On a dynojet, my friends GTA was showing 5500 rpms.. In all reality, the dyno-tuner said it was really at 4500 RPMs. My question is how "off" are the stock tachs really? If you install an MSD shift light to come on at 5k, is it really coming on at 5k??

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91 Formula 1LE 1 of 46
305 TPI 5speed
1LE/G92/WS6

Paxton SN92 supercharger(SN93 upgrade), Ford SVO 24#injectors, Crane AFPR,SLP airfoil, ported/polished plenum, March alt. pulley,Crane Gold 1.6rrs,MSD coil,MSD6AL (5200rpm limit),Holley 9mm wires,Ac delco R43ts plugs,custom chip,Bosch O2sensor,SLP 1 5/8" headers,SLP catback,shortened shifter,3:73s w/Auburn posi,170* t-stat,JET 195* fan switch, Macewen white face gauges, Autometer gauges, Zoom hi-performance clutch.

 

Are you sure you had the dyno tuner set for the engine you were testing? I know that the factory tach is not totaly accurate but this is only by 50rpm or so.

 

compared to my Autometer tach with shift light, my tach is about 200 rpms fast. So i would imagine that there not too far off- definately not 1000 rpms off. Sounds like something was wrong somewhere.

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My 1986 Z28
GM350, T-5, 3.23's, lots of go fast goodies, 4th gen Firebird interior etc...
85 IROC w/ 1364 miles!

You'll always find what you've lost in the last place you look

R.I.P Dale Earnhardt

 

I recently got another 140 MPH IROC instrument cluster; when I put it in my car, I was idling at 1200 RPM instead of 800, cruising at 2800 at 40 in 4th instead of 1900, etc. So they can be hugely inaccurate.

When I swapped my original circuit board for the other one, readings returned to normal.

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"So many Mustangs, so little time..."
ICON Motorsports

 

i have a 5500 chip in my MSD and it gets kicked out at 5300.

 

My 84 tach's dead nuts accurate according to the MSD rev limiter. I've checked it at 3000, 5800, and 6000.

Now, if I only I could say the same for the POS tach in my LS1....


------------------
"Beware he who would deny you access to information, for in his heart, he dreams himself your master"

'84 Black Camaro ZZ4 M5 (V-6 in a former life) -- street beast
'91 Medium Blue Metallic Z28 A4 (UB-HSTRY's old ride) -- currently broken
'99 Navy Blue Metallic Firehawk #120 M6 (99.44% stock) -- daily driver

 

Mine is slow by about 200. Thats why I installed an autometer!
bsa

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"If they do not run, Then they will die." - Stonewall Jackson
#3
'87 IROC, 355 TPI, Converted to SD, 700R4, B&M Megashifter, 3.42, Trick Flow Twisted Wedge Heads, Edelbrock Intake, Accel Runners, Comp Cam, Crane Gold rockers, Lucas 23# injectors, Edelbrock headers, Flowmaster 3'' exhaust, Mallory Ignition, Transgo Performance Shift Kit, Corvette Servo, 2,000 Stall Torque Converter

 

My ZZ4TPI is off about 200 rpm according to my Autoxray scanner. Its 200 rpm higher so its like a built in 200rpm rev limiter, if I'm paying attention to the tach! . But my MSD is set at 6000 and I've bumped it once.

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Rob P
89RSconvtZZ4TPI
Edelbrock Hi-Flow Intake
SLP Dual Cold Air Intake
1 5/8" Headers
Semi-Siamesed Runners
MSD6AL/relocated MAT/ AdjFPR/IROC frnt@rear swaybar/wonderbar/steeringbox/alum drvshaft/ Alston SFC/3:23posi disc rear/MAC LCA/H.Adams Panhard Rod/KYB struts/ shocks/
92Z28convt5spd
71Impala convt 402BB
BETTER DRIVING THRU SUPERIOR HORSEPOWER!

 

Factory instruments (except the voltmeter) might as well not even have graduations, just green and yellow bars to indicate safe and unsafe ranges. About 10% variation in indication from actual is very common.

------------------
Later,
Vader
------------------
"What a Day..."
Adobe Acrobat Reader

 

When I'm at 6000 RPMs according to my tach, my Diacom and the dyno said I was at 5400 RPMs.

------------------
West Coast GM Shootout 2001!
1991 Camaro Z28
5.7L 5-Speed (originally 305)
13.23 @ 107.62 MPH
Southern California
Member: SoCal 3rd Gen F-Bodies
Webmaster: SoCal F-Bodies
-=ICON Motorsports=-

 

Vader's right (as usual) about the calibration of factory gauges. I know someone who works for a gauge manufacturer. He told me that GM, F@rd, etc pay about $3-5 for a tach! You get what you pay for!

RB, i think you put a V6 tach in (will read 1/3 higher +/- normal inaccuracy)

 

mines off by 500 according to my diacom and autometer tach

 

I've been told about 200 rpm's. When I had an AutoMeter Monster Tach in my 92, it was only off by 50...maybe 100 rpms at the most-guess it all depends

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92 Z-28
305 -5speed
160 thermo.
K&N Air Filters
B&M Ripper
14.7 @ 96mph (stock run)

 

My Diacom reads 300 rpms higher than the tach.

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1989 IROC-Z 5.7L
NOS 150HP kit
ProBuilt 700R4, PI Vigilante 2800 stall lockup
Baer Brakes 12" Sport System

 

My tach is quite accurate, within 50 rpm of my Diacom readings. And my speedo is bang on. My temperature guage is another thing and as for my fuel guage, it may as well say "Got gas and got no gas".

 

I thought about that too Merf, you're right that exchanging them that way would produce that error; however this tach was out of a 87 IROC, in the cluster with the 145 MPH speedo; the part # on the little circuit board was the same as the one in my original 83 tach, except it was rev. C instead of rev. B or something like that. The PC board itself was the thing that made it read 50% high.

I wonder how the guy I got it from was able to put up with that for however long it was doing that...

In any case, it is a good example of how far off they can be sometimes. The factory seems to regard such things as "trim: items, not instruments; so as long as it looks cute and moves up and down when the engine changes speed, I think they think they've done their job.

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"So many Mustangs, so little time..."
ICON Motorsports

 

No wonder i can't set the base idle at 600. Time to get a good tach.

[This message has been edited by 84TransAm (edited May 04, 2001).]