930 handling problem under acceleration
 

I take my car for 1 hour fast driving run as is so dry today (my first dry ran with mu new 930 !) so like to have some fun.
I found that under acceleration my car like to slightly move left especialy when I accelerate from 85 -90 mph up. I checked all tire pressure and front is 2.05 bar (30 psi)
rear left 2.95 (42.5 psi) right 2.75 (40 psi) so 5% difference. Question it cold be by this difference or I should to check any other thing?
I have LSD (40%) and -1.0 camber front and 1.5 camber rear.
From other site it accelerate like beast....like it!

Do you know if its been aligned lately, these cars are very sensitive to being out of alignment.

I'd fix the air then if it's still there, check your alignment on all 4 wheels..
If that doesn't work, try a fat girl in the passenger seat.
Also Where are you ?? in the states cars tend to move slightly to the right due to road slope..

I have made alignment week ago. All factory spec exclude camber that I move negative as mention in first post. I will see what happen with add pressure in right tire.We have not as perfect roads as in germany but I proof my car on Highway so it is not slope issue. With slop it turn right or left depends od slop...
But seriously will see what happen when I will add air. I will check ride high both side too as I didn't do that during alignment process.
:confused:

I have had a similar sensation due to underinflated tires. Driver's side rear tends to loose a little each week. I once let it get down to the mid-30s, and could feel a strong yaw while accelerating. Airing back up to 44 psi made the handling under acceleration come back to neutral. Don't know if just a couple psi loss would do the same, but it's pretty cheap to try that fix first, isn't it?

What type of tires are you using. The tire pressure seems pretty high.

I wonder if your limited slip would enhance the difference between tire pressure? Your right rear is softer and would hook up better so the limited slip transmits more power to that side?

Just a thought.

I'm with gumba.

Are you running stock tire sizes? If so, start with the recommended pressures (cold).

I am going on 225x45 and 255x40 on 17". I find morning that I have rear and front high (counting lip fender to ground) 25" all around so I need to up front end by 0.5" correct? But that could not be my issue of left moving I think so, or it could?

No....lower front fenders gives you more downforce and makes the car look aggressive.

You can post some pictures of your car and show us the ride height. We can give you an idea if it looks good.

The ride height probably has nothing to do with your car pulling to one side.

The recommended tire pressure is 29 front & 34 rear, cold. I run 31 front and 34 rear. Your tire pressure can increase 6-10+ lbs depending how hot they get.

Stock tires, for the 930, like a pretty high rear psi...which always surprised me. I think it's because the factory intended those pressures to accomodate high speed driving, as well as to create a little more front bias in the weight distribution, thus reducing the oversteer tendency.
Curious, here's what Firestone has to say (I'm sure their data base comes straight from Porsche):

1987 Porsche 911 Turbo

Front / Rear / Both Size Speed Rating Front Inflation (PSI) Rear Inflation (PSI) Standard / Optional
Front 205/55VR16 V 29 N/A S
Rear 245/45VR16 V N/A 43 S

I personally run 32 and 42, front/rear respectively.

Where did you get those numbers? The owners manuals for '73 S and '79 930 both spec out 29 front and 34 rear. Regardless of what tire your using Porsche seems to spec out a 5 lb. front/rear pressure difference. Maybe the factory spec is different for the later 930's. I would confirm that with the owners manual. A 10 lb. front/rear difference would have a noticeable impact on how the car handled.

My owners manual says the same as the info that came up from Firestone (but for the rear, which I think it quotes 44 psi...I would have to check).

So I add air and... is better only little move under heavy acceleration (high boost) but acceptable. So I have 29 psi front and 43 psi rear just like sticker show. I also go on 0.4" up with front suspension set up. ( I was to low front and about 0.5" different between left and right side) Tomorrow will proof lower pressure in tires let say 29 and 38 psi.

Check your suspension points to be sure nothing is broken, also check for dragging brakes on the left side.

If your jacking up the corners, you may want to get it corner balanced. Going by height alone is a guesstimate at best.

Torsion bars and sway bars could be off a click allowing the car to lean/rise more one side to the other...

Re-check your toe at the rear using strings. Trust, but verify.
When I first got my car, it pulled hard to the right under acceleration and to the left on a downshift. Diagnosis: <zero> toe on the right rear wheel (left was -1 degree) and the left rear tire was down on air by15 lb.

if its only while accelerating id have your lsd checked....when they go bad the car will torque steer

when they go bad what is the failure mode.....do they lock creating a solid axel or slip creating effectively an open diff?
Just interested.

they slip and one axle gets no power

I drive several times with new front tire and still the same if not accelerate you could take out hands from steering wheel and is going perfect straight. But if I hard accelerate it turn right , worn out right rear suspension ?
I have taken today my 911 S and it goes like new car under heavy acceleration, so what it could be?:confused:

Nope. Failed LSD is just a "open" diff. LSD is actually nothing than "open" diff with pack of clutches in-between.

Clutches are not the ones transmitting the power in straight line unless one wheel starts slipping.

When on boost, rear axle will squat. In his case, I suspect:

1. Uneven tire pressure
or..
2. Rear torsion bar out of index one notch
or..
3. Parking brake cable getting tightened when car squats
or..
4. Something in rear suspension is detached


Rear suspension arms have anti-squat geometry. If one arm goes up more than other, that wheel will get more toe and start to steer the car out of straight line.

I would check torsion-bar indexing and do the corner weight.

Hopefully it is the air pressure.

Sounds like this issue started after an alignment. That the alignment followed stock alignment settings except as to camber.

The 930 suspension differs from a normal 911 in sever ways (anti squat, less rear camber loss under compression, wheels extend further from the center.) As noted above, they are sensitive to alignment settings and there sport setting is different I believe than would be used on a 911.

Do you know what settings they used? What is your fender to ground measurements and what is the angle of the front tie rods relitive to the front A arms like at rest?

Rear camber effects height and corner weighting a bunch on a 930. You might try the tripod method of as a quick check on the corner balancing. Jack the car from its standard jack point. Remove the two wheels on that side. Lower to stock height all around. On the wheel side, measure to height to center of torsion bars. Put the wheels back on, go for a drive and then do the same thing on the other side. All this should be done after first checking air pressure.

Per the factory, the 930's allowable corner balance weight is less than the 911's.

Best of luck.

I just back from 300 mil trip with new rear tires (dunlop SP 9000) and... I am surprise!
No handling problem under acceleration and free of vibration. So it was tires ! Could you imagine that tires could make such problem! I had have strong side moving (right ) under heavy acc. and now my 930 drive like new car! I spent 15 minutes with old tires and they look pretty good but it must be cord damage somewhere. I am so happy that problem is gone this way.SmileWavy

Congrats!!!!!!!!!!!!!