Hey all, I believe the V6 tach will show the V8 higher than it really is - for every 6 ignition pulses, it will think 1 revolution, correct?

Is there a way to make the V6 tach work (doing an LT1 swap). Or do I need to find a V8 tach from someone that swapped over to 4th gen gauges? I just want to keep the dash looking fairly stock instead of swapping out gauges if possible. Like the retro look.

 

You are correct that it will read incorrectly. I also swapped an LT1 into a car that originally had a v6. Here is a link to instructions that I used to modify the tach.
https://www.thirdgen.org/forums/elec...h-v8-tach.html

 

Ok thanks for the info. Nice to know that it can work without too much $$.

It seems like there would be a known resistor value to modify the readout of the V6 gauge to read close on the V8 as referenced in the other thread. Doesn't have to be perfect. I don't have a good way of getting the actual RPM, unless I hook up a temp. Autometer gage or something. I suppose with the right software the LT1 computer should tell me what RPM it's really at as well. I'll need that anyway to tune it, and get rid of the emissions codes etc..

Anyone know what resistor value would be close?

 

I have used a rheostat in some applications to dial the tach in perfectly after eliminating the carbon calibration resistor

 

My V6 tach ('89 Firebird) was modified by a friend and it is dead nuts on until I accelerate hard and then it can't keep up. It gets way off in the high rpm. I suspect it has something to do with the RC constant of the circuit and it just won't ever respond like I need. Never bothered me on the street because anything I do is over in 1 or 2 seconds, but it was a real issue at the drag strip because I couldn't figure out when to shift. Ended up buying a Speedhut tach this winter (an entire gauge set actually).

 

IIRC the V6 tach puts out 3 pulses per rev vs. the V8's 4 pulses per rev....... if that's true than using the 'wrong' tach would be about 33% off.


I installed a LT1 in my 84 and used a V6 Cluster/Tach so the Yellow/Red lines were more realistic & closer to my actual LT1 RPM limits. In order to do so I added a Dakota Digital signal converter. I could have simply used a V8 cluster/tach ( not possible for me to swap the only the tach ) but my tach flashes when over Yellow ( 4500RPM ) and it's hard to get an accurate reading when the thing is flashing !

In your case; you can either pay someone ( or learn yourself ) to "mod' your original Tach, buy a replacement V8 cluster/Tach, or buy a signal converter.