MAD!! gear swap

[1993]

I completed the 5th gear swap, i swapped in the 5th gear from an mp2 into my mp3 in the sc2 for better mileage on the interstate and to keep the engine from screaming at 70 mph, now the guys over on turbosaturns say it was a waste of time and wont help, and it will actually be harder on the engine not being revved so high, what gives? That doesnt sound right..

[ProDarwin]

Woah woah woah. I did not say it was harder on the motor. I said there is more load. You are at a higher throttle position at the same cruising speed, but at a lower engine speed.

You may see a mileage increase, you may not. I'm just saying the gains will be almost nil, but you will see a noticeable performance decrease in hilly terrain.

[1993]

Darwin, wasnt referring to you..But since youre here, i watched what i was doing all the way home from work and it actually takes less throttle to hold it at 70, it almost just coasts along where before you had to hold the throttle a little ways..on most inclines at 55 even it doesnt slow down but only about 2 mph, it actually surprises me that it didnt hurt the power that much, but flies along at 65-85 like a caddy!

[xupthree60]

so what the difference b/w mp2 and mp3? im guessing that mp2 was more geared towards millage and came on older cars? either that or the mp2 was in sx-l and mp3 in sx-2?

[ProDarwin]

Originally Posted by xupthree60

the mp2 was in sx-l and mp3 in sx-2?

bingo.

it DOES take MORE throttle position.

[xupthree60]

so are all the gears geared more towards millage or just 5th? it there any other difference other than the gears?

[eljefino]

chill out a little.

One thing about gasoline engines is they're most efficient at near max power. At max power fuel enrichment goes down but this is at above 80% throttle angle or so.

To make your engine "lug" in a zone with a mostly open throttle, max EGR flow, and no enrichment is a great way to get economy.

Besides, there's always 4th. The biggest bummer in your swap I would expect to be the decision of whether to downshift or not, b/c of a big ratio change.

I'm one of those hosers who winds up in 3rd on my MP2 sohc to cruising speed then I skip 4th right into 5th so that wouldnt' be an issue for me.

Post back when you compute gas mileage.

xupthree60, the DOHC makes power from 4000-6500 RPM and not much under 4k so it was given a 'close ratio' MP3 transission with lower gearing and less difference between the gears. If 1993 shifted his modified transmission at redline in 4th (rather fast that would be) it would land in 5th below the engine's power peak and be less fast and less fun. It has been argued among DOHC owners on flat land that they could use a taller 5th gear.

Which is why some new cars have 6 speed manuals.

[madpogue]

Does anyone else remember the Mitsubishi twin-stick? It was a four-speed manual tranny coupled to a two-speed differential. In effect, 8 forward gears (and two reverse!). Showed up in the Colts and Plymouth Champs of the late '70s and early '80s. 'Course, you'd usually drive in only one range or the other, but yeah, I would still like the response and hill-climbing of my MP3, but the flat-land (of which we have little here in WI, granted) cruising efficiency of an MP2 fifth. And I don't think I'd like that tall top step between an MP3 fourth and an MP2 fifth.

[wolfman]

Originally Posted by madpogue

Does anyone else remember the Mitsubishi twin-stick? It was a four-speed manual tranny coupled to a two-speed differential. In effect, 8 forward gears (and two reverse!). Showed up in the Colts and Plymouth Champs of the late '70s and early '80s. 'Course, you'd usually drive in only one range or the other, but yeah, I would still like the response and hill-climbing of my MP3, but the flat-land (of which we have little here in WI, granted) cruising efficiency of an MP2 fifth. And I don't think I'd like that tall top step between an MP3 fourth and an MP2 fifth.

YEP. The second stick had a STAR (for performance) and an E for economy.

In E, they would not get out of their own way, and God help you if it was a Colt wagon! In STAR, it would use up the engine RPM wise very quickly between gears, and was SCREAMING like a banshee at highway speeds. The twin stick system in those cars altered the FINAL DRIVE RATIO (IIRC)

My favorite econobox of that era was the Toyota Corolla FX16. (Basically a Corolla with the MR2 engine under the hood. Talk about a sleeper!)

[madpogue]

Originally Posted by wolfman

In E, they would not get out of their own way, and God help you if it was a Colt wagon! In STAR, it would use up the engine RPM wise very quickly between gears, and was SCREAMING like a banshee at highway speeds. The twin stick system in those cars altered the FINAL DRIVE RATIO (IIRC)

Yeah, that's how I recall it; the "Star/E" shifter went to the differential. 'Course, wasn't the usual procuedure to shift up thru the gears in "Star", then throw 'er into "E" as the final top gear? (Come to think of it, this is how you usually ride a mult-speed bike that's got a front derailleur.)

My favorite econobox of that era was the Toyota Corolla FX16. (Basically a Corolla with the MR2 engine under the hood. Talk about a sleeper!)

That was the car (a red one, no less) in which my sister got a glove-box-ful of speeding tickets. It protected her, though; she 720-rolled it in the median ditch on I-75 btw. Flint and Pontiac (on one of those tight curves). Pushed out what was left of the windshield and climbed out, walking.

[Post terminated by the SaturnFans thread hijack detection system]

[wolfman]

Lower RPMs = less fuel consumption

The PCM will seek stoic (max efficiency) based on a steady throttle setting (at any position) and in relation to what it is seeing via the O2 sensor

Lower RPM's = less fuel consumption

Your numbers will vary.

I'll (too) be interested to see your actual MPG numbers.

I think they will be higher.

[cheapybob]

Not that I'm any authority on this kind of swap, but I'd be willing to bet you get 3-4 mpg better on the highway, all other things being equal.

The reason is that you have a motor that is going to use air/fuel at 14.7 to 1 as it spins. The more it spins, the more air and fuel it uses. Your gearing change will cause it to spin fewer RPM's per mile. The load on the motor in 5th will be the same as anyone with an SOHC, but you have a motor that puts out more hp, so it certainly won't be worse than an SOHC. The torque curve on a DOHC might have it putting out more torque higher on the curve, but I don't think I'd worry about it if I were you other than to not shift to 5th under 35 or 40 mph. You can tell when the engine is running too slow for the load and "lugging". Just don't run it at so low an RPM as to lug it and you'll be fine.

[1993]

Wolf, are you saying it should get better mileage now?

[amazinghl]

1993, Why are you here asking?

Go out, drive and do the math!

You have THE car to find out if you're gonna get higher gasmilage or not.

[ProDarwin]

Originally Posted by cheapybob

The more it spins, the more air and fuel it uses.

NO! RPM and air/fuel use are unrelated. The throttle plate is what controls the amount of air/fuel used. At a certain speed, it will require X HP to move down the road, regardless of the speed of the engine. At lower RPM, the throttle will be open MORE to achieve this same HP output, which is a good thing, because it reduces the pumping loss of the motor, making it more efficient. However you've also moved farther out ot the efficiency range of the cams/head by dropping RPM.

Again, you may see an increase you may not. If you do, I'm betting it will be minimal, and probably not worth the sacrifice in power.

[cheapybob]

I agree he should take it out and try it. Theory is nice, but test results are what you use to figure out what's wrong with the theory. You get the real answer at the pump when you fill it.

[twosaturns]

it will probably be the same benefit as just not going as fast. but, by putting in a taller 5th, he 'should' be able to go the same avg speed as before, just spinning a little less.

[cheapybob]

PS: the SC2 I gave my daughter will probably need a clutch this summer and I'm thinking of doing either a 5th gear swap or just getting a trans out of an SOHC.

If I do, I'll do a before/after test run with my scangage so we can find out the actual result.

How hard was it to swap the 5th gear? What parts did you need?

[sdowney717]

how about using taller tires in the rear.
This would lower the final drive gearing, less engine RPM per mile traveled.
Anyone tried this yet?

[madpogue]

In the rear?