Robert, first let me thank you again for your IR advice a while back.
Under the conditions that I'd typically shoot IR (i.e. near sunny 16) the exposure times really aren't bad with this film/filter. This was 15 seconds at f/22 under relatively low light. If I didn't have so much foreground to keep sharp and it were brighter I could have opened up more and used a shorter exposure.
This was with the 89b filter and ISO 3 (i.e. a 7 stop filter factor). I also tried with an 87 filter and two more stops exposure (60 seconds) and got no exposure at all -- a completely clear negative. So I think I'd need to open up at least 5 more stops to get any exposure with the 87.
I looked at a color shot I'd taken at the time. The top of the sky was quite blue and the sky lower down cloudy. So interestingly this dark stormy sky is actually blue sky, rendered dark by the IR filter, whereas the clouds are reflecting some of the deep red and IR light from the sun off to my right.
I can't think of what the cut off is for the 87, but keep in mind, it could simply be beyond the ability of the film. If that is the case, you will get no image at any amount of time.
Yes, just looked at the 87 filter. You might have to over expose an order of magnitude to get anything if it's possible at all. It's just too close to the cut off.