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Tachometer vs HEI Limiter

General engine tech -- Drag Racing to Circle Track

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BobbyB
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Tachometer vs HEI Limiter

Post by BobbyB » Fri Jun 26, 2020 3:50 pm

New engine mated to an old ford c4 transmission.
Using Petronix D72000 hei module with built in rev limiter.
New engine was broken in on an old duraspark ignition and ran about 1000 miles without problems using duraspark.
Installed HEI on car and set hei rev limiter to 6500 rpm. Car runs great and has run great for 3000 miles on new engine.
Car has an air shifter activated by the shift light from the 30 year old Mallory tachometer. It has worked great for years.
Set shift rpm to be about 5900 rpm...shifted perfectly about 20 plus times...never really looked at the tach.
Decided to up shift point to 6100 ... seemed to run against the rev limiter before the shift occurred.
Pertronix says the hei limiter should be +or minus 50 rpm from set point.
Reset hei rev limit to 6400... it acts the same.
Plugs look good but changed them any way. Acts the same. Checked wiring and added an extra ground. Acts the same.
I have played around with lowering the shift point to 5000 and even 4000. The shift point works to within 100 rpm based on me and my son watching the tach.
I tried comparing the Mallory tach to a cheap tach with both attached to the negative of the coil together,they did not agree well at all. I doubt the cheap tach.
The Mallory tach seems to act like it always has.
How can I verify that my Mallory tach is reading true and that my shift light is reading true?

superpursuit
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Re: Tachometer vs HEI Limiter

Post by superpursuit » Fri Jun 26, 2020 10:27 pm

Send the tacho out to be checked and / or calibrated.

77cruiser
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Re: Tachometer vs HEI Limiter

Post by 77cruiser » Sat Jun 27, 2020 12:05 pm

Can it be set to no limit, just to try?
Jim

BobbyB
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Re: Tachometer vs HEI Limite

Post by BobbyB » Sat Jun 27, 2020 1:05 pm

77cruiser wrote:
Sat Jun 27, 2020 12:05 pm
Can it be set to no limit, just to try?
Yes, but I don’t want to risk trying it because I wont really know the engine speed. If I do that, I will be believing the tach is reading correctly and that the HEI rev limiter is wrong.

I will probably send the HEI module back to Pertronix and have them be sure the rev limiter is working correctly.

With confidence in the HEI rev limiter I can live with the tach and shift light reading a little low, though I would really like to have everything read correctly.

I was hoping someone would tell me a really easy way to verify the tach reading but maybe there is no easy way.

I don’t know who I could send the tach to to have it calibrated.

Thanks for the reply’s.

BillK
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Re: Tachometer vs HEI Limiter

Post by BillK » Sat Jun 27, 2020 1:46 pm

Try turning off the rev limiter and shifting at 6100 manually ? See if the engine will go to 6100 that way.
Bill Koustenis
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Waldorf Md

www.enginerepairshop.com

BobbyB
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Re: Tachometer vs HEI Limiter

Post by BobbyB » Sat Jun 27, 2020 2:19 pm

BillK wrote:
Sat Jun 27, 2020 1:46 pm
Try turning off the rev limiter and shifting at 6100 manually ? See if the engine will go to 6100 that way.
Thanks Bill, I tried that and it would not go past about 6000 according to the tach. It acts like the HEI rev limiter is set on 6000 when it is set on 6400.

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Re: Tachometer vs HEI Limiter

Post by BillK » Sat Jun 27, 2020 8:17 pm

BobbyB wrote:
Sat Jun 27, 2020 2:19 pm
BillK wrote:
Sat Jun 27, 2020 1:46 pm
Try turning off the rev limiter and shifting at 6100 manually ? See if the engine will go to 6100 that way.
Thanks Bill, I tried that and it would not go past about 6000 according to the tach. It acts like the HEI rev limiter is set on 6000 when it is set on 6400.
Well that certainly sounds like a problem with the rev limiter. I guess the only way to check the tach is maybe borrow another one and try it.
Bill Koustenis
Advanced Automotive Machine
Waldorf Md

www.enginerepairshop.com

Geoff2
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Re: Tachometer vs HEI Limiter

Post by Geoff2 » Sun Jun 28, 2020 5:10 am

Most of the HEI modules I have seen with adjustable rev limiters that use a screwdriver slot for adjustment are a potentiometer - a variable resistor. Most use a cheap carbon track as the resistance medium & it can change their value [ ohms ] with age & heat. Hence setting the rpm at a particular setting only to find the limiting occurs at a different rpm to what was set.

BobbyB
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Re: Tachometer vs HEI Limiter

Post by BobbyB » Sun Jun 28, 2020 8:38 am

BobbyB wrote:
Sat Jun 27, 2020 2:19 pm
BillK wrote:
Sat Jun 27, 2020 1:46 pm
Try turning off the rev limiter and shifting at 6100 manually ? See if the engine will go to 6100 that way.
Thanks Bill, I tried that and it would not go past about 6000 according to the tach. It acts like the HEI rev limiter is set on 6000 when it is set on 6400.
CORRECTION!!! I did NOT turn the rev limiter off when I tried manual shifting. The rev limiter on the HEI was set at 6400 And the engine would not rev past 6000 according to the tach. Sorry for the misstatement.

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