Will I need a new tune??
 

My first post but I know some of you from other forums. Anyway on to my issue.

First the truck info/bolt-on mods I've done so far.

97 XL 4.2L V6 4r70w 3.08 Non LS, General Grabber UHP 265/70r16s, AF1 CAI, ASP UDP, Cherry Bomb Vortex SIDO catback, Maggie Hiflow Cats, Troyer tuned 94 performance.

Now the issue I have is after lowering the rear 2" with drop shackles the truck has developed a very slight bouncy feel. Had them check the pinion angle and it's only 0.5 degrees off which is ok. My mechanic took off the inspection cover of the rear diff and the spider gears badly pitted, one even has a visible crack and they have waayyyy too much play in them. I was hoping to put this mod off until next year but seeing that crack makes me nervous. So I've ordered a Tru-Trac LS and 3.73 gears.

My question is since my Troyer tune was made for the 3.08s & open diff will I need to get a new tune or could I make the changes with the Xcal3? I'd normally stick with the same tuner but I don't want to drive 10+ weeks with the wrong tune.

I am not to familier with the xcal3 but it should have a "custom option" menu where you can change the gears and save the change with out effecting the custom tune.
If you have the manual it should tell you in there.

Lars

These are the only things it will let me change.


Global Spark
WOT A/F
Rev Limit
Idle Speed
WOT Shift
Shift.

There's very limited things you can mess with for the 97s.

Then I have to say you DO want new tunes. Sorry!

With the new gears your speedometer and odometer will be "off", which is no HUGE deal, but your shift points aren't right either. They're commanded by a combination of speed and throttle position. So, since the truck's speed is wrong, they happen at the wrong time.

I'm sure it's all something you could live with, but I imagine you tuned your truck for a reason, and that reason is not "halfway performance".

But, I've just had a thought, which may negate everything I've said above. Your truck MAY have a more "direct drive" if that makes any sense from the speed sensor to the speedometer. I'm thinking now that the PCM doesn't even get into the act in your MY truck here, and a tune may not be able to adjust the speed. But, when you get the new axle, they can install different gears on the speed sensor so that it will send correct speed and distance information to - "wherever" (sorry, I'm not really sure how your system works - I know it's not a mechanical cable, but I don't think the PCM controls the speedo.

However, the PCM and the speedo DO get their speed information from somewhere, so all you have to do is make sure that information is correct. If it is, then your tune is fine with the new gears and you won't have any speed or distance errors.

Find out from the shop doing the swap what happens with the speed and distance indications.

- Jack

Yes it looks very limited.


Lars:2thumbs:

Lars, I went back and changed my mind. :shrug: See what I added. And, I think the latter part is more correct. I think this is why he has no speed correction capability with this unit.

- Jack

Oh
So a new gear requires a new speedometer gear to compensate for them?
97xl will have to look for a speedometer gear correction kit.

Lars

Yup! I think that is the RIGHT approach. I vaguely remember reading something about this in the not so distant past, when my brain cells were more active. And, I'd think this would be a trivial addition to a gear mod, wouldn't you? I'd be surprised if the installers of the gears, didn't include it in the work order.

- Jack

They must have let it slip their minds also?
Good catch:1toast:

Lars :2thumbs:

I know I can't adjust the speedo with the programmer, needs to be done by swapping out the VSS gear. Just wondering if I'll need a new tune to make the shift points correct. Last thing I wanna do is go messing with the shifts in the programmer and make things worse. It should be liveable until I can get new tunes. Right now the speedo gear is set for stock tire size of 235/70r16 and since my winters are 255 and summers are 265 it was already off to begin with but that didn't seem to make a difference with the tune(when I had the tune made I put 255s as the tire size on Troyers order form.) Guess I'll just wait till PHP is open again and find out the details.

Thanks for the info.

Thanks for reminding me of the name. If the VSS gear makes your speed correct, then the shift points should also be correct. I'd say install a VSS, with the new gears, that matches the tires you use most of the time and live with the slight error that will be present with the others. Looks to me like you'll have only a 2% error between the 255s and the 265s.

- Jack

Ok just so I have things clear in my head...the shift points are controlled by the speedo...the VSS gear in the truck right now is the stock one so I know the speedo is off but if I change the VSS to match say my 265s that will correct the speedo error and since the shift points are based on the speedo then the truck should shift at the proper point without having to modify the tune??

Yes, almost. The part throttle shift points are controlled by speed AND throttle position. At a fairly open throttle, the shift is delayed to a higher speed, and vice versa. But yes, having the correct VSS for the tires AND gears will make the shifts occur where they should.

- Jack

Ok thanks for clearing that up. This mod is already costing me big $$ and I was hoping not to have to spend more to get new tunes.

Well the rear end has been installed and my speedo is waayyy off...when I know I'm going close to 60 kph(based on tach rpms) the speedo says 80ish. The trutrac has a whine to it whenever you let off the gas. My mechanic said to bring it back in a week so he can check to make sure its ok. Now to get that VSS gear.

Once the VSS gear is corrected, the shifts will all return to normal, at least based on MPH. However, they may still be a little high RPM-wise since you've now changed your RPM/MPH ratio. This isn't necessarily a bad thing, but might not exactly be what you're looking for... especially if you're looking for fuel economy.

If you can provide old gear/tire combo and new gear/tire combo, I can run it through the calculator and let you know just how much to expect your RPM to change. Of course, you've probably already noticed the difference. :)

Thanks Bill,

Old:

3.08 open diff 265/70r16 tires 30.61"

New:

3.73 LS 265/70r16 tires 30.61"

Update. I spoke with Eaton directly and they said 99.9% that the reason I've got the noise is because they used synthetic diff fluid. They said only use conventional. Was told it will make a small amount of noise when turning but other than that there should be no noise if the gears are setup properly. Also that its possible to take the exciter gear from the old rear diff and install it on the new one so the ABS will work. I called the shop today to make sure they still had my old parts, which they do, so this should be getting fixed tomorrow.

The gear change with no tire size difference equates to a 21% difference.

I've attached a really neat calculator that will take tire size, rear gear ratio and transmission ratios to provide an RPM per MPH value for each gear. Hopefully it's useful.

Take care.

Thanks Bill, that gives me a better picture. Right now when the speedo says 120kph(74.5 mph) the tach is only at ~2000, with the old gears the tach would read ~1800-1900.

Update:

Took it back on Tuesday. Mechanic took it for a drive and said yeah its too loud and there is a clunk when it downshifts. Took it apart again, swapped over the exciter gear from my old carrier so now I have ABS again, put in the conventional 80w90 diff fluid and tightened things back up...what a difference now, no clunks and no noise, except if you turn a corner and get on the gas it clicks but thats normal for transferring power to the other wheel. I can now get the tires to squeal on dry pavement from a dead stop in a straight line, with the old setup I didn't get so much as a squeak. Feels like a new truck. The backlash is 0.010". Now I just need to get that new speedo gear and this mod is complete.

If the speedo gear doesn't fix the shift points I'll be calling you guys for some new tunes. I mean I love my Troyer tunes I just don't want to wait 13 weeks to get them.