View Single Post
Old 11-28-2007, 01:43 PM   #9 (permalink)
Batman Junior
MetroMPG's Avatar
Join Date: Nov 2007
Location: 1000 Islands, Ontario, Canada
Posts: 22,472

Blackfly - '98 Geo Metro
Team Metro
Last 3: 70.09 mpg (US)

MPGiata - '90 Mazda Miata
90 day: 55.97 mpg (US)

Even Fancier Metro - '14 Mitsubishi Mirage top spec
90 day: 70.75 mpg (US)

Appliance car - '14 Mitsubishi Mirage ES (base)
90 day: 72.89 mpg (US)
Thanks: 4,017
Thanked 6,904 Times in 3,581 Posts
How, exactly does changing cam lift & duration improve low end torque?

I'll be the first to admit I'm no a cam expert. Everything I've learned, I learned online... recently. Such as:
  • Lower lift & duration, reducing overlap, and advancing cam timing for more low end torque has apparently been understood for a long time.
  • Here's a description of an "economy cam" (designed like the XFi's to shift the torque peak to a lower RPM) in a special "efficient" 1960's era Olds 4-4-2 engine. It has the same effect: lower HP, shifted torque peak (so you can operate the engine more usefully at lower RPM), better fuel economy:
...the camshaft is the key. Intake duration is reduced from 286 to 250', and from 286 to 264' on exhaust. Overlap is reduced from 58 to 36', and lift is reduced from 0.472 in. on the 4-4-2 camshaft to 0.435 on the special ... camshaft.

The vital point of maximum torque is dropped from 3600 to 2600 rpm. - source

I can't defend the decisions of the engineers who made the XFi cam. But the proof is in the pudding - the XFi gets better fuel economy. And I suspect the laws of physics were respected.
Project MPGiata! Mods for getting 50+ MPG from a 1990 Miata
Honda mods: Ecomodding my $800 Honda Fit 5-speed beater
Mitsu mods: 70 MPG in my ecomodded, dirt cheap, 3-cylinder Mirage.
Ecodriving test: Manual vs. automatic transmission MPG showdown

has launched a forum for the efficient new Mitsubishi Mirage - fuel efficiency info for Geo Metro owners - electric car conversion on a beer budget
  Reply With Quote