milkmandan

I've been having a problem with the car losing revs under heavy braking. It shows up in 2 particular corners on this track, both braking down to around 35-40mph after a 4->3 downshift into right hand, downhill, off camber corners. You can see the revs drop in 2 places in the OBD2 datalog below (one is marked with the blue dot on the datalog, and you can see where the other occurs in the color map of the track at the dark blue zone):



As I brake, it feels like the clutch is still pressed in and the rev's drop down to around 1900, and once the braking is over, I've turned in and gotten back on the gas, it feels like something is slipping (either clutch or tire) as the revs rise to where they should be (around 5000ish). If I'm hamfisted enough with the throttle, there will be some oversteer as the revs catch up.

Here's a video of it happening in the corner highlighted above:


You can see I brake, downshift, get back on the gas again for a short little burst between corners, and then brake again when which the revs drop all the way down, and then I'm pretty sure I'm spinning the right inside tire as it catches up.

Car setup information:
6 speed trans with 4.3 Torsen
Supermiata 4-puck unsprung clutch and lightweight flywheel, 1.6 size
900/500 Xidas with helpers
Racing Beat 1.125" front sway, outer hole
5x Racing rear sway, middle hole
949 Race alignment (-3.5f/-3.0r, 1/16" total toe rear)
4.5" front pinch weld height, 4.75" rear pinch weld height

Ideas I've thought of but don't make complete sense:
- weak pressure plate on the clutch?
- Torsen sucks?
- poor cross weights (car has not been cornerweighted, I don't have a set of scales unfortunately)?
- not enough droop travel on the right rear tire?

Thanks for your help!

sshamrockk

Are you sure you aren't lifting up a wheel? I wanna say I'v read that the Torsen pretty much turns to an open diff with one wheel in the air. So is it possible that one wheel is coming up and the brakes are slowing down the lifted wheel causing your RPM drop, then when the wheel starts to contact the road rpms climb back up?

MX5RACER

That sure sounds like a one-wheel-peel! You sure you still have a functioning Torsen? Also, have you checked your droop-travel. It could be that under full braking load, you are lifting a rear wheel due to not having enough droop-travel. Jack the rear of the car up (Using jackstands, of course), remove one end link of the sway bar and let the suspension drop all the way and measure from the lower control arm to the ground. Now support the lower control arm and remove the lower shock bolt and let the suspension drop and measure from the lower control arm to the ground again. If you have a large difference between these two measurements you may need longer shocks or more travel in your current set-up.

A Torsen will 100% go open if you lift a wheel. I have seen way back in the old CS autocross days, with too much rear rebound holding the suspension up and getting wheel spin as the suspension returns to ride height.