So once I got my 2056cc built the tach could no longer keep up and just flailed around worthlessly... Especially when running an autocross. So I finally got around to trying the 2200uF trick and now it's over damped... Sigh. Maybe I'll try a different variant of capacitor and see what happens.

Anybody else have a recommended type of capacitor for the application:

http://en.wikipedia.org/wiki/Types_of_capacitor

Maybe halving the farads will help?

My EE friend thought that changing the type is probably a better bet. I have a rather large, polarized, electrolytic style - like you would use in audio. He was thinking of trying a non-polarized style.

It moves kind of like the old, mechanical ones you see on 20's and 30's race cars. Very jerky and then "sticks".

Where did you get the info on the capacitor? I thought that the capacitor changed the RPM factor (like changing it from reading a 4 cyl to an 6 or 8 cyl). A large electrolytic one is not necessary. You can get a small capacitor at Radio Shack for a few cents and that should do it. The capacitor is basically acting as a clip on the RPM wave form. Like turning down the volume. Right now you are turning the volume down too far so a lower farad cap would give you a smaller time constant thus allowing more of the RPM signal through. You can see where changing the cap could dope the signal enough to stall it and make the needle rise slowly enough to match the RPM of a 6 or 8cyl.

I would think that gutting a universal tach and putting the guts into your case with your face and needle would be a more cost effective and time wise endeavor.

It's just what the EE stock room on campus had... I've grown accustom to not really having to read the tach as I have a Mallory 6AL (rev limit) and can feel the engine really well.

That being said it would be nice to have something functional there... I'll try another, smaller cap first and see what happens.

IIRC, the 2200uF was the one used to convert to a V8 tach.

I have heard of people greasing the needle shaft with some semi viscous lube to slow its bounce. This may also be a quick and simple solution for you. It would dampen the needle movement without changing the value.

Mine also bounces in the 2056. I just figured it was a function of me needing to be smoother in my driving - as the smoother I am, the less bouncy the tach is.
Zach

I used a 2200uf cap that i pulled off of an old motherboard i had laying around, there are usually a bunch of them surounding the CPU. Works nice and smooth now however i also modded my tach to take the lower voltage output from a spark box.

same here

more aggressive I drive, more it bounces

I'm getting the feed off my Mallory 6AL. How or what mod's did you make?

I'm not sure if this helps (free advice, take for what it is worth). I jumped a resistor to get the tach to accept the signal from an EDIS module. It also worked with a the tach out line from an SDS ECU. I haven't measured it but, I suspect both of the fedds I used were 5V.

Dave

Jumping a resistor makes some sense as well. It is like opening up the volts after it is doped down by the cap. However, it could lead to escaping magic smoke

Magic smoke is still contained (knock on wood).

The Tach is dampend with a pin coming off of the meter movement that rides in a small amount of grease or silicone. After time this dries and no longer functions.
I have cleaned the old grease out and used vasoline to replace it. It works great the trick is getting the right amount and put in the right place.
I had done a write up on the old Yahoo porsche chevy group on how to do this and recalibrate the tachs for a V8 or 6cyl.
With that being said I really think the best thing to do is shit can the 35 year old tach and get one from BIGMARK with up to date movements

Bob

My tach justo bounce then i start the car one day and it didn't work. I have a good sense of where the rpm is at so i dont really pay attention anymore.

Had a similar problem and found the connection on the capacitor was not good. Cleaned it up, put a new connector on and works like a charm now...

Remember that capacitors add in parallel. You can go from a 2200 uF cap to 2400 uF by wiring a 200 uF cap in parallel with the original cap. If they are electrolytic then they have a polarity and should be wired the same, + to + and - to -.

I just installed a 2315 engine in my ride and have the same bouncy tach issue. Need to get it dampened out so I'll be adding 200-400 uF at a time until I get the needle response I want.

Steve

After my PCA event on Sunday I noticed it's really not all that bad. It's actually quite functional in real world conditions. It still can't keep up with blipping the throttle and it also appears I didn't reset the needle quite right. It's almost 500 RPM off, so I guess I need to open it up anyway.

Someone needs to invent a variable capacitor for dampening purposes... I'd mount it outside the housing. Dreams...

Just send it to North Hollywood or Palo Alto and tell them it bounces too much. They will fix it.

Some things are just worth the cost to have fixed.

Thanks, but with the Mallory 6AL doing the rev limiting my money is better spent on tires and suspension right now (all things being equal).

There are variable capacitors. You could mount one on the outside

Holy Crap! I was just making that up! I had no idea they were real...

http://en.wikipedia.org/wiki/Variable_capacitor

That was part of the tuning mechanism of old school radios

If you are running carbs then...

I cured my bouncing tach by grounding the relay board in the engine bay - see the classic thread http://www.914world.com/specs/SirAndyCarbFuelPumpRelay.php

Just run the wire from the driver's side front connector to ground - suggest you use black wire rather than red since it is a ground.

I use the unfused +12v connection to run my fuel pump, oil cooler fan and accusump relay - with fuses of course, and red wire.

I also cleaned the body ground connection next to the relay board in the engine bay.

Of course, I also added a 2200 uF capacitor across the movement before solving the problem. And I got a nice clean tach glass as well.

The right capacitor is not an exact science. What it does can be seen below. The oscillatory response is what the stock tach does since it is under-damped. The step function is what you want but is basically unattainable.

The critically damped response is the one that gets to the desired level as fast as possible without overshooting.

The overdamped response which is what you seem to have just takes too long to get to the right RPM.

Lowering the value of the cap should help. I tested mine before reassembling the tach.

I'm guessing some of the reason for my tach needing a lower cap value is that it's getting it's feed from the tach output of the Mallory 6AL vs. the coil.

I am using the 6AL as well. Check out the pic that Dave Hunt posted, that is the modification i am talking about. The signal coming from the 6AL is not as strong as it would be off the coil. Jumper that resistor and see if it helps. With the 6AL, the resistor bypassed, and the 2200uf cap across the windings, mine is nice and smooth.

-James

Will do. Might pull it back out of the dash tonight.