What speed are you really going?

[macfly]

There has been much written about this, but I think this chart prepared by Grease Monkey is the definative work on the stock Z8's mph/rpm ratios.

Below are his calculations

*Take the RPM shown on the tach, divide by the transmission gear ratio, then divide that answer by the differential ratio.
*Set that number aside.
*Now divide 63,360 (inches in a mile) by the circumference of the rear tire in inches and set that number aside. *Finally, divide the first number by the second number and multiply by 60.

You now have the speed you are traveling in MPH.

[Z8doc]

until you feel it in the seat of your pants!

This was a very quick photo while on Day 1 of my trip to Rendevous with everyone at the Meet in the Middle in Santa Fe. This was on a West Texas Highway south of Throckmorton, TX on my way to Albany, Tx to meet up with the Texas group before heading west. Time was about 7:15pm on May 31st.

Long straight, flat and fast--- it was surreal and amazing!
[Calculated using the formula above at 158.03 but with the increase in tire size due to centripedal force going that fast -- more like 165 as I was at the "buried" point for about 20 seconds and still accelerating as I had 1300 rpm still left -- Z8 is equipped with Dinan software, Quaife, and 3.64 gears).

[Gammaman]

Just curious Jeff, was the top up or down at that speed? If down how was the buffeting? Thanks, must have been way cool.

[Z8Mania]

Excellent!

[Z8Bob]

With the Quaife 3.64:1 the table must be recalculated as this table (GM did the calcs and I rendered the table) is for the stock 3.36 differential. At 6000 RPM in 6th gear, stock wheels, you were making 158 MPH with the Quaife.

I have attached the revised sheet in PDF.

BTW if anyone wants the Excel version, drop me a note & I'll email it to you.

[Z8doc]

the tires do enlarge the faster you go so with the tires being larger and covering more ground (the speed sensors do not take this into account) you can only estimate the speed and it should be faster than the calculated amount -- I estimate 6000 rpm for 20 seconds or so to be around 165 --- but hey -- what ever the speed, it is still fast as hell and a hell of a ride!!!

[Z8Bob]

You are no doubt correct about the tire expansion. I don't know exactly how much there is at 150-160 MPH. BTW there are now tires in prototype stage that do not expand. Primary focus of the research has been for jet aircraft tires, where the rapid spin-up and expansion on landing is a major cause of tire failure. Apparently they have adapted an embedded weave that contracts under centrifugal force (ala Chinese Finger Puzzle). I'm sure we will see this technology in race/high performance tires well.

My purpose in attaching the schedule above was to point out the difference between the table for the OEM 3.36 differential and your (and my) Quaife 3.64

[hapc]

I was wondering why people are changing to a lower (numerically higher) gear ratio. I honestly feel that first gear is already too low geared. The car is at red line before I am under way. As a comparison, first gear in my GT is virtually identical to second gear in the Z8. With an even lower gear in the Z8, low gear would almost be useless. I could evaporate the tires and be at red line at a low speed. Clearly I understand the increase in rear wheel torque caused by the 8% ratio change, but is it actually worth this change? Years back I did a similar gear change to my 95 NSX and I was actually disappointed. Are those who have made the change happy with the lower gear and how do you use first gear? Are you increasing the red line RPM and turning the engine faster? I?m just curious about the thinking.
Hap

[thegunguy]

You have to be a little more careful under full acceleration in 1st with the 3.64 rear-end, but better tires (than the OE RFTs) help everything stay under control. For me, the Z8 is not a dragster, and as such, I spend very little time in 1st gear. For "enthusiastic sporty" driving on back-roads, etc. I spend most of my time in 3rd and 4th. The lowered ratio makes the acceleration of the car much more elastic (both up and down). On the highway, the new gears aid in passing with or without having to drop a gear. The big sacrifice is top speed (something like 184 vs 198), but most of us will rarely go over 150. So, this is no real sacrifice on US roads.

I am absolutely pleased with the 3.64s, and yes, I have the Dinan software that raises the redline so that the sift-points are roughly the same as stock. Other than Andrew (who had them in a previous car), everyone that I know that has the 3.64 is pleased with the change.

[macfly]

But I found the stock set up suits me well, and I like a lazier, more touring oriented set up for my car, especially as it isn't my track car. Curiously the oem gearing in 2,3, & 4 was also a perfect match for my local tracks when I was tracking the my two previous Z8's, and that also desuaded me from wanting to mess with the gearing.

I have an LSD, which I'll fit once the car is out of warranty, but I'll leave the final drive ratios as stock.

[hapc]

Thanks for your thought on the low rear end gears and the LSD. I have neither in my car and the car seems a little low geared in low, but it is clearly a beautifully drivable car as is. I do not race the car but I do, on occasion, drive it vigorously over some interesting roads. My son and I did the Z8 driving experience and I noted that the tires that they used were much better that the OEM tires. My Z8 is an 03 and has 8500 miles and is not yet in need of new tires. If I keep it, (it will be inspected in 2 days, Monday 7/10) I will put better tires on it, but I think that I will leave the gear ratio as is.

By the way, is the Z8 stable at speeds over 150? Years ago I owned a 930 Porsche that I modified and it was very flightily at 160 or so, but both of my NSX?s, a 91 and a 95, were stable flat out at something over 170. I am sure that my GT will do fine at speed, but I wonder about the dynamics of the Z8. It is a great GT car and is comfortable at 120+ but I wonder about 150+. Sorry to change thoughts in the middle of a post!
Regards all
Hap

[thegunguy]

It's probably attributable to its weight, but the Z8 is very stable at triple-digit speeds. However, after seeing how the OE RFTs can separate internally without any warning, I HIGHLY recommend that you change tires before attempting any high speed runs, or even runs on the highway for that matter...

Actually, every BMW I've ever driven has been amazingly smooth at high speeds. Too bad this may be my last.

[macfly]

but I have only done really high speed runs with the hard top on. My approximate maximums are ~140 top down, ~ 155 soft top up, ~190 hard top on. This last one was done on a closed road event in the Nevada desert in a Dinan chipped car. I was able to hold the car at just below the redline in top for several minutes on an arrow straight road. I have heard it said that at these kinds of speeds the car isn't too stable if you are on the autobahn where you are also negotiating turns due to the odd aero effect, but it felt fine going in a straight line. BMW also experimented with this aero-wing.

[hapc]

Thanks for the report. I have not run mine over about 130 and I don't think I could tech at the Silver State run for real speed. By the way, I would suggest that someone run with at speed with a GPS system. I have a couple of nice Garmins and may try it myself. I do not believe that tire growth will overcome tire slippage. When putting several hundred HP to the ground at speed, there will be a few percent of slippage. I know this is seen on the dry lake or salt flat runs and I would bet that there is some on a paved road. Years ago, in a stock bodied 56 Chevy, I could run about 125 and by going on and off the throttle the tach would move 2 to 3 hundred RPM at 6500 or so. It had large Blue Streak Sports Car Special tires on it ? rated for speed!

With regards to tires, I think that right after the BMW inspection I will replace mine. At the Z8 experience they were using Michelins, which were much better than the OE tires. My GT came with an inflation kit and no RF tires. I will buy one of these and take my chances. So friends, how about recommendations? I do not compete with the car and I want a compromise tire. Something that is safe at speed with reasonable traction, and wear, but not lots of tire noise. Of course it will never see rain or snow. Am I sking too much? Share experience please. Thanks all.
Hap

[Dogsbreath]

Hey Hapc

I have been running Dunlop Sport Maxx tires and am very pleased. I highly recommend these.

Dave P

[thegunguy]

Hap,

If you want to keep the OE wheels and tire sizes, your choices are limited. The currently favorite in the OE size is the Bridgestone S-03 Pole Position. However, this model appears to be in the process of being superceded by the RE050 Pole Position. To date, I don?t know of anyone who has the RE050 PPs, but they are suppose to offer improved performance over the S-03.

I know there are other models mentioned in the Tire/Wheel section, but I am pleased with the S-03s.

BTW - I cannot wait for BMW to deny someone the fix for using lighter and more plyable non-RFTs. Just wait - it will happen.

[hapc]

Thanks very much.

I will stick with the stock wheels and I have a couple of tire recomendations.
Well, maybe I will try the new ones and let everybody know how I like them. And your are probably right about BMW and changing tires. Oh well, lets see how this shakes out.

Hap