After my blowers been in, I've noticed my RPM tach is very jumpy under WOT. What could cause this? I have a rev limiter in (MSD pill)set at 5500, and one in the PROM at 6200.

But the tach starts acting funny past 3k...

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91 Formula
305 TPI 5speed
1LE/G92/WS6

Paxton SN92 polished kit, SLP airfoil, ported/polished plenum, March pulleys, Crane AFPR(43psi),Crane Gold 1.6rrs,MSD coil,MSD6AL,MSD boost-master,Holley 9mm wires,Ac delco R43ts plugs,custom chip,Ford SVO 24# injectors,Bosch O2sensor, Bosch in-line pump(w/stock pump), SLP 1 5/8" headers,SLP catback,short shifter,3:73s w/Auburn posi,170* t-stat,JET 195* fan switch, Macewen white face gauges, Autometer gauges, Zoom hi-performance clutch.

 

Ryan,
though I'm not certain I would tend to think that the ignition is breaking up an arcing in the cap. Pull your cap and look for signs of carbon tracking. How does it run under boost?
Chuck

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87 Formula 350 13.94@96.67
98 Z28 1LE 13.15@107.59
1LE Owners Association

 

Hey Chuck, the car runs great under boost. The cap, rotor, plugs, and wires are all new. And I run an MSD6AL. I have no idea what this could be
Ryan

 

My tach is jumpy around idle. Some ignition boxes need a tach signal amplifier to work. The square wave 12v on the tach lead is a little weaker than the stock signal.

Lots of places sell the amplifier but it would probably be better to go with the same brand. This may or may not be the prob, just my .02

 

You need a tach adapter for the 6AL

 

It never used to do this until I installed the blower and 24# injectors though

 

if you had a spark problem, then I'd think you wouldn't be running so great under boost conditions.

I have seen fluctuations in charging cause a tach to be jumpy. A tachometer is much more sensitive than your voltmeter. A failing alternator or regulator can cause this problem.

 

I do have an underdrive pulley on the alternator if that matters at all.

My battery voltage is 14.2, my fuel pump voltage 14.4. So I don't think it is a charging problem really. Maybe though.

 

nah, I didn't say low voltage,
but fluctuating voltage..
too quick for a voltmeter to pickup, but a tach will.

 

hmm. interesting.

Oh well. It doesnt look like it's affecting performance. I can always verify the RPMS with my auto-xray.

Thanks for the help ODB