How Steady Should Idle Be?

[Bruzilla]

My 440 has been running just fine, but I put a digital tach on this weekend. This is the first time I've used a digital one, and I'm used to the sweep needle variety. These needles have always jiggled a little, but I never gave it much thought as long as it stayed right about where it should be, but the digital ones are a bit more revealing.

My digital is showing changes in idle speed of about 10-20 or so RPM over time, sometimes higher, sometimes lower. Should it remain constant, or are these variations normal and I've just never registered them with the sweep tachs?

 

[1967coronet440]

Normal, I have the digital tach in my srt. Never noticed how much they actually change since the needle about stays put, mines constantly changing within 20-30 rpm maybe. Drove me kinda crazy at first but I'm used to it now.

 

[oldbee]

ITs fine.

 

[Speedbird]

He said jiggled.

 

[Bruzilla]

He said jiggled.

That gives me an idea for new type of tach... where's Richard Cranium!

 

[moparjohnny]

That gives me an idea for new type of tach... where's Richard Cranium!

i have read on this forum that new radio speakers have more of a magnetic field on them and they cause your tac to jump,they are close to together and some said you should put some barrier between the two,i am going thru prop wash build and i think he did something with to guard against that,i have not found it yet,because he installed the new speaker at the dash for his roadrunner build.

 

[rumblefish360]

The digital tachs (& other gauges/equipment) are super sensitive and quick to respond to any and all changes. Down to the most minor degree. What your seeing is normal engine behavior. The engine never rotates at the exact same speed for any length of time. And that is due to several factors from firing order, 18536472 is not the best order, engineers have admitted this, amount of fuel coming from the carb, more and less due to engine poise, vacuum, runner lengths, plenum size & design, carb size and jetting, the day's condition, hot, cold, dry, humid, rainy .... electronic interference, distributor mechanical fluctuations...

You get the idea....

 

[WileE]

Show us what you mean by "jiggle"

 

[YY1]

I have a "ScanGuage II" OBD II interface in my P/T daily.

It has a digital tach built in.

Even that new of an engine varies as stated about 10-30 RPM.

 

[Speedbird]

That gives me an idea for new type of tach... where's Richard Cranium!

He also said revealing.
There is no "revealing" where RC is, apparently.

 

[WileE]

I do not think of a tach, when the words "jiggled" and "revealing" are mentioned in the same context.

 

[rumblefish360]

I think something like this, but, I could be wrong...
https://media4.giphy.com/media/11R9HQz9EYnLz2/giphy.gif

 

[Bruzilla]

I think something like this, but, I could be wrong...
https://media4.giphy.com/media/11R9HQz9EYnLz2/giphy.gif

BINGO!

 

[mopar 3 B]

I can keep track of a needle without reading numbers. Spinning digital has never appeared practical.
A digital spinning gun has saved me from more than a few nasties where if a needle had been used the results would have been quite different.
Just my opinion.

 

[moparwacko]

If you're seeing only 20 RPM difference, congratulations, you're better than all new cars. Opening the hood will cause more disruption.
Ok, now let's move on to more exciting stuff.
Jiggling, revealing, and now you add spinning? I can see this going blue before long.

 

[kiwigtx]

I think something like this, but, I could be wrong...
https://media4.giphy.com/media/11R9HQz9EYnLz2/giphy.gif

That there is some good numbers on the JPM (Jiggles per minute)