Normal RPM for LT1 at 100 kph on highway?
09-18-2016, 04:41 PM
#1
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Normal RPM for LT1 at 100 kph on highway?
My 1995 Z28 RPM gauge reads about 2800 at about 100 kph. Is this normal. Don,t know much about the car, I just purchased it. It seems to be revving a little to high. Is this normal. would a shift kit do this? It seems to shift into first really hard. It is automatic.
09-18-2016, 07:02 PM
#3
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Re: Normal RPM for LT1 at 100 kph on highway?
17 inch tires. not sure if anything was done with the gears. There is no 3 gear.
PRND D in a circle and then 2 . would a shift kit do this? Not sure what is one with it. New to me. How would I know if it had a shift kit by observation
PRND D in a circle and then 2 . would a shift kit do this? Not sure what is one with it. New to me. How would I know if it had a shift kit by observation
09-18-2016, 09:41 PM
#4
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Re: Normal RPM for LT1 at 100 kph on highway?
A shift kit would not affect RPM -- only shift firmness and the point at which the shifts occur.
Based on the fact that your shifter shows a 2 but not a 1, I can tell you have 2.73 rear gears, not the 3.23 gears that come with the "performance axle ratio" option package.
The shifter should be in the D closest to N for normal driving. That allows the transmission to use 4th gear, which is an overdrive. At 100kph in 4th with the torque converter locked, you should see about 1550 rpm.
If you're in 3rd (the second D), that's a 1:1 gear. At 100kph with the converter locked, this would be around 2200 rpm.
If you were in 2nd, you'd be running 3600 rpm at 100kph, so I don't think that's it.
If your have the wrong D selected, that would be the problem. If not, the clutch bands that engage 4th might have failed, or your torque converter lockup might not be working. It's also possible that the rear gears were upgraded to a higher ratio by a previous owner.
Note that the tachometers in some of these cars are known to read high -- I've seen some that read as much as 1000 rpm higher than actual. Does the engine sound like it's spinning at 2800 rpm? Note that it's the gauge, not the sensor, that is responsible for this. The best indicator of this problem is the indicated RPM at idle. If it's a normal 750 or so, this is probably not your issue.
Based on the fact that your shifter shows a 2 but not a 1, I can tell you have 2.73 rear gears, not the 3.23 gears that come with the "performance axle ratio" option package.
The shifter should be in the D closest to N for normal driving. That allows the transmission to use 4th gear, which is an overdrive. At 100kph in 4th with the torque converter locked, you should see about 1550 rpm.
If you're in 3rd (the second D), that's a 1:1 gear. At 100kph with the converter locked, this would be around 2200 rpm.
If you were in 2nd, you'd be running 3600 rpm at 100kph, so I don't think that's it.
If your have the wrong D selected, that would be the problem. If not, the clutch bands that engage 4th might have failed, or your torque converter lockup might not be working. It's also possible that the rear gears were upgraded to a higher ratio by a previous owner.
Note that the tachometers in some of these cars are known to read high -- I've seen some that read as much as 1000 rpm higher than actual. Does the engine sound like it's spinning at 2800 rpm? Note that it's the gauge, not the sensor, that is responsible for this. The best indicator of this problem is the indicated RPM at idle. If it's a normal 750 or so, this is probably not your issue.
Last edited by JakeRobb; 09-18-2016 at 09:50 PM.
09-19-2016, 11:44 AM
#5
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Re: Normal RPM for LT1 at 100 kph on highway?
17" is the wheel diameter. That does not control RPM..... it's the outside diameter of the tire. What size tires are you running?
There are codes for the trans that do not turn on the SES light. You can only find them if you scan the PCM for codes. Some of the codes inhibit 4th gear, keeping the trans in 3rd. Other inhibit torque converter lockup, which would have a minor affect on RPM.
Look at the Service Parts label for the RPO code for the rear axle ratio that was in the car when it left the factory. RPO "GU2" = 2.73:1, RPO "GU5" = 3.23:1. Look at the tire label in the door. Stock tires were either 235/55-16 (26.1" diameter) or 245/50-16 (25.7" diameter). If the PCM is still programmed for the stock rear axle gear and stock tires, and they have been changed, could account for the problem.
From my experience, typically the stock tach reads 100-200 RPM higher than actual engine RPM.
Using my 4L60E spreadsheet, the RPM for various combinations are:
2.73:1, 26.1" tires, at 100 KPH/62 MPH:
3rd (1.00:1) = 2,180 RPM
4th (0.696:1)= 1,517
2.73:1, 25.7" tires, at 100 KPH/62 MPH:
3rd = 2,214
4th = 1,541
3.23:1, 26.1" tires, at 100 KPH/62 MPH:
3rd = 2,579
4th = 1,795
3.23:1, 25.7" tires, at 100 KPH/62 MPH:
3rd = 2,619
4th = 1,823
Even if you were running 4.10 gears, 4th gear RPM would only be 2,279
There are codes for the trans that do not turn on the SES light. You can only find them if you scan the PCM for codes. Some of the codes inhibit 4th gear, keeping the trans in 3rd. Other inhibit torque converter lockup, which would have a minor affect on RPM.
Look at the Service Parts label for the RPO code for the rear axle ratio that was in the car when it left the factory. RPO "GU2" = 2.73:1, RPO "GU5" = 3.23:1. Look at the tire label in the door. Stock tires were either 235/55-16 (26.1" diameter) or 245/50-16 (25.7" diameter). If the PCM is still programmed for the stock rear axle gear and stock tires, and they have been changed, could account for the problem.
From my experience, typically the stock tach reads 100-200 RPM higher than actual engine RPM.
Using my 4L60E spreadsheet, the RPM for various combinations are:
2.73:1, 26.1" tires, at 100 KPH/62 MPH:
3rd (1.00:1) = 2,180 RPM
4th (0.696:1)= 1,517
2.73:1, 25.7" tires, at 100 KPH/62 MPH:
3rd = 2,214
4th = 1,541
3.23:1, 26.1" tires, at 100 KPH/62 MPH:
3rd = 2,579
4th = 1,795
3.23:1, 25.7" tires, at 100 KPH/62 MPH:
3rd = 2,619
4th = 1,823
Even if you were running 4.10 gears, 4th gear RPM would only be 2,279
09-20-2016, 09:21 AM
#6
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Location: St. Paul, MN
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Re: Normal RPM for LT1 at 100 kph on highway?
I have a 95 Z28 with automatic and 2:73 gears. My tire size is P245/50R16 with a 25.71 diameter.
Here is a photo of my speedometer and tachometer at approximately 66 mph (106 kph)
The transmission is in the D closest to the N during this photo. Hope this helps a little.
Here is a photo of my speedometer and tachometer at approximately 66 mph (106 kph)
The transmission is in the D closest to the N during this photo. Hope this helps a little.
Last edited by gaedbo; 09-20-2016 at 09:29 AM.
09-20-2016, 10:53 AM
#7
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Re: Normal RPM for LT1 at 100 kph on highway?
To get an accurate RPM, you really need to hook up a scanner and read the RPM directly from the PCM. That eliminates the error in the tach.
09-21-2016, 08:39 PM
#8
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Re: Normal RPM for LT1 at 100 kph on highway?
I'm sorry it's off subject but you freaking have nice interior lol!!!!
09-22-2016, 10:54 AM
#9
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Location: St. Paul, MN
Posts: 194
Re: Normal RPM for LT1 at 100 kph on highway?
Thanks for the nice comment on the interior. The light colored piece on the passengers seat is a Fabric Softner Sheet that I place in the car thru the winter time while it's stored in the garage to help prevent odors. (Minnesota Winters)
09-26-2016, 01:28 PM
#10
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Re: Normal RPM for LT1 at 100 kph on highway?
I just checked my RPM/Speed with the Scan94/95 software.
At 66 mph I'm actually supposed to be at 1647 to 1677 RPM on the Tachometer.
At 60 mph the Tach should be at 1490 to 1520 RPM.
Scan94/95 sure is helpful !
At 66 mph I'm actually supposed to be at 1647 to 1677 RPM on the Tachometer.
At 60 mph the Tach should be at 1490 to 1520 RPM.
Scan94/95 sure is helpful !
02-09-2017, 10:47 AM
#11
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Re: Normal RPM for LT1 at 100 kph on highway?
Just replaced the gears from 4.10.s to 3.23 hoping it would drop the RPMs. It is still around 2600 rpm at 100kph. It doesn.t seem to change into 4th gear. what could be doing this? Rear tire siz is 275/40R17.
02-09-2017, 12:54 PM
#12
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Re: Normal RPM for LT1 at 100 kph on highway?
Have you scanned it for codes? If not, read the second paragraph in my post #5 above.
But, if you had 2,800 RPM with the 4.10's, with 3.23 it would drop to 2,200 RPM. How did you determine your car had 4.10's?
Tach could be screwed up. Put a scanner on it and read the RPM from the PCM. See my post #7 above.
But, if you had 2,800 RPM with the 4.10's, with 3.23 it would drop to 2,200 RPM. How did you determine your car had 4.10's?
Tach could be screwed up. Put a scanner on it and read the RPM from the PCM. See my post #7 above.
02-09-2017, 02:08 PM
#13
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Posts: 13
Re: Normal RPM for LT1 at 100 kph on highway?
Unable to read codes. I have a OBDII reader and the 95 has a OBD1 with a OBDII connector. Don't know what GM was thinking.. The car doesn't seem to be going into 4 th gear. I removed the existing ring and pinion and replaced it with 3.23 gears. Any advice would be appreciated.
02-09-2017, 04:28 PM
#14
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Re: Normal RPM for LT1 at 100 kph on highway?
You can read the codes with a Free Scanning Software from this website created by Gary Doug.
Click on the link below !
https://www.camaroz28.com/forums/com...95-lt1-874306/
Read some of the information before you click on the link to download it so you will know which download to select, and how it works.
The link to download is from mediafire.
You will need a cable and a laptop to use it. One of the places you can get a cable from is http://www.aldlcable.com/
Injuneer the Super Moderator is able to read and diagnose the file if needed.
Click on the link below !
https://www.camaroz28.com/forums/com...95-lt1-874306/
Read some of the information before you click on the link to download it so you will know which download to select, and how it works.
The link to download is from mediafire.
You will need a cable and a laptop to use it. One of the places you can get a cable from is http://www.aldlcable.com/
Injuneer the Super Moderator is able to read and diagnose the file if needed.
Last edited by gaedbo; 02-12-2017 at 10:09 AM.
02-09-2017, 09:09 PM
#15
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Re: Normal RPM for LT1 at 100 kph on highway?
Sounds like it had 3.42's before, not 4.10's. 3rd gear/25.7" tires/100 KPH with 3.42's would be 2,779 RPM, compared to 3rd gear/25.7" tires/100 KPH with 3.23's being 2,619 RPM. Pretty much what you experienced.
Sounds like someone installed the rear axle assembly from a manual trans car. 94-02 6-speeds all had 3.42 gears. You need to program the new gear ratio into the PCM if the PCM was programmed for the 3.42's. Otherwise the speedo will be off by about 5 or 6 KPH low at 100 KPH.
What GM was thinking with the mismatch on the 16-pin connector was their plan to move to an interim hybrid version combining parts of OBD-1 and OBD-2. This was nicknamed OBD-1.5 by the public, but not by GM. They installed it on the LT1 in the 94 and 95 Corvette, but apparently never got around to it on the 95 F-Body. Some codes on the Corvette are 2-digit OBD-1 and some are P 4-digit OBD-2.
Sounds like someone installed the rear axle assembly from a manual trans car. 94-02 6-speeds all had 3.42 gears. You need to program the new gear ratio into the PCM if the PCM was programmed for the 3.42's. Otherwise the speedo will be off by about 5 or 6 KPH low at 100 KPH.
What GM was thinking with the mismatch on the 16-pin connector was their plan to move to an interim hybrid version combining parts of OBD-1 and OBD-2. This was nicknamed OBD-1.5 by the public, but not by GM. They installed it on the LT1 in the 94 and 95 Corvette, but apparently never got around to it on the 95 F-Body. Some codes on the Corvette are 2-digit OBD-1 and some are P 4-digit OBD-2.
