V8 tach output for 6 cylinder tachometer
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.
Quote:
Originally Posted by Project GatTagO
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)
