swapped a single cam trans into a twin cam Saturn
 

So, a freak transmission failure forced me to replace the tranny in my SL2. Apparently the gears were feeling claustrophobic so it decided to blow a 3” hole in itself with the diff pin. :eek: Since I had to replace it anyway I decided to swap it out with a trans from a single cam car. For those that don’t know Saturns, the single cams had wider spacing of ratios and a much taller overdrive (not to mention an OD 4th as well). I had been thinking about doing this swap for some time in the hopes of lowering my cruise rpm (I do a lot of highway driving) and, hopefully, gaining some mpg’s; this failure just forced the issue.

The swap itself is VERY straightforward; it literally bolted right up, all the sensors hooked strait up and it requires no modification of any sort. :thumbup: Saturn seems to be one of the precious few carmakers in the last 30 years that made an honest effort in trying to make their vehicles “mechanic friendly”. A complete transmission swap can be done in 4-6hours by a single, reasonably competent, shade-tree mechanic. I’m confident that if I had to do it again I could start first thing in the morning and be done by lunchtime; it really is a piece of cake. :thumbup:

Anyway, my impressions thus far:
-Why didn’t Saturn offer this combo from the start? The twin cam engine and that single cam manual play very nicely together. There is still plenty of passing power available (although passing in 5th is an exercise in patience-dropping to 4th solves that :turtle: ) and the RPM at a 60mph cruising speed have dropped from just over 2500rpm to just barely 2100. The car positively “glides” along and only a steep hill would require a downshift at anything over 45/50mph.

-The ratios feel generally well matched to the power characteristic of the twin cam. The close ratios of the old tranny felt “sportier” but the tall gearing of the new trans doesn’t create any problems in terms of “not having the right gear”. There seems to be a break point around 30-35mph where 5th is just almost too tall but even in 4th (which is also an overdriven ratio) the rpms are barely 1500-1600 at those speeds; in 5th it's more like 1000-1100.

-MPG’s do indeed seem to be up. My winter-time mileage has been hovering right at 37mpg (+/- .5mpg-I tend to be very consistent). My first tank with the new trans netted me 40.2mpg (equal to my best summertime numbers). It’s still a little early to call a mpg gain but it would appear as though I’ve easily picked up a couple. :cool:

Overall, I’m happy with the swap. If this one blew up tomorrow there would be another single cam tranny going in. For my highway driving the relaxed feel of lower rpm’s just makes for a nice ride and it would appear as though there is a mpg bonus to it as well. :cool:

Thanks for posting, Captain Chaos. And congrats on the success.

When I swapped in the transmission with the taller final drive into my Firefly/Metro, I had the exact same initial response: the car felt so much more "relaxed" at cruise. It does require more downshifting out of top gear, of course, but I don't mind at all. It's a manual transmission, and I like using it!

I wish automakers would offer a highway "nerd gear" option for people who really want to save fuel and aren't bothered by the idea of downshifting for power when needed.

I'd like to feature your mod on the home page. Any plans to post a fuel log? Something so people could see some of your "before" & monitor the "after".

I don't suppose you have the ratios for the before/after?

I agree 100%. I'm fully convinced that the lower mpg rating on some modern cars vs. the auto-tragic versions is due to very poor ratio selection. The option for more mileage biased ratios would put whatever car offered it to the top of my wish list.
The real kick in the teeth is when, like my Saturn, the ratios are already available and require no mods what-so-ever to add but the mfg just doesn't offer it.

A fuel log for before/after comparison could be tricky for a couple of reasons:
1) I keep a mental log myself (I check mileage at every fill-up) but I never write it down. I do know that my summertime mileage "before" would float right around 39mpg, with the occasional 40mpg tank when conditions were very good (no wind- clear sailing). Winter time has me right around 37. I've had this car for several years now and I'm confident in those numbers. My driving is also VERY consistent (~400mi/wk to/from work, very little side driving) so it's very easy for me to track mileage gains/losses. However, anyone reading this is just taking my word for it and they have every right to be skeptical (I would be).
2) My odometer just recently decided to stop working so tracking mileage for anything other than to-from work (which I KNOW to be exactly 78.2 miles) is impossible until I fix it. I’m hoping to get on it here sometime soon but in all honesty it’s not a top priority at the moment.

As far as the ratios, here’s a pic of the chart I have.
There’s also a site with them (as well as the ones for the auto versions) here:
Different Racing

If anyone has questions about the swap or anything I'd be happy to help. :thumbup:

not sure if it still happens, but the LT1 and LS1 F-bodies and corvettes got a .5:1 6th gear when equipped with a T56 transmission. more often than not, that was paired with something like a 3.45:1 final drive too. they basically idled along on the highway. it's why the real-world highway MPGs of those vehicles are so much better than the EPA test would imply.

another interesting thing to note is that the corvette back then was known to reach close to, if not at, 200MPH..... in 5th gear. 6th was used entirely for MPG purposes.