Saturn S-Series 5th Gear Swap
Hello -
I am copy-catting Multiades' great post : Taller top gear makes a real difference http://ecomodder.com/forum/showthrea...ence-2748.html I did my 5th gear swap back in September 2007. It was based on this saturnfans.com thread : MAD!! gear swap - 04-27-2006 http://www.saturnfans.com/forums/showthread.php?t=76116 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 : http://img222.imageshack.us/img222/8437/01coverwo7.jpg 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 : http://img222.imageshack.us/img222/4...5thgearco1.jpg This is my original "performance" 5th gear in place after my mechanic took off the cover. NEW MP2 5th gear being installed : http://img144.imageshack.us/img144/8...5thgearop3.jpg Here is the MP2 5th gear during installation. Fits like a bug in a rug. Gear Ratios for 91-02 http://www.differentracing.com/tech_...gearratio.html http://img144.imageshack.us/img144/6...rratioscc4.jpg 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%. CarloSW2 |
Good post. I'm going to twin yours and the other one and link to them from the home page.
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Wow great mod! Are those straight-cut gears? I wish mine was so easy, but it will almost certainly involve pulling the trans :(
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JohnnyGrey -
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The other virtue of the Saturn S-Series manual tranny is that the 5th gear is accessible without dropping the rest of the transmission. That's what makes it so fun. PS - Just to be obvious, you need both of the MP2 5th gears (the 43 teeth gear and the 26 teeth gear) in order to mesh properly. CarloSW2 |
Perhaps you mentioned it, but how much did the gears cost?
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Arminius -
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I did see this gear set on ebay for $80 for both gears. I think the most economical way to get these is to go to a junkyard, find an S?1 with a manual tranny, and remove the 5th gear yourself. I posted my mod in saturnfans.com and someone named Nagyfasz copied me by going to the junkyard : MP2 5th Gear Swap into MP3 Online ... http://www.saturnfans.com/forums/sho...d.php?t=105555 CarloSW2 |
nice i heard my honda 5th gear swapp is the same on the 3rd gen hondas..
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Are there any people doing 5th gear swaps on civics?
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ZachUA -
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CarloSW2 |
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So in essence, the cliff notes version is "I'm making 5th gear taller to reduce RPMS on the hgihway" ?
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koihoshi -
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CarloSW2 |
is it a safe thing to do?
if you can't hack your tranny, the concept of "dubs" wheels cramming the wheel wells full is a decent option, or just bigger wheels and tire. Taller rubbers on little wheels actually hurt more than gain. It is a tall tire tale (haha). Most well disciplined trannies have non-straight gear and change quite dynamically to this one simple move.
I can use my own example of a old subaru, 15 inch wheels were difficult to find, but ,anyway, it came with tiny 13 inch wheel tires (23.2 inch diam). The same rpm, given the japanese wonder tranny, does 80 instead of 65 with 25inch diam..engine runs out of power first. This also increases safety, a thought for you tall gear swappers and low tire ratings. Seems the saturn went to a higher ratio for a reason.Alot of times, more than not, higher ratios OEM mean a weakness someplace, even though engine has enough power to go much taller geared. I learned the hard way... Are the wheels the same as the saturn with tall gears as the one getting the swap? |
bgd73 -
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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, :o . 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. ... My current tires are one-size-up from stock : http://ecomodder.com/forum/showthrea...ires-2642.html I think the thing to keep in mind regarding tires is that I am *not* doing the Dub thing. I consider the Dub thing to be an aesthetic choice that consciously sacrifices safety/engineering for the sake of style. By going "one size up" on my tires, I think I am pushing one design parameter in my car "by one notch". I don't consider this to be a drastic change. CarloSW2 |
Did the Cruise control ever get sorted out?
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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. |
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To threadjack further: I Just found out that the taller ninth-gen Corolla 5th gear (from the five-speed) fit the xA/xB. When I some day have to replace my clutch, I may do this. The limited clearance under the hood may prevent use of a puller to get the gears off, apparently. |
I used to have an HF 5th gear in my crx si, I remember having one tank at 50 MPG but most of them were mid 40's but this was before I was into hypermiling, I still had the si final drive, I had put the long gear in just to get back and forth to the autocross. I still have the gears if anyone is interested in buying them, I know I have them somewhere burried in my garage.
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In the pickup community, a lot of guys have tried bigger diameters tires and the result is always the same: lower MPG.
The rotational inertia of the wheel tire assembly goes up with the square of the diametyer. Every time you accelerate the vehicle you have to not only translationally accelerate the vehicle but also you have to rotationally accelerate all the rotating machinery including the wheels and tires. So everytime you pull away from a stop bigger tires have the same effect of alot more weight. Very few guys can drive so as to recover all that energy by coasting. This is why very high-MPG cars come with preposterously small wheels and tires. |
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Does anybody know of 5th gear changes or parts that would be availalbe for a 2002 nissan sentra XE, 5spd 1.8 base model. Yes, I just picked it up to allow the semi-retirement of the suburban:(
I asked in some other forums and nobody had a clue. |
Cfg83, looking at the BFSC map it appears to me that the sweet spot is around 70 mph/2500 rpm. Which is nearly identical to a sohc, peak torque for me is 2400rpms, 114lb ft. I've read in the past that that the dohc makes within 3lb of the sohc until after 3500 rpm.
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lovemysan -
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CarloSW2 |
CFG I was merely stating that from a BSFC eff standpoint 70mph is best. However I find that 38mph is best for maximum eff. LOL. You should be able to hit 43-44mpg at 70 though. Of course there are a lot of different variables here that I don't understand how to compute.
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Dust -
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CarloSW2 |
lovemysan -
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ROUTE : 10 West -> 710 South -> 105 West -> 405 South -> Exit Freeway => Roughly 27+ miles SCANGUAGE "CURRENT" Readings : Code:
SLOW FAST Code:
Degrees F : 77.00 76.00 CarloSW2 |
RPM too low???
I'm not dead sure, but i think my civic dx is around 3000 @ 60-65, with only 100hp. My question is my new Nissan Quest runs 2200 @ 80mph and around 1600 @ 60. Is this lugging the engine? Or will it downshift before enough lugging occurs. (5spd auto). I guess it has a little more torque (255) than the civic too, maybe that helps.
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Admittedly, while everyone here would like a top gear tall enough to run a flying mile on the salt flats, most people want to be able to accelerate at a pace quicker than 'glacial.' Hence, at 80mph, my xB is running at something like 4200rpm. Bigger engines tend to make a lot more torque at lower rpms, so they can be geared much taller without impacting performance. The upside is better fuel economy than if shorter gears were chosen. |
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PS - nice salt flats reference. :) |
Awesome thread. Been toying with the idea of oversize tires...does this gear change have a similar effect on the speedo readings?
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Nope, speedo is not effected.
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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). |
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55-65% Load (perhaps 18in/Hg vacuum) at 2500rpm might be ideal. With your actual tires, speeds should be as follows: Code:
RPM MP2 MP3 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. Quote:
What, in case you have to go really fast because your wife is in labor? :p -Chris |
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But I was curious of your experience. |
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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. |
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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 |
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. |
lovemysan -
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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 |
I had to replace the dogbone motor mounts and saved $120 using prothane inserts. This made the car less tolerant to low rpm load.
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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? |
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