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Posted by: TGM Jan 13 2007, 07:08 PM

My Hyfire box is set at 6500 for rev limit but is kicking in at around 5750 rpm's according to my new autometer tach. Here is some background:

Stock tach was cutting out around 6K (needle drops to zero) then after a few more seconds at full throttle the rev limiter would kick in. I concluded that the stock tach was reading fairly accurately. Tach is wired to tach output from MSD box not the coil. Stock tach worked fine at lower revs but exhibited the typical excessive bouncing. Everything I read suggested replacing stock tach with Autometer but now the rev limiter is activating at 5750.

Should I hook up diagnostic tach to check the new autometer? Will this type of tach work with an MSD box? Sure as heck don't want to over rev my new Raby 2270. What do you guys think?

T.G.

Posted by: Aaron Cox Jan 13 2007, 07:11 PM

so is it a mallory hyfire box? or an MSD 6al? similar but not the same....

the tach driver on my hyfire DIED. a clean signal can be had by tapping into the white? wire from the dizzy to the box...this tells the box when to fire....

see if that makes any difference.....

and your box is set for a 4cyl etc right?

autometers are pretty accurate

Posted by: TGM Jan 13 2007, 07:32 PM

yeah its a mallory. checked the switches for 4 cylinder and dial for rev limiter and everything is ok. I'll try sending the distributor signal to the tach instead. I guess I could also wire up the VDO unit and see how they compare.

Posted by: Aaron Cox Jan 13 2007, 07:50 PM

you TEE into the trigger signal to the box....

it is setup like this now

dizzy white wire ---- some wire on the box (forget color and its cold outside)

so now it goes white wire --|---- some wire on the box, and | is the piggyback spade wire to the tach... i

Posted by: Mueller Jan 14 2007, 01:21 AM

QUOTE(TGM @ Jan 13 2007, 05:08 PM)

Posted by: TGM Jan 14 2007, 12:19 PM

I've now wired up the tach using the tach output and the signal from the distributor and both work the same. I also wired up two VDO tachs from my parts stash and both agree and read about 300 rpm higher than the autometer at 3000 rpm. I'm starting to question the accuracy of my new tach. The rev limiter seems to activate when the engine ceases to generate power which given the cam should be around 6500 rpm not the 5800 indicated by the autometer tach. Oh well I guess its time to go buy a diagnostic tach or digital timing light as suggested by Mueller.