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#1 |
< 69 Blazer
Join Date: Mar 2004
Location: Nevada, TX
Posts: 293
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Tach reading way too high ?
I bought a tack/air pressure dash from a swap meet early this year. It came from a big truck and there is an idiot light on the fuel gage for 'tandem axle'.
I replaced the air pressure gauge with a vaccume gauge and put a normal bezel on it and put it in my blazer. I hooked the tach up to my HEI (tach spade). The probem is that the RPM reading is WAY too high for what the engine must really be turning. I have a TH350, NP205 , 3.07 gears and 33" tires and it was reading close to 4000 rpm at 70mph (if I remember right). Why is my tach reading way off like that? It returns to 0 when off and reads about 700rpm at idle (but I'm guessing it really idleing around 500rpm). Could I have a 6-cyl tach? If that could be the problem did 6-cyls come in big trucks with tandem axles? ![]() |
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#2 |
No,I DON'T have Tourett's
Join Date: Mar 2005
Location: Reno, NV
Posts: 556
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most early "big" trucks were 6 cyl
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'72 K5 CST 350/350 Daily Driver '69 GMC3500 Camper Special CST Flatbed 396/400 Upgraded to show truck '69 GMC 2500 L6292, 3spd beater |
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#3 |
Senior Member
Join Date: Sep 2004
Location: CANADA
Posts: 1,681
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most i have seen here are 8cyl
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#4 |
Senior Member
![]() Join Date: Apr 2000
Location: madison indiana
Posts: 3,839
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it will be stamped on the back of the tack 6cyl or 8 cyl. and there is a screw on the back also, it for adjusting the tack reading, try playing with it to see if you can get it to read correct, but first i would hook up a know good tack to check the one you got.
jamie
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jamie72 72 BB cheyenne -- Restoration getting started.. member #807 http://www.myspace.com/whitetailobsession www.antlertalk.com |
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#5 |
Senior Member
![]() ![]() Join Date: Jan 2005
Location: CA
Posts: 5,527
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Engine RPM = Gear Ratio x Speed x Transmission Ratio / Tire Diameter / .002975
Engine RPM = 3.07 x 70 x 1 / 33 / .002975 Engine RPM = 2188 You're right. It is reading way, way too high. If this is being caused because it's a 6-cylinder tach, then it should be reading 2188 x 8 /6 = 2917 RPM, which is still quite a bit lower than what you're seeing. Something else must be wrong. |
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#6 |
its all about the +6 inches
Join Date: Jan 2001
Location: Hilliard Ohio
Posts: 2,693
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yeah, stock tachs sucked 30 years ago.
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#7 | |
< 69 Blazer
Join Date: Mar 2004
Location: Nevada, TX
Posts: 293
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Quote:
I'll pull it out and see what the tach says on the back. If it is indeed a 6-cyl, how hard is it to replace it? (if I have to buy a new tach to put in this, then I'll have too much money in the darn thing ![]() ![]() |
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#8 |
Senior Member
Join Date: Nov 2003
Location: Brookings, SD
Posts: 10,497
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Even if it is a 6 cyl tach you may have enough adjustment with the screw to get if pretty close.
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Some people are like slinkies, they aren't good for anything, but you can't help but smile when you see one tumble down the stairs. |
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