What causes tach spikes to occur @ certain RPMS?

shaodome · Post by **shaodome** » Sat Nov 27, 2004 5:07 pm

For the last week or so i've been battling a tach spike that occurs under light load around 4k rpms. Being that thanksgiving came, I've got a cold, and the weather is crap I haven't had much time to fiddle with it yet.

I was just wondering why the tach spikes would ALWAYS occur in the same place and only under light load?

Bernard Fife · Post by **Bernard Fife** » Sat Nov 27, 2004 5:22 pm

shaodome,

I'm guessing here (and I'm sure someone will correct me), but I think this is the result of a couple of factors:

- some sort of resonance occuring in the circuits (i.e., one or more of the circuits involved is acting as a sort of antenna or oscillator), (this is why they are at the same rpm),

- and the engine requires more voltage to fire at light loads (where the combustion chamber density is low and the mixture is relatively lean), raising the coil voltage, making it more likely to give problems, (this is why they are at light loads only).

If I recall correctly, one of the biggest challenges with lean-burn engines is trying to light the fire with the lean mixtures and low 'effective' compression.

Have you tried the Dave cap?

Lance.

Mike_Robert · Post by **Mike_Robert** » Sat Nov 27, 2004 5:52 pm

lance wrote:shaodome,

I'm guessing here (and I'm sure someone will correct me), but I think this is the result of a couple of factors:

- some sort of resonance occuring in the circuits (i.e., one or more of the circuits involved is acting as a sort of antenna or oscillator), (this is why they are at the same rpm),

- and the engine requires more voltage to fire at light loads (where the combustion chamber density is low and the mixture is relatively lean), raising the coil voltage, making it more likely to give problems, (this is why they are at light loads only).

If I recall correctly, one of the biggest challenges with lean-burn engines is trying to light the fire with the lean mixtures and low 'effective' compression.

Have you tried the Dave cap?

Lance.

Lance, I fully agree with premise #1. BTDT. I would recommend that the tach line be separated as far as possible from the high voltage parts of the ignition system.

However,

, I'm nearly certain that higher pressure nonionized gasses are more difficult to fire a decent spark through. I remember seeing a spark plug tester some years ago (high school shop, 1975?) - it was a small chamber with a glass window which allowed you to see the business end of the spark plug in question (which you screwed into a threaded hole of said chamber). An adjustable air pressure regulator providing pressure to this chamber would allow you to see the quality of a multivibrator/coil (no jokes here!) generated spark decrease as the pressure was increased. If there was no spark after a certain pressure the plug was defective. That said, I understand that lean mixtures are difficult to ignite and require more spark energy, stratified charge, etc., tricks to get them to light. I accidentally found earlier this week that a rotary can idle, very roughly, with an A/FR of 17:1 and an advance of >40 degrees at 1200 rpm. The timing light showed me the error of my way!

-Mike

whittlebeast · Post by **whittlebeast** » Sat Nov 27, 2004 6:29 pm

My current setup includes a 20k resistor in the tach wire, the Dave cap trick and oddly enough some code (ie *.s19 file) are more resistant than others. Ask the EE and code guys to explain. I am just a knob turner type HEHE

Andy

shaodome · Post by **shaodome** » Sat Nov 27, 2004 6:41 pm

The Dave cap is on my list of things to try.....but it is STILL raining outside....Sunshine State....yeah right

Thanks for all the info!

Bernard Fife · Post by **Bernard Fife** » Sun Nov 28, 2004 9:20 am

Mike,

I'm nearly certain that higher pressure nonionized gasses are more difficult to fire a decent spark through.

That could certainly be, and it may be that it's harder to ignite the leaner, less dense gases, but not harder to get the spark through it.

Thanks for the correction!

Lance.

spada · Post by **spada** » Sat Dec 04, 2004 4:59 pm

Jordan (shaodome) are these tach spikes actual tach spikes, or just megasquirt rpm spikes? i ask this cuz of course megasquirt rpm changes would be the fault of the crankshaft sensor, and actual tach spikes would come from the tach circuit off the coil.

anyways, i get a very similar tach spike. me and shaodome have the same car, and same megasquirt setup for the most part, basing our rpm signal off our crankshaft sensors in the distributor.

sorry for thread jacking, but this may help both of us. i get a spike at idle.... 600-700rpm. verified in mslvv, it's a jump to 1300rpm for like .1 seconds. this obviously changing the fuel and spark delivery for that spot, andi get a quick stutter at idle. it's not as noticeable since i put the msd blaster2 coil on, but i can see it still happening in my megatune, and my o2 voltage will go from a constant .75volts at idle, down to around .4v real quickly, then back up to .75 once the signal evens out again .1 seconds later.

would this be a wiring problem? or possibly a hardware glitch? or maybe it's just common with our sensors.

vic · Post by **vic** » Tue Dec 07, 2004 7:44 am

See this topic for clarity on tach signal spikes.
http://www.msefi.com/viewtopic.php?t=34 ... t=fighting