Saturn S-Series 5th Gear Swap

[Daox]

Nope, speedo is not effected.

[Multiades]

Great writeup. Better than mine! =)

I think the results would be more dramatic if you were more rash and drove at higher speeds (before and after) on the highway. Might also make more of a difference with significant aero mods so that you could be in 5th w/o lugging (also speed-dependent though).

[Multiades]

Originally Posted by cfg83

My DOHC has a different BSFC map than the SOHC :

http://ecomodder.com/forum/15008-post4.html - Post #4

I don't have the BSFC map for the SOHC. I should find it. However, even if I did find it, I wouldn't know how to interpret the differences, .

Originally Posted by cfg83

My current tires are one-size-up from stock :

http://ecomodder.com/forum/showthrea...ires-2642.html

It would be pretty hard to find the optimum speed to drive at for highway mileage, but assuming you were able to reduce Cd on your car enough that aero drag was less of an issue, I bet putting it near the center of the sweet spot on that BSFC graph would be about right.

55-65% Load (perhaps 18in/Hg vacuum) at 2500rpm might be ideal.

With your actual tires, speeds should be as follows:

RPM		MP2	MP3
2000		58	48
2100		61	51
2200		64	53
2300		67	56
2400		70	58
2500		73	61
2600		76	63
2700		79	65
2800		82	68
2900		85	70
3000		88	73

I somehow doubt you can do 73mph efficiently in a car that isn't shaped like a needle -- looks like the MP3 gear might be better in some ways for that. The BSFC "sweet spot" seems to extend down to 2000 with room to spare, so maybe your best mileage will be at 60ish.

If the car feels absolutely powerless on flat highway at 2k that's probably too low. Aero would help.

50mph is probably too slow though.

Originally Posted by cfg83

In terms of safety, under an emergency, I would have to downshift to a lower gear no matter which 5th gear I have, so I don't consider it to be a critical issue.

What, in case you have to go really fast because your wife is in labor?


-Chris

[itjstagame]

Originally Posted by Big Dave

Hey! Don't forget about me!

I swapped the R&P in the rear axle of my F-350. I went from a 3.73:1 (2000 RPM @ 70 MPH) to a 3.08:1 ratio (1700 RPM @ 70 MPH).

My MPG improved from 20 MPG to 23 MPG at the time. A 15% improvement in MPG is nothing to sneeze at.

How hard was this, any pics? I've thought about it on my truck, but since it's a 4x4 I'd need something more like a 2 speed rear, one for normal, summer, econ, 2wd driving and one for before I engage the 4x4. I don't think it's possible for me to just buy a 2speed rear though or make one, so I guess I'll just not do that mod.

But I was curious of your experience.

[lovemysan]

Originally Posted by Multiades

It would be pretty hard to find the optimum speed to drive at for highway mileage, but assuming you were able to reduce Cd on your car enough that aero drag was less of an issue, I bet putting it near the center of the sweet spot on that BSFC graph would be about right.

55-65% Load (perhaps 18in/Hg vacuum) at 2500rpm might be ideal.

With your actual tires, speeds should be as follows:

RPM		MP2	MP3
2000		58	48
2100		61	51
2200		64	53
2300		67	56
2400		70	58
2500		73	61
2600		76	63
2700		79	65
2800		82	68
2900		85	70
3000		88	73

I somehow doubt you can do 73mph efficiently in a car that isn't shaped like a needle -- looks like the MP3 gear might be better in some ways for that. The BSFC "sweet spot" seems to extend down to 2000 with room to spare, so maybe your best mileage will be at 60ish.

If the car feels absolutely powerless on flat highway at 2k that's probably too low. Aero would help.

50mph is probably too slow though.





What, in case you have to go really fast because your wife is in labor?


-Chris

I can tell you that unloaded the car has a modest amount of usable accel from 50mph in 5th. On ramps are FE killers. Loaded full of family you pretty much set the throttle to 20% and wait for accelleration to occur.

From a strictly efficiency perspective the engine is most eff at 2500rpms. My understanding(small brain here) is at max efficiency its producing the best ratio of BTU's used vs work produced. Unfortunately at 73mph a lot of work is wasted on aerodynamic load.

I believe the best FE is attained around 39-42mph for the MP2 equipped saturn. I have one tank at 62mpg, where at didn't really exceed 50 mph.

[Multiades]

Originally Posted by lovemysan

I can tell you that unloaded the car has a modest amount of usable accel from 50mph in 5th. On ramps are FE killers. Loaded full of family you pretty much set the throttle to 20% and wait for accelleration to occur.

From a strictly efficiency perspective the engine is most eff at 2500rpms. My understanding(small brain here) is at max efficiency its producing the best ratio of BTU's used vs work produced. Unfortunately at 73mph a lot of work is wasted on aerodynamic load.

I believe the best FE is attained around 39-42mph for the MP2 equipped saturn. I have one tank at 62mpg, where at didn't really exceed 50 mph.

Yeah, you're right in general about slower being more efficient, but I was talking about driving in top gear at a useful speed. At 39-42mph I would think 5th gear would be useless.

It's always a compromise. Could we get better hwy mileage with P&G at a very slow speed? Yep. But most of us will probably sit and drive at a tolerable highway speed, loading it more on the uphills and coasting down the back side when convenient.

Realistically, I probably wouldn't drive slower than 50mph for any amount of time on the highway, and with the right gearing, wouldn't feel too badly about doing 60.

-Chris

[lovemysan]

5th gear is good from:

22mph accellerating downhill
25mph accellerating level
28mph accellerating uphill

It will cruise comfortably down to idle in 5th but will require a downshift at 22mph to accellerate on level ground. I have found that anywhere above 30mph is more efficient than 25mph.

I've never been able to make P&G work for my saturn. Its hampered by its slow accelleration I guess. And it also is typically loaded with people.

[cfg83]

lovemysan -

Originally Posted by lovemysan

5th gear is good from:

22mph accellerating downhill
25mph accellerating level
28mph accellerating uphill

It will cruise comfortably down to idle in 5th but will require a downshift at 22mph to accellerate on level ground. I have found that anywhere above 30mph is more efficient than 25mph.

I've never been able to make P&G work for my saturn. Its hampered by its slow accelleration I guess. And it also is typically loaded with people

Those sound the same for me now that I have an MP2 5th gear. I don't have people as much as I have crap in the car. My car is like a swiss army knife, so I don't want to remove the stuff. If it was clean on the inside, then maybe my numbers would be 1 to 2 MPH less than yours.

I used to use P&G like mad, but the MP2 5th gear is as gutless as your Saturn's nickname. In LA traffic, *not* using P&G is easier because I don't stick out like a sore thumb. It's just so much less *work* to get good MPG.

CarloSW2

[lovemysan]

I had to replace the dogbone motor mounts and saved $120 using prothane inserts. This made the car less tolerant to low rpm load.

[jasonm]

On a RWD car, should you be able to get similar gains by switching the final drive ratio?

ie, on a 240sx, would changing the final drive from 4.08 to 3.54 have a significant impact, assuming no other changes?