Diff Upgrade-How Did You Decide on Your Diff Ratio

[Fresh1179]

Hey All,

The time has come for me to service my differential. I will likely purchase a diff from Diffsonline, I believe. Regardless, how did you come to your final decision regarding your diff ratio? I know the most common upgrades are 3.91 and 4.10, but I am trying to figure out if I should base my ratio choice based upon my home track. I understand that would result in a "compromise" on other tracks, but if %50-%65 percent of my days are spent on VIR Full Course, wouldn't it make sense to purchase a ratio that best fits that track configuration? Honestly, the stock ratio works pretty damn well for our cars there. Any advice is appreciated. I am just trying to make sure I think this through before I commit to a relatively expensive and relatively "stationary" upgrade. Thanks for your time.

[Obioban]

I think stock is perfectly matched to the engine, so I retained stock when going to my WaveTrac.

Works pretty well on most tracks, too-- generally the top of 4th pretty well aligns with the straights. I'd rather stay out of 5th, as much less likely to money shift if only going between 3 and 4, where no sideways forces needs to be applied to the shifter.

I had 4.10s in my track only s50b32 e36 M3, and hated them. Felt like I never stopped shifting. 5th was required. Lots of corners where I either had to go in with too few RPMs or shift the car mid corner. Worse.

[OldRanger]

I just did the same, and chose to retain stock ratio - even on a car that is primarily track. I'm new to track time in cars.

My thoughts:
1) I'm happy with the track experience I get with stock ratio
2) I still drive my car to the track (40 miles).
3) I'm not racing - only HPDE/DE for the challenge/fun and don't particularly feel the need to gain tenths of a second here or there - so I don't see the value for the cost ($1,300 - $1,600)

If there was no upcharge - I probably would have opted for the 3.91.

[foolio]

Stayed with stock myself as well. Also went with Diffsonline. I put a Wavetrac in other than a rebuilt stock diff.

[Feffman]

I have Drexler diff that initially had 4.10 but it ran out of gear on big straights (Road America, Road Atlanta COTA, etc.) so swapped to 3.91. I like the 3.91 as it allows me some launch out of the corners, but I don't run out of 5th gear.

Feff

[elbert]

For VIR, I decided to stick with the 3.62.
With the 3.62, I can leave it in 3rd for T1 through Snake.
I'll hit the rev limiter on the front and back straight in 4th gear as I'm about to enter the braking zone, so I'll usually shift into 5th at 7500 to 7800 rpm to preserve the engine.

With the 3.91, I might hit the limiter before T3 and definitely in that mini straight after Roller Coaster. And of course need to shift into 5th on the big straights.

I would check your data, and see where you would need to shift. I'm too lazy to shift where a 3.91 would put me.

This is what I calculated for max speed per gear:
3.91
3rd 94 mph
4th 128 mph

3.62
3rd 102 mph
4th 138 mph

ps Fresh1179 , I sent you a PM.

[bigjae46]

Tire size will be a big factor. A tire with the same OD as stock might be OK at VIR. You might top out in 5th with a smaller but more common 275/35R18. Also have to factor in the possibility that you might find speed at certain places. What used to not require an upshift may now require one.

I've ran a 3.62, 3.73, and a 3.91 with 265/40R17 tires and a ZF320. At my home track, not sure either one made much of a difference. Hard to tell, but I'm faster out of the corners with the 3.91 but some of that is negated with an extra upshift. When it is hot, I can stay in 4th because my car is a little down on power. I have to shift to 5th when its cooler. I used to have a 2-3 second delta between summer and fall temps. Now that has narrowed a bit (probably right at 1.5 to 2 seconds) which I think means the upshift when its cooler negates some of the power advantage.

I recently drove at Motorsport Ranch in Cresson, TX on the 3.1 mile configuration. Half of the track is right around 90-110mph which is right near the 3rd to 4th upshift. With the 3.62 and 3.73 it was straight forward, 3rd gear in a downhill section where I really need the power to rotate the rear end. Now with the 3.91, I'm in 4th and I have to carry a lot more speed to get some rotation. In some places it is just not going to work. AND there are two shorter straights where I either bounce off the rev limiter or upshift and then downshift in less than a second. I also have aero so I carry more speed than an non-aero car. My buddy with a 4.10 in an E46 and no aero doesn't have the same issue.

I'd guess that a fast, flowing track like VIR, a 3.91 would be advantageous. If you drive lower speed, twisty tracks then a 3.91 might force extra shifts at points on the track which will cost you more time.

I think gearing is less important - its the limited slip unit that is they difference maker. But if I had to choose a great all-around gearing it would be a 3.73 or 3.85. A little extra jump out of the corner and might force an extra shift per lap but I never ran into or close to that issue with a 3.73. For cost and availability, I'd choose a 3.85. I sold my 3.73 gearset...I'm already regretting that.

[Mpassioned]

Highly reccomend Diffsonline. Dan has built like 5 different diffs for me over the years and I would not hesitate to use him again if the opporunity arose.

For a race car, Longest straight, just hitting redline in tallest gear may be optimal. For my S52 5speed car at VIR, I ran a 3.73 which was perfect everywhere except I would have to feather the throttle at the end of the back straight. There, I was out of revs a little too far out from the braking zone. To mitigate that now I am running the 3.62.
When I tracked my E46, I thought stock gearing was fine for what I was using the car for, part street, part trackdays. This was at Summit and VIR.