MTAS

OK, as you can see by my list of mods I have 4.10 gears and 18" tires in the rear that are roughly 2" taller than stock, which basically makes the effective gear ratio a 3.90 or so. On a Mustang dyno, the car put down 283RWHP. According to Mustang Dyno's web page FAQ:

Q:
Why does the dyno show a power loss after a differential gear change (say a 3.42 to 4.10 gear)?
A:
The Chassis Dynamometer measures power that reaches the tires, not Engine Power. Therefore, any changes you make to your drivetrain may result in a change in your chassis Dynamometer results. A steeper gear ratio will provide the following,

1. Higher peak force to the ground
2. Higher average force to the ground
3. Lower top speed (MPH)
4. Higher peak acceleration rate in all gears
5. Lower peak horsepower

The reason you lose your peak hp is based on higher acceleration rates, increased gear mesh/losses, higher tire losses. These will affect your peak horsepower number in a negative fashion. Losing approx. 5% is expected when changing to a significantly steeper rear gear. Engine power was not reduced, power to the ground was. In exchange, you have more force available which is what allows you to accelerate faster off the line. If you enjoy the feeling of Gs forces as you accelerate, you made a wise choice. Do not be discouraged by the peak power numbers—they are just a reference.

So here's my question: do I take 283 + (.05 x 283) = 297 to get a better estimate of the actual RWHP?
I know dyno don't mean jack to some on here, but to a lot of us non-racers they do mean something. Thanks for any info!

don_w

Why would you do that??? RWHP is what it is. If it was measured (actually, calculated) at 283rwhp, then that is what it was at that point in time. If you're going to estimate, then why bother getting on a dyno at all?

MTAS

I don't know really, just kind of wondering out loud. Yeah it doesn't make sense to estimate a number from a number that is generated from the dyno But thanks for the reply any way

MusicMan66

Don is correct -- the HP numbers are what they are. What you are trying to figure is what the RWHP numbers would be without the gear change. And although your equation is backwards, the number is close because the difference is so small. To figure it, use Y - (Y*0.05) = 283, Y being the RWHP before the gear change. Solving for Y, we get Y = 283/0.95, giving you 297.89. (I can't believe I remember this stuff -- it's been at least 20 years since I've done any algebra!)

MTAS

Thanks Steve - that makes sense to my finite mind

don_w

And that assumes that this statement is accurate and applicable in this situation:

Losing approx. 5% is expected when changing to a significantly steeper rear gear.