Follow up to Sibling Rivalry

Thanks to everyone who responded to my first set of questinos, I have some more to ask the experts...

To answer one of the questions, I am running 215/45 ZR17 on the front and 255/40 ZR17 on the rear. I have noticed that around 100 mph or so I have a little shake, which I don't care for and did not remotely anticipate. I recently had my tires balanced and I still have the same small shake??

Regarding cams, a couple of questions. I have heard "S", Solex, "E", GE40, GE60 and C2 cams are all worthy considerations. Does anyone know the pros and cons of each? If I just had my cams changed, approximately how many hours of labor should I expect a mechanic to charge me?

It appears the cams, may be a good bang for the buck. I assume I would want my webers re-tuned for the new cams? Any necessary ignition changes?

Finally, I don't really understand the 7:31 ring and pinion in my 915 transmission point made by Warren, I assume it is changing the gearing somehow, but what difference will that make?

Thanks again, I am starting to see that red 993 in my rear view mirror......

The 7:31 gearing is exactly what you say, lower gearing -- you're putting more revolutions to work at lower speeds, changing the spectrum of power distributed from the engine to the wheels from, say, 0-160 mph, down to something more like 0-140 mph. The trade-off, obviously, is in top speed -- but the engine can work more efficiently at more realistic (0-120 mph) speeds. On a track, or in a quarter mile, you're not actually adding any more power with this change, but you're allowing the car to use its power somewhat more effectively.

Another way to understand it is the difference between a (hypothetical) three-speed manual transmission, and a six-speed. With the same power going to the wheels, your three-speed box is going to have to wind through a longer (and less 'focused') power band to go from 0-100 than your six-speed will. You'll be able to wind the engine out more quickly with the additional (and that means lower) gears.

That's my layman's explanation, at least.

------------------
Jack Olsen
1973 911 T (3.6) sunroof coupe

[This message has been edited by JackOlsen (edited 01-07-2001).]

There are two ways to look at the improvements of going to the 7:31, and it's important to see both. One is just the mathematics of speed and rpm. This change raises the rpm, at a given speed and gear selection. It also moves the gears a little closer together, so that there is more selection at 'realistic' speeds. I consider the 7:31 to be favorable for this reason.

But then there's the other reason. A 7:31 will make your car pull harder in every gear. Warren has the math, but this change may make 5th pull harder than 4th used to. And third and fourth would be wicked. I've heard that the 'seat of the pants' assessment of this change is striking indeed.

I don't know whether your rivalry is a straight line drag race or a dance through the twisties. In a drag race your SC probably does not have a prayer. This kind of change makes the car a much more ferocious tiger in the twisties, and this is the game you could lay title to.

------------------
'83 SC

Gene,

The significance of changing your 'stock' 915 ring & pinion, at 3.875 (8:31) ... to 4.429 (7:31) without making any 1st-5th changes, is 14.3% MORE TORQUE available in every gear, and at any rpm level in your engines power band! I will send you a couple of gear charts for comparing rpm vs. speeds, calibrated for your tires 'nominal' diameter specification ... your actual speeds could vary by a few percent, depending on your rear tires' actual rolling radius.

Before choosing cams, I suggest you get a copy of Bruce Anderson's Porsche 911 Performance Handbook, because it has engine specs in tabular form. I think you can forget the 'E' cams, as they are so close to what your SC has, so as to be not worth the effort and expense of changing them. With GE-60 or 'S' cams, the ngine will be decidedly 'peaky' on the torque curve, though a pleasure to drive with 7:31 gearing in your 915! All of the other, less 'energetic' cams would be OK with your stock SC gearing, but I would not recommend the GE-60 or 'S' cams without making the ring & pinion change, because closer gearing is needed with peaky engines!

If you happen to know anyone that has a pre-'73 911S that you can drive to compare the engine's response and torque curve to your own car, that would be good, too.

------------------
Warren Hall
1973 911S Targa

Your tire shake problem can be caused by many things. Alignment, ballance, bearings, flat spots, bent rim, tire out of round, rim out of round, etc. I have a slight shake from 60-65 mph. After looking at the obvious I took the rims and tires to a tire shop (always take your wheels off yourself) and put them on a computer ballancer. All were in ballance, but as they spun you could see one tire was not round! It was very slightly out of round, but only caused a harmonic shake at 60-65 mph. Hopefully your problem will be easier to track down.

For the tire shake I have my tires balanced on the car.

I used to have vibration over 80 MPH and was really getting frustrated until a buddy told me about on the car balncing. It costs $25 per wheel/tire and realy does the trick.

They first take the wheel off and spin balance it to make sure the wheel is good and the tire is not lumpy from being a garage queen. Then they put the wheel back on the car and torque it down, with an actual torque wrench... Not one of those blasted impact wrenches. Usually places that torque with those impacts never re-ckeck the lugs and who knows what torque those are geared for. Probably 150 lbs! Enough ranting... Then they bring a computerized cart with a spinner on the front to go against the sidewall of your on the car tire. The guy brings the wheel up to about 60-65 MPH and there is some optical sensor that can detect wobble in the wheel. From there the guy adds/moves/subtracts weigh as needed until the wheel is balanced with rotor to make a smooth circulating mass that should not wobble on the highway.

I can verify that the car will remain smooth upto 115 MPH I do not have enough clear and safe real estate to go any faster where I live. I cannot wait until summer for a good road trip to open her up a little more!

Find a high quality tire shop and they can help you setup your car. PepBoys just ain't gonna do it for this type of thing.


------------------
Adrian Pillow
1979 911 SC
1966 VW Microbus
PCA - Peachstate Chapter

Amen to that, Pillow. A few years back I worked (as a salesman) at a national chain tire shop (I won't say the name, but the initials were NTB), and can verify from first-hand experience the fact that impact wrenches are about all that are used in most discount shops-they don't really care about you, or your car-they just want to get as many people and cars through the doors as possible. As a result of working there, and seeing the kinds of things that went on behind the scenes, I only subject ANY of my cars to dealer or shop repairs when absoloutely neccesary-(that means only for factory recalls(just got my Jeep back from the dealer for 3! recalls at once-they put on new rotors but didn't bleed the brakes OR torque the wheels right!) or work that I simply don't have the tools for-like auto tranny work.) I change my own oil, rotate my own tires, do my own tune-ups, track down and repair any electrical malfunctions-if you REALLY like your car, and care about it, learn all you can about it and do as much work as you possibly can yourself-caveat emptor. Gene, your vibration problem sounds like either an out-of-balance tire, or an out-of-round tire-they ARE 2 different things-(though they can be combined.) Remove the wheels and tires yourself, take them to a GOOD shop, then WATCH them put them on the spin balancer and look for wobble or out-of roundness yourself while the wheel is spinning- a handy trick is to drag a tool (pliers or something) across the tread while it's spinning-out-of-roundness will be easily felt. One or more of the tires may have an internal belt separation-sometimes you can see the deformity in the tread! Good luck.

------------------
Clay Mcguill www.geocities.com/the912guy