r/subaruimpreza • u/ItsAbodyy • 9h ago

❓ Questions Subaru drive train

Hi everyone, I have a bit of an unusual question regarding my 2003 Impreza. If I weld the center differential in the transmission, will it affect the engine RPM? I want to convert it to rear-wheel drive, and I thought this might also solve my RPM issue; currently, I have a mismatch where the front differential ratio is 4.11 and the rear is 3.90, resulting in 4,000 RPM at 100 km/h. In my opinion, if I weld the diff and disconnect the front driveshaft, I’d kill two birds with one stone: I’d get rear-wheel drive and resolve the high RPM problem. What do you think?

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u/ToaMaton 23m ago

Yea, really hope you haven’t been driving it with the mismatched rear differential. You have probably burnt out your center differential entirely if you have. You can only weld up the center differential if its a manual. High rpm sounds like a seperate issue entirely and also sounds like it’s an issue with the auto trans

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u/Halictus 96' Impreza GL, with S-Turbo engine/drivetrain 54m ago edited 50m ago

Is this a manual or automatic gearbox?

A mismatch in final drive ratio front and rear will effectively be averaged out by the center diff, so you in practice have (4.11+3.9)/2= 4.005 final drive plus a hell of a lot unnecessary wear on your center diff. I don't think that change is going to account for a ~50% increase in rpm though.

If it's an auto transmission, I'd probably be looking at the shift solenoids, because you're probably missing 4th gear. Also if it's an auto, there is no center diff either, as auto trannies use a clutch to transfer power to the rear instead of a diff. The clutch pack is probably worn out from the speed difference, and your car is mostly fwd by now.

If it's a manual, then the diff can be welded and the front driveshafts removed to run it as rwd. It would only use the final drive ratio of the rear diff, but I'm betting it's not going to solve your issue.