very smooth through the RPM range.... all with a blue coil. I have an ignitor coil to go in also......

But...as we all know...tach no workie with CDI....

and...as i read here....
914world.com writeup on how to mod a tach....

finished soldering. now for reassembly...

AA

i suck at taking pics....

but... you jumper the initial signal resistor according 'agrump'

update again...

tach mod works!..also.... late model black center tachs require no mod...they work right out of the box (black center vs silver center gauges)

Aaron,

When do your classes start?

QUOTE (Headrage @ Aug 26 2005, 02:37 PM)

Aaron, When do your classes start?

sept 19th - why?

That really is an Andy class picture...

Specifically, what's going on here is the old tachs depend on the fact that the pulses coming off the coil are spikey from noise (up to 30v or so), and there's a resistor at the signal in to limit these spikes. A better way would've been to use a Zener shunting the spikes to ground, but a resistor will do. The mod is to bypass this resistor, so the clean 12v square wave from the CDI boxes will trigger the tach. You'd have to do the same if you used the tach signal from an aftermarket ECU.

The later tachs don't have this problem, perhaps because they went to a shunt diode instead of just using a resistor, which will work with either the 30v spiky signal OR the 12v square wave signal. No doubt they were responding to complaints from people trying to use aftermarket CDI boxes.

Glad the mod worked for you.

I had problems using a hyfire with a bosch blue coil. I know others have done it without a problem but I ended up blowing up two boxes before giving up on the coil. It would work for about a week and then it would start crapping out. The coil I had had the built in resister from aircooled.net. The coil would get hotter then hell even without the hyfire, it may have been defective. Good luck!

QUOTE (agrump @ Aug 26 2005, 05:20 PM)

Glad the mod worked for you. I had problems using a hyfire with a bosch blue coil. I know others have done it without a problem but I ended up blowing up two boxes before giving up on the coil. It would work for about a week and then it would start crapping out. The coil I had had the built in resister from aircooled.net. The coil would get hotter then hell even without the hyfire, it may have been defective. Good luck!

that sucks.....

it worked for 6 months straight with the mallory unilite.... (which is said to be tempermental if a coil is not the correct ohmage 1.4-3.0 ohms)

i have a pertronix flame throwe 45k volt 1.4ohm coil sitting here..... should i try that?

Dean -

also - the tach mod works great....except at idle. tach jumps around and usually sits at 2000 rpm at idle, but any throttle and it is dead on!

deciding if ill keep that tach, or harvest a late model tach guts....

My blue coil worked fine with my unilite as well. Jake said he runs one of his cars with a hyfire and blue coil with no problem. I talked to Mallory about it and of course they recommended one of their coils. The guy I talked to did mention that he was working with someone else having problems with a blue coil. Check out Mallory's site, they have a tech forum that may have more info.

Not sure about the tach at idle, mine worked fine.

Awesome aaron! Im glad to see it worked out for you. That tach looks purdy in your car

Have you driven the car with the CDI? Does it sound/feel any different?

QUOTE (MattR @ Aug 26 2005, 05:55 PM)

Awesome aaron! Im glad to see it worked out for you. That tach looks purdy in your car Have you driven the car with the CDI? Does it sound/feel any different?

alot smoother.... no hesitation....

gee...imagine if i just jetted it right

Nope went with a monster tach from Autometer. Best 100 bucks I ever spent. Set the shift points and it is dead nuts accurate. Using Petronics flame thrower coil.

QUOTE (lapuwali @ Aug 26 2005, 01:41 PM)

The later tachs don't have this problem, perhaps because they went to a shunt diode instead of just using a resistor, which will work with either the 30v spiky signal OR the 12v square wave signal. No doubt they were responding to complaints from people trying to use aftermarket CDI boxes.

James, Do you mean later 914 tachs or contemporary Porsche tachs? Would my 76 tach have the shunt diode?

I am such a doofus when it comes to electrical stuff!

QUOTE (Joe Ricard @ Aug 26 2005, 06:04 PM)

Nope went with a monster tach from Autometer. Best 100 bucks I ever spent. Set the shift points and it is dead nuts accurate. Using Petronics flame thrower coil.

Joe what ohmage coil???

i have a 1,5 ohm flame thrower coil to use...

Damn Aaron, you have been a busy beaver with your teener! Your BLOG is very cool! Check out my tach, crank fire = no problemo....

QUOTE (Aaron Cox @ Aug 27 2005, 07:07 AM)

QUOTE (Joe Ricard @ Aug 26 2005, 06:04 PM) Nope went with a monster tach from Autometer.  Best 100 bucks I ever spent. Set the shift points and it is dead nuts accurate.  Using Petronics flame thrower coil. Joe what ohmage coil??? i have a 1,5 ohm flame thrower coil to use...

IDONNO, Ran great with points and now runs greater with Mallory, triggered by compufire. No resistors in line anywhere. most of the writing on the coil is worn off.

QUOTE (maf914 @ Aug 27 2005, 05:19 AM)

QUOTE (lapuwali @ Aug 26 2005, 01:41 PM) The later tachs don't have this problem, perhaps because they went to a shunt diode instead of just using a resistor, which will work with either the 30v spiky signal OR the 12v square wave signal.  No doubt they were responding to complaints from people trying to use aftermarket CDI boxes. James, Do you mean later 914 tachs or contemporary Porsche tachs? Would my 76 tach have the shunt diode?

I'm bumping this one in the hope that someone knows about early versus late tachometers.

QUOTE (maf914 @ Aug 29 2005, 08:58 AM)

QUOTE (maf914 @ Aug 27 2005, 05:19 AM) QUOTE (lapuwali @ Aug 26 2005, 01:41 PM) The later tachs don't have this problem, perhaps because they went to a shunt diode instead of just using a resistor, which will work with either the 30v spiky signal OR the 12v square wave signal.  No doubt they were responding to complaints from people trying to use aftermarket CDI boxes. James, Do you mean later 914 tachs or contemporary Porsche tachs? Would my 76 tach have the shunt diode? I'm bumping this one in the hope that someone knows about early versus late tachometers.

Sorry, didn't see this. "Early" 914 tachs (w/ silver center button) v. "late" 914 tachs (w/o silver button). The changeover was in '74 or '75. ('76 = '75, they were all made in '75, just leftover inventory).

I'm guessing on the shunt diode, btw. It seems to match what people see, but I've never pulled one apart to verify what's inside.

a little update.....

car runs fantastic. BUT.... every once in a while i get an intermittant NO-Spark and the engine begins to decel, and then a second later refires with a hellacious backfire.

checked all the connections. 2 people ive spoken to suggest a bad ground, or a relay that controls the ingition (skline had a no ignition relay failure also )

my neg, battery terminal did not have a place to hook up the negative lead from the CDI, so i grounded it to the engine block..... could this be the issue?

James, Thanks for the reply and info. I'm hoping my 76 tach will be able to read the signal from a Mallory Hyfire box.

QUOTE (maf914 @ Aug 29 2005, 10:57 AM)

James, Thanks for the reply and info. I'm hoping my 76 tach will be able to read the signal from a Mallory Hyfire box.

i tried a late model tach (74-76) and it worked fine......

Oh, yeah! That's good news. Thanks Aaron and James.

QUOTE (Aaron Cox @ Aug 29 2005, 10:56 AM)

a little update..... car runs fantastic. BUT.... every once in a while i get an intermittant NO-Spark and the engine begins to decel, and then a second later refires with a hellacious backfire. checked all the connections. 2 people ive spoken to suggest a bad ground, or a relay that controls the ingition my neg, battery terminal did not have a place to hook up the negative lead from the CDI, so i grounded it to the engine block..... could this be the issue?

help me on this....

all of a sudden, no ignition...and a second later it refires....and makes a huge backfire...and drives great

Move that ground to the chassis instead of the block.

QUOTE (Travis Neff @ Aug 30 2005, 09:18 AM)

Move that ground to the chassis instead of the block.

thats what im going to do today....