GRT sport EX ..resistors in p-lead

I am installing the 7"GRT sport EX. Plan is to obtain GRt tach signal from the mags P-lead. I notice 2 sets of resistors in package, heatshrinked with spade terminals on either end, 27K ohm and 39k. Instructions mention both of them, in different places, but no criteria on deciding which to use.
Hmmmm. Anybody have advice?

Lycoming 0-320 carbed. standard mags.

Thanks, Don

EIS 4000

Forgot to mention..GRT Sport Ex WITH the EIS 4000 engine monitor, which of course the subject of the question.
Any advice welcomed!
Don

FWIW, I just installed a Sport SX and EIS 4000 and used the 27K. All is well.

I had 30K resistors for the Dynon and left it for the EIS. Has worked well for me with no issues.

I was thinking the resistor was to provide a current source to the input but from their manual...

"A 27 k Ohm resistor must be connected between the instrument and the mag P-lead as shown in figure 7 to protect the mag from being turned off by a failure in the instrument."

27k is the one you want, 39k is a throwback to an earlier version:

http://www.vansairforce.com/community/showpost.php?p=909176&postcount=6

I suggest putting the resistor somewhere along the lead wire where you can secure it firmly. Mine was originally just prior to the p-lead terminal, sorta hanging in the wind, and the vibrations were not very good for the solid wire lead out of the resistor itself, failed after 100 hours or so. I mounted the replacement resistor 8" or so back down the line, firmly attaching the crimped leads on either side of the resistor to the wire bundle running along the engine mount.

I have my resistors right at each mag switch (per Aeroelectric Connection). P lead goes to resistor then resistor connected to switch.

FOR GRT's benefit...

Ahhh..Thanks men!
Catmandu, thanks for the link to Katies post. kinda curious I just ordered and received the whole thing from GRT, and they still send both..oh well. Also kind curious that statement of hers is not in the manual..it oughtta be if they are going to supply both..anyway..I certainly understand your logic in moving the resistor "down line" a bit. makes good sense. In fact I think I will splice mine in aft of the firewall, giving me another place to disconnect behind the panel...hmmm..maybe....