Brett928S2

Hi All

Here is the scenario...

I am going to try a TOP SPEED RUN in the UK....ON AN AIRFIELD...

These runs are very hard to get access to in the UK ...and I want to hit 190 MPH...

Now to my problem....

It will be a rolling start around a bend at around 50 mph when I will pass the start beam...at the start of the 1.25 mile straight runway..to end beam ...with a half mile shutdown area at end...

Sooo...

I have a 1986 S2 Auto...4.7 (the last of the S2s with the S4 suspension and brakes) WITH Nitrous....it will be pushing around 480 hp...so the power isnt a problem..I HOPE LOL

The gearing is my problem with which I would like your help on....

I already have fitted 18 inch 996 wheels.......with low profile tyres...and 4 speed Auto....

My THOUGHTS were ...for this top speed run I really need TALLER tyres for the rear end to INCREASE my gearing for a higher top speed....

I am happy to buy the TALLEST tyres I can find .....

But WHAT size ????

Grip is unimportant....these are purely for increased top speed...

Any mathematicians here that could work out what size I need please ????

Or any of the distance racers ..who may have done this ???

All the best Brett

PORSCHE 928 S2 AUTO V8 4.7 LTR 1986 IRIS METALLIC BLUE WITH X-PIPES ,RMB ,PROMAX CHIPS, KICKDOWN SWITCH.K&N.NITROUS KIT 150 HP.
PCGB UK 928 FORUM MODERATOR - Email managingdirector@pchealthcare.co.uk

taffelman

Some guessed theoretical figures (use as example)

Stock wheel = 245/45-16"
Stock topspeed = 165 Mph?

Wanted speed = 190 Mph = 15,15% speed increase

If we assume you will hit the same max RPM you need wheels wich have 15,15% longer "rolling radius" than the stock wheels.

You could calculte it or find a online wheel calculator wich will tell you that 265/50-18 would be a perfect match with 15,2% longer "rolling radius".

In THEORY it might add up, buts its up to you to prove it, :-)

Note! not sure if the factory numbers for topspeed is based on speedo readings or radar meassuring, and we all know speedo's are not accurate (my speedo show about 7-8% more than my GPS navigator).

Garth S

Take a look at the gearing charts in the back of the Owners Manual: eg, a S4 with 2.20 rear gear and a 4 sp will do a theoretical ~275Km/hr (171 mph) @6000rpm on a 245.45R16. The latter tire has a nominal 24.7" diameter.
Therefore, at 6000 rpm, the tire od must increase 11.1% to achieve 190mph .... or be 27.4". Most all 18" performance tires are < 26", with the 235/50ZR18 being the exception at 27.2": with these, a few more revs over 6K, and you've got it made ..... if your gear is 2.20.

michaelathome

With 25" tires, 6000rpms is 164mph, 7000 rpms is 190mph.
http://members.rennlist.com/flyingdo...r%20ratios.htm

jorj7

Brett,

If your 86 S2 auto has the 2.36 final gear ratio then you can get to 202 mph
with 25" diameter tires, but you'd need over 500 rwhp to get there. At a
little over 6,000 rpm the car would be at 190 mph, and would need about 430
rwhp. If you have the optional 2.54 final gear ratio then 25" diameter tires
will get you to 190 mph at 6,500 rpm and would require a little over 432 rwhp.
Most 18" tires 245/40 - 315/30 will be 25" or larger diameter. I would recommend
that you get (Y) rated tires, 186+ mph speed rating, or at lest the regular Y rated
tires (up to 186 mph). Also you didn't say if your 480 hp was at the wheels. If
not, then you're talking about ~384 at the wheels, and you wouldn't have enough
power to overcome the drag without doing some aerodynamic improvements.

FlyingDog

Oops, I missed that this was an auto.

mark kibort

the holbert car ran 180mph at nardo, and if the car had the same hp i tested, then thats 285rwhp.
of course, who knows how long it took for the test .

190mph would require about 50-75 more hp and 200 would require about 200more hp.

Bills right on his calculations, but the main issue is having the hp at the top speed you need. you could need smaller diameter tires to make it all happen.

Ill have to plug it all in the simulator. is the gear box, an auto 2.XX ratio?

Mk

Vilhuer

Software is your friend. Try these three.

Speed in different gears.
http://www.bmw850.de/GearSpeed.zip

For that you need to know rear tire circumference. This next one will give it based on sizes you give it. For 225/50-16 it's 1984mm and for other stock sizes very close to it also.
http://www.miata.net/garage/tirecalc.html

George already gave you estimate how much hp you'll need. If you want to calculate what different tire sizes and rear diffs will do to top speed this third software will give all the answers when input numbers are correct.
http://www.cartestsoftware.com/carte...ds/CarTest.zip

jorj7

Mark,

You need to factor in that the S4 has a .05 better drag coefficent then the S2. Also
I'm sure they ran the Holbert car without mirrors and a few other details that they
don't mention...

Vilhuer

Also if tire size is increased drag coefficent will worsen because rear end will lift up unless suspension is changed also. Lifting also front up by using matching set of rubber will help only partially.

ew928

Would folding in the mirrors help any?
Check out Fifth Gear and Top Gear episodes on their high speed runs, Brett.
Not sure how long is your runway track but street cars all seem to have a hard time
getting up to final speed in the short alloted runway before hitting the braking zone.

All the speed you want and no speed cameras.
Ernest (NYC)

Brett928S2

Hi

Just looked in my owners handbook...rear axle = 2.357 OR 2.538

It does not say why there is an "OR"

Anyone know know which mine is ??

All the best Brett

Brett928S2

Hi

Thanks Erkka that Bmw zip was brilliant

ok....here are the results....

With a 2.357 rear end and my CURRENT tyres..(255/35/18) top speed at 6200 revs = 195.8 mph

With a 2.538 rear end and my CURRENT tyres..(255/35/18) top speed at 6200 revs = 181.8 mph

So.....how do I find out which diff I have ???

Any help much appreciated

All the best Brett

Vilhuer

Easiest way is to drive 181.8 mph. It engine revs under 6200 rpm you have 2.357:1 diff.

If you don't want to do that, next best thing is to check your gearbox type from stamping in it. Reason for two different gearings were that 2.538:1 (IIRR) was optional in ROW 16V automatics at '85-86 while 2.357:1 was standard. Or was it other way round.

FlyingDog

He could lower the ride height to help with that. A runway won't have too many bumps to worry about bottoming out on.