MrDave
Veteran Member
- Joined
- Sep 5, 2002
- Location
- 1300 km NW of nowhere
Came across the BMEP chart from a different post,
and I've been thinking.
Looking at the BMEP with HP values:
If I'm currently at 2750 rpm at ~100 kph on the highway
(I have a totally different tranny mounted compared to
the regular TDI), so let's assume it's taking 50hp to drive
the car that speed.
If I were to install different gearing that would take me
down to 2250 rpm for the same speed, assuming the same 50hp
to drive the car along, my BMEP values change from 225 to
205, following the 50hp line. That's a difference of
~9.75%. So my fuel economy should increase ~9.75%?
Following further, dropping the rpm to 1750 would give me a
bmep of 197, for a difference of 13.6%, so my economy should
increase 13.6%.
But looking at the BMEP curve, is engine is right at
it's power limits there? Looking at where the bmep limit
for that rpm, there's not a whole lot of HP left for passing
or dealing with a headwind.
Am I reading this right?
This is, of course, assuming that it takes 50HP to move the
car along at that speed and disregarding the differing driveline losses due to different engine speeds.
If considering driveline/engine losses, assuming a constant 'x'hp loss per revolution, then 2750 rpm is 2750x, and 1750 rpm is 1750x,for a difference of ~36%, meaning
that the engine power requirements may drop an assumed
10hp, so the car would require 40hp to drive it, leaving a
bmep of 199, or a difference of 11.5% in the bmep, so 11.5%
increase in economy, and another 10+ hp left for passing?
Am I on the ball here, or out to lunch?
-Dave
and I've been thinking.
Looking at the BMEP with HP values:
If I'm currently at 2750 rpm at ~100 kph on the highway
(I have a totally different tranny mounted compared to
the regular TDI), so let's assume it's taking 50hp to drive
the car that speed.
If I were to install different gearing that would take me
down to 2250 rpm for the same speed, assuming the same 50hp
to drive the car along, my BMEP values change from 225 to
205, following the 50hp line. That's a difference of
~9.75%. So my fuel economy should increase ~9.75%?
Following further, dropping the rpm to 1750 would give me a
bmep of 197, for a difference of 13.6%, so my economy should
increase 13.6%.
But looking at the BMEP curve, is engine is right at
it's power limits there? Looking at where the bmep limit
for that rpm, there's not a whole lot of HP left for passing
or dealing with a headwind.
Am I reading this right?
This is, of course, assuming that it takes 50HP to move the
car along at that speed and disregarding the differing driveline losses due to different engine speeds.
If considering driveline/engine losses, assuming a constant 'x'hp loss per revolution, then 2750 rpm is 2750x, and 1750 rpm is 1750x,for a difference of ~36%, meaning
that the engine power requirements may drop an assumed
10hp, so the car would require 40hp to drive it, leaving a
bmep of 199, or a difference of 11.5% in the bmep, so 11.5%
increase in economy, and another 10+ hp left for passing?
Am I on the ball here, or out to lunch?
-Dave