10Krpm / 6-cyl chrome dot tach build, DIY conversion from early -4 donor tach Options

[Retroracer]

So this post builds on, and references Tim’s (timothy_nd28) awesome post here:

http://www.914world.com/bbs2/index.php?sho...719&hl=tach

…which details the steps necessary to improve the original VDO tach performance and provide better transient and damping response, coupled with a more universal input. If you’re not familiar with this DIY tach upgrade, best read that first.

Given the flexibility and performance upgrade of retrofitting the SunPro/Bosch innards into the VDO casing, I took on the challenge of seeing if a 10K racing tach could be constructed around an early 914-4 tach housing – with a target cost of $100. Read on if this piques your interest; move on if you don’t think this sort of thing is necessary or sensible.

A 10K tachometer was an essential part of the GT cars (and other Porsche race cars too), which typically had no redline area indicated under the glass, such that the actual redline could be indicated with a marker or tape, to suit the particular engine build being run for that event. Original 10K VDO units appear for sale occasionally, but the prices are usually prohibitive.



I ordered the Bosch version of the SunPro via Amazon, and disassembled it using Tim's post as suggested:





There is some metal fabrication necessary to get the donor VDO fascia to mate up to the new core. This is not difficult, the only thing you may not have around is the M2 tap.



Getting the right fascia: So I toyed with idea of using a drawing or graphics package to replicate the 10K tach face, then somehow print a design on to vinyl, trim it, then glue that over the existing -4 face. Then I realized that I was never going to get round to doing that, so looked at other options.

I ended up buying a printed and finished insert fascia from these guys:

https://www.heiler-tachodesign.de/Tachomete...ithout-red-Area

Not a cheap way to get a result, but they a) offer multiple choices to match various Porsche VDO tachs and other gauges, b) the quality of the fit, paint and graphics are exemplary and c) without the talent and tools for “rolling your own”, there are very few other options. They also allow you to pick custom colors and lettering if you so desire. It took about 2 weeks to arrive, I also ordered a 914-6 style temp gauge replacement at the same time, total cost for both gauge faces including shipping was about $93.



So, $100 target blown already - the tach core is around $35 - but still in the ball park.

[TravisNeff]

Great job Tony! I appreciate the how to. Now to understand what this generator thingy is and what does it do.

[Retroracer]

Now to understand what this generator thingy is and what does it do.

Travis - good point, I didn't make this clear: so part of the reason for writing this up is that if anyone wants to build a 10k tach, they now don't need to worry about the signal generator stuff; if you use the same Heiler fascia and change the R9 resistor (per Tim's original thread) on the core to 220K, you just need to worry about getting the needle absolute position correct - which is easier.

- Tony