the nice looking graph.

Once entered in the program, it looks like this:


The generated torque curve seems to duplicate the behavior of the stock twins very well, which is a good sign.


And now, the driveline!

Finding the gear ratios for the cars mentioned in Part 1 was (thankfully) easy. I kept the final drive ratio the same as stock (4.1) in all cases, because as i mentioned earlier, it isn't part of the gearbox but of the differential. This should level the playing field enough to declare a clear winner. Driveline efficiency was set at 80%, roughly standard for a 5/6 speed synchronized manual with trans-axle drive to the rear wheels.
Here is the list of ratios:


Next its play time!

So time to start the first massive post, or series of posts. As per the title, i'll be looking at the effects of this swap on the 7.. without actually doing it. Effectively I'm going to simulate the results using a few equations and calculations i learned at school, with the help of a nifty little program called Excel Dyno.

This is a simple spreadsheet program with lots of the more common engineering formulas already embedded into it. Things like a simple distance vs time graph, as well as gradients, traction and shift points are automatically calculated based on the data supplied. So the program is only as accurate as the data that's given to it, a daunting premise given that I don't actually have a car to the record data from and the data gathered from the net has no accuracy guarantee.

Nevertheless, part 1 is dedicated to collecting said data.

If you're interested in doing this yourself to your own car, Excel Dyno is available here:
http://offroadvw.net/exceldyno/

All credit goes to Wes for this uber little big spreadsheet.


Why?

Strictly speaking there is no real reason to swap the standard gearbox for another one. The stock 5 speed is sturdy and reliable with a very pleasant shift feel. However. There are advantages to having an extra gear, the most important of which is cruising speed (engine speed) given by the last gear ratio and the final drive combined. Cars generally spend a lot of time on the freeway (or Autostrada in Italian). My 7 will be no different. I have lots of experience with 5 speed cars on the freeway, and every time i drive one, i keep asking myself if the world would be better without them. The 7 isn't a parsimonious car to start off with, so anything i can do to improve its real world performance without changing the feel of the car is absolutely worth it.
Swapping a gearbox is very expensive and time consuming. Using a simulation is the best way to actually gauge the improvement before spending valuable time and considerable money on the swap.


Which box?

 There are 3 options to choose from that have already been attempted.

Tremec competition box
Toyota Supra box
Aisin AZ6 (Honda S2000 Mazda MX-5 RX-8, Toyota Altezza box)

of these 3, the Aisin is definitely the most palatable. The Toyota box is overkill, as i never plan to produce four figure horsepower numbers and it is considerably heavy. Tremec boxes are extremely costly, rare to find in H pattern configuration and are generally available with very short ratios.
The AZ6 on the other hand, is widely available from a scrapyard in good nick, as there are no less than four cars to pull it from. Each car also has unique gear ratios which range from short to relatively long.
Note: changing gearbox has no effect on final drive. The final drive gear set is contained in the differential and must be substituted separately. I will be throwing different final drives (fd?) into the simulation to see which combination of gearbox / final drive is most effective, both with the new and stock transmission.


And now that you've read all that, on to Part 1.1! lol

Well its been a while, but the research is ongoing!

New template to the blog, i didnt like how the last one looked / worked. This one is much better. Still a little work necessary before it is 100% complete, but so far so good.


Also, I have been working on two very VERY large posts regarding the plausibility of a gearbox swap and an engine swap and have been studying properties of some racing radiators to quell the wankel's propensity to overheating.