I just received the Viair 400P today. I spent a good amount of time reviewing the various portable compressors and for the price, this seemed to be the best option. The one I purchased has an automatic shut-off to prevent overheating. With everything stored inside the nice carrying case (multiple pockets) it measures 11" wide x 13" long x 8" tall. It comes with a tire inflator / deflator with pressure gauge, 25' coiled extension hose with quick connects on both ends and 2 additional replacement air filters. The tire inflator has a lever that allows you to lock it down on the valve stem. It has a duty cycle of 40 minutes at 72F, so that should be more than plenty for airing up four tires. I believe it only weighs about 10 pounds.
It seems very well built and is incredibly easy to use. It shouldn't take you more than one minute to hook it up to the battery and have everything ready to go (leave the engine running). It also should have more than enough length to cover all four tires on an SMB.
I realize that an onboard compressor is a more optimal solution. However, if you are not going be constantly airing up & down during a given day, this will be more than adequate. The only other downside to this is that you will probably need to let it set for several minutes after use to let it cool down before packing it away. However, this is definitely a less expensive solution to onboard air and is much easier to service/replace if something goes wrong. We will probably never be airing down/up more than once a day, so a portable compressor more than fits our needs.
I purchased it from 4x4 Group Buy. Here is the link to the specific one I bought:
http://www.4x4groupbuy.com/store/via...45-p-5579.html. Shipping was $25 (flat rate) and no sales tax.
Viair claims the times below. Independent reviews I've read have either matched or done better than this. None were slower than the claims made by Viair.
Begin/End PSI 31" X 10.5"
0 To 30 PSI 2 Min..28 Sec.
15 To 30 PSI 1 Min. 25 Sec.
Begin/End PSI 33" X 12.5"
0 To 30 PSI 4 Min. 00 Sec.
15 To 30 PSI 2 Min. 00 Sec.
Begin/End PSI 35" X 12.5"
0 To 30 PSI 4 Min. 50 Sec.
15 To 30 PSI 2 Min. 20 Sec.