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.

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.

efnfast · 06-30-2017

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

Mystic ZR-1 · 06-30-2017

Quote:

Originally Posted by efnfast

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 · 06-30-2017

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

Mystic ZR-1 · 06-30-2017

Quote:

Originally Posted by efnfast

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 · 06-30-2017

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?
😋

06-30-2017	#1
Ccmano Join Date: Jun 2006 Location: Sparks, NV Posts: 2,852	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	#3
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	#5
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	#6
efnfast Join Date: Nov 2011 Location: Dunbarton NH Posts: 7,293	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.

06-30-2017	#8
efnfast Join Date: Nov 2011 Location: Dunbarton NH Posts: 7,293	Re: Discovered a bonus with my recent purchase Doug, I'm only running 3.73's. My top end is in 5th gear.

06-30-2017	#10
Mystic ZR-1 Join Date: Jan 2010 Location: Mystic CT Posts: 2,607	Re: Discovered a bonus with my recent purchase 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? 😋