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RPM doesn't seem right

Bstoker87

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After completing a manual swap on my uncles 70 Cuda and driving it I've noticed what i believe is a RPM discrepancy.

3.55 rear gear, 27" tall tire, .68 over drive gear at 65mph the car is at 3k RPM but feels like its about 2k.

At idle it sits around 1,200 RPM but acts like it wants to die and sounds like it's really at 550-650 RPM.

Nothing on the engine was changed during the swap, the car has a MSD 8387 ready to run distributor and a MSD blaster coil.

I'm new to these cars but was wondering if the stock guage could be that far off calibration or if more likely a signal issue?
 

moparleo

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The factory tach ( I am assuming a Rally dash) may not be compatible with that ignition system. The factory tachs are easily modernized with an electronic internal conversion. These tachs were not that accurate eve when new.
You can't always just swap parts in an electrical system and assume everything will just work together.
Simple answer is to buy/borrow a modern tach and just hook it up and test it against the cars tach. How do you even tune the car with out knowing Rpms and such ?
The installation instructions state that the rpms may not be accurate with all tachometers.
I would contact their Tech department for answers. It is free and you paid $$$ for the distributor.
 

Bstoker87

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Yes on the Rallye Dash, I've done 0 tunning to the engine, that was done by my uncle's who installed the distributor and coil.

I have a newer Auto Meter tach i planned on hooking up but was looking to see if anyone has ran into this possible issue before.
 
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Challenger RTA

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As moparlo said: not compatible. some aftermarket tachs have a 4-6-8 cylinder switch to change the resistance. The factory tach or distributor may need a balance circuit. Or the set up just might not work. contact tech department to see if applicable.
 
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Bstoker87

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I ended up hooking up a Auto Meter tach and here are the reaults.

Car- 1,100/ Auto Meter 800
Car- 2,000/ Auto Meter 1,250
Car- 3,000/Auto Meter 2,000
Car- 4,500/ Auto Meter 3,000

My uncle's are happy to know the difference but at this time don't plan and doing anything but driving the car.
 

Challenger RTA

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Ok now you may be getting into a voltage issue. Where did you connect the Auto Meter Tach? Good and bad connections voltage limiter. some MSD coils were for race only.
 

Bstoker87

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Auto meter was connected directly to the battery for power and the grey tach signal from distributor.
 

Challenger RTA

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An other weird thing I experienced. check the distributor cap inside and out for corrosion. I seen the tach read wrong from that.
 

fasjac

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After completing a manual swap on my uncles 70 Cuda and driving it I've noticed what i believe is a RPM discrepancy.

3.55 rear gear, 27" tall tire, .68 over drive gear at 65mph the car is at 3k RPM but feels like its about 2k.

At idle it sits around 1,200 RPM but acts like it wants to die and sounds like it's really at 550-650 RPM.

Nothing on the engine was changed during the swap, the car has a MSD 8387 ready to run distributor and a MSD blaster coil.

I'm new to these cars but was wondering if the stock guage could be that far off calibration or if more likely a signal issue?
You could just hook up a digital timing light and check the RPM.
( if you have one )
 

Sinitro

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Note the OE factory tach is not compatible with later after-market transistor ignition modules,
it may read but will be inaccurate. The tach has to be matched to the iginition module..

Just my $0.02....
 
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