V8 tach output for 6 cylinder tachometer

[V8 Supra Builder]

I did a search for this information and could not find it using keywords Tachomter and Tach.

I am looking into adjusting the tach output for my 98 VCM due to putting the motor into a 6 cylinder BMW. The stock setting for high & low resolution is 6/6. What is the proper adjustment to make the tach output the same as a 6 cylinder motor? I know the LSx output is seen as a 4 cylinder by tachometers. Nothing else on the car uses the tach output.

EDIT (reduced to only 4,6,8 cylinder tachs for clarity)
Did some math & research & some more math, this looks to be correct:
The number of crankshaft medium resolution pulses (24x pulses) to hold the tach output high (should match {G1205} "Tach Pulses Low").
The number of output pulses P is calculated as P=24/(N*2) where N is the value of this calibration.

Sample values for this calibration for a 4,6,8 cylinder tach:
For 8 pulses (4x2), use N= 3 (Note- 8 cylinder tach setting)
For 6 pulses (3x2), use N= 4 (Note- 6 cylinder tach setting)
For 4 pulses (2x2), use N= 6 (Note- LSx 8 cylinder & 4 cylinder tach setting)

N= 6 same as you see in HPT (for Gen 3 motors)

Pulse Calculator: V8 tach reads 8 pulses (4x2) per revolution, so 24/8=3. 6 cylinder reads 6 pulses (3x2) per revolution, so 24/6=4. 4 cylinder reads 4 pulses (2x2) per revolution, so 24/4=6. These are the "N" numbers for above that are plugged into the HP Tuners Tach Output fields.

EDIT #2 (2017, 58x wheel, experimental math)
I've been out of the swap game for a while and no longer have a VCM to test this on, but see there have been a number of questions since I posted this. The above was for a Gen 3 engine, which uses a 24x wheel. Later engines (usually 2005+) use a 58x wheel, so we have to use different numbers. I downloaded the HPT 3.4 demo for some reference info, and used the included 2010 V8 (E38) Camaro file.

The first thing that is different on the Engine>Tach Output screen is in addition to the 2 boxes for tach resolution (High and Low), you have the following option:
Type: Serial, Frequency, or Crank

Per the below thread, use Crank.

Note: Despite any of this, some cars need a pull up resistor to operate the tach, my Supra w/ a Gen 3 engine needed one to work. Here's a page from Autometer on how to do that, and they offer an external adapter if it does not work. This post talks about using a potentiometer to figure out what size resistor you need.

So after being a little ticked off this morning about the whole tachometer ordeal I decided to make some calls. I called my buddy John Parsons and he gave me a quick run down of Ohms Law. So I decided that instead of getting a collection of various resistors I would get a 10K Ohm potentiometer and see what works best. So I hooked up the potentiometer to the tach and set it to the 10K setting. Fired up the car and the tach seemed to work fine at low RPM. I then kept reducing the resistance until the tach stopped responding. The I zeroed in on the lowest setting where the tach responded smoothly. I then turned off the car and measured the resistance across the potentiometer. It rea 3.9K Ohms. So I gave myself some headroom and installed a 4.7K Ohm resistor. So far so good. The tach does seem to flutter ever so slightly at about 800 RPM, but there are no random fluctuations to zero. I think that will do it.

In lieu of reinventing the wheel, I see this issue was addressed in another forum. I used that data along with a spreadsheet I wrote to update my original work. In summary, that member found using a high/low setting of 14/15 was needed to accurately drive an Autometer tach in 4 cylinder mode. If you're using an adjustable aftermarket tach, setting it for 4 cylinders and using this setting should get you going. If you're using an OEM tach in a swap, keep reading, but you may want a tach adapter.

After a lot of work, I found a formula that will get the settings as close as possible. Due to the 58 tooth wheel, 100% accuracy is not possible for 6 and 8 cylinder tachs. If this level of accuracy is needed, a tach adapter could be used. Autometer and Baker Electronix sell them. I ended up using a $50 adjustable tach adapter from the latter on my Supra.

Here's the formula I used for HP Tuners-
High/Low= split value of:
Desired pulses= # teeth / (high + low)

In the E38 example above, 58 teeth / 2 desired pulses= 29. We have to as evenly as possible split this value to get the proper number of pulses. In his case, 14/15 worked perfectly. The numbers for 6 and 8 cylinder tachs are 19.33 and 14.5, so there is no way to evenly split them in whole numbers.

The closest I got for a 6 cylinder was 9/10, which returned an error of 1.75%.
At 1,000 RPM it would read 1,018
At 3,000 RPM it would read 3,053
At 6,000 RPM it would read 6,105
That would probably be accpetable for most people.

The closest I got for a V8 was 7/7, which gives an error of around 3.6%.
At 1,000 RPM it would read 1,036
At 3,000 RPM it would read 3,107
At 6,000 RPM it would read 6,214

58x Summary
8 cylinder tach= 7/7 (3.6% error)
6 cylinder tach= 9/10 (1.75% error)
4 cylinder tach= 14/15 (100% accurate)

[Baldturbofreak.com]

Very Nice info, this should become part of the Stickie's

[LSxpwrdZ]

I know this is a very old thread but I believe this should be added to the help documentation to clearify what the tach output resolution high/low actually does.

[Snyper]

This is old but whats the difference between Serial, Frequency, and crank?

[04colyZQ8]

anyone no what a 2004 colorado is looking for tach wise? I am putting in a Gen IV V8 based off 06 SSR E40 pcm I don't know what to set the tach output too? And there is no tach setting on the tune file for a colorado viewed wiht hp tunners

[jakebdb56]

I know this is a very old thread but I believe this should be added to the help documentation to clearify what the tach output resolution high/low actually does.

Totally agree.

Hate to bump an old thread, but there's some really valuable information here. With so many people doing swaps now and even the new HPTuners commercial has a swap featured in it, it would be nice to have these things outlined as in what they do and how they work. This data here will help with going V6 to V8 in a LS vehicle, but for other things it would be cool to have the data. Just my .02

[Rinkrat456]

Bump for good info that should be at the top or at a sticky.

[BlazinDorito]

I got a 1996 GMC K1500

I installed a 2005 8.1L 496 and the tach dont work would someone know the setting value on the tach output

Thanks

[PitchblackC3]

Should be 3/3 for a 8 cyl output

[WS6TA99]

Good stuff. i installed a ls1 into a 97 v6 firebird. what should my settings be? any help is appreciated!

[1duffman]

I got a 96 Tahoe with an 2000 LQ4 and set the tach to 3 and 3 and still have nothin. Any ideas???

[Chrome_Rust]

do you have red connector pin 10 (white wire) going to your tachometer? I have done several OBS swaps and never had an issue with the tach.

[Dirthead Racing]

Gen 3 LQ4, should it be 3/3 or 6/6. It's currently 3/3 and my autometer tach is a couple hundred rpm higher than hptuners is reading out. If I read the first post correctly, Gen 3 should be using 6/6?

[Smoky68]

Dirt Head did you leave your on 3/3, I have a 2005 LQ4 and mine is set on 3/3 also...just curious if I should change it to 6/6 and would it make a difference other than my tach output

[Dirthead Racing]

I'm pretty sure I have to do the resister thing. I haven't worked on it in a couple of years. Within a couple hundred rpm has been close enough for the couple times I've driven it. I still want to get it figured out but it'll have to wait until the kids are older.

[Spoolnsquirrels]

So I have tried to adjust my tach which is a stock s10 v6 cluster I have tried to adjust from a 3/3 to a 6/6 and on some settings it is dead in at the low rpm and some settings it's off at idle but dead on at 3000 any help please!

[V8 Supra Builder]

So I have tried to adjust my tach which is a stock s10 v6 cluster I have tried to adjust from a 3/3 to a 6/6 and on some settings it is dead in at the low rpm and some settings it's off at idle but dead on at 3000 any help please!

I have updated the first post. If you're using an engine with the 58 tooth wheel (Gen 4, mostly 2005+), then try 9/10. It won't be 100% accurate but will be <100 RPM off on a 6 cylinder at most RPM readings. See also the linked voltage divider info if there is a signal problem.