Discovered a bonus with my recent purchase - ZR-1 Net Registry Forums

Ccmano · 06-30-2017

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.

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

WARP TEN · 06-30-2017

Quote:

Originally Posted by 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.

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

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.

Paul Workman · 06-30-2017

Quote:

Originally Posted by WARP TEN

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

WARP TEN · 07-01-2017

Quote:

Originally Posted by Paul Workman

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

KFoster · 06-30-2017

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

KILLSHOTS · 06-30-2017

Quote:

Originally Posted by 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

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

WVZR-1 · 06-30-2017

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.

Ccmano · 06-30-2017

Quote:

Originally Posted by 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.

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

Ccmano · 07-04-2017

Quote:

Originally Posted by 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

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

efnfast · 06-30-2017

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

06-30-2017	#1
Ccmano Join Date: Jun 2006 Location: Sparks, NV Posts: 2,842	Discovered a bonus with my recent purchase 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. 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 __________________ 90'ZR-1 #1461 Red/Black Ported Intake, Degreed Cams, OBX Headers & Exhaust, Fidanza FW, Secondary Delete, Custom tune, C6 GS Wheels. Sold 08 C6 Z51 Crystal Red Metallic Sold Vararam Intake, Tune, Sold 05 C6 Z51 Red/Black Sold 90'ZR-1 #1723 Black/Gray Sold but not forgotten 91' Z51 L98 White Vert. My First, you always remember your first. Sold.

06-30-2017	#5
KFoster Join Date: Jan 2015 Location: Decatur, AL Posts: 93	Re: Discovered a bonus with my recent purchase 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 __________________ 1990 ZR1, 1987 Mustang GT (since new), 1973 DeTomaso Pantera, 1987 928S4 Passed on: 1984 930S (gray market), 1988 Mustang Coupe Trooper car(drag)

06-30-2017	#7
WVZR-1 Join Date: Oct 2004 Posts: 1,889	Re: Discovered a bonus with my recent purchase 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.

06-30-2017	#10
efnfast Join Date: Nov 2011 Location: Dunbarton NH Posts: 7,534	Re: Discovered a bonus with my recent purchase Top end in sixth gear with 4.10's is about 270 mph, so unlikely you could reach that.