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bgd73 05-14-2009 10:23 PM

natural aspirated manifold pressure
Here is a topic that no doubt belongs here...
My background has nothing to do with this type of science, but common sense follows me like a dog...

I came across this vid got me thinking.

I am a fan of the carburator, being unformally educated, I cannot explain myself. the above vid is part of the answer, and I further pursued the subject of manifold pressure at wikipedia.

Manifold vacuum - Wikipedia, the free encyclopedia

I disagree with 90% of wikipedia, I hope at least someone reads it and finds a similar problem I had with thier discussion.

I have an old subaru. In much disbelief, the 2300 pound car (more now with steel and weld) has a swap of 13 inch steel wheels to 15 inch alloys, t-rated tires and they are gnarly rally directional tread (nokian) at 160 dollars a piece. A racing team would use them in gravel.They make noise every trip, I swap them for nothing. :|

The car gets 40mpg on a bad day, I have climbed it to 60....
while in the fast lane of the highway all the way from central maine to southern mass. 10 gears in the dual range transmission, 1781cc and 280cfm carb. It is no miracle to me.

I have concluded it to be a super atomizer with manifold pressure exactly correct for the given displacement utilizing the carbs vebturi with zero loss...and even its own engine cooler.
It adds to the defense: Turbo charged engines are by no means an economical way to get a small engine to be big.
There is one problem with "super stoichers" has to be a boxer to sustain success. All inlines and v engines have a fault of uneven thermal dissipation for all the cylinders . A boxer is even all the way around. It adds to my belief in a real car is extincted.

Electric Frenzy 05-14-2009 10:43 PM

I want some of what you're smoking.

Bicycle Bob 05-15-2009 12:52 AM

The Wikipedia entry looks fine to me.

trebuchet03 05-15-2009 03:26 PM

I disagree with any statement that says "zero loss" - I'm also really confused as to what you're trying to say. It seemed to culminate into turbo=bad (to which I say thermodynamics can disagree).

As for the video.... I'm not following why it's pertinent to an intake manifold. Yes, this machine and an IM have vacuum - but on a significantly different scale.

An intake manifold at 25 inHg = 635 torr
That machine can pull down to 1 torr

PV=nrT prevails :p

theunchosen 05-18-2009 08:22 AM

yeah I didn't get where that was going either. yeah they are talking in the range of about 4-5 absolute PSI when it boils.

Your IM won't stay at those PSI (in the vid) for very long without choking up.

Edited Wiki page. Indicated that some modern cars didn't have throttles. Diesels are the only production vehicles that have no throttling valve. It also mentioned atkinson had no manifold pressure and used variable valve timing instead of a throttle to achieve this. I've never heard of an engine using variable valve timing to replace a throttle. . .? Diesels replace it by injecting only the fuel they need. Prius and other hybrids definitely still have throttle bodies and manifold pressure(all engines have it unless you have an open intake header in which you can see the injector squirting fuel onto the valve and hoping it gets sucked in.)

some_other_dave 05-18-2009 01:36 PM

There are a couple of gasoline-powered cars that do not have traditional throttle plates. There was a BMW M3 engine, I think, that did not. They actually mucked with when or how much the valves opened to draw different amounts of air into the combustion chamber. That gave them less intake restriction than a car with a throttle plate, because the only restriction was the manifold pipes and the valves themselves.

I am guessing tha they didn't stick with it because it was too complex to be cost-effective, and possibly also due to the difficulty of precisely measuring how much air was going into the motor.

Anyway, turbocharging is only bad if you don't need the power the turbo provides. If you need it, then turbocharging is good.


theunchosen 05-18-2009 06:24 PM

I encountered that engine but I am pretty sure they dropped it as you said because they could not get the air flow exact and it burned lean-rich each stroke. In which case it might fail smog inspections depending on how good your valve system is today.

Other than that and a few people tossing ideas out there so far throttleless engines aren't peing pitched.

Turbos are useful. They allow a much smaller engine to produce more power. It drastically increases the engines power to weight which matters in very small vehicles. That said the avg production car weighs well over 1 ton so saving a hundred pounds on engine isn't going to heavily impact FE. They do it for performance reasons, because its cheaper to build a block with x horsepower and charge it to y than build one that can produce y NA.

some_other_dave 05-21-2009 04:21 PM

Also, if you want to market a car that has 200 HP, you can either build a moderate-sized engine that makes 200 HP, or you can build a much smaller engine that only makes that power under boost. Guess which one typically uses less fuel to cruise down the freeway?


theunchosen 05-21-2009 05:03 PM

Yeah SoD,

If the driver is just cruising at low HP he might stay out of boost completely and has a much much smaller displacement(almost always means higher FE) sicne he doesn't need that extra hp at that moment.

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