I'm talking to the guys at Trailtech about their Vapor computer. For those who are not familiar with it, it's a dash replacement for motorcycles that's quite inexpensive (about $120), reliable, and has all the essential readouts (speed, tach, and engine temp) plus some other useless stuff. I'm trying to find out if they could produce a model just a little larger, with appropriately adapted sensors, to work with cars. I've got a small following interested so we're pursuing it. My idea at the moment would be to make the thing read off of the drive shaft, but I'm needing help with math to see how well that would work.

What I'm needing to know is how fast the drive shaft is spinning per mile per hour on a TII (I assume that's got the shortest gears). This is assuming stock tires. I'll need to find out for all the other models, too, possibly, if the shaft isn't turning to fast to allow a reliable signal.

Thanks in advance!

 

The drive shaft speeds are measured using Hall effect sensors.

 

By "drive shaft" do you mean the propeller shaft or the driveshafts?

Also, did you want to calculate the "drive shaft" speed at a particular engine rpm or vehicle speed?

If you give me that information then I can give you a speed. However, note that the unit will need user input to adjust for the differential ratio and tire size if the sensor is on the propeller shaft, or the tire size if the sensor is on one of the driveshafts.

Auto Meter makes a lot of cheap tach/speedos:
http://www.autometer.com/cat_gaugesearch.aspx

SPA Technique makes an excellent-quality tach/speedo that has a lot of nice features:
http://www.spatechnique.com/product_...page.cfm?cat=3

The best bang for the buck in full-function dash units is AIM. These budget units are rapidly replacing the expensive brands in high-stakes GT racing.http://www.aimsports.com/
Picture in a 2Gen street car:

 

I think the issue here is that the speed may be too fast for a cheapie sensor.

 

I've always called the driveshaft the gadget that connects the transmission to the dif.

I belive S4 TII's have the shortest gears in the tranny and rear end...but if not, that's what I need. I need the driveshaft RPM per mph on the shortest gear ratio combination available on a stock FC.

The Aim pieces are really, really amazing, but even the cheap piece is getting into the $1000~2000 range. That's a good buy for racers with big investments in their sport, but hard to swallow for amateurs with $250 cars. What I'm looking at is not meant to be a lap timer or anything...there's no datalogging. It's just straight real-time what's going on...and, unless adapting sensors becomes problematic, it'll be well under the $200 area.

I like some of the autometer combos, though...how do they pick up speed? I would love to just get rid of my stock gauges all together and put in good dials...I've just always been stumped on the speedo.

 

haha I hate shopping...I want the speedo, tach, water temp, and oil pressure in this style...

http://www.autometer.com/cat_gaugede...earch&gid=3903

That's just a mere $800 bones lol

 

Unfortunately, Mazda calls that the propeller shaft.

I still don't understand what you are asking, so here is the formula to figure it out yourself:

V = (C*RPM)/(1056RnRf)

V = Vehicle Speed in mph
C = Tire circumference in inches (A stock S4 TII 205/55-16 is about 78.15639")
RPM = Engine RPM
Rn = Transmission Gear Ratio (See the FSM for gear ratios)
Rf = Final Gear (2.90, 4.10, or 4.33)

If you need to figure the propeller shaft speed, it is 2.95" dia for TII or 2.24" for NA. Some of the early NA's had a thicker shaft, probably closer to the TII size, but I am not sure.

That is half the amount that I spent on my gauges.

If you don't have much money, then I suggest that you just pick up a used gauge set at the junk yard if yours are not working.

 

I tried the used cluster thing lol...the only S5 found locally in a salvage yard the guy wants $350 for the cluster. Just can't see spending that. It's more than that, though...in my particular case the car has sat and there is a LOT of corrosion on all the contacts...makes it hard to chase ALL the bad contacts. The others interested just want something different.

I'll try and elaborate. What I need is the "propeller" shaft RPM at a given speed. I think I sorted it out from your equation, though.

A tire with a circumference of 78.15639" will have to make 945.795984 rotations in one minute to be traveling 70mph.

With a 4.33 TII rear end, the "propeller" shaft will have to be spinning 4095.29661rpm in order for the car to be travelling 70mph.

Tranny ratios and engine RPM aren't relevant in this as the propeller shaft, half shafts, and wheels will remain constant to vehicle speed regardless of what gear the tranny is in.

Does that math seem accurate?

 

Keep in mind there is no stock 4.33 TII rear end. The stock gears will give you a 4.10 final drive.

 

I thought the tallest was 4.10, but EA posted a 4.33...he's more knowing than I so I assumed he knew something else I didn't. It's not a debate, though...if the math is correct adjusting between the 4.33 and 4.10 is pretty simple.

 

I don't know what rear end you're running (wasn't it a BMW rear?), but if you go with anything from an FC, you'll have 3 options: 3.90, 4.10 & 4.33. But the 4.33 is from the NA GTUs model, which is fairly rare. And yeah, changing a single variable is simple.

 

The BMW thing got scratched...I just sold it. I wanted something with a 13B, I just stuck with the RX-7...and I'll keep that mentality until I'm ready to stick an LSx in something...then I'll stick with the rx7 lol.

I'd have to change a couple of variables to suit the TII over the GTUs, I think...unless the GTUs came with a TII tranny. Either way, it's an easy adjustment and, either way, it's just as useless. The sensor can't pick up 4000 pulses per minute.

 

Geez, I got an extra S4 cluster for about $20.

Yup, that's it. Now you can calculate for any differential ratio, stock or otherwise.

If you want more math, here is how to calculate tire circumference. Manufacturers may not adhere directly to the dimensions written on their sidewalls, and wear and distortions under driving conditions may cause variances, but this can get you in the ballpark:
C = ¶(Rim Diameter inches + 2(Aspect Ratio x (Treadwidth mm / 25.4)))

For example, 205/55-R16 side wall marking:
Treadwidth = 205 mm
Aspect Ratio = 0.55
Rim Diameter = 16 inches

You can get the 4.33 ratio with the stock NA GTUs ring gear M066-27-110A or with the Mazda Comp TII ring gear P092-27-110B.

The GTUs had NA parts. There is a lot of misinformation running around the internet due to the technically-flawed advertising that touted the GTUs as the "TII without the turbo", which it wasn't.

Hehehe, I figured it was a cheapie POS sensor. Just put it on one of the driveshafts (halfshafts), assuming it can take a little bouncing around. The LSD doesn't move much, so it may be pretty stable if mounted on the outer ring (large-diameter portion of the halfshaft where it mounts to the LSD).

 

The big issue with one of the halfshafts is the distance from the cluster...be a long cable. They actually emailed me back about some gadgetry that may adapt to the stock speedo drive...which would GREATLY simply everything.

 

Yeah, they may have something like the Auto Meter hall effect sensors that could plug into the transmission.

Oh, also just as a correction, I think the 4.33 ratio is actually 4.30. Mazda Comp publishes it as 4.33 so it sticks in my head that way, but I seem to remember that the MT pinion has 10 teeth and the ring gear is either 41 or 43 giving a 4.10 or 4.30, with the NA AT pinion having 11 teeth matched to the 43 gear giving it 3.9090909_. Either that or Mazda Comp is right and I am just putting too much thought into it.

 

Everything that I've looked at is 4.33....but I've never been in either so it's hard for me to speak with certainty.

Yer smurt, though...I just started another thread with a much more pressing issue that I would like some advice on, if you don't mind.

 

I am not a mechanic, so unfortunately I don't really have much input on those troubleshooting issues because I have not come across them myself. Sorry about that.

 

it's alright...I appreciate the help with the numbers and other odds and ends.