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Old 03-31-2008, 03:30 PM   #7 (permalink)
LostCause's Avatar
Join Date: Feb 2008
Location: California
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Thunderbird - '96 Ford Thunderbird
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Originally Posted by MetroMPG View Post
That's a mixed up perception that I see often. High vacuum at cruise is definitely the good guy.

There's a difference between lowest fuel consumption and best engine efficiency. With a typical engine at light loads, you'll probably get your best MPG well outside of your engine's best efficiency (BSFC) area.
I think the goal is to never operate at light loads.

I like analogies, so here's one about a business.... During busy times (acceleration) a business operates efficiently by working its vast group of employees hard. During low times (cruise) it works efficienctly be cutting its workforce and working the remaining team hard...not by spreading the meager workload.

A small engine operating at its BSFC and sized perfectly for cruise will outperform an oversized engine running high vacuum.

The reason I stand behind this notion is that you can see the automotive industry heading that way...small "undersized" engines that use turbochargers, electric assist motors (aka hybrids), turbocompounding (scania diesels), ethanol injection (MIT research), or just plain neutering (Metro Xfi, CRX HF) to gain fuel economy.

Why does Honda use lean burn, why did BMW try to replace the throttle body with VVT, and what is the value of a WAI? I think vacuum is a pure loss that serves nothing more than an indicator of engine load. I'm willing to be convinced, though...

- LostCause
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