i've worked on a lot of them including dynojet 248, dynojet 448, with an without friction brake loading and/or eddie current loading, mustang, the austrailian bolt-on deal, some other off-brand dual-roller types, etc.
First and foremost, if you're buying a dyno to be able to compare power numbers with other dyno shops and/or other entities, buy the same dyno they're using or you'll be forever wondering which dyno is correct. The do not read the same between brands. Within one brand, they are very close.
I would strongly recommend not buying a dual-roller type. I've seen a few catastrophic tire failures on high horsepower cars, and simply don't like the way the car tries to climb the leading roller when the roller diameter is small...they walk around, it's scary, i don't like it.
Across all the dyno's i've used, my personal favorite tune-shop dyno is the dynojet 248 with the big locomotive brakes and friction loading control add-on. The reason is durability. Large tire ....... Patch, the fact that the mass of the drum alone is enough to accomplish enough load to get useful data without a load cell (simpler, more reliable), the bearings and brakes are hugely overbuilt (thus never requiring maintenance), and the friction loading (dragging the brakes) works just as well as the eddie current stuff for most tuning without the expense or complication. For a tune shop, this is a real work horse that will last forever, and i wouldn't be at all afraid of buying any used 248 if you can find one...they're that durable. If you want to see one, rpm in rhonert park, mustang ranch in santa clara, apex motorsports in santa clara, and others have them.
That said, if you're trying to do analysis under varying road conditions or wanting to do track load simulations, or other extended durability testing under load (say, and hour at realistic varying loads at 150+mph...road race durabilty testing), this isn't the right machine. You'll want a dyno with a large eddie current load cell like the 448 or the mustang, or the bolt-on austrialian deal.
The austrailian bolt-on deal drives me nuts. It takes too long to set up a car. For a tune shop, this is a pain in the butt. If you ever want to have a dyno-day on your dyno and run a dozen cars an hour...you won't be doing it with this type of dyno. If you are looking for something to bolt to a car and do development on occasionally, this is probably the best solution due to footprint, cost, and features. If you havn't seen one of these, speedmerchant in san jose has one. So does the shop next to griggs racing at infinion.
The mustang dyno is a good machine, but the numbers don't jive with dynojet which seems to be more popular and more accepted. If you're going to have people get numbers off your mustang dyno then go to a dynojet, you'll be asked repeatedly why the numbers don't match. 10-20% difference is common. Aside from that annoyance, it's a good machine with good load cell capability and interesting software.
Hope that helps,
byron
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