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 01-01-2009, 08:13 PM #4 (permalink) Christ Moderate your Moderation.   Join Date: Nov 2008 Location: Troy, Pa. Posts: 8,919 Pasta - '96 Volkswagen Passat TDi 90 day: 45.22 mpg (US) Thanks: 1,369 Thanked 430 Times in 353 Posts The problem with using a 4 cylinder tach in a 3 cylinder is that the tach reads output from the ignitor and does math.. x/4 = RPM. Lets say x is 4000 on the 4 cylinder, your formula is 4000/4 = 1000 RPM (idle... ) Now, lets assume you put that cluster in your car, and don't change anything about it. 4000/4 = 1000RPM for a four cylinder, but for the 3 cylinder, the actual formula should be: 4000/3 = 1333.33 So basically, your engine is revving 1330 RPM when your tach is displaying 1000 RPM. The difference won't be linear either... it's a curve. 16000/4 = 4000, 16000/3 = 5333.33. Now, if you can change caps or resistors to make it do the math correctly, then you're set to go, but just using it without modification will definitely yield an incorrect reading. __________________ "¿ʞɐǝɹɟ ɐ ǝɹ,noʎ uǝɥʍ 'ʇı ʇ,usı 'ʎlǝuol s,ʇı"