Question about my Tach - The 1947 - Present Chevrolet & GMC Truck Message Board Network

xtreme80 · 11-04-2007, 08:09 PM

I just put a new tach in the truck, and I'm not completely sure if it's accurate or not...

I did a WOT run in the truck, and according to the tach, it shifted first to second at 4000 RPM. Shifted from second to third at 4200 RPM.

Here's the engine I have...
http://www.jegs.com/webapp/wcs/store...55609_-1_10763

Now either the tach is inaccurate, or it's shifting too early, because the specs on that engine show 290hp at 5200 RPM, and I'm about 1000 below that according to the tach. I've got it hooked to a main power source in the fuse box, and I've got the actual RPM wire hooked to the "tach" terminal on the dist. cap. It also shows it's idling at about 500rpm in gear, which I think is a little lower than it actually is. I could be wrong though..

ChaseDale3and8 · 11-04-2007, 10:56 PM

Is this an aftermarket tach or a factory one? If it's an aftermarket, it should have a switch on the back of it for a 6 cylinder or 8 cylinder. It may be on the 6 cylinder setting and just need to be changed. I used to have a Sun tach in my 82 years ago and that's how that tach was.

If it's a factory tach, it's very likely that it came out of a 6 cylinder truck, and that's why it's reading off as the 6 cylinder and V8 tachs are different. ...Joe

DBLSPRT68 · 11-05-2007, 12:44 AM

If you put in a factory tach, do this. Turn your ignition switch to on, don't start it; your tach should read 0. If it dosen't, your tach is off/bad. I have one good tach in my truck, but have three more that are off/bad

David

xtreme80 · 11-05-2007, 10:34 AM

It's an aftermarket sun pro tach, and I already made sure it's on the 8 cylinder setting, but I guess it wouldn't hurt to double check that

71swb4x4 · 11-05-2007, 12:04 PM

I think I am missing something. Why would the specs on the engine determine when your tranny will shift? Wouldn't the tranny control the shift points? How does a tranny decide when it will shift?

Pyrotechnic · 11-05-2007, 12:59 PM

The transmission doesn't care what your engines power band is. Wide open throttle shifts are going to happen whenever they are set to in the transmission.

xtreme80 · 11-05-2007, 02:46 PM

Quote:

Originally Posted by Pyrotechnic

The transmission doesn't care what your engines power band is. Wide open throttle shifts are going to happen whenever they are set to in the transmission.

I know all of this.

What I'm getting at is that if the tranny is set to shift at 4200rpm, well then, the engine is capable of much more than that.

11-04-2007, 08:09 PM	#1
xtreme80 Wishin' I had more money! Join Date: Feb 2007 Location: Eugene, OR Posts: 1,242	Question about my Tach I just put a new tach in the truck, and I'm not completely sure if it's accurate or not... I did a WOT run in the truck, and according to the tach, it shifted first to second at 4000 RPM. Shifted from second to third at 4200 RPM. Here's the engine I have... http://www.jegs.com/webapp/wcs/store...55609_-1_10763 Now either the tach is inaccurate, or it's shifting too early, because the specs on that engine show 290hp at 5200 RPM, and I'm about 1000 below that according to the tach. I've got it hooked to a main power source in the fuse box, and I've got the actual RPM wire hooked to the "tach" terminal on the dist. cap. It also shows it's idling at about 500rpm in gear, which I think is a little lower than it actually is. I could be wrong though..

11-04-2007, 10:56 PM	#2
ChaseDale3and8 Registered User Join Date: Jun 2007 Location: Springfield Mo. Posts: 1,086	Re: Question about my Tach Is this an aftermarket tach or a factory one? If it's an aftermarket, it should have a switch on the back of it for a 6 cylinder or 8 cylinder. It may be on the 6 cylinder setting and just need to be changed. I used to have a Sun tach in my 82 years ago and that's how that tach was. If it's a factory tach, it's very likely that it came out of a 6 cylinder truck, and that's why it's reading off as the 6 cylinder and V8 tachs are different. ...Joe __________________ I'm not speedin'! I'm qualifyin'!

11-05-2007, 12:44 AM	#3
DBLSPRT68 Registered User Join Date: Nov 2000 Location: Ft. Mill, SC USA Posts: 931	Re: Question about my Tach If you put in a factory tach, do this. Turn your ignition switch to on, don't start it; your tach should read 0. If it dosen't, your tach is off/bad. I have one good tach in my truck, but have three more that are off/bad David __________________ PROJECT TRUCK: 1985 SWB C10 Silverado Black/Charcoal, 1990 V2500 Suburban, 2018 Suburban, 2005 GMC 2500HD CCSB, 2014 Toyota Camry SE, HAD.....1968 Camaro RS/SS Family owned since new; 350 3 speed SOLD

11-05-2007, 10:34 AM	#4
xtreme80 Wishin' I had more money! Join Date: Feb 2007 Location: Eugene, OR Posts: 1,242	Re: Question about my Tach It's an aftermarket sun pro tach, and I already made sure it's on the 8 cylinder setting, but I guess it wouldn't hurt to double check that

11-05-2007, 12:04 PM	#5
71swb4x4 Senior Member Join Date: Nov 2003 Location: Brookings, SD Posts: 10,497	Re: Question about my Tach I think I am missing something. Why would the specs on the engine determine when your tranny will shift? Wouldn't the tranny control the shift points? How does a tranny decide when it will shift? __________________ 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.

11-05-2007, 12:59 PM	#6
Pyrotechnic Registered User Join Date: Feb 2007 Location: Austin, TX Posts: 3,930	Re: Question about my Tach The transmission doesn't care what your engines power band is. Wide open throttle shifts are going to happen whenever they are set to in the transmission.

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