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Would anyone mind explaining a little about this mod to the engine illiterate?
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caveat: i'm no expert on cams/valve timing - learning as i go. please all, feel free to jump to this thread with corrections. last thing we need is more psuedoscience presented as "fact" for all cars in all situations...
1) start with the assumption that valve lift & duration is a compromise between low and high rpm performance. the manufacturer tries to find a sweet spot somewhere in the middle which satisfies everyone.
2) one way around this compromise is to offer variable valve timing, with both low rpm efficiency and high rpm power - e.g. vtec, vvti.
3) the economy cam this guy makes apparently shifts the powerband lower - it sacrifices high end performance for low rpm performance
4) the cam is machined for lower lift and shorter duration. the result is slightly higher compression, because normally, the intake valve is not fully closed as the piston begins its compression stroke.
evidence of #4 comes from a compression test done on a 1.0 motor before & after the cam swap...
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A hot compression test on the engine before the MPG cam:
#1 - 182, #2 - 178 and #3- 180 lbs.
The same test after the new cam was installed: The hot compression test was:
#1 - 195, #2 - 185 and #3 - 190 lbs.
The compression increase is probably because of shorter duration which should help the low and mid-range power. (source)
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the idea is similar to adding a turbo. more power means you can shift up at a lower RPM and improve MPG through reducing your average RPMs (lowering internal friction). the tester's anecdotal support for this is that he can do 2nd gear "roll-outs" from stop signs much easier with the MPG cam than with the stock cam.
the million dollar claim:
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in 5 tanks of fuel, the economy cam alone showed an improvement of 8%-9% in fuel mileage. (source)
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however tank-to-tank testing is fallable, so you have to take that claim with a very large grain of salt. i personally wouldn't accept it as "proof" of the cam's effectiveness.
e.g. if he did the cam swap in the spring, his next 5 tanks would have been affected by increasing ambient temperatures - fuel economy at 70F can easily be 8% better than fuel economy at 50F, all else being equal.
and guess what - i just checked the dates on the postings i quoted above and the swap reported in march 05 and the "results" were posted in may 05.
so i'll reserve judgment on this until i hear from the SLC metro owner who is scangauge equipped and who can measure results more accurately.
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