Spec Boxster Build

[thstone]

Originally Posted by Greg Holmberg

I think I could get those 5 more HP if I replaced the cats with straight pipes.

First, thanks for posting your dyno sheet. Always good to see real data!

Yes, I would expect at least a 5hp gain from removing the cat's. If you drive the car on the street, you'll have to re-install the cat's every two years to pass the CA smog test but its not that much work.

Originally Posted by Greg Holmberg

I wonder how 197 at the wheels translates to HP at the crank. I think one loses about 15% through the gearbox and tires. So 197/.85 = 232 HP at the crank?

15% is the standard estimate for driveline loss.

Remember that the engine is spec'd from the factory at 201hp. That might be a little conservative so my thought is the power from a good 2.5L engine probably ranges from 203-205hp.

So how does a 205 hp engine (at the crank) make 197 hp at the wheels with a 15% driveline loss?

205hp * 0.15 = 31hp driveline loss
205 - 31 = 174 hp at the crank

Now, add back changes that add horsepower;

+8 hp cat back muffler
+8 hp removal of cat's
+4 hp under drive pulley
+2 hp cleaned/calibrated injectors

174 + 8 +8 + 4 + 2 = 196 hp

The exact HP values for the muffler, removal of cat's, UDP, and calibrated injectors might vary a bit from those shown above. Also, we used a "standard" 15% for the driveline loss and that might be slightly higher or lower than the actual loss.

Originally Posted by Greg Holmberg

Is torque also lost through the gearbox, or only HP?

Yes, as Anker said, torque will scale the same as horsepower.