I've done about 500 miles of motorway driving this week in my 99' RA. It's revving at about 3500-4000 rpm when driving 70-90mph. That's alot higher than I'm used to driving a diesel but it is what is because of the close ratio box. It's pretty smooth but around 77-80 mph the car vibrates. I just had all the wheels balanced which smoothed it out alot at this speed. On part throttle it's ok but if I floor it it gets really rough. I had tracking done but not a full check of camber etc as I want to get the P1 springs (hitting bump stops on some rough roads with only me and the dog in it) but I dont think it's related to balancing. Could there be a fault with fuel starvation underload? It doesnt seem like it's struggling but it's not smooth. The exhaust is an STI back box with straight section. I still have the original which has a small resonator and a cat. maybe the standard mapping needs this exhaust fitted to work properly? Cars standard apart from: 17 inch wheels, lowered springs and the straight section exhaust.
Any ideas?

 

Just wanted to add that that the sound from the exhaust gets boomy under full load (pedal pushed fully down) when I'm trying to accelerate hard in 5th from say , 70mph. If I get to 80-85 and ease off the pedal it can say there without the boom. Boost pressure related I think.

 

Warped discs maybe causing vibration ?. When I hit 70-80 and brake I get a little vibration because the discs are on their way out. Might be worth checking.

 

will check. There is a bit of judder when braking. Comes and goes. They are grooved and drilled. were the originals solid and un-grooved?

 

Vibration might be due to the prop shaft support bearing.

 

is it in front of the diff or back of gearbox? i havnt got under it yet so sorry for silly reply! How much?

 

My discs are ungrooved mate

 

ok. It seems thats the discs get warped quite alot from what I read? Is it any particular brand of grooved and drilled discs that are the cause? Yes propshaft bearing could be a reason. I'm now also think that the springs being too short is a cause. Perhaps they make the car squat too easily when under hard instant acceleration changing the angle of the shaft too much? I must get underneath this car

 

I'd go for a good brand of disc. I'm gonna get brembo discs and hope that they last a bit longer

 

Had a look yesterday. It's looking like bad lower front ball joints at the moment. propshaft bearing looks very good. The rear inner driveshaft boots have splits so perhaps they are worn now?? Discs are brand new front and back and 4 pot calipers fitted (dont know if 99's came with these??)

 

99 impreza turbo models came with subaru 4 pot calipers.

 

Quick update for anyone who cares!
After replacing the lower ball joints (shot) and shortening the exhaust back box hanger the vibes at motorway speed have gone (for now)....