OK, so which of you guys has 4.08 gears and likes them for road courses?

BGTV8

You are running a 6MT and if you are a track junkie, then the answer to your question is based on whether you are likely run out of rev's in 6th at any track you currently visit.

I have attached copy of my gear ratio/speed sheet based on several tyre diameters so you can play with the options, BUT I reckon our Phillip Island circuit is going to match most of the high terminal speed circuits available in the US, and I don;t use 6th at PI and still don;t run out of revs heading into T1 which has a downhill approach, AND I am using 18" wheels with "little" 275/35R18 rear tyres (overall diameter about 650mm). I have a set of 265/35R18 to try next (I have gone square on this tyre recently, having used 265 on the front and 275 on the rear previously).

Now with 4.08 gears and my tyres, my Vmax with the OEM gear-box ratios and my own rev-limit of 7000rpm (as I do not trust the OEM oil-pump at sustained high revs) in 5th falls to 210kph and 6th is still 264kph, so I reckon that drag above 210kph is still going to affect the engines ability to continue accelerating the car a whole lot more. My car seems to be drag-limited at around 235-240kph as that is the best Vmax I have ever logged at PI (and that was with a 30kph following wind).

If I had more torque (SC or TT), then I would certainly go for it.

If you remain NA, then there will be some benefit for sure as in the lower gears the torque multiplication effect will be greater and for a given amount of torque, the engine will accelerate the car more quickly, so you WILL see gains, but I am not sure how significant.

Use the XLS to check your corner entry speeds with old and new diff ratio and establish whether your gear shift points will be radically out of phase with the way you currently drive your tracks, and make a decision based on that.

NOTE: The XLS is embedded in the DOC file attached as the forum would not let me upload an Excel sheet. Enter your tyre size in the boxes on the top row and it will re-calculate. I know that the rolling radius is a little less than that which the sheet calculates so you can adjust that if you like, and I also know that not all tyres respect the sizing but this will give you a reasonable idea at least).

RB

Attached Files

Z34 Speed in Gears.doc (44.5 KB, 6 views)

GSS138

Thanks GB, I will definitely take a look at the spread sheet. On most of the courses I do, I never see even close to top of 5th. I know I will have the downside of an annoying shift here and there into 5th instead of topping 4th. But the door swings both ways- I will also get to the top of 4th faster. AS for anytime in the immediate future1-2 years the car will remain NA.

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