solid axle swap dyno results

[lachj1]

I just got back from the dyno. I wanted to see the difference between the IRS and solid.
The last dyno I did in the fall was 419.9 rwhp / 396 trq.
My best today was 424.0 / 405 torque.
The conditions were about identical, temp and all.
The only other changes I made were going to a stinger catback from the steeda catback.
Someone earlier posted about a significant gain in hp's with their swap. I'm using 3.73 gears and noticed only 2 hp and 3 lbs torque average overall.

 

[PhillyCobra]

lachj1,

Were you using 3.73 gears in your old IRS, too?

I think new rear end gears take some time to mesh optimally. I wonder whether you might see a few more HP after the new rear end breaks in. Did you use a new rear axle, or a used unit?

I assume all your numbers are SAE corrected?

 

[lachj1]

Sae corrected, they are.
My old Irs was completely stock.
The solid axle was just built, about a 100 miles ago with strange axles and eaton posi.

 

[sambandit]

Gain HP and loose weight. Pretty Cool!

 

[PhillyCobra]

lachj,

You lose some HP on a dyno when you are in a lower gear (rather than 4th for instance) or have a lower rear end, as the engine will accelerate to max RPM quicker, resulting in more inertial losses to speed up all the rotating mass. I know that sounds weird, but the quicker you accelerate the rotating mass of the drivetrain, the more power is absorbed (as power is a measurement of work over time). This is one reason you dyno in a higher gear (in addition to the fact that 4th gear is direct and thus has less frictional losses than other gears). Someone did an analysis of the magnitude of this effect on modularfords.com several months ago, and it amounted to something like an extra 20 or 30 hp loss if you dyno in 1st rather than 4th. Thus, you will lose a few HP with your lower rear gear, and I bet you'll gain a few after it breaks in. There's tremendous fricition in the ring and pinion (as proven by the need for a differential cooler in open track running).

So I would guess that the switch probably would give more like 10 HP with the same rear gears and after everything is broken in, but that's just a guess!