1st high speed SC run = disgust
 

I had a crotch rocket come up behind me this morning on the way to work on a wide open country road. I was doing about 75 so as he went by I quickly attemted to down shift to 4th (which of course resulted in a momentary 915 moment of disgust). After 4th was solidly engaged, I mashed the throttle and off I went to briefly chase the bike (I know most crotch rockets are stupid fast, I just felt like getting on it for a brief high speed run). After 120 (which is the fastest I had gone in this car to date) my '79 SC felt very squirrely and at 130 w/ tach reading about 5900, I decided to put an end to this nonsense and let off.

Can anyone tell in the pictures below, if my front valance/whale tail/ride hieght combination look suspect.

I recently put on new Avon M500 tires, turbo tie rods and checked the suspension. The suspension is set at 0 camber front, -1 camber rear and toe-in (set between parallel strings)
front and rear leading edge of 16" wheels toed in 1/16" compared to rear portion of wheel.

I rarely drive at these speeds but this car felt like crap compared to my '87 951 (it was running out of steam and available rpm to redline also). Big disappointment after spending the last year and lots of money trying to square this car away.
http://forums.pelicanparts.com/uploa...1119440432.jpg http://forums.pelicanparts.com/uploa...1119440485.jpg http://forums.pelicanparts.com/uploa...1119440533.jpg

You are running a turbo tail with a Carrera front chin. That should not be the cause of the hi-speed behaviour of the car.

There is another recent thread about hi-speed stability and I think the cause was the tires... I'll try to find it and post the link... it might give you some clues.

here is the link. Good luck

http://forums.pelicanparts.com/porsche-911-technical-forum/227282-high-speed-stability-issues-diagnostics.html

I do not know the difference between a Carrera/SC/Turbo valance. I assumed that the rubber lip on mine was appropriate for the tail that a previous owner had obviously installed. Is my front valence all wrong for my car set-up?

Did you have the car corner balanced?

Was the car lowered correctly WRT torsion plates and bump steer? Rear bushings OK?

Are the tires broken in? Are they balanced correctly?

Maybe some negative camber in the front would help.

I'd recommend a corner balance and full alignment. I also run a little more negative camber.

On a consistent basis I take my car over 130 (at the track) - I don't have any spoilers and my car is rock solid. You have something wrong with your car.

No corner balance, but the alignment is set at where I stated previously. The tires have about 1000 miles on them now and are balanced. It is hard to explain, but the car just felt very light as though it would have darted off the road if I relaxed my death grip on the wheel.
I do not need to drive at these speeds obviously, but it would be nice to know if my tail/valance combo or my toe settings are all wrong.

Tail/valance are fine. I was just pointing out that the tail is a Turbo as opposed to a Carrera. At those speed it produce a marginally higher downforce.

I agree with the other comments... I'd check the balance and allignment again first.

You have too many variables to conclude one specific issue. I agree with everything said so far. Since 911's are rear engine and have little weight on the steering, issues that cause the front end to become light (improper ride height, ride rake, alignment, weight distribution, suspension stiffness, etc) will be immediatly felt in the steering. Your car seems to be sitting correctly, but that not saying at a 130 they rear suspenion might be getting compressed by the turbo tail and lifting the front end. I think most 911's without some suspension stiffening do get light over a 100. I've never had any trouble with mine up to 145. I just wish I could get another 50mph out of it;)

When the car was lowered did you install the bump steer kit?

It might just be a wrong impression, but is the body height in the front higher than in the back? Like I said, it could easily just be me this morning.

Did you check the rear toe when you had it between the strings?

Might be worth a look.

I did not lower it (I got it this way) but there is no bump steer kit installed. I have noticed a slight bump steer issue on rough roads, but it is not too bad. This morning's incident was on smooth pavement. Also the car w/ these tires exhibits no tramlining issues. If I remember right, FWIW the front measured 24.75" and the rear measures 24.25".

Your car looks quite similar to mine, turbo tail and correct front spoiler for an SC (I'm not sure what someone meant by the "Carrera front" comment). While I would certainly never exceed the listed limit, I have driven mine well in excess of 120 and it was very solid and still "pulling" strong. The only thing different about mine is I'm running about 1.5 degrees camber both front and rear (I wouldn't think that would contribute to very much difference in stability in a straight line though). At a 1000 miles your tires should be "broken in". I would suggest re-checking your alignment.

Cordially,

Jerry M
'78 SC

The rear toe-IN was set the same as the front (about 1/16" difference between front of 16" wheel and rear of wheel). I had always assumed toe-OUT causes dartiness maybe my slight toe-in at the front w/ rigid turbo tie rods is not a good thing?

Bottom line is that if your objective is to go fast and out run other vehicles you have the wrong car. Get a corvette, or if you have tons of cash, a 996TT. My neighbor has a Techart 996TT, and it is wicked fast, has all wheel drive and is very stable at triple digit speeds. And unlike our older 911's, it will beat just about everything in a stoplight duel. Though I would love to have one of those, I love my SC and it is just fine for my purposes.

That might be true but it's not the point. These cars are stable at 150mph when they are set up properly. There must be something wrong with Tim's car.

Actually, "a death grip on the wheel" is a bad idea. You're losing out on a lot of the sensations that the front tires are trying to impart to you. I remember Hurley Haywood slapping my hands and insisting I lose my own "death grip on the wheel." He uses his fingertips. Of course he is Hurley Haywood...

Sephan

I agree with Matteo - If anything my car gets more stable as the speed goes up - up above 135 or so the front end gets a little light (or maybe it's my death grip on the steering wheel) but never darty. He either has something worn - or something out of adjustment. has the car ever been hit Tim?

I agree, but if it is a street car only and drives great below 110, I wouldn't worry about a lot of tweaking to make it more stable at 130, as you should not be there often or for very long on the public roads. :)