...

Go Back   Lateral-g Forums > Technical Discussions > Transmission and Rear End
User Name
Password



Reply
 
Thread Tools Display Modes
  #1  
Old 05-14-2021, 02:12 PM
vet57air's Avatar
vet57air vet57air is online now
Member
 
Join Date: Feb 2021
Location: West coast Canada
Posts: 90
Thanks: 33
Thanked 97 Times in 44 Posts
Default 8l90

Have an Gen V LT4 8L90 auto in my 66 C10. Have put 75 test miles so far. Noticed when in full automatic at city speeds (30mph) the engine rpm is about 1000 rpm. My rear tires are 31" tall and rear gear is 3:73. The engine does it but rpm seems low?
Reply With Quote
  #2  
Old 05-15-2021, 03:45 PM
Mean Buzzen 6 Mean Buzzen 6 is offline
Member
 
Join Date: Jul 2015
Posts: 24
Thanks: 2
Thanked 1 Time in 1 Post
Default

Do you have it in overdrive?
What gear is it in driving in the city?
Reply With Quote
  #3  
Old 05-15-2021, 05:15 PM
dhutton dhutton is online now
Senior Member
 
Join Date: Nov 2006
Location: Mountain Springs, Texas
Posts: 1,761
Thanks: 727
Thanked 270 Times in 166 Posts
Default

Quote:
Originally Posted by vet57air View Post
Have an Gen V LT4 8L90 auto in my 66 C10. Have put 75 test miles so far. Noticed when in full automatic at city speeds (30mph) the engine rpm is about 1000 rpm. My rear tires are 31" tall and rear gear is 3:73. The engine does it but rpm seems low?
Who did the tune? Did you give them the tire diameter and rear gear?

Don
Reply With Quote
  #4  
Old 05-15-2021, 06:41 PM
vet57air's Avatar
vet57air vet57air is online now
Member
 
Join Date: Feb 2021
Location: West coast Canada
Posts: 90
Thanks: 33
Thanked 97 Times in 44 Posts
Default

Quote:
Originally Posted by Mean Buzzen 6 View Post
Do you have it in overdrive?
What gear is it in driving in the city?
The 8L90 has drive (full auto) and manual (I use paddle shifter). Near as I can tell at 30 mph it is in 5th. I seems to like to be around 1000 rpm at pretty much any speed until your at highway speeds in 8th and rpm goes up with speed. This is a crate package and all "tuning" was done by GM.
Reply With Quote
  #5  
Old 05-17-2021, 02:47 PM
dontlifttoshift's Avatar
dontlifttoshift dontlifttoshift is online now
Senior Member
 
Join Date: Oct 2012
Location: Beach Park. IL
Posts: 881
Thanks: 8
Thanked 111 Times in 68 Posts
Default

Right, but the ECU doesn't know what tire diameter or gear ratio you have. Consequently the shift strategy is based false information. It's close for a lot of applications but with a 31" tire it won't shift the way it should.
__________________
Donny

Support your local hot rod shop!
Reply With Quote
  #6  
Old 05-17-2021, 05:35 PM
dhutton dhutton is online now
Senior Member
 
Join Date: Nov 2006
Location: Mountain Springs, Texas
Posts: 1,761
Thanks: 727
Thanked 270 Times in 166 Posts
Default

Quote:
Originally Posted by dontlifttoshift View Post
Right, but the ECU doesn't know what tire diameter or gear ratio you have. Consequently the shift strategy is based false information. It's close for a lot of applications but with a 31" tire it won't shift the way it should.
I was thinking the same thing. The ZL1 uses 2.85 gears with a 27” tall tire so it’s similar to 3.73 gears with 31” tall tires if I did the math right. Got me wondering what the Connect and Cruise tune assumes.

Don
Reply With Quote
  #7  
Old 05-17-2021, 06:07 PM
raustinss raustinss is online now
Senior Member
 
Join Date: Dec 2013
Posts: 775
Thanks: 36
Thanked 63 Times in 51 Posts
Default

id have to agree , not that it matters but my 17 silverado with 3:42 gears isnt even that low lol
Reply With Quote
  #8  
Old 05-19-2021, 06:30 PM
vet57air's Avatar
vet57air vet57air is online now
Member
 
Join Date: Feb 2021
Location: West coast Canada
Posts: 90
Thanks: 33
Thanked 97 Times in 44 Posts
Default

Quote:
Originally Posted by dhutton View Post
I was thinking the same thing. The ZL1 uses 2.85 gears with a 27 tall tire so its similar to 3.73 gears with 31 tall tires if I did the math right. Got me wondering what the Connect and Cruise tune assumes.

Don
The install docs for the LT4 8L90 specify 3:08 to 4:11 gear and 26" to 30" tall tire. Yes I went with a 31" tire but assumed the transmission only sees the driveshaft rotation as an end result? Funny my speed to the Dakota speedo was within 5 mph first time out.
Reply With Quote
  #9  
Old 05-19-2021, 06:38 PM
dhutton dhutton is online now
Senior Member
 
Join Date: Nov 2006
Location: Mountain Springs, Texas
Posts: 1,761
Thanks: 727
Thanked 270 Times in 166 Posts
Default

Quote:
Originally Posted by vet57air View Post
The install docs for the LT4 8L90 specify 3:08 to 4:11 gear and 26" to 30" tall tire. Yes I went with a 31" tire but assumed the transmission only sees the driveshaft rotation as an end result? Funny my speed to the Dakota speedo was within 5 mph first time out.
Ya I think the rear gear ratio sort of cancels out the tall tires.

Don
Reply With Quote
  #10  
Old 05-19-2021, 08:20 PM
130fe 130fe is online now
Lateral-g Supporting Member
 
Join Date: Jan 2005
Location: Jacksonville, FL
Posts: 756
Thanks: 152
Thanked 31 Times in 20 Posts
Post

Do you have HP Tuners? You can change it that way in the gear/tire wizard in the tune. You are using the Gen V connect and cruise pcm correct?
__________________
Chris
1968 Camaro SS
LS3/T56
Full of DSE and Holley parts
Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT -5. The time now is 04:33 PM.


Powered by vBulletin® Version 3.8.11
Copyright ©2000 - 2022, vBulletin Solutions Inc.
Copyright Lateral-g.net