Discovered a bonus with my recent purchase

[Ccmano]

While driving the (new to me) Z recently I noticed that the rpms seemed high for any given speed in all gears. I was informed by the seller that the differential had been rebuilt in the last two years but never asked if he changed the ratios. So I conducted an experiment. I noted the rpms at 60mph in 4th gear (1:1 trans ratio) I was running at 3300 rpm. My rear tires are 25 inches tall. Using the calculator below:

http://www.crawlpedia.com/rpm_gear_calculator.htm

It appears I have 4:10 gears. :dancing

Stock 3.42's should have gotten me 2750 rpm and 3.73's should be 3000 rpm all else being equal.

So... unless the tach is very wrong, my first real mod is already done.
Cool
H
:fahne:

[WARP TEN]

While driving the (new to me) Z recently I noticed that the rpms seemed high for any given speed in all gears. I was informed by the seller that the differential had been rebuilt in the last two years but never asked if he changed the ratios. So I conducted an experiment. I noted the rpms at 60mph in 4th gear (1:1 trans ratio) I was running at 3300 rpm. My rear tires are 25 inches tall. Using the calculator below:

http://www.crawlpedia.com/rpm_gear_calculator.htm

It appears I have 4:10 gears. :dancing

Stock 3.42's should have gotten me 2750 rpm and 3.73's should be 3000 rpm all else being equal.

So... unless the tach is very wrong, my first real mod is already done.
Cool
H
:fahne:

Great discovery. If you do have it is the best mod for your car to improve performance noticeably without doing major work. You can also simply "count" the axle with the car in neutral, handbrake off, one rear wheel on the ground so it doesn't spin, one rear wheel jacked up. Make a mark on the driveshaft you can see, a matching one on the diff housing, and one on the wheel. The drive shaft should turn 4.1 times for each wheel revolution. If it only turn 3.4 times, stock ratio.

[KFoster]

I'm thinking of going to 4.10. Not that I ever will top the car out, but I still don't want to reduce the top end potential that got the car all that hype back in the day. Anybody have any opinions on how it impacts top end? Will it pull to redline in 6th with 4.10s where 5th is fastest gear stock?


Sent from my iPhone using ZR-1 Net Registry (http://r.tapatalk.com/byo?rid=90383)

[KILLSHOTS]

I'm thinking of going to 4.10. Not that I ever will top the car out, but I still don't want to reduce the top end potential that got the car all that hype back in the day. Anybody have any opinions on how it impacts top end? Will it pull to redline in 6th with 4.10s where 5th is fastest gear stock?


Sent from my iPhone using ZR-1 Net Registry (http://r.tapatalk.com/byo?rid=90383)

Oh boy, here we go! You're opening the floodgates now...

[WVZR-1]

OP - I assumed maybe the same when I first got my '90 but I had been told it was original! The '90 tachs are known to be very inaccurate and I discovered it was stock after using the ATC panel to calculate RPM and I connected a Tech1 to confirm that I actually had a 3.45 and the cluster needed attention.

When you turn the KEY TO RUN with the car "not running" if the tach needle isn't parked on 0 you have a tach error. Needs to park on 0 - not close - on 0!

Use the ATC if it's a C68 car and calculate RPM using it for confirmations.

[efnfast]

Top end in sixth gear with 4.10's is about 270 mph, so unlikely you could reach that.

[Mystic ZR-1]

Top end in sixth gear with 4.10's is about 270 mph, so unlikely you could reach that.

I musta been dozing off when you did 270...
That wasn't thunder and lightning?
It was the sound barrier!!!
😋

[efnfast]

Doug, I'm only running 3.73's. My top end is in 5th gear.

[Mystic ZR-1]

Doug, I'm only running 3.73's. My top end is in 5th gear.

Missed out on that too....
😋
Was really tired.
😋

[Mystic ZR-1]

According to Marc's chart, with stock 3:45s
at 7500 in 6th the car would go 338!
I'm assuming that's with the top on and windows rolled up?
I have trouble getting over 300?
Maybe I should roll up the windows?
😋

[Paul Workman]

Great discovery. If you do have it is the best mod for your car to improve performance noticeably without doing major work. You can also simply "count" the axle with the car in neutral, handbrake off, one rear wheel on the ground so it doesn't spin, one rear wheel jacked up. Make a mark on the driveshaft you can see, a matching one on the diff housing, and one on the wheel. The drive shaft should turn 4.1 times for each wheel revolution. If it only turn 3.4 times, stock ratio.

One leetle problem dar Bob! Fergittin 'bout the positraction? Gonna be tough rotating the driveshaft with one tire on the ground! Best both wheels off the ground and counting the driveshaft rotations per one wheel rotation.

I

[Ccmano]

OP - I assumed maybe the same when I first got my '90 but I had been told it was original! The '90 tachs are known to be very inaccurate and I discovered it was stock after using the ATC panel to calculate RPM and I connected a Tech1 to confirm that I actually had a 3.45 and the cluster needed attention.

When you turn the KEY TO RUN with the car "not running" if the tach needle isn't parked on 0 you have a tach error. Needs to park on 0 - not close - on 0!

Use the ATC if it's a C68 car and calculate RPM using it for confirmations.

Good point and as I wrote that was assuming the tach was correct. I'll be checking that next. I have to say that based on my perception of engine rpms I'm inclined to think it has the 4:10's but we shall see.
H
:fahne:

[BigJohn]

I have some 5:13's!

[efnfast]

Big John, is that what they run in the Callaway? 5.13's? Wow.

[XfireZ51]

Hans,

If u have GPS, u can use that to confirm.

[WARP TEN]

One leetle problem dar Bob! Fergittin 'bout the positraction? Gonna be tough rotating the driveshaft with one tire on the ground! Best both wheels off the ground and counting the driveshaft rotations per one wheel rotation.

I
Good point, Paul. As I recall from my (very) old days summer job as a track mechanic at Ford's Dearborn test track, it was not a problem but I don't recall truly if we dealt with posi axles back then. I do know that with open rears we had to have an assistant hold the other wheel while we counted the axles. So if you are right that both need to rotate, then jack up both wheels and let them rotate together. That should work I would think. --Bob

[WVZR-1]

Hans,

If u have GPS, u can use that to confirm.

GPS can only be used to confirm Speed & odometer - I still don't get the u vs you - LAZY?

This isn't a TEXT exchange - it's a "conversation"!

[BigJohn]

Big John, is that what they run in the Callaway? 5.13's? Wow.

I don't think they would fit in a corvette!
The 5:13's are in my plow truck.

[Ccmano]

Good point and as I wrote that was assuming the tach was correct. I'll be checking that next. I have to say that based on my perception of engine rpms I'm inclined to think it has the 4:10's but we shall see.
H
:fahne:

Had a chance to take her out today, with the laptop attached and Datamaster running.

Bob, you were right, tach is off considerably. The speedo is correct. At 60mph the correct rpm is about 2800. Based on the formula that is right for a 3.54 diff ratio.

Also noticed that's the tach is really off at high rpm. When I shift at 7+k on the tach I'm barely at 6k in reality.

Anybody have any thoughts on how to get the tach fixed?
H
:Eagle:

[WVZR-1]

Had a chance to take her out today, with the laptop attached and Datamaster running.

Bob, you were right, tach is off considerably. The speedo is correct. At 60mph the correct rpm is about 2800. Based on the formula that is right for a 3.54 diff ratio.

Also noticed that's the tach is really off at high rpm. When I shift at 7+k on the tach I'm barely at 6k in reality.

Anybody have any thoughts on how to get the tach fixed?
H
:Eagle:

KEY ON - engine not running the needle isn't parked at 0 either. That's the quickest "first check"!

There are a couple possible ways to correct the tach. In the past few years there's very likely a much improved procedure than what was used for years before.

Mine was actually done by an GM/ACD cluster facility that I had a relationship with at the time. My entire cluster and display was confirmed. The tach correction has been a DIY for many but much depends upon the persons talent.

I don't know how expensive a "for hire" today might be but if the "for hire" included a check/correction of the entire cluster that could be a very good way to proceed.

I sent a circuit board to PW many years ago for him to sort out a project he had at the time.

[Ccmano]

It looks like Haibeck offers the tach correction service. Still not cheap at $225 plus shipping. I wonder if there are other alternatives.
:USFlag:
H

[-=Jeff=-]

Batee, but it is DIY

http://www.batee.com/corvette/dcrg/services/9096_analog_ic_adj_zr1.html

$49.95

[tf95ZR1]

http://www.zr1.net/forum/showthread.php?t=17728&highlight=tach+correction

[XfireZ51]

Hans,

Follow Paul's DIY fix Ted has posted above. Also, for shifting up top, just install a shift light. Won't matter what the taxh says.

[-=Jeff=-]

there is a group buy of the harlan light for registry members

[Paul Workman]

I sent a circuit board to PW many years ago for him to sort out a project he had at the time.

So YOU"RE the one! I'd forgotten and had wanted to thank you. Well, (belated) THANK YOU for that board!

Hans, the procedure Ted's link takes you to works, except for the fact the pictures are being held hostage by Photobucket. No matter. I still have the board and I can attach the original pictures, if I can find them, or simply re-shoot the pictures and attach them.

The part you need is the ceramic resistor board from bantee.com, and I would prolly buy the board (chip). OR, you can buy the potentiometer and some resistors and have at it. (If you can't find THE exact value resistor, a combination of series and/or parallel resistors can be assembled to get the right value. I can help you with that. But, just buying a new ceramic resistor board (chip) would be a lot easier!)

[Ccmano]

Thanks guys. I saw the one from Batee.com on eBay. Looks like a plan and another winter project.
:salute:
H