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Old 06-04-2008, 03:22 AM   This thread is in the EcoModder Project Library | #1 (permalink)
cfg83's Avatar
Join Date: Dec 2007
Location: Southern California
Posts: 5,864

1999 Saturn SW2 - '99 Saturn SW2 Wagon
Team Saturn
90 day: 40.49 mpg (US)
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Saturn S-Series 5th Gear Swap

Hello -

I am copy-catting Multiades' great post :

Taller top gear makes a real difference

I did my 5th gear swap back in September 2007. It was based on this thread :

MAD!! gear swap - 04-27-2006

My mechanic didn't want to do it at first because he normally farms out transmission work, but thanks to all the information I got at saturnfans, I described the process to him and he was able to do it, no problemo. He did have to pop the cover again to make an adjustment, but I just drove home and was super happy with it. Here are some pix :

Transmission Cover :

As other (saturnfans) have said, you don't have to remove the transmission to do the mod. The 5th gear is under the transmission cover seen above.

OLD MP3 5th gear in place :

This is my original "performance" 5th gear in place after my mechanic took off the cover.

NEW MP2 5th gear being installed :

Here is the MP2 5th gear during installation. Fits like a bug in a rug.

Gear Ratios for 91-02

It turns out that the 1998-2002 years had the best economy 5th gears for the MP2 transmission.

Some number crunching -

MP3 5th gears : 37*27 teeth => 27/37 = 0.7297 => 0.730 = The MP3 5th gear ratio seen above.

MP2 5th gears : 43*26 teeth => 26/43 = 0.6046 => 0.605 = The MP2 5th gear ratio seen above.

Dividing ratios : 0.730 / 0.605 = 1.2066 => ~21% reduction in RPMs => 21% gain in MPG (maximum).

When I first drove home at close to steady-state 50 MPH, on a stretch of freeway that I recalled seeing ~40 MPG, I was seeing about ~50 MPG on my ScanGauge, so this fit the 20% increase I was predicting.

But, this 20% is an ideal gain because I am pushing the engine away from it's optimum power band (BSFC). That's the way I think of it, anyway.

There is an associated power loss, but it runs fine at 35 MPH on the flat in the new 5th gear. I have to be lighter on the throttle, but I'm already Hypermiler-Granny trained in that respect anyway, so no biggee. With the old gear, on the flat, I would say that the absolute bottom of the gear would have been about 20 MPH. Today, I think that is now closer to 25 MPH.

Right close to home, I always spend time "lugging it" uphill for about the last 3/4 mile. With my old 5th gear, I could just hold my own at 30 MPH going up the hill. Now I feel the loss in power so I need to use 4th. But, since that is such a small component of my commute, I don't really care.

As other (saturnfans) have said, I did lose my cruise control. The ECU/PCM is none too pleased at this state of affairs, but I don't need it.

Benefits : I am getting better MPG on city streets because I can just "cruise in 5th" and be driving close to idle RPM most of the time. It's *easier* to get better MPG.

Detractions: I used to P&G a lot because my sporty 5th gear could do a really quick pulse followed by a long glide. Now, with the economy 5th gear, the pulse takes longer. I think it is also less efficient because I have moved away from the BSFC sweet spot (right!?!?!?!). I think that because of this, the economy 5th gear (for me) is a wash on the freeway. I still get great MPG on the freeway and I still P&G, but I don't have to work as hard. If I was not already using the P&G technique, the MPG gain would have been closer to the ideal 20%.



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