Speed at WOT ?

[DarkAngel]

Hello

Just wondering,
I've drived my 'Z a little hard this week-end, and i remark that i could only reach 100mph on third at ~7000rpm

Refering to this table, it should be 120mph with stock 3.45 (at 7000 on third)http://i185.photobucket.com/albums/x220/6PPC_bucket/tech%20files/ZF6MPH345vs410gearratio-Haibeck_zps837f03ef.jpg

My car is totally stock (except B&B catback)

On this video (not from me), the guy reach also 100Mph and not 120
http://www.youtube.com/watch?v=E0Egyp4N-uA

My car works fine i think and it revs until 7000
but do i have to worry about something ?

Is 100MPH on third at 7000 normal or not enough ?

[BigJohn]

Tire Diameter would make a differance!!!

[XfireZ51]

Based on the chart and assuming your speedometer is correct, it looks like you have a 4.10 gear. I wouldn't be surprised by that since 4.10 gears are a popular upgrade for the ZR-1. Have you checked the accuracy of your speedo?

[DarkAngel]

the tires are same as stock
315/35/17 and the speedo is accurate (tested with GPS)
i don't know about gear ratio but i guess it has not been changed

[WVZR-1]

You don't mention the year of your Z either. If it's a '90 and has a tachometer error then yes you would be convinced that the Z has had a rear gear installed. Use a scanning tool and check the RPM's from the ALDL to confirm tach operation. All years could have the tach error but it's frequently found on the '90 and less frequently the '91.

I'm guessing that's your error situation!

Drive it with the scan tool attached and do the same or similar runs!

Check the rear axle ratio. It's discussed in this thread and I mentioned a couple different ways to calculate it and the latter using degrees or minutes of rotation was confirmed by the OP.

http://www.zr1.net/forum/showthread.php?t=21653

[We Gone]

http://www.youtube.com/watch?v=ktdywT5EsiQ

[DarkAngel]

You don't mention the year of your Z either. If it's a '90 and has a tachometer error then yes you would be convinced that the Z has had a rear gear installed. Use a scanning tool and check the RPM's from the ALDL to confirm tach operation. All years could have the tach error but it's frequently found on the '90 and less frequently the '91.

I'm guessing that's your error situation!

sorry, i forgot to mention the year
it's a '90 ZR1
but i've checked the speed with a GPS and it seems pretty accurate

i don't have all the history of the car but it has been kept for a long time in a private collection ; it has really few milleage (17000miles) so i would be surprised that the gear ration would have been changed :neutral:

[WVZR-1]

http://www.youtube.com/watch?v=ktdywT5EsiQ

Had he understood what exactly what he was doing he had it solved with his "first" drive-shaft rotation. With a locking differential the one tire revolution and counting the pinion rotations is used most frequently. While rotating the drive-shaft if a person is doing it by himself you have to do the crawl a couple times. Marking the pinion deflector and using the degrees or minutes it can be done by one person with just a reach under for the marking of the pinion deflector.

[WVZR-1]

sorry, i forgot to mention the year
it's a '90 ZR1
but i've checked the speed with a GPS and it seems pretty accurate

i don't have all the history of the car but it has been kept for a long time in a private collection ; it has really few milleage (17000miles) so i would be surprised that the gear ration would have been changed :neutral:

I'll wager the "tach error" !

I have never used the C68 display to check for tach error but I would guess it could be done. Drive at a known display on the C68 and see what the tach displays. Will that work? I would think but I've never tried.

I believe you would do X 25 for the C68 display in 6 for RPM's. A C68 display of 80 would be 2000, 120 would be 3000, 160 would be 4000. I believe that's correct. I'd appreciate knowing.

Access is shown in the C68 section of the FSM but I believe you just depress fan up/down (same time) to access the display, fan up or down until you get to 6 and then press the "auto" button for the display of that parameter.

[We Gone]

When I bought the 90 ZR-1 I had I got a great deal on it as the owner thought it had serious issues with the motor due to lack of power. Only problem the tach was just over 2k rpms off....

[Hog]

Had he understood what exactly what he was doing he had it solved with his "first" drive-shaft rotation. With a locking differential the one tire revolution and counting the pinion rotations is used most frequently. While rotating the drive-shaft if a person is doing it by himself you have to do the crawl a couple times. Marking the pinion deflector and using the degrees or minutes it can be done by one person with just a reach under for the marking of the pinion deflector.

OR simply put the tape marker on the inside of the tire.

[WVZR-1]

OR simply put the tape marker on the inside of the tire.

Yep!!!

[LancePearson]

Watching the videos it seems to me that the speed each gear runs up to 7-8000 rpms is faster than a 3:45 rear end. I relate since I also have a 2008 TBSS with LS2 that came from GM Performance group with a 4:10 rear end so I have a direct comparison of two over 400 hp engines and a 3:45 and 4:10 rear end. I'd guess you have a 4:10 rear end in the Z from watching the videos of the one that gets to 100 mph in third gear. I get to 125 mph in third gear in my stock 1991 Z with 3:45 rear end. Speed to get through each gear seems more like a 4:10. Check it.

[XfireZ51]

Watching the videos it seems to me that the speed each gear runs up to 7-8000 rpms is faster than a 3:45 rear end. I relate since I also have a 2008 TBSS with LS2 that came from GM Performance group with a 4:10 rear end so I have a direct comparison of two over 400 hp engines and a 3:45 and 4:10 rear end. I'd guess you have a 4:10 rear end in the Z from watching the videos of the one that gets to 100 mph in third gear. I get to 125 mph in third gear in my stock 1991 Z with 3:45 rear end. Speed to get through each gear seems more like a 4:10. Check it.

w my 3.45s and 17" wheels, I would got through the traps in 3rd gear at ~7000rpm with a trap speed of 118 or so.

[Paul Workman]

My money is on the voltage divider on the cluster board. The divider that biases the op amp for the tach is printed on a ceramic substrate. It is exposed to the air, and over time deteriorates.

By comparing the analog tach reading to the the digital rpm value on a scanner connected to the ALDL, it is easy to determine if the tach is reading correctly.

It is my understanding that over time and exposure to air, the resistance value changes, resulting in the analog tach reading to creap up. (Mine was reading well off the scale when it was at only 7000 rpm.)

Check that first. If tach calibration is the issue, "there is an app for that!"


http://i185.photobucket.com/albums/x220/6PPC_bucket/tech%20files/cut4and10-2.jpg (http://s185.photobucket.com/user/6PPC_bucket/media/tech%20files/cut4and10-2.jpg.html)
http://i185.photobucket.com/albums/x220/6PPC_bucket/tech%20files/1kpotwithpair100kresistors.jpg (http://s185.photobucket.com/user/6PPC_bucket/media/tech%20files/1kpotwithpair100kresistors.jpg.html)
http://i185.photobucket.com/albums/x220/6PPC_bucket/tech%20files/greenside-potattachmentshown.jpg (http://s185.photobucket.com/user/6PPC_bucket/media/tech%20files/greenside-potattachmentshown.jpg.html)
P.

[XfireZ51]

... By comparing the analog tach reading to the the digital rpm value on a scanner connected to the ALDL, it is easy to determine if the tach is reading correctly...


The ALDL datastream has a tach ceiling of 6300rpm which is the limit of the L98
datastream. Granted the rpms change rapidly at that point, but if you want to check tach error v the datastream, do it at a point below 6300rpm. There is raw data for rpms which can be output with a change in the ALDL datastream, but you will need to convert that to rpm.

[WVZR-1]

The ALDL datastream has a tach ceiling of 6300rpm which is the limit of the L98
datastream. Granted the rpms change rapidly at that point, but if you want to check tach error v the datastream, do it at a point below 6300rpm. There is raw data for rpms which can be output with a change in the ALDL datastream, but you will need to convert that to rpm.

There is "nothing" that needs to be checked in excess of 6300 RPM - if it's an issue with the tach the display from the scanner will document it in the complete RPM range. It's NOT as complicated as you seem to imply, there is either a tach error there is NOT.

I would guess that likely 95% of the '90 cars if checked likely have an issue OR they've been corrected. How many are aware? Who knows.

[mike100]

stock rev limiter is 7100 rpm. I would suggest revving it until 7100 rpm and then the ecm will cut the ignition. Compare tach reading to known actual rev limit of 7100 rpm.

[WVZR-1]

stock rev limiter is 7100 rpm. I would suggest revving it until 7100 rpm and then the ecm will cut the ignition. Compare tach reading to known actual rev limit of 7100 rpm.

There's no need to do that. That would seem to be abuse that isn't required!

[DarkAngel]

Thank you everyone for all your quick answers :)

for tach limiter, i have a chip so i don't know if i still have the rev limit at 7100
furthermore, i don't really want to hit the rev limit :-S

[WVZR-1]

Thank you everyone for all your quick answers :)

for tach limiter, i have a chip so i don't know if i still have the rev limit at 7100
furthermore, i don't really want to hit the rev limit :-S

A scanner if you have one from the ALDL and I'd be interested if you decided to do the C68 display for a possible validation. Do you have a FSM for the car? I could probably email you the procedure from the FSM if you don't.

In the FSM it should be found 1C1-1 through 1C1-5 for the '90 and for other years on similarly numbered pages I'd think.

[Paul Workman]

The ALDL datastream has a tach ceiling of 6300rpm which is the limit of the L98
datastream. Granted the rpms change rapidly at that point, but if you want to check tach error v the datastream, do it at a point below 6300rpm. There is raw data for rpms which can be output with a change in the ALDL datastream, but you will need to convert that to rpm.

No, not that complicated. The (static rpm) % error can be corrected at a reasonably lower rpm - I used 3000 - and one can get the % error down to eyeball accuracy i.e., close enough -so that it will track across the rpm range.

The bigger tach error is due to mechanical lagging as a function of rapidly increasing rpm - especially in low gear at WOT. (So, a lot could be said for the original C4 digital tach displays, far as mechanical lag (the lack thereof) goes.) So, obviously the shift light is a far more accurate (and maybe safer) way to avoid analog errors when rowing the gears.

P.

[DarkAngel]

A scanner if you have one from the ALDL and I'd be interested if you decided to do the C68 display for a possible validation. Do you have a FSM for the car? I could probably email you the procedure from the FSM if you don't.

In the FSM it should be found 1C1-1 through 1C1-5 for the '90 and for other years on similarly numbered pages I'd think.

i have check in the Corvette manual that i have but i didn't found the section you are refering to
if you can send it to me, it would be really nice :)

does 4.10 gear ration came stock out of the factory in '90 ?

[We Gone]

1990 - 1996: HVAC Diagnostic Codes

On 1990 to 1996 Corvettes equipped with RPO C68 (electronic climate control), it is possible to read the HVAC system diagnostic codes from the climate control panel on the dashboard.


1. To enter the diagnostic mode, press the fan up and down arrow buttons at the same time until -00 appears on the digital readout.

2. One you have entered the diagnostic mode, you can scroll through the various parameters by using the up and down fan buttons.

3. To view the value for a particular parameter, press the "AUTO" fan button.


Note: If you see a "-" next to a parameter value (example: "-105"), you must add "100" to that value to obtain the correct value for that parameter. Therefore, the correct reading would be 205 for this example.
Parameter Description Values
0 System Faults
(one of these will be set if your air conditioning controls have an LED flashing) 00=No Faults
01= Temp Door Motor Circuit Fault
02= Temp Door Motor Circuit Fault
03 = Ambient Sensor Open
04 = Ambient Sensor Short
05 = In-Vehicle Temp Sensor Open
06 = In-Vehicle Temp Sensor Short
07 = Solar Load Sensor Open
08 = Solar Load Sensor Short
09 = Low Freon Detected
10 = UART Failure
1 Temp Setting 60F to 90F
2 In-Car Temp Sensor 10 = HOT -130 = 230 = COLD
3 Outside Temp Sensor 10 = HOT -130 = 230 = COLD
4 Sun Load Sensor Max light Max Dark
L98 38 183
LT5 115 -110 = 210
5 Ignition System Voltage 0 = 9 Volts -155 = 255 = 16 V
6 Engine Speed (RPM divided by 25)
7 Vehicle Speed
9 A/C System Mode 00 = OFF
01 = Re-Circulation
02 = A/C
03 = Bi-Level
04 = Heater
06 = Defrost
07 = Vent
10 = Manual Re-Circulation
10 Blower PWM 0 = 0 Volts 128 = 14 Volts
11 Program Number 00 = COLD -155 = 255 = HOT
12 Mix Number -155 = 255 = COLD 00 = COLD
16 Coolant Temperature In degrees C
17 Solar Correction 114 = Max light 128 = max dark
30 Stored Full Hot Value 0 to 50
31 Temp Door Travel Range 100 to 200
34 Temp Door Position Requested 00=Full HOT -153=253=Full COLD
35 Compressor On Time 0.1 second increments
36 Number Of Times Below Critical Time
37 Software Version Number

[WVZR-1]

does 4.10 gear ration came stock out of the factory in '90 ?

The 4.10 ratio doesn't come from the factory for any year C4 Corvette

It looks like "We Gone" has you covered on the C68 diagnostic and information procedure. If you've other questions you can see the answers are generally just minutes away. The tach error when you confirm that is what it is won't be a difficult repair for you either. There are multiple DIY threads available for that.

[DarkAngel]

The 4.10 ratio doesn't come from the factory for any year C4 Corvette

It looks like "We Gone" has you covered on the C68 diagnostic and information procedure. If you've other questions you can see the answers are generally just minutes away. The tach error when you confirm that is what it is won't be a difficult repair for you either. There are multiple DIY threads available for that.

yes reponse are really quick:handshak:
thanks again everyone for your reactivity =D>

[XfireZ51]

No, not that complicated. The (static rpm) % error can be corrected at a reasonably lower rpm - I used 3000 - and one can get the % error down to eyeball accuracy i.e., close enough -so that it will track across the rpm range.

The bigger tach error is due to mechanical lagging as a function of rapidly increasing rpm - especially in low gear at WOT. (So, a lot could be said for the original C4 digital tach displays, far as mechanical lag (the lack thereof) goes.) So, obviously the shift light is a far more accurate (and maybe safer) way to avoid analog errors when rowing the gears.

P.

I may be guilty of providing too much information. Since a scan tool had been suggested, I simply wanted to make people aware of the fact that the ALDL data will only display RPM up to a value of 6300. So if you were trying to compare the ALDL to what you were seeing on the tach and using, say 6500rpm, it won't be shown on the scantool. I think I even said, use an RPM lower than that to come up w an error factor. I have my WB and shiftlight connected to the diagnostic plug near the firewall. That will show actual rpm when using a shift light or the WB logging software.

[DarkAngel]

ive made a quick test this we
at 2000rpm on the tach, correspond 66 on the A/C : 1650rpm
at 3000rpm, 108 : 2700 rpm
at 4000rpm, 140 : 3500 rpm

this is approx because i've made the test alone and it was hard to look at the tach, look at the A/C, maintain the rpm and look at the road :-D

i will make other test another time and keep you informed :)

[WVZR-1]

I'd say you've confirmed the tachometer error and unlike a speedometer error it's not going to be at a "fixed percentage". Did you happen to make an observation "at idle"? Maybe now that an error is suspect it would be motivation to find a scanner and do it again using the ALDL as a source.

How responsive did the A/C display appear to be to your application of throttle? Considerable lag or maybe rather responsive. I've never actually used it.

[DarkAngel]

at idle it was between 28 & 32 : 700/800 rpm
the A/C display is not immediately responsive but not that much laggy also

[ALLTHECAMS]

Im having this same issue and found this thread thru a googl e search. I have a 1991 ZR-1 with 68k miles. It has Borla exhaust so its possible it has 4.10s. With the tach reading 7000 rpm I'm at ~100mph in 3rd gear. I did the HVAC test and the tach is off ~300 rpm but it doesn't seem like 300 rpm would make a 20mph difference. I guess the only way to be sure is jack the rear up and count driveshaft revs?

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[XfireZ51]

Do you have a GPS? Compare that to your speedo.

[ALLTHECAMS]

Did that too. Speedo is accurate. Also, the shift light does come on when the tach reads 7k.

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[XfireZ51]

At a steady 70mph ctuise, what's ur rpm?

[ALLTHECAMS]

At a steady 70mph ctuise, what's ur rpm?
About 1900 rpms.

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[dredgeguy]

I have 4.10's and at 70 MPH I am about 1900 RPM.

[ALLTHECAMS]

I have 4.10's and at 70 MPH I am about 1900 RPM.
The tach reads 1900. I checked using the HVAC readout and engine speed at 70mph is 1650. :(
What controls the shift light? The tach or the ecm?

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