Can someone post up what the exact shift points are for a stock GM tune 2.73 rear. I want to know both RPM and MPH, and any tables if you have them.

Also, what tables would you change when going from 2.73 to 3.73?

Thanks.

 

TTT,
Someone must know this.

 

Here are the tables from my 98 stock tune. It's an Excel file zipped, since we can't upload *.xls.

For your conversion due to gears, use the percentage obtained from:
Correction = 1-[(New ratio)/(Old ratio)].

In your case, that would be -0.3663 or 36.63% decrease.

Your new shift speeds would be:
New shift speed = (Old shift speed) + (Old shift speed)*(Correction)

For example, converting a 100mph shift speed for 2.73s to the desired shift speed for 3.73s:
Correction = -0.3663
New shift speed = 100 + (100*-0.3663)
New shift speed = 63.37mph.

 

Thanks,
Helps a lot

 

FYI,
I think your calculations are incorrect.

For the 2.72 to 3.73 example, it is indeed a 36.63% increase, or better looked at as 1.3663x(old ratio).
To calculate the new MPH, you need to divide by 1.3663, effectively 73.19% of the old MPH. Your calculations use 63.37% (1-36.63%), but I believe that is wrong.

You have to be careful when your talking about increases/decreases when using percentages. If I'm wrong, please someone chime in but I'm pretty sure I'm not.

Anyway, finally got to reprogramming my entire shift table using the stock numbers as a baseline (since unfortunately, my stock numbers are long gone). Car shifts beatifully right before red-line at WOT and gone is the jerkiness of the 1->2 part throttle shifts.

 

First, good to hear that the car is running well.

Second, the method I posted was taken from the EFILive corrections for the speedometer (also applied to shift points). The method I posted yields the same results as their correction routine, which I think is unlikely to be incorrect (although possible).

If your car runs correctly using what you have described, that's odd, but it runs, so stick with it.

 

Hmm, maybe that's why my car hasn't been running to its optimum performance?

I switched to 3.73s and left stock shift points??

I have a Diablo Predator I can change the MPH shiftpoints..

So Ryansm1, I can't remember the stock shift points, but it shows them in the Predator...but just for example...if the 1->2 is 40mph....you say I do 40 + 40*-0.3663 ..... my calculations come out to about 25mph??

So with 3.73 gears, it should shift a lot sooner or later in MPH?

 

I also checked it against EFI lives routine, plus also my engineering background (not saying in the least that I'm an expert). Plus also against the guidance on the HP tuners forum. All seem to agree with me.

Here's my methodology.
With 2.73 gears, the 1->2 WOT shift should be at 40mph. 40mph comes out to ~4400 rpm (just used a gear/speed calculator). That translates to a wheel rpm as 4400/2.73/3.06=526.7064wrpm. Tire diameter of 25.66->circumferance of 80.6132in/revolution.
Just checking all the numbers that comes back out to 40mph. So the gear speed calculator was correct. (always like to back calculate stuff I find on the internet).

Changing that ratio to 3.73 is 36.63% more, but you have to be careful when you use that percentage. It's only valid to increase numbers, not decrease. It's much better to use strait decimal units.

By your method, you'd adjust the mph by 63.37%, or approx 25mph. 25mph/60min=.416m/min*5280ft*12in=26400in/min divided by the circumference=327.49wrmp times the new gear ratio (3.73) and 1st gear tranny ratio (3.06) = 3738RPM
You've change the engine rpm shift point down almost 700rpms!

My my method.
You'd divide by 1.3663 (effectively 73.19% or a 26.81% decrease) and it gives 29mph.
29mph=30915.6in/min --> 383.5wrpm --> 4377RPM. Right back to that 4400 RPM shift point (give or take a little for rounding errors).

Coincidentally, increase 29mph by 36.63% and you get right back to 40mph. See how you can get tricked by using percentages the wrong way. I had this hammered into me way back in the college days. 2.73-->3.73 is a 1.3663 change, so strictly multiply or divide by 1.3663, forget the percentages.

I'm certainly not trying to start anything, but I'm pretty sure my numbers are correct. I just want to make sure everyone gets thier car right too.
I'd be happy to for someone to point out where I might be wrong.

In any event, thanks again for the table. Helped a lot.

 

Yes, you need to adjust your WOT mph limits. I also did mine in a predator, and also ran the results through EFI live (<-long story).

I set it this way
1->2 29mph
2->3 65mph
3->4 125mph (125 is the max value in the predator)
2->1 26mph
3->2 61mph
4->3 115mph

Your car shifts on the mph value at WOT, since a 3.73 gear car spins the tires much slower, it takes more engine revs to do the same speed. If you mph value is set too high, the engine will rev to it's rev limiter and the tranny will never be commanded to shift.

 

Yikes. Stock shift points with such a dramatic shift change will likely leave performance on the table. I posted the stock 2.73 shift points earlier in the thread, if you want to see them.

Changing to a higher (numerical) gear ratio would, as nj02vette stated, require the engine to turn at a higher RPM to maintain the same wheel speed.

nj02vette:

I understand what you are saying, however, the EFILive routine does not agree with you, at least, not for me. I checked with my stock tune, and (provided all other variables constant), using the speedometer/shift point correct routine from 2.73s to 3.73s results in a 1-2 shift from 40mph to 25mph.

I do see where you are saying to simply divide by the ratio of the new gears to old, and that seems logical to me as well. I will think more about this disagreement after work and post up.

Regardless, the shifts are done in combination between engine RPM and vehicle speed. If you recall, the 1-2 shift in a stock 2.73 F-body occurs right around 50mph (6000rpm), not 40mph; your back calculations show the shift point would be at 4400rpm, too.

I am not sure how the shift point is determined and takes place, as obviously, shifting at 4400rpm would be far too early. I would guess that both conditions must be met (40mph and 4400rpm) for WOT shift, but more weight is placed on the engine speed over vehicle speed. This might explain why (if true) jetlag is having performance issues: it is unlikely for him to meet both conditions for the shift.

 

Oh, I just put up 40mph to use in the formula, I couldn't remember offhand what it really was..just wanted an example.

This stuff is confusing...I don't know what to do...both of you sound right, but then say each other is wrong..haha

 

For part throttle shifts, only the TPS and mph value affect shifting. For WOT, I believe your correct that both conditions must be met, but I also hear there is a lot of leeway in that it does not seem consistant due to tranny lag and other factors.

Still, my intuition is telling me how to adjust the numbers. I only had access to EFI live for a brief time, but on thier website, they have an example problem for how to change shift points. I used this as a guide as well, and using both methods, the only one that matched thier result was to do it the way I did.

We'll figure this out! As you said before though, the car is running great, and that's the important thing.

 

OK, so I the bearings went out on my 02-Z/28 with 2.73's in it and since the shop was in there replacing stuff I figured, "why not bump up to 3.73's!" So with that being said, my question is this: I pretty much have all of the bolt-ons you can get (incl. custom tune from James @RWTD via Predator), still running stock h/c, tc, tire size, etc. I am planning on using the shift points above that you have but, did you change the RPM at all? Or did you leave that alone? (Its been so long since I have had to use my Predator I am trying to recall if you even can with it?!?) Thanks in advance.

 

Guys WOT the ecm uses BOTH RPMS AND MPH for it's wot shift routine. It has to hit one THEN the other to get permission to shift to the next gear..

The main thing you would be worried about would only be the normal shifting tables not at WOT, to bring the shift points back to normal shifting rpms.


EFILive shows you at the bottom of your screen the Current RPM value of your mph shift points. So you can easiely see if it can or needs to be fine tuned.


Also, you can download the efilive software for free at www.efilive.com and check out your tunes for free if you just download one from the holdencrazy bin site and play around with the calculators and see what your stock settings are.

 

Thanks, you have a pm.