Any reason why?

It gives you a true HP number because of the gearing.
In a lower gear you'll get lower numbers, in a higher gear you'll get higher numbers

That's a huge misconception IMO. You do know your diff ratio is not 1:1 right? And your wheel diameter alters both speed & torque. If you want to know "true hp" pull the engine out and strap it to an engine dyno. That's why dynos are a tuning/comparison tool not a "how much power is my engine making" tool. It gives a good idea but should be not taken religiously IMO.

The real concern should be, do you really want to be doing close to 150mph on rollers without the air flow to keep the tires temps down? :)

For the amount of time your at this speed tire temps are not even a consideration.

Of course your differential is not 1:1, you just want a direct drive or as close as possible through your transmission to get the most accurate numbers. But yes a dyno is a tuning tool.

If your concerned about wheel diameter changing your HP numbers, use a better dyno such as a eddy current dyno not inertia.

The 1:1 is a "industry" standard, every tuner/dyno shop I know along with any tuning classes you can take will tell you to dyno in a 1:1 ratio or as close to it.

That is true, but what I'm trying to say is, even if you dyno at 1:1 gear ratio, your torque is still multiplied by 3.3 through the diff. So the whole attempt to get "accurate engine ratings" is now off anyways, so you might as well tune in a slightly lower gear. When tuning my car on the dyno we spent at least 2 hours on the dyno, and had to stop every now and then to get fluid temps to cool down so it's safe to assume the tires were also getting too hot.

You will get slightly higher numbers in a higher gear, but you're also making the motor work much harder.

4th gear for both AT and MT is fine and so long as you do all the dynoing in the same gear you can make meaningful comparisons from pull to pull.

Went in Saturday
 

OK so I had the K&Ns put in a few weeks ago. My fuel economy actually seems to have dropped from that mod for some reason. I'm afraid I may have actually lost power :confused: But I do like the subtle hiss. Saturday I took it in and had the Berk HFCs installed in the morning, then took it to Jotech in the afternoon in Garland to have it Uprev tuned. They tuned it and compared it to my baseline (with CBE).

Before the tune I was only at 301 hp. So those two mods only gave me 3 hp. Not what I would expect from what I've read. After the tune I only went up to 309. Torque was 261. Now Saturday was warmer by about 26 degrees, but less humid. I don't think the temp would account for more than a few hp though.

All power discussion aside, the car sounds a lot meaner now.

It just occurred to me though, I did not ask him what gear he did the dyno pull in on Saturday. If he did it in 4th the numbers would make much more sense. I'll probably never know. What I am thinking is I may take it down and do the 1/4 mile at the local racetrack. That should be a good measuring stick.

Comments are welcome...

Most tuners I have seen will tune in 4th gear, rather than 5th which is the 1:1 ratio.

So the numbers seem low. But what you want to see is the difference in power throughout the entire band, not just peak.