Has anyone changed the rear end on an auto C5 from the factory 2.72 or 3.15 to a 3.42? I understand that since only the 2.72 and 3.15 ratios were offered for my car, the Hypertech programmer will detect that, and not allow me to change the selected ratio in my computer to 3.42 or beyond.

Anyone have any experience with this?

 

Just out of curiosity, why does the programmer care what gear ratio you have? If I'm not mistaken, the speedometer is independent of the ratio, and I wouldn't think shift points would care about anything other than rpm's...

 

WOT shift points DO CARE about the vehicles speed in addition to RPMs.
Shifting will occur ONLY when both criteria is met.

 

The computer needs to know what ratio the rear end is so it can process the correct rpms for odometer speed display correct transmission shift timing.

 

My speedo will not be correct because the Hypertech will not allow me to tell the computer that the ratio is a 3.42.

 

You will need a tune with a different programer ie ls1 edit, or similar.

 

Just had it done by ECS. Went from 2:73 to 3:42

Chuck

 

If you do a search on this forum, you'll find that this is one of the most popular mods. Mike Mercury has an excellent DIY write-up on the swap, along with tips and myths about the change.

Incorrect, the vette does not use the ratio to calculate speed, however, changing WOT shift points is very important, otherwise it will throw codes and not shift optimally. I've heard if you drive in 3rd gear, it won't throw codes, until you can get it tuned.

 

The Hypertech won't recognize the axle change because on a C5 when you change gears there is no speedometer error---The sensor pick-up ring is not on the output shaft of the transmission but rather on the output shaft of the transaxle---you can can install any gear you want and the speedo will be correct--However on an automatic you will have to adjust the shift points both for WOT and Part throttle shifts--Best way is to scale all the tables by whatever % the change is--For example a 2.73 vs. a 3.42----your would need to subtract 25% on the upshift & downshift tables (2.73 X 1.253 = 3.42)

 

... about the Hypertech... which is a problem on the C5. The Hypertech will do everything needed for rear axle ratio changes.... except for... the altering of the TCC Slip Table. And because of this, a C5 A4 with a non factory rear axle ratio will eventually set the P1870 error code and place the tranny into the limp mode.

The Hypertech has this "bug"... and as far as I know - it was never corrected.

more gear swap info here:

http://mikemercury.home.att.net/342.htm

 

Not trying to rob thread but have one question . Have 98 with 2.73 . Next week having diff changed out to 3.42 @ tampa shop. I have 160 therm which I have changed fan settings w hypert 3. Since the shop will have to change shift points will I lose my settings for fan,shift firm, etc. Can I still use hypertech to change settings at later date without deleting shop tuned shift points for 3.42 diff..

Any replys appreciated Ian

 

I think its best if you reset everything back to stock, then have the shop set everything for you. I'm not sure, but I don't think the hypertech will like playing with another tune. Then you can either store the "new" factory settings, or sell the unlocked unit.

 

 

I read your detailed page on the diff change but I can't see where the computer senses a higher RPM than usual that causes a p1870 Code--On a 3.15 gear 2000 RPM is hit at 70 MPH and with the 3.42 you will hit 70 mph at about 2100 RPM--BUT you can turn 2100 with the 3.15 as well---The RPM's are irrelevant because you can hit any RPM with ANY gear ratio so why would it matter ?--the only thing that changes is the MPH---The TCC slip table is measured in RPM slippage--The torque converter doesn't care what the MPH is or where it is hit--only the amount of slip--
Also the ECM has to hit 2 paramters in order to command a WOT shift--One being the "shift at RPM" and the other "shift at vehicle speed" The Hypertech only addresses 1 of these and leaves the other out-NOT the right way to change WOT shift points--Not doing both will sometimes cause a shift to be missed and put the engine onto the Rev-limiter instead--And sometimes it will alter the downshift by making it downshift when applying only minimal throttle pressure--( going up a slight grade and having to give it more gas that causes an annoying
downshift--where before it would stay in locked TCC mode)
You don't have to agree with me and certainly have a right to your opinion as do I---

 

The difference is that you are guessing... and I (and all the others here) are not. From your response, it's clear that the TCC Slip Table and the P1870 code is soooo over your head, that you haven't a clue.

but there is no "opinion"... there's facts. And all the common-sense thinking and rationalizing on your part - can't change the facts.

http://www.ctechradio.com/temp/c5-p1870.doc

you keep on believing your "opinion", and please... PLEASE proudly post this opinion on all the C5 boards.


.

 

To Make A Long Story Short, I Bought My Car, Someone Had Put A 3.42 Rear End In It. I Tried Hypertech, Due To Shift Points. It Never Worked Right And I Went Back To A 2.73 Which Was Stock ... No Problems Now Just Cost A Bunch To Get It Right.

 

What a shame I'm so looking forward to putting 3.42 in mine. Can't legally go 170mph around my town,and I dont track it. Did you try shop tune for shift points .I went to ck yesterday after posting here and theyll do a street tune and all settings for 199.00. Nice clean shop with plenty of equip to do job. I'm hoping for no problems

Ian

 

LS1 Edit (newer version) and HP Tuners supposedly automatically adjusts the TCC Slip Table when selecting a rear axle ratio higher than the 3.15 on the A4 C5 models.

The Predator takes care of this when you first load in the Performance Tune Program (well; at least the 1999-2002 model of the Predator. The newer ones may handle it differently).

The Hypertech fails because they didn't further investigate the differences between F-Body LS1's and C5 LS1's PCM program. The HPIII doesn't know there is a TCC Slip Table... and that's why it fails when changing rear axle ration on the A4 C5. It flat-out doesn't know to change the values in that table.

Hypertech has admitted to the problem, but said they weren't issuing an update. This was the data from about 18 months ago. Maybe they changed their mind... I haven't kept up with the HPIII recently.

Here is the archived post abotu the HPIII:

http://forums.corvetteforum.com/showthread.php?t=944545

 

I bought my 98 Vert with a 3:73 that the previous owner had put in it. Had a heck of a time with it at first with hard shifts, etc. Took it to West Coast Corvette and they told me whoever had it prior had jacked up the tuning SO BADLY by tweaking individule aspects (ie tweaked it here, tweaked it there)..that the car was fighting itself. They tuned it with the 3:73 and it's awesome!!! It ended up giving it way more hp than when I bought it. I just replaced the gears a week ago (previous owner didn't wear them in properly) and it's absolutely awesome now!!!!!!! Don't let anybody tell you it can't be done.

 

Actually I was just trying to be polite--What I said is not an opinion and not a guess as you put it but is true. I base this on the fact that I am a pro tuner and have tuned over 250 cars and have had extensive training with EFILIVE and Moates in high performance applications---What are you talking about ??"all people on this forum" ?? I have NEVER heard anyone ever express such a ridiculous explanation as you !!! and there is no table called the TCC slip table--It's called the PWM table(pulse modulation table)--Get your terminology correct--obviously learning and studying is soooo over your head.--Oh yeah I clicked on your silly link--IT doesn't exist--Your explanations can't be followed by a trained person because you talk is circles and really never explain anything but just point to other peoples words--he-said-she said--My explanations can be followed and make sense because they are true--You are just offended because finally someone who knows better questioned your self proclaimed expertise--I tried to teach you something but you chose to pound your chest --Sorry if i offended you It wasn't my intention thats why I stated it as an opinion at 1st--but instead of letting it go you chose to act like a spoiled brat and start a war--Bring it on Einstein.

PS The reason Hypertech can't scale any shift point table greater than a 3.15 is because their shift point MPH table doesn't go far enough to accomodate the MPH's needed in scaling any greater ratio--AND when you can't scale the shift point table far enough the trans will start to skip shifts and put the engine onto the rev-limiter--then shift.That casues the trans to overheat and slip--hence the p1870 code and the limp home mode--EXAMPLE going from a 2.73 to a 3.42 is a 25% increase----so you would have to subtract25 % more MPH to the 2-3 shift point to correct it--The stock 2-3 shift @ VSS table is usually set at 70 MPH
70 X .25= 17---You would have to subtract 17 MPH to correct that shift point --The HPP3 won't go that far--