Terry Burger

iTrader: (4)

I noticed something kind of funny when I switch from 3.23 -> 4.10 gears, that my WOT shift points did not really change. They were previously set at 6400-6500rpm with the 3.23s and when I put the 4.10s in (but before I had the PCM corrected) it still shifted around 6400rpm@wot. Is it possible that the stock 2001 pcm shifts@wot based off RPM rather than mph?

Terry Burger

iTrader: (4)

Colonel, I'm not sure that what you're saying is right. Before I fixed the speedo with the 4.10 gears @ 6600rpm my speedo read 94mph at the top of 2nd when the car shifts. After I fixed the speedo it now reads 78mph at the top of 2nd when the car shifts. If the shifting is MPH based you'd think with 4.10s (and stock 3.23 speedo programming) the car would shift out way early, around 5200rpm. My only solution so far is the shifting must be RPM based.

Colonel

Your MPH was thrown off by the same amount as your gear change. That's why it still shifts correctly. It still *sees* the same mph at the same rpm as it did before.

Now, your shift rpm will be changed slightly by reduced load on the converter at WOT. With numerically higher gears you will have less slippage at the shift point which means that your shift will happen at a slightly lower rpm than before. This is the opposite of what happens when you go to a less efficient converter.

Colonel

"If the shifting is MPH based you'd think with 4.10s (and stock 3.23 speedo programming) the car would shift out way early, around 5200rpm."

At that point, the speedo was off by just as much as what the gearing change was, obviously. Thus, the ECM knew not the difference.

What I'm saying is that after the gear change but before the speedo correction, your speedo said 94 MPH at 6400 RPM, as you said. I'm also saying that before your gear change it read 94 MPH at 6400 RPM. Check a gear calculator to confirm this (I already did.)

The ECM didn't know the difference. <img src="gr_stretch.gif" border="0">

12secSS

iTrader: (30)

With 26" diameter tires, 2nd gear (1.63) at 6400 rpms with 3.23 gear ratio will be 94 mph. If you swap out the 3.23 to 4.10 the mph will be 74. This is simple machanical conversion. All transmissions use rpm and throttle position (and/or vacuum) to determine the shift point. This is because the trans needs to know the load of the vehicle before shifting to a higher gear. Remember, the computer does not see anything after the rear of the trans. Basically, it doesn't know if the differential gears were swapped or not, so it continues to calculate the speedo using 3.23 as the ratio.

Gear Calculator

[ January 17, 2002: Message edited by: MRZ28HO ]</p>

Terry Burger

iTrader: (4)

Thanks Chris. <img src="gr_images/icons/wink.gif" border="0">

CHRISPY

2001's shift based on rpm <img src="gr_grin.gif" border="0">
Cheers,
Chris