Tach doesn’t match rev limiter

[scottiec]

Hey guys!

Been having an absolute blast with my finished car and have started to notice something. My speedhut tach (the ff gauges) doesn’t seem to match up with my pertronix ignitor rev limiter that lives under my distributor cap. I have it set to 6500 rpm but it comes in at 6000. I tested it at 5500 and again my tach read 500 rpm lower at 5000.

Any ideas here? Was there any sort of calibrating I was supposed to do with the tach?

As a side note, if you have a dart 363 with about 450 wheel, a rev limiter is a good idea lol

[Rdone585]

I've never really heard of a tach being off unless the signal feeding it wasn't good. Are you sure there is good continuity to ground?
As a side note, I have learned that just because your ignition system has a rev limit set at 6500, doesn't mean your engine is built with all the correct components to rev that high. One of my engines was built with a certain manufacturers cam that claimed to go to 6500. it would get to 6500 in neutral, but under load it would only do about 5400 before it would appear to rev limit. Turns out the cam design has issues and the engine just couldn't rev that high. My (next) engine builder told me he researched that using an engine dyno and had many conversations with the cam manufacturer. All said and done, the cam just didn't meet the design specs to rev that high. So the one in this build was a custom design from the builder, but oddly enough from that same manufacturer. This one revs to 7000 with no issues. But that was first confirmed on the engine dyno before he would let me set it that high.

[Jim1855]

Maybe, it's just possible, and I have no confirmation of this but... It's possible that the tach doesn't respond to revs as quickly as the rev limiter does.
Jim

[GoDadGo]

Hey guys!

Been having an absolute blast with my finished car and have started to notice something. My speedhut tach (the ff gauges) doesn’t seem to match up with my pertronix ignitor rev limiter that lives under my distributor cap. I have it set to 6500 rpm but it comes in at 6000. I tested it at 5500 and again my tach read 500 rpm lower at 5000.

Any ideas here? Was there any sort of calibrating I was supposed to do with the tach?

As a side note, if you have a dart 363 with about 450 wheel, a rev limiter is a good idea lol

Hey ScottieC,

I'd check your connections; however, my guess is that one or both parts are off just a little bit.
Consider getting a good quality multi-function timing light because those display the RPM's.
After that, I'd just set the rev-limiter accordingly and go have some fun.
If your bottom end is all forged, 6,500 RPM shouldn't be an issue.

Good Luck Fellow Dart Stroker Buddy!

Steve

NOTE:..I'm still set super low since (4,500 RPM) because wheelspin is an issue in 1st, 2nd & 3rd.
...........My issue is small tires (285's/40-17) with 3.73 gears behind my 383 SBC.
...........Redline is 6,500 so I'm working my way up as I learn how to drive it.

[NAZ]

The 500 RPM variance is not enough to assume you simply have selected 6-cylinder or 4-cylinder when setting up the tach. As Jim suggested above, it could be the tach has enough lag that the rev limit is reached before the tach senses the 6,500 set point if you are testing it by flashing the RPMs to the rev limit. But that's easy to check by either slowly reving the engine to the set point or holding it there for a several seconds once the rev limiter engages. You should really determine which is accurate and that's as simple as hooking up another tach which can be a timing light or other instrument that has a tach function. That way you can at least adjust the rev limiter correctly but if the tach is displaying inaccurate info that would hurt my brain enough that I would either correct it or replace it with one that is much closer than 500 RPM.

Typically on a street car, the limiting factor on RPM is the valve train. A limit of 6,500 passes my reasonableness standard but if the rev limiter is off by 500 that could could mean the RPMs could reach either 6,000 or 7,000 before the limiter engages. And 7,000 could be trouble for a hydraulic lifter and heavy valve train with typical lower rate springs. So IMO, unless your peak HP is at 5,500 RPM or less it's important to get this resolved if you need to run that engine at or near 6,500 RPM.

[johnnybgoode]

My setup does the same thing. MSD dist. Crane Hi6 box, Autometer tach. 414W. Motor stops making power after 5500 so I set the rev limit at 6000 on the box and forget about it. I always figured the tach was a bit slow. Scott

[scottiec]

Thank you all for the responses. For some reason I wasn't getting notifications that this was being responded to. Apologies for the late response.

Bottom end is forged and ready to rpm. Valve train limitations bring me to 6500. I had a custom cam made around my specs, and will be confirming everything on the dyno shortly. I was looking into purchasing a new timing light anyways since mine is a little to old school. I'll let you guys know what I find.

[scottiec]

quick thought actually - Since the tach measures the amount of time the coils cracks off a spark, I wonder if I need to change something since I switched to this pertronix ignitor 3 set up which is multi-spark. So it is actually firing a plug multiple times, that changes the amount of pulses the tach sees per revolution... hmmmm

[GoDadGo]

quick thought actually - Since the tach measures the amount of time the coils cracks off a spark, I wonder if I need to change something since I switched to this pertronix ignitor 3 set up which is multi-spark. So it is actually firing a plug multiple times, that changes the amount of pulses the tach sees per revolution... hmmmm

A call to the folks at Pertronix may confirm your theory because your comment seems logical from this wrench swinging banker's perspective.
Please keep us posted since this may be happening to other forum members who are using the same ignition system.