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scca_ita
We tested our 915 spool for a long period in our road / track / autoX car.
This is a 170 hp, 2200 lb car. Either 205's all round, or 205 / 225 depending on our activities.
On the street the spool was no problem, but we did see more front tire wear.
On the AutoX it was all or nothing. Very high levels of commitment required and often 170 hp was not enough to maintain a balance of oversteer on the throttle. For an experienced driver there were times when it was good (TTOD at one event) but for a less experienced driver without the range of driving skills at her disposal, it was often too much inherent understeer to overcome. Both drivers always left foot brake and this is a major advantage with the spool.
On the track, we started with a balanced car and found we had to lower rear ride height and stiffen front ARB to restore the same balance as the car had with the 40% ZF diff. The advantage with the spool was very significant, more stability on the brakes, more entry and mid corner speed. I like to have the car with a mild US (at the limit) in the exit and this was easily maintained with the spool - same feel as the 40% ZF.
On the track the spool was definitely faster.
Some of the fastest PCA GT4 cars in the country are running with spools, these are 300+hp cars with 12" slicks, so their package is much different to ours.
After we completed testing the spool we returned to the ZF diff, this time set at 80% - hoping to take advantage of what we learned from driving the spool. I did one AutoX like this and removed it, and set it back at 40%. At 80% the ZF Diff was less predictable than the spool.
For track work I would not hesitate to use a spool, even if I drove to the track (in fact we did at that time) but for a track / AutoX car I would go for a 40% ZF diff.
Regards
Hayden
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