72-CLASSIC_RIDE Posted April 17, 2016 Share Posted April 17, 2016 Spent this Gorgeous Day looking over some things on the Monte that needed attention. The main one was trying to determine if Factory Tach is reading accurately. I used a Dwell / RPM Meter to track what engine is doing and comparing it's readings against those of the Tach. What I discovered is the Engine RPM does not coincide with Factory Tach information. Incrementally, as the Tach needle rises, the Tach to Meter comparison separation widens as RPM's increase, Tach reads 2000 RPM while the Meter readings at 1800 RPM, a difference of 200 RPM higher. While I never reved engine higher, it can be assumed that the range widens between the two as engine is further accelerated (based on initial observation). So when I have been consumed by false speed/odometer readings all that time and since resolved them, they pretty much told on the Tach but I needed the additional proof just to confirm. I might have to compare engine RPM vs. Tach RPM again and establish those performance metrics at lower/higher revs to understand how to judge what the engine is really doing and based on that judgement, when to relax. What I wanted to ask (but also wanted to share my story), is there something else I should be looking at and or is there a method to calibrating our factory Tachs? And did I say what a Glorious day it was today! Well, Sunday and Monday look to be in the mid / upper 80's according to the weather man. Looks like Kick Back weather the next couple or so days.......................... Doug Quote Link to comment Share on other sites More sharing options...
Sam (Bones) Posted April 17, 2016 Share Posted April 17, 2016 This is a very common problem with the factory tach! A couple of years ago I did some research about recalibrating them, there is a resistor in them that goes bad, some were replacing it with a reostat so it was adjustable Pain in the pooper of a job since you need to drop the dash to get the tach out, it will be easier to just replace the tach Quote Link to comment Share on other sites More sharing options...
72-CLASSIC_RIDE Posted April 17, 2016 Author Share Posted April 17, 2016 I agree Sam. Will the dash have to come completely out to replace Tach? Considering it's a 402 CI now vs. the original 350 CI, where should the Tach redline at? I think this was lightly discussed a couple years ago. If I end up changing out Tach by removing the Dash (again), might as well be correct for engine CI. Doug Quote Link to comment Share on other sites More sharing options...
Sam (Bones) Posted April 17, 2016 Share Posted April 17, 2016 Don't go by it should have this or that resline, it's been proven again and again that you could have either one in any engine, just the assembly line screw up You have to at least drop the dash down to get the cluster out Quote Link to comment Share on other sites More sharing options...
72-CLASSIC_RIDE Posted April 17, 2016 Author Share Posted April 17, 2016 Is there anywhere on the Build Sheet that identifies the Tach type? Thanx for the info Sam. Incidentally, you were the one who started this ball rolling. You were questioning if my rear axle was a 331.1 Ratio and considered a 353 / 373 axle was more in line with my numbers. As I had the axle apart and had it rebuilt, I counted teeth and did the math to determine it a 331:1. So the only thing left was start checking the indicators for correctness. So, at least I have it all out on the table, reduced to this issue . I might just replace it when I have to remove Dash in the future. Doug Quote Link to comment Share on other sites More sharing options...
Sam (Bones) Posted April 17, 2016 Share Posted April 17, 2016 it should say which tach or gauge package came in your car Quote Link to comment Share on other sites More sharing options...
Sam (Bones) Posted April 17, 2016 Share Posted April 17, 2016 the thing about using RPM's/MPH/gearing with an automatic trans is the torque converter, you can not get an accurate reading because they are not a direct connection like a clutch, they is always some slippage Quote Link to comment Share on other sites More sharing options...
72-CLASSIC_RIDE Posted April 17, 2016 Author Share Posted April 17, 2016 Yeah, I can see that. I probably won't mess with it until I have a reason to pull Dash. My Build Sheet showed a number (405) as my Tach. Not sure what that means but it is on the Build Sheet! Also, a member commented on his feeling that the small block would be more apt to the higher RPM gauge because they (factory Specs) show factory performance operate at a higher RPM. So I checked an older Motor Repair Manual and looked up motor specs and compared Factory Specs. In almost every case (that I looked at) showed the small block at a higher normal operational RPM than the Big Block engine(s). I also get what your saying about inadvertently or even purposely installing incorrect redline Tach. As an example: My 72 Monte originally a 350 CI engine and had the 5000 Redline Tach installed. Initially, I just assumed Bigger Motor, Higher RPM and appear to have it backwards. Appreciate your follow ups Sam. I believe I can now make an informed decision when the time comes to tackle this. Doug Quote Link to comment Share on other sites More sharing options...
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