: tach

05-11-06, 09:37 PM
I have a 1980 coupe dev. with a 368 cu.in. I would like to put in a underdash tach., how involved is this and where would I make the connection?. tnx.

05-11-06, 10:02 PM
tech tips is NOT for questions

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05-11-06, 11:26 PM
Most aftermarket tachs are easy to install. They usually come with a 12V power cable, a ground cable, and a lead that attaches to the coil. Some come with a lead that goes to the light switch as well. You just pick the spot where you want it, mount it, and the wiring is easy.

Night Wolf
05-12-06, 12:03 AM
Very easy. Mine was mounted to the column. black is ground, red is +12v, white is illumination (wired into the light switch, it dimmed with the lights) and green goes to "Tach" on the distrbutor.... hardest part was fishing the wires thrut he fire wall... the tach on my '79 was a blast! I did pull that, but can't find a spot to put it on the Coupe.....





The redline was an unoffical 4,200-4,400RPM... when in drive she would shift into 2nd at 3,900RPM, around 55mph and into 3rd around 3,400RPM, around 80mph... if manually held out to 4,300RPM or so, it would do over 60 in first and top out at 113 in 2nd..... which is why if you leave it in drive, you are not getting the full performance... yes, at 4,200RPM its way outta its powerband, but it won't push 3rd that far, or atleast that fast... I got 3rd to 105 and it was pretty steady, 113 was in 2nd and there was still a bit more, but the revs were getting up there....

Man, I miss that car... gosh... never thought I would miss the 425 and 2.28 gears so much but damnit I do... all I can say is, atleast I had lots of fun with it and lots of memories were made.... I'll never forget the 1,400 miles in 36 hours... that car was THE best highway cruiser I have ever been in.....

05-12-06, 09:38 AM
That seems like such a low redline, but when you think that it's a 7 liter pushrod V8 that never was meant to be a high revver then it makes sense. What did the 500 and 472 redline at?

P.S. That 425 really sounds nice with that aftermarket air cleaner setup you had!

Night Wolf
05-12-06, 10:57 AM
All BB Cads "redlined" around 4,000-4,400 RPM... if anything, the 425 would rev higher due to low stroke. The valve train is the weak link on these engines, with an upgraded valve train they can see 5,000-5,500 RPM pretty easy. I would personally have no desire to go higher then that... these things are tuggers, not revvers. They can sit at 2,000RPM all day and pull a house. I would always enjoy watching the tach on the '79, just because its so different then cars of today... merge onto the on ramp doing 30, hit the gas right a bit right before the secondaires open up, it'll downshift into 2nd and the RPM will jump to 2,000, keep it at that throttle input (not floored) and the car will accelerate very briskly 30.....40.... now its at 2,100RPM....50.....60... now its at 2,300RPM and it shifts into 3rd, then the RPM would drop to 1,900 and it would keep pulling.

The only way to get it above ~2,000 RPM when in drive would be to floor it and open the secondaries.. but it dosn't need to. Driving around under 2,000RPM in that car was about 70% of the feel of wide open, just because all the torque, and the gearing.

When driving over the mountains coming down to FL, there was all the steep hill signs for trucks to downshift, when I crossed it in the '79, it was at night, but I remember where I was... these were steep grades and I had the cruise control set at 75, my friend and I were crankin some music... when the time came to go up the mountian, 7 miles of doing up, I was watching the tach, it went from the 2,000RPM from normal ground, to about 2,200RPM while going up the hill... I coudln't believe how that steep hill didn't even bother the car at all... just for fun I downshifted into 2nd and put it to the floor, and it put me in the seat.

The 425, 2.28 gears and '79 DeVille werent the best for drag racing from the factory, but I tell ya, it was a very neat combination, just what it would allow the car, and engine to do, like maintain a very nice RPM (torque peak) at a high cruise speed without over drive.

All it had was a 14" Edelbrock open element air cleaner, I liked the sound of it too... only a slight suction sound at part throttle, but open the secondaries and.... well, you heard it :).

Once the secondaires opened, it brought new life to the car, more power, downshift and alotta sound... it was fun to drive around with people for a long time not openeing the secondaries (I knew just where that point was) then when they would least expect it, while making a turn or going onto an on-ramp, I would floor it around 30.... it would surprise everyone :).

05-12-06, 11:28 AM
Redline is determined more by the cam profile than the bottom end. Even a 472 or 500 could probably go past 4500rpm with exploding, just like your average 350 chevy will go past their usual 5000-5500rpm redline.

But, like you said with the stock cam profile you are just making alot of noise to take the engine up that high. You'll go faster by shifting and moving back into the powerband.

Night Wolf
05-12-06, 02:42 PM
Yes, the "redline" is generally determined by the valvetrain...

I was just saying, the valve train on any BB Cad is the weak link... in comparison to the rest of the engine, its is the weakest link by a long shot (in durability, not making power) Get a more heavy duty valve train in, and thats where you can start getting the power boosted nicely.

I woudln't want to push and BB Cad much past 4,400RPM, its asking way too much.... ONCE I ran first gear to 4,600RPM.... I had the music cranked up loud and did a wide open throttle run from a stop.... what I didn't realize was that I put it into low, as I kept my foot into it, a bit later I got worried that it wasn't shifting when it should be... I could still hear the engine, but mostly just felt the way the car was accelerating... it wasn't until I looked down at the speedo, saw it approaching 70, looked at the tach, saw it beyond 4,500RPM, and looked at the shifter indicator and saw the needle right over Low...... I was really pushing it that time, and I would never go that high again. 4,400 was pushing it... Unless I was crazy enough to verify the top speed, in 2nd gear, I would usually manually hold it back into 2nd until 4,100RPM then bump it into 3rd, when I was in the mood to push it, which was often....

If you put a tach in any BB Cad, I woudln't go over 4,200RPM all that much. a 472 or 500 may even be lower due to the increased stroke.