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Making the Tach Work: 58X vs 24X, 3 PPR vs 2 PPR

Unfortunately for me, all the older LS based engines use a 24 tooth reluctor wheel to keep track of the crankshaft speed and position (LS3 uses 58x. Lucky you!). The Porsche engine uses a 58X reluctor, meaning I cannot just plug in the GM sensor into Porsche DME. The other option is to use the tach output signal directly from the ECU. Unless both engines have the same number of cylinders, this will not work without some modifications to the signal. In general this is what the output is for each engine layout:

  • 4 cylinder : 2 PPR (pulses per rotation)
  • 6 cylinder: 3 PPR
  • 8 cylinder : 4 PPR (in case of GM engines, it’s 2 PPR)

I did some research on this and came across a 944 thread where the OP changed out some resistors in the cluster to adjust for the signal difference. I thought that I could possibly use this approach myself, so I took apart the cluster. Things are much more complex in the 986 unit, the entire cluster is just one huge PCB and I really wouldn’t want to mess with it even if I knew what I was doing. The only easy option that’s left is to buy a converter from Summit Racing for about 90 USD, but I’m not made of money, so I wanted to tackle this myself.

I plugged in an Arduino board to the tach wire and sent a 3 PPR pulse signal for which I knew a corresponding RPM just to check if they would match. Indeed, the tach input is 3 PPR.  If you want to find the tach wire, it’s a purple/green, and you can access it at pin 9 on the OBD plug, pin 15 on the white cluster plug, or I believe pin 17 on DME plug #4.
The way to find the pulse length is pretty straightforward. Lets use 7000 RPM for example:

7000 RPM = 7000/60 RPS = 116.7 RPS (rotations per second)
There are three pulses per second at 50% duty cycle, which looks like a square wave. While there are 3 pulses (peaks), there are also 3 valleys, hence the total number of intervals is:
116.7RPS * 6 intervals/rotation = 700 intervals/s
interval = 1s/700intervals/s = 1.428 milliseconds or 1428 microseconds


Just to be clear, the cider box is not mine 🙂

I then took out the cluster and brought it home. You have to hook up all the term 30, term 31 pins on the black plug to positive terminal and term 15 to negative to supply power. The tach signal wire goes to pin 15 of the white plug. I used one Arduino to simulate 2 PPR signal and the second Arduino to modify the signal. Here is the code that I have so far; it seems to work Ok without any noticeable delay. The key is to keep the code as short as possible, or else your intervals will get all screwed up. The micros() function relies no interrupts to keep count, so when you call an ISR, the counter stops. Please let me know if you have a way to improve this or approach it from a different angle.

Intake and Engine Cover

The other day I finally finished the intake. These are the parts that I used:

  • Cobra intake elbow
  • 3.5″ 60 degree silicone elbow
  • 3.5″ aluminum tubing (1/16″ wall thickness, and I believe 9 inches length)
  • 3.5″ hump coupler
  • 3.5″ grommet
  • Aluminum intake box that I made from 0.3″ sheet
  • K&N RU-3130 filter from Amazon

I still have to wrap it in some insulation tape to keep the heat out and mount a couple of sensors, but this is pretty much what it looks like.

V8 Boxster

Custom intake box

The cobra intake elbow does not fit under the existing engine cover, so I had to make a small cutout for it and then fabricate a small cover to protect the top from engine heat.

Quick update

I had a bit of an accident that set me back a week or so, but I’m back on track now. Below I posted some recent car pics.

I bought some sheet aluminum for $20 and made a firewall cover. I wish I had fitted the seats first because there isn’t enough leg room for me at the moment. I will have to add a bit of an angle to the cover in order to move the seats farther back. I mounted the hand brake housing just above the “tranny tunnel” just to simplify things. Putting it where Renegade does is a pain, and it requires twisting the wire a little more than I like.

I also got a new intake manifold due to some serious defects in the old one. A 1-3/8″ freeze plug from Napa worked great to plug the EGR opening.

V8 Boxster Swap

The car is 95% done at this point. I’m just waiting on a couple of intake parts and need to get the exhaust made. I am also looking to install a fire suppression system in the trunk.

The big things to do now is get insurance, safety, and emissions done.

V8 Boxster Interior

V8 Boxster

V8 Boxster


#v8porsche #v8boxster #boxster #porsche #lsx

A video posted by Boxster LS (@porschelsx) on

A bunch of random stuff I forgot to post about

Starter Motor:

I picked up an $50 starter from a local scrap yard which came off a 2005 Silverado. The Kennedy Engineering adapter plate was designed around LS1 starter, so I had to make some modifications. The moulding in the dome is a little bigger I guess… I spent a good hour filing away, but eventually I got it to fit. The reason I got a truck starter over LS1 is because it’s cheaper and I heard LS1 starters  tend to crack.


I didn’t want to start the engine without priming it, so I purchased a drill pump at Princess Auto for $15. One end of the hose clamps onto the filter threads on the oil pan and the other into a jug of oil. This pump has no problems reaching 45 PSI. I stuck a gauge on my remote mount oil pressure switch hose (I will eventually post part numbers and pics for this part).

As for the tranny, I called Porsche about gear oil, which they quoted me at $55 a liter!!! I ended up purchasing some Swepco 201 which is manufactured locally, but surprisingly difficult to find.

Engine Cover:

The engine cover still fits, but it had to be cut a bit to make room for the throttlebody.


Rennline Floor Boards:

Rennline hooked me up with a wicked deal on their racing floor boards. I really didn’t want to put carpet back in this car, so these things are great.

Corbeau Seats:

I also gotta thank Corbeau for hooking me up with a deal on these FX1 Pro seats and rails. My friend started working on the brackets the other day while I was making brake lines.

Cracked Rear Subframe:

I noticed a crack on the rear subframe side section where it mounts to the body. It seems like it was put together with a bent stud by the previous owner that caused it to crack. While taking it off, one of the lower control arm bolts got stuck inside the bushing and pretty much pulled it apart. This was a huge pain in the ass, but at least I got a sweet deal from Woody for the replacement.


Pretty much all the brakes lines are gone on this car. I tried making some hard lines out of the copper-nickel stuff, but had a hard time getting them to flare properly. I ended up buying some poly vinyl fluoride coated metal lines from NAPA. This stuff is great, they flare perfectly and are easy to bend. I would also recommend Lisle tube pliers if you plan on doing any kind of brake line work.