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.

[bahnzai]

Tony,
What would it take to reset the Zero & 6K positions like this (with Zero at 6 O'clock position)?
Thanks,
Danny

@Retroracer

[Retroracer]

Tony,
What would it take to reset the Zero & 6K positions like this (with Zero at 6 O'clock position)?
Thanks,
Danny

@Retroracer

Danny - couple of key things to consider here, and a couple of clarifications needed:

1) what model VDO tach will you be modifying? e.g. 914-4, 911, etc

2) what engine are you running? six, four, flat eight....!?

The first point affects the position of the warning light guides on the metal casing. I chose the Heiler fascia to match exactly the positions of the "<", ">" and "full beam" light guides in the early -4 casing I was using. I wanted these to match up, as moving the guides around seemed like an extra level of modification hell that I wanted to avoid! Heiler seem to have a bunch of choices to suit various donor casings, which helps. Note also that in my stack up, I dropped the Heiler fascia on top of the VDO original to maintain the outer lighting "lip" which allows the back light bulbs to illuminate the gauge face without having obvious gaps (when viewed from the front). Even more important for the cut outs to match precisely.

So that was an underlying assumption for my thread that was not explicitly stated. The fascia you show in the post (for instance) will not match the light guides in a -4 casing - so you're in unknown (light guide modification) territory if that's your plan...!

The second point is related to the component mods needed on the Bosch tach core. Its relatively easy to set any zero position - just a case of placing the needle on the shaft at the correct angle. The resistor change sets the correct translation from the number of ignition pulses coming in, to the correct RPM being displayed.

Perhaps a numerical analysis helps the concept: at 3000 RPM, the crank will be rotating at 50 revolutions per second - or 50Hz. Each cylinder fires once every 2 revolutions (4 stroke engine!) - so in a 4 cylinder car the tach pulses will be at 100Hz (i.e. 4 x 25Hz); for a 6 cylinder the input to the tach will be 150Hz (6 x 25Hz). The Bosch core essentially converts this input frequency to a defined swept angle; and tweaking the R9 value therefore matches the input pulse frequency (which is dependent on how many cylinders you have) to the swept angle to match the markings of the fascia you choose.

Hopefully that all makes sense... feel free to PM me if you have specific questions

- Tony