E36 Street mod autocross diff rules clarification

[guymandude]

Can anyone help me on the street mod SCCA rules.


E36 m3

Can I change ratio? To a standard ratio. 3.38 from an auto car in a manual car.

Clutch increase? 3 clutches

[matthugie]

Yep, all of that is legal in both Mod and Street Prepared. That ratio seems a bit short though, from what I've read most SM E36's run a 3.15.

[jakermac]

Differential in Street Modified is wide open. You could mount the aluminum housing IRS diff from a Ford Explorer with whatever LSD you want and still be legal. In SP, you'd need to run a diff housing and ratio that came from a North American market E36 M3 (US market if you want to run in the US since there were some euro 94 M3s here in Canada).

Ratio depends on a lot of things. Do you want to compromise your local site performance on the off chance you go somewhere that runs high speed courses? What diameter tires are you using? What rev limit are you using? I currently run the 3.23 ratio with 275/35-17 (24.8" dia) tires and a 7,800rpm rev limit with my Getrag 325is trans with a 2.52:1 2nd gear. That combination is good for 71mph based on number crunching (http://www.apexgarage.com/tech/gear_ratios.shtml). I rarely hit the rev limit on my local courses (Pitt Meadows airport, Vancouver, BC). I have run at SCCA Champ Tour courses in Packwood, Wa and I don't think I've hit the rev limit there though I'm sure I've been close a couple times.

So you have to decide if you're willing to take the hit from having the better punch off the corners from the 3.38FD with the possibility of either riding the rev limiter on a fast course somewhere or having to shift back and forth from 2nd to 3rd. I personally try and avoid that like the plague. I suck at downshifting from 3rd to 2nd in an autocross environment. I can do it perfectly 100% of the time on the street. I can do it perfectly 10% of the time when autocrossing. Things are just happening too fast when autocrossing. And you can most likely only do 2 things really well while driving on the autocross course: Brake and turn, brake and downshift, downshift and turn. Try to add the 3rd thing, and something's gonna get screwed up! For me, it's always the downshift.

And for the record I'm switching to the 3.38 ratio (68mph) during my next diff build though I may tweak my rev limit for a few more MPH.

[jbrannon7]

On my car in autocross the 3.23 is just about perfect. I just touch the rev limiter in 2nd on many courses but seldom have to use 3rd.

[guymandude]

My car has a 3.15 right now.

The courses at my club can be pretty fast. My engine is a euro 3.0L (that's why I'm in Street mod) it redlines at 8600. I'm on the Rev limiter allot.

Others from my club are going to 3rd. I don't want to have to shift down to second from third but even with the 315 I should probably go to third anyway.

I've been tinkering with a crusty 338 that I got for cheap.(winter project). I am going to try this diff out. Maybe I would need a 2.93 if I want to stay in second.

[jakermac]

Is that at the Spokane site? That's crazy fast - 80mph. What size tires? How sure are you that it's a 3.15?

[guymandude]

Deer park airport Spokane. I try not to turn. Tire size 245/40/17. The car came with 3.15 but I haven't verified that that's what's still there.

[jakermac]

What RPM does the engine spin on the highway at 120kph (75mph) in 5th gear? And is that a ZF trans?

[guymandude]

It's a ZF transmission. It's been so long since I've driven it. I'm not sure what RPM's at hwy speed

[jakermac]

With my Getrag (same 5th gear as you 1:1) and the 3.23FD, I'm at about 3,200 in 5th at 120kph. However, like you it's been a little while since I drove the car. I'll be driving it again here in the next couple days so I can confirm that number. You should be at about 3,100 if it's truly a 3.15.

[jakermac]

So I can now confitm that at 120kph indicated, my tach is indicating approx 3250-3300.

[guymandude]

3200 at 120. Im going to stick with this ratio for now, since i can win with it. I am considering a 1 way or 1.5 way trust rings. I guess driving style and preference determines what is best for that