SS RRR

iTrader: (2)

So glad I have a manual car!

quik95lt1

iTrader: (7)

in reality 96caprice is correct........if you DO NOT change anyting in the computer including NOT correcting the speedo the car will still shift exactly the same......if you correct the speedo and DO NOT correct the shift points.....then it will be off........

look at it this way (i know these numbers areent accurate but lets use them for demo purposes)

lets say our test parameters are.....

7000rpm shift point
6' run out on our tire
3.06 1st gear raio
3.73 gear which we will swap out to a 4.56 in the second part of this....

TsRPM=Tailshaft RPM
TrRPM=TireRPM
Erpm = Engine RPM
Scalar= Speedo Scalar that ECM uses to calculate mph from VSS (my mehtod is for demo purposes actual calcculation is far more complex)


so....7000 Erpm /3.06 (1st gear ratio) = 2287 TsRPM
2287/3.73 (rear gear ratio) = 614.3TrRPM
614.3 * 6 = 3685.8ft/min = 41.88mph

so for a speedo scalar we have
2287TsRPM/41.88mph = 54.60 Scalar

so the pcm is programmed with a 54.6 scalar the speedo is correct and it shifts at 2287TsRPM which is 41.88 mph and 7000rpm.....




now your buddy came out and kicked your teeth in with his LS1 and you swap in 4.56's to catch him...........but you DO NOT change your tune AT ALL......

so....7000 Erpm /3.06 (1st gear ratio) = 2287 TsRPM (NOTICE THIS PARAMTER HASNT CHANGED)

2287/4.56 (rear gear ratio) =501.53TrRPM
501.53 * 6 = 3685.8ft/min = 34.19mph

so for a speedo scalar we have
2287TsRPM/34.19mph = 66.9 Scalar


but you never changed the speedo scalar in your desperate attempt to beat your buddy.......the computer still calculates thinking its got 3.73's.....is your speedo off your "actual" speed........hell yes.......but does the PCM have any idea......nope........the pcm says your doing 41.88mph still at 7000rpm regardless of you actually doing 34mph according to your fancy gps on your dashboard.......the pcm sees that 41.8 and bam it shifts........


now lets say you change the pcm scalar but never change the shift points....well now ya got a problem......

lets say you change that scalar so now at 7000rpm the pcm tells your speedo its doing the correct 34 mph........your shift tables are still at 41mph......unless you got a really good valve train or a rev limiter your going to be picking up pieces............



that should clear all this mumbo jumbo up........plain and simple.....if you leave it alone and deal with the incorrect speedo its all good.......but if you do decide to correct your speedo by changing the gear ratio in the tune.......change ur shift points also.....simple math will show you how much

3.73/4.56=.817 so you need to decrease your shift points and tcc lockups by (1-.817= .183*100= 18.3%) 18.3%.......41.88*.817= supprise supprise 34.21.....so you just scale it by -18.3% and you are good to go........

however due to the added accleration rate of the gears now in 1st gear I add 2% to the first gear calculation and 1% to the second gear to allow the ECM, solenoids and trans extra time to shift because its time is greatly reduced now because of the acceleration rate of the rpm of the engine due to the lower gear......just a little tuning tip

hope this helps

FAD2BLK93

Damn...my 700r4 is looking pretty good right now.

WS Sick

Yep if its not corrected the shift will happen at the same mph, the mph just will be off.

Thats what I said.

Forgot to add, RPM when in D has no bearing on the shift. Manually shifting, yes RPM has a say.

Sorry , at work my edit button does not exist..lol (firewall) anyway In Tunercat, when you change the scaler to compensate for your new gear ratio, it atumatically changes the MPH in your shft tables to reflect your new gears.

Edit again, yes if you change nothing and your WOT shift is at 7000 it will still be there. Part throttle and tooliong around will be annoying as crap (think waiting for shifts or being in OD in less than 100 feet from a start) but if you just don't look at the speedo on WOT your shift will happen at 7000 RPM.

ramair 1

iTrader: (2)

Thanks quick95 and everyone else for the input on this .I will be finding someone or some way to set the correct ecm tune before i start driving it. The car is only driven about 500 miles a summer so i dont care to wait till its set up right.

F0x Slaughter

Makes perfect sense.

97 6speed z

It doesn't, and that is what causes the shifting problem! As WS Sick said the shift points are determined by vehicle SPEED (i.e. mph). When you change to a deeper gearset (i.e. a numermerically higher ratio), for any given number of driveshaft rotations that the VSS is "reporting" back to the PCM, without re-programing, the PCM "thinks" the vehicle is going FASTER than it actually is. This makes the trans shift EARLIER than it normally would.

So VERY true, and because the speedo is off ..... the PCM again "thinks" the car is going faster than it REALLY is, and the trans shifts early.

WS Sick ...... you are TOTALLY correct here!

Again, for a change to a DEEPER rear gear ...... the speedo will read HIGHER than the vehicle is ACTUALLY going, without any re-programing!

And (again) that is exactly what causes the shift problem. Think of it this way, after a change to a deeper gear the VSS "tells" the PCM the car is going, let's say, 30 mph in first gear and it's "time" to upshift to second gear, the trans does upshift at the PCM's direction, but, the car is actually going only, say 18 mph. The upshift comes TOO early with respect to TRUE vehicle speed.

Again, the PCM does NOT know a change occurred back there in the differential, sooooooo ......... it bases transmission shift points on erroneous MPH readings.

Again ...... it doesn't! That's what causes the problem!

Again ....... right on WS Sick!

No it won't! Believe me .... we took and A4 car with original 2.73 rear gears in it, and swapped them up to 4.56's. to make a bracket race car and without re-programing the PCM .......... the trans would upshift to third gear by 23 mph during light throttle/cruising.

F0x Slaughter

If you dont change anything 6000rpm is 41mph to the PCM regardless of gears. We just went over this.

97 6speed z

I have NO idea what information you are trying to convey here???? When my M6 car is in FIRST gear at an engine speed of 6,000 RPM ........ it's going about 35 mph, when it's in FOURTH gear at 6,000 engine RPM ........ it's going about 115+ mph.

What do you mean 6,000 rpm is 41 mph to the PCM regardless of gears???

F0x Slaughter

I was throwing out a number.

245/50/16 tires on a4 with 3.23 is 44mph at 5700rpm in 1st gear.
3.73 is 38mph.

VSS uses tail shaft rotations and a scalar formula in the PCM to tell the speedo what the mph is. If you change the gears in the axle it doesnt affect the VSS. So if you do not correct the speedo, when you hit 5700rpm the PCM thinks the car is going 44mph and will shift, in reality the car is going 38mph. But as discussed the PCM will have no idea the car is doing 38mph because the scalar in the PCM tells it that you're doing 44mph.

Since shifts are dictated by mph it doesnt matter what gear or tire size you have as long as you don't correct the speedo.

If you correct the speedo and don't change the shift point you will need to spin 6600rpm if your shift point was 44mph, with 3.73s.

Also a stock m6 with 3.42 and 245/50/16 tires at 6000rpm is doing 50mph. In 4th is doing 134mph. Remember the tach on the dash is about 200-300 rpms high.

Also a4 with 245/50/16 and 2.73 is doing 55mph at 6000 in 1st. With 4.56 is doing 33mph. The speedo still thinks its doing 55mph though.

97 6speed z

First off, Thanks! for clarifiying what you posted above. I now think (?)I know what you are trying to say.

The fallacy of your logic however, lies in thinking that the trans will shift based on engine RPM. What WS Sick and I are saying is that the PCM commands transmission shift points based on MPH.

Let's use this VERY simple example:

An A4 car is properly programed to make a 1-2 transmission shift at 25 mph which occurs at exactly 1,000 driveshaft RPM, with it's 2.0 to 1 rear gear ratio. So in this first case 25 mph happens at 1,000 driveshaft rpm with those 2.0:1 rear gears, and the TRUE vehicle speed is 25 mph. (Because, again, everything is properly programmed in the PCM for those 2.0:1 rear gears, and a VSS input to the PCM of 1,000 driveshaft RPM).

Now we JUST change to a 4.0 to 1 rear gear and do NO re-programing whatsoever to the PCM. Now again at 1,000 driveshaft RPM, the VSS, once again, "tells" the PCM we're at 25 mph, and the PCM, (based on NO change in either it's programming or any different input signal from the VSS), commands the transmission to ..... do the 1-2 upshift!!!! Which it does do at a TRUE vehicle speed of only 12.5 mph. See, the trans upshifts TOO early because it's programed to shift at 25 mph ....... which it "thinks" it is at, but ....... it isn't. After this upshift we are now in 2nd gear at a TRUE vehicle speed of only 13 mph!!!

97 6speed z

Yes it does! And so does the PCM, which now was originally programed to be waaaaay into 2nd gear at 55 mph, but ....... the car is ONLY truly going 33mph ....... right?

F0x Slaughter

Also remember though the shifts are dictated by TPS% also.

Here are the stock tables.
1>2 2>3 3>4 2<1 3<2 4<3
0.0 11 20 30 9 18 27
6.3 12 22 34 9 18 27
12.5 15 28 44 9 19 27
18.8 20 36 54 9 19 27
25.0 25 46 64 12 19 27
31.3 29 54 74 12 20 30
37.5 34 64 84 12 22 35
43.8 38 72 94 12 27 43
50.0 40 76 104 12 31 49
56.3 42 80 114 12 36 56
62.5 44 84 124 12 42 65
68.8 46 88 160 12 51 72
75.0 48 92 160 12 57 78
81.3 49 94 160 12 68 152
87.5 50 96 160 12 68 152
93.8 52 98 160 40 90 152
100.0 52 98 160 40 90 152

2000rpm is 15mph so you would have to have the TPS as 12.5% for it to shift at 2000rpm with 3.23 gears. Now if you switch to 3.73 gears, 15mph would be 2200rpm. 2000rpm with 3.73 is 13mph. So it would shift 3mph lower with a 3.73 gear as opposed to 3.23 gears.

55mph is 2nd gear at 3800rpm.

So yes I see where you are coming from. A huge gear change from 2.73 to 4.56 is a huge difference and would require a PCM before drivable.

A 3.23 to a 3.73 wouldn't its only a 3mph difference.

Should you get a tune yes, but it wouldn't be necessary.

97 6speed z

F0x Slaughter, congratulations you .... got it!!!! Just as you cited above when you switch to deeper rear gears, (in your example from 3.23 to 3.73 gears), the transmission shift points occur at lower actual or true vehicle mph ...... with NO PCM re-programing, (of course).

primaryfitness

If anybody has a hypertech that will fit my 1996 formula they want to sell they can message me with a price...