Could they do it with 400? Certainly not. However, 375 - Unleashed (47) - Comp (12) - Top Alky (11) = 305... a far cry from 400.
We average 52 seconds per pair, including downtime. 2 time trials, plus 10 min of cooldown time between each of the last 3 rounds (EVERY class, so we're building in and accounting for an EXTRA 3-1/2 hrs of downtime for cooldown, etc.)
S/ST 44 1hr 48m
S/G 57 2hr 12m
S/C 72 2hr 37m
SS 33 1hr 32m
Stk 43 1hr 48m
TS 23 1hr 12m
TD 32 1hr 25m
--------------------
13 hours
Long day? Yep, but they absolutely could do it. As a matter of fact, they have. Carolina Dragway 2007, 286 cars, Race #1 done in 12hrs (including Jet cars, wheelstanders and other 'show' stuff they booked in), Race #2 (1 time trial) done in 7hrs 45min. (I think 2006 was bigger, but can't find stats) And given *3* days, it not only could be done, but done comfortably.
I announced at the Div. 2 Bracket Finals last year... 400 cars. We ran 5 races in 5 days.
Like Myron said, why doesn't everyone just flood the IHRA events and show them how they "can't" do it.

LOL (Please, nobody mention the 2009 World Finals!)
