Tach reading higher than actual RPM.

G72Zed

Veteran Member
Sep 8, 2015
4,405
Canada
Some of those aftermarket distributors have a selection for 4/6/8 cyls.

Some of the aftermarket HEI distributors are not worth more than the cardboard box it came in (internal electronics even less).

Failure rate on the cheap "aftermarket" HEI is a known fact, some are good, others are just pure POS. We now keep a complete known/quality HEI at the dyno shop just for this reason.
 

TTR230

Veteran Member
Aug 1, 2020
676
Burlington, Ontario
How would that affect the tach reading the wrong RPM?

The op refers to engine speed in relation to a certain road speed to judge that the tach must be reading wrong. If the speedo gear is wrong the speedo could read optimistically or pessimisticlly , potentially making some the data op is using faulty.
 
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camarochevy1970

Veteran Member
Jan 23, 2003
8,166
Pittsburgh, PA
The op refers to engine speed in relation to a certain road speed to judge that the tach must be reading wrong. If the speedo gear is wrong the speedo could read optimistically or pessimisticlly , potentially making some the data op is using faulty.
But for the tach to be off by double (OP estimate) would would also have to be way off on the speedometer, not just a little. If tach is saying 7k rpm, you would know what a SBC at 7k rpm sounds like versus 3500
 

Twisted_Metal

Administrator
Staff member
Lifetime Gold Member
Feb 26, 2004
31,884
Bloomington, MN
If it reads 4000 when it should be 3000 RPM (and reads 2000 @ 1500)...
Somebody swapped a 6-cylinder tach into it.

What does the face look like?
I have one that originally came in my 80 (229 V6) and the red line is different from a V8 tach.
 

TTR230

Veteran Member
Aug 1, 2020
676
Burlington, Ontario
But for the tach to be off by double (OP estimate) would would also have to be way off on the speedometer, not just a little. If tach is saying 7k rpm, you would know what a SBC at 7k rpm sounds like versus 3500
Yeah, I'm not arguing, I was just laying out why I think copo asked the question. I think that there needs to be some definitive measuring done with a handheld laser tach or some other means to confirm the engine speed. Seat of the pants isn't a great way to figure this out.
 

TheReelEss

Member
Oct 17, 2022
70
If it reads 4000 when it should be 3000 RPM (and reads 2000 @ 1500)...
Somebody swapped a 6-cylinder tach into it.

What does the face look like?
I have one that originally came in my 80 (229 V6) and the red line is different from a V8 tach.
You make a fine point about the possibility of a 6 cyl tach. I'm working right now. But when I get the chance I'll take a pic and post it.
 

Rosster

Veteran Member
Gold Member
Mar 1, 2020
1,105
You mean aftermarket tachs? The distributor has 8 wires on it. But this is a stock tach and of course, it came with a V8.
Nope I mean the ignition boxes used with some distributors are able to be set for 4-6-8 cyl. many of the msd ones had this feature
 




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