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opendeck251+stiInternals
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I'll be the first to ask....


Why? I see no benefit to this swap. Other than the slight gearing advantage in the 4.44 axle ratio. It just means you'll be at higher rpms all the time for the same speed.


~Josh~
 

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I kill threads!
2000 Ver 6 STI - 1996 Gravel Express
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3,762 Posts
It's not a swap to do a swap for performance. He's doing it to replace a bad tranny. He's just verifying if it'll work fine, due to easy access of an RS tranny.
 

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GF OBS-T, GM RS EJ20, SF EJ20
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I'm getting old, but I'm pretty sure the OBS 4eat has a 4.11 FD, 5 speed OBSs has the 3.9 FD.

Generally speaking for Subarus:
If the 5-spd has a 3.9 Fd the 4eat will have a 4.11 Fd
If the 5-spd has a 4.11 Fd the 4eat will have a 4.44 Fd

There are exceptions like the Legacy SS's - the 5-spd & 4eat both have a 3.9 FD...
 

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1997 OBS
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646 Posts
wait, so the final drive in a rs 5 speed is lower than a OBS manual? im a little confued. ive owned a 99 rs 5 speed and a 95 L 5 speed. the L was almost exactly 500 rpms lower than the rs in every gear. so what final drive is in each?
 

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1997 OBS
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646 Posts
ok, thatd be the 500 rpm difference. if i wanted to lower the final drive in an rs, thus gaining mpg, would i have to do a whole tranny swap or just the rear end?
 

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1995 Impreza L FWD coupe
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295 Posts
Tranny and rear-end. They have to match. I am not sure as if it would really help any with fuel mileage. It will take more to get the car moving where the gears are taller. It all comes down to what type of driving you do mostly. If you are cruising the highway for 30 minutes then it would but if your slowing down and speeding up all the time it won't do a thing for you.
 

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1995 Impreza L FWD coupe
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295 Posts
You must have the 1.8 in the L. That is the difference. 21-22 with a 2.5 is decent. 1.8 run around 30(ive gotten 35 once), 2.2s run 24-28, 2.5s run lower 20s. 500 rpms might get you .5 mpg.

Here is the reason behind the small amount of change.

When a speed change occurs the engine introduces more fuel then is needed to allow for more energy to be created. The longer you push the throttle the more fuel you will burn. Flattening the peddle on take off every time will result in lower mpg. Rolling into it will limit the amount of excess fuel resulting in better fuel economy.

The more backroads and what not you drive the more this has an effect on mpg's. If you cruise the highway the engine needs a minimal amount of fuel to maintain the speed.

This is why it wont matter much.
 

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1997 OBS
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646 Posts
huh, i thought it injected the same amount of fuel throught the entire rpm range, which would mean its burning the same amount of fuel per stroke no matter rpm. which would also mean higher rpm=more fuel used
 

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1995 Impreza L FWD coupe
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No. Injectors open for a designated amount of time to let a certain amount of fuel into the cylinder. It does require a bit more to run at higher rpm's. It still takes a good amount more to run a 2.5 then it does a 1.8.

There are some engines that introduce fuel more then once but those are diesels and thats another discussion.

All in all, keep the trans that came in it. Unless you do have better reasons then fuel mileage.
 

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I kill threads!
2000 Ver 6 STI - 1996 Gravel Express
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3,762 Posts
If the engine always squirted the same amount of fuel into the cylinders, then you'd always be at the same rpm. The only thing making the crank spin is the fuel being ignited. Less fuel, slower it spins. More fuel, faster it spins.
 
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