DragStuff

Anyone know how I could fool the map sensor input on my 7531 box into accepting a signal for rpm driveshaft so that I could plot is along side engine rpm?

I talked to MSD about it at PRI and they said NHRA wouldn't allow them to build it to accept external signals, other than the MAP sensor. So he didn't really say it couldn't be done, but didn't offer any suggestions either.

Craig,

What you would need is a frequency to voltage converter. This would probably give good results at drive RPM over 1000, but the low RPM readings would not look very good due to the mismatch in sample rates between the converter and the logging in the ignition.

Regards,

Randy

Thanks for the info Randy. Do you offer such a converter for sale or know of a good source for a reliable one?

RPM under 1000 isn't a concern. Its never under 1500, even at idle. Launch at 3500 off the two step, flashes to 5100, shift it at 7200, through the traps at about 6800.

RPM under 1000 is a concern Craig, your driveshaft rpm will be under 1000 for probably the first .4 to .5 seconds

Doh! What was I thinkin!

Not one of my brighter moments.

Thanks.

Craig,

I don't know of one that has the correct input and range to do the Driveshaft RPM. Have you thought about a basic data logger or the Auto Meter 2 channel tach to get engine and driveline RPM?

Randy

Beyond wrote:RPM under 1000 is a concern Craig, your driveshaft rpm will be under 1000 for probably the first .4 to .5 seconds

But cant that be corrected using a multiple Magnet pickup collar instead of a Single. would just require some math to figure out the RPM of the Driveshaft, but it may correct the rpm at which it would be acurrate

Strange has Magnet pickup collars with as many as 4 magnet I think, but that would still only drop the accuracy to 250 rpm. But that would be working well at only around 4-6 mph even at 1000 RPM it would be working well at under 20 mph

A 4 magnet collar at 500 rpm would provide 33 samples per second. If the logger is recording at 100 samples per second (a reasonable minimum for good Drive RPM data) the data would look like stair steps. The problem is due to the conversion method and the sampling of the second conversion. The first conversion would change the analog voltage at every trigger input, holding that level till the next sample. The second conversion assumes that each sample is a valid measurement at that time when it was really a measurement at some time prior to the sample it is reading. In this example, the logger would read the same value 3 times in a row, thinking that the driveshaft speed did not change for .03 seconds.

This data could be usable if you remember it is incorrect, and interpret it correctly. However, there is another problem. At low RPM, magnet spacing is not too critical, but at higher RPMs, unless the magnet spacing is very very close, the converter would need to switch to reading a full revolution or there will be errors, depending on which magnet pair was measured. For example, at 6000 RPM, each degree of spacing error will show up as a difference of 20 RPM.

The correct instrument for measuring RPM, updates with each trigger pulse from the collar and interpolates the reading between each trigger, thereby eliminating the stair step appearance, and providing much more accurate readings.

The bottom line is, yes you could do this, and it would provide a close approximation to the actual RPM. The difficult part would be finding an existing product that was setup to do it well.

Regards,

Randy

What specs for the converter do I need to be looking for?

Thanks.

Craig,

The input frequency would be about 0 to 700 Hz for a 4 magnet collar at 10,000RPM. If you use the same sensor we use, the input would be 12v rectangular waveform with about 95% hi, 5% low pulses. For the output, you need to know what input range the MSD expects (probably 0 to 5 volts). The details as to exactly how the converter work are also important. If you find one that looks reasonable, let me know what it is and I will see if the function is what you need.

Regards,

Randy

Thanks again for your help and expertise. I'll do some searching and let you know if I find something that looks close.

Thanks again.