Yes - you need a 6 cyl code plug, but beyond that you should be accurate with your setup since you have the same swept area as a 325 and are using the total swept area to indicate rpms (via your 325 paper face).
The inherent problem with using a e30M3 cluster is obviously the code plug, but then once you take care of that you will be reading high rpm's since a factory M3 tach registers to 8k in the same swept area a 325 can only indicate 7k. So you will be off something like ~12.5% because of the math involved with total swept area by needle and the difference between 7K being maxed out and 8K being the max.
The needle should read accurate in your setup - mine won't read correct (I'll be reading ~12.5% high on rpm's) until I either do what you did and swap faces - or what I plan to do is to get the signal to the tach decreased by roughly 12% so I read lower and slightly closer to correct.
I haven't researched it fully, but I'm thinking a ~12% increase in ohm resistance on the voltage going to the tach will solve the problem.
DISCLAIMER :
**I haven't even looked at the cluster yet, so there may be no basis that the last sentence is true or even possible, but it's my starting ground. So don't quote me on that yet.**
---Ben
The inherent problem with using a e30M3 cluster is obviously the code plug, but then once you take care of that you will be reading high rpm's since a factory M3 tach registers to 8k in the same swept area a 325 can only indicate 7k. So you will be off something like ~12.5% because of the math involved with total swept area by needle and the difference between 7K being maxed out and 8K being the max.
The needle should read accurate in your setup - mine won't read correct (I'll be reading ~12.5% high on rpm's) until I either do what you did and swap faces - or what I plan to do is to get the signal to the tach decreased by roughly 12% so I read lower and slightly closer to correct.
I haven't researched it fully, but I'm thinking a ~12% increase in ohm resistance on the voltage going to the tach will solve the problem.
DISCLAIMER :
**I haven't even looked at the cluster yet, so there may be no basis that the last sentence is true or even possible, but it's my starting ground. So don't quote me on that yet.**
---Ben
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