Ok, here's the breaks as I understand them from the NHRA rulebook.
11.49 to 10.00 - 5 pt. rollbar required. Max car weight = ? I'm going to use 4750 here, from Greg's earlier note about the max weight allowed for a sub-10 sec NHRA cage. (They allow a 5 pt rollbar in cars between 11.5 and 10.0 as long as the body / firewall / frame is stock and complete.)
135+ OR 9.99 and quicker = NHRA 8 pt cage, up to 8.50 ET (around 160 MPH). NHRA 3-year certification tag required, max weight 4750.
8.49 and down (161+) requires an SFI 25.5 spec cage, max weight 3600.
So, we've got 'stages' of required in-cab protection as follows, based on a kinetic energy threshold.
Rollbar required at anything over: 11.49 ET (Approximately 118 MPH) @ 4750 lbs.
Equivalent KE: 71,136,162 lb-ft
NHRA 8pt Cage required at anything over: 9.99 ET OR 135 MPH @ 4750 lbs.
Equivalent KE: 93,109,492 lb-ft
SFI 25.5 Cage required at anything over: 8.50 ET (Approximately 161 MPH) @ max 3600 lbs.
Equivalent KE: 1,000,000,000 lb-ft
If you notice, there's a couple gaps - if a 4750 lb car is running 8.51 @ 160 MPH, it's going to be packing a lot more KE (1.31 bill. lb-ft) into it's NHRA 8pt cage than a 2800 lb Pro Stock Truck running 174 with an SFI full chassis (0.91 bill. lb-ft).
In reality, there aren't any 4750 lb cars running those speeds. BUT, the SAME ENERGY (1,310,000,000 lb-ft) is generated by an 8000 lb truck running just 123 MPH.
The atttached graph tries to illustrate these 'breaks' or brackets, using a given kinetic energy as a dividing line.
The white area is the equivalent of running sub-118 MPH @ 4750 lbs - kinetic energy at or under 0.71 billion lb-ft. Notice it's capped at 118 - any MPH over that will put you under 11.49, the break for requiring at least a bar. Notice how it drops as weight increases to the right. An 8000lb truck generates enough KE to need at least a bar by 91 MPH.
The green area is the 'Roll Bar' zone, with kinetic energies between 0.71 and 0.93 billion lb-ft. Notice it's also capped at 135 on the left - that's the NHRA breakpoint for needing a full 8-point cage.
The yellow area is the '8 Point Cage' zone, with kinetic energies between 0.93 and 1.31 billion lb-ft. Notice the sloped dash line - that's the KE line for a 3600lb car @ 161 MPH (8.50 ET). I didn't use that as the upper limit for the 'cage' area because of what I mentioned above - the mythical 4750 lb car running 8.51 @ 160, legally. That car's equivalent KE line is the upper boundary of the 'cage' area.
Anything in red crosses over into SFI territory - and there's nothing that's really representative, as their chassis specs are capped (currently) at 3600 lbs.
So, what can we take from the graph?
8000 lb trucks running between 91 - 104 MPH need a bar. 104 to 123 needs an 8 point, NHRA certified cage. Over 123 and the basic NHRA 8pt cage likely won't cut it.
6000 lb trucks running between 105 and 120 need a bar. 120 to 142 needs aan 8 point, NHRA certified cage. Over 142 and the basic NHRA 8pt cage likely won't cut it.
And so on, by weight.
Attached a table with values every 500 lbs, so you can see where your rig fits in.
So, back on topic: for guys running
sub-11.5 (call it faster that 120 MPH):
If the truck weights more than 6000 lbs, it needs a full 8 point NHRA certified cage to be on par with their current rules / weight limits, regarding equivalent, worst-case kinetic energy. (4750 lbs @ 135 MPH = 6000 lbs @ 120 MPH).
Ref:
http://www.nhra.com/contacts/tech_faq.html