Opinions/Exp on Diff Ratio - need to change mine

[bmellott]

Anyone interested in giving me there opinion/expereince on Diff Ratio
Currently:
Z3 3.0i (red line is 6500, I don't like going much passed 6000)
Current diff 3.46 40% lock (Org 3.07 torsen) which rep's a 12.7% change
Average tire dia 24.5

Basically I am running out of gear/rpm entering some areas on some tracks. Example WGI shift to 5th out of the esses and having to do some coasting into the bust stop; what can I say Im out of RPM top end on the 3.0
So... I am considering either a 3.15 or 3.23
Thoughts/opinions?

thx
bill

[///Madman]

Do you have access to the Excel spreadsheet that calculates MPH/RPM for varying final drive ratios/gear ratios/wheel diameter? If not, send an email to me at polanie@directcon.net and I will send it to you. It is probably available online, too, if you want to search for it.

I went from a 3.23 to a 3.73 in my M Roadster and also have the problem with running out of gear at certain points on certain tracks. A 3.64 might have been better for me.

[S.Lang]

Posts like these always make me wonder. I mean, BMW gave you 6500 RPM, use it! Why don't you like going past 6000? The engineers at BMW left a pretty good safety factor when they set the rev limit at 6500.

It just seems that not going past 6000 rpm is at odds with tracking a car. I mean, how do you give diff ratio advice to someone who is not currently using all that BMW gave them engine-wise? I mean, when you say you are "out of rpm", are you REALLY out of RPM, or are you talking about your short-shifting habit?

Anyway, here's a link to diffsonline's ratio/speed spreadsheet:

http://diffsonline.com/techinfo/index.shtml

[bmellott]

Ok too these points:
1.) yes used the online calc
3.07 @ 6K 142mph / 6.5K 154.4mph
3.15 @ 6K 138mph / 6.5K 150.5mph
3.23 @ 6K 135mph / 6.5K 146.7mph
3.46 @ 6K 126mph / 6.5K 137mph (Currently)

I pull 5th as I track out of the esses onto the back straight
1.) Sure I guess I could push it to 6500 heck why not just go to the max of 7000
it's SW'd I'd go, it has gone..still run's
Let's say that I drive it to every event as I do not own a trailer/truck and etc etc time. Let's also say as an engineer I understand the engineering prinicpals and I choose to leave myself a safety margin/continued operating margin.
Im DE's here not racing.
Let's also consider do you think the BMW engineers really design the the motor for a lot of 6500~7000 activity or for the intended most often used power band day to day use?
Heck I'd bet the thing might spin to 8K without much problem..but day after day? (ok it's only say 30 or so DE a year)
If' Im short shifting it's so I not use up my saftey margin

Ok "out of RPM" is a realative term to each their own. For me for as much as I track the car and etc. I'd rather call my "red line" 6K with a saftey to 6.5K

[ScottL]

I think they do design the engines to run up to red line all the time. I have a 200,000 mile M20 that seems perfectly happy up there all day long. I take it to just below the rev limiter on almost every gear except during warmup.

(Watch, I'll spin a bearing next time out).

I don't think the motor should be pegged for sustained intervals nor should it be wound up that high without a load. But if the safe running limit for revs was lower than what your tachometer says, I think they'd set it lower - especially given the fact that the everyday driver has no clue about such things.

Disclaimer: I'm not an internal combustion engineer but I played one on TV.

[///Madman]

I will give you my experience of running at Redline for a long time. I drive a 98 M Roadster with a few performance goodies (Schrick Cams etc stc) and a 7200 redline. I have 125 track days on the car with an average of 5 sessions at 20-30 minute sessions per day. I live at Redline or on the way to it! I often run in 95-100 degree weather. For nearly 8 years I drove to and from my track events (now trailering). Your engine, with good maintenance, can handle just about anything you can throw at it (just don't money shift).

[S.Lang]

The way I see it is yes, I think these motors could spin 6500 rpm all day, every day. But we're not talking daily use, unless you do DE's every day!

The occasional trip to redline, whatever that RPM is, is fine. To change diff ratio to a lower ratio, which is going to negatively affect acceleration in daily use, seems silly to me when the reason for doing so is to eliminate VERY occasional high rpm use on a race track.

But hey, it's your car! Sorry for the off topic.

[JClark]

6500rpm with your current diff is plenty on the back straight. You wont be going past 137mph without some more power. Then keep short shifting the rest of the track. One time every 2 minutes isnt going to hurt it. There are guys taking their daily drivers to redline every shift point at every track day.

Normally I'd say "yeah just flog it to death, I do it all the time" but since my last motor let go with you sitting shotgun, I'll keep it to myself.

[bmellott]

6500rpm with your current diff is plenty on the back straight. You wont be going past 137mph without some more power. Then keep short shifting the rest of the track. One time every 2 minutes isnt going to hurt it. There are guys taking their daily drivers to redline every shift point at every track day.

Normally I'd say "yeah just flog it to death, I do it all the time" but since my last motor let go with you sitting shotgun, I'll keep it to myself.

Thanks
It's not so much as I am short shifting the rest of the track as I am worried about being so close to red line if I push it.
The diff is good every where except on long straights etc.

SO what happend to the motor that time?

[Kos-motate139]

By comparison, I used to do open road racing events where I'd spend 35 - 40 minutes around 6500rpm in my '97 M3. Oil Analysis before and after showed no unusual wear, with just a stock engine and Mobil 1.

I might worry in an old unmaintained engine, but if you're on top of oil changes and get it analyzed to see what the rpm effects are you'll be able to prove if 6000, 6500 or 7000 is an issue or not.

[magnetic1]

buy a 6 speed? Maybe buy a Honda/Acura and drive a few thousand below redline.

[Damon in STL]

I say buy stickier tires and carry a higher gear through the corners if you are RPM limited!

I just bought the "Trackvision" software to use with my IQ3 and video. I've now added the data acquisition info to vids for a couple of different tracks and was watching some of it this past weekend. It's interesting to watch where I'm carrying more speed (by watching the speedometer on screen) and using the higher gear while just maintaining throttle.

You may find, as you get faster.....you may want the old diff ratio back!

Oh, and I've got the RPM on screen now and I'm always between 4K and 7K....right where the s14 should be.

Damon in STL