OT why ar hemis powerful?

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Willson wrote in part:

How are the above all true if turbocharging is, as explained well by Nate, a kind of supercharging?
- Don Klipstein ( snipped-for-privacy@misty.com)
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snipped-for-privacy@manx.misty.com (Don Klipstein) wrote in
From http://en.wikipedia.org/wiki/Turbocharging;
The objective of a turbocharger is the same as a supercharger; to improve upon the size-to-output efficiency of an engine by solving one of its cardinal limitations. A naturally aspirated automobile engine uses only the downward stroke of a piston to create an area of low pressure in order to draw air into the cylinder through the intake valves. Because the pressure in the atmosphere is no more than 1 bar (approximately 14.7 psi), there ultimately will be a limit to the pressure difference across the intake valves and thus the amount of airflow entering the combustion chamber. This ability to fill the cylinder with air is its volumetric efficiency. Because the turbocharger increases the pressure at the point where air is entering the cylinder, a greater mass of air (oxygen) will be forced in as the inlet manifold pressure increases. The additional oxygen makes it possible to add more fuel, increasing the power and torque output of the engine.
the "efficiency" gained is VOLUMETRIC efficiency. "size-to-output efficiency". meaning the motor acts like it is of much larger displacement.
turbocharging uses the wasted energy of the hot exhaust to compress the intake charge,while supercharging is a parasitic drag all the time.
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Jim Yanik
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snipped-for-privacy@manx.misty.com (Don Klipstein) wrote in

I think we agree,then.
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Jim Yanik
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A turbocharger is also parasitic. It raises the exhaust pressure, thereby raising the pressure inside each cylinder during the exhaust stroke. Ain't nuthin' free...
There are two advantages to a turbocharger that I can think of. Fewer moving parts means less friction (basically one). It can run at higher speeds that make for more efficient compression.
If any given engine is more fuel efficient with the turbo engaged, it most likely is only because that engine was designed to run with a turbo and runs inefficiently without it.
But I could be full of it too.
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Larry Fishel wrote:

One slight advantage to the turbo is that you can run a much less restrictive exhaust because exhaust passed through a turbo makes less noise than exhaust coming straight out of a set of headers. Even straight pipes or just glasspacks can make a turbo engine quiet enough to avoid notice from the cops unless you really get on the loud pedal.
nate
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Who said Nate is right?
EJ in NJ
Don Klipstein wrote:

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(Amazon.com product link shortened)
I still have my copy, although it's undoubtedly outdated by now... (hard to sell textbooks for electives that only a handful of people take every year...)
Actually anyone reading this thread that really wants to understand this stuff and can bear to wade through lots of really engineer-y dryness might want to see if you can get your hands on a copy
nate
Ernie Willson wrote:

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GAS ENGINE HEMIS IN THE 60'S-70'S COULD HAVE 12:1 COMPRESSION RATIOS. TODAYS GAS CAR/ TRUCKS WITH HEMISPHERICAL COMBUSTION CHAMBERS ONLY RUN ABOUT 8:1 COMPRESSION RATION.. IT TAKES RACING FUEL TO BURN IN A 12:1 ENGINE OR THE ENGINE WONT HARDLEY RUN..
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On Mon, 26 Jan 2009 08:32:46 -0500, against all advice, something compelled snipped-for-privacy@webtv.net, to say:

My Audi runs at 11.25:1 on 92 pump gas.
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wrote:

Without any knocking ever? I too thought that would be hard. Is there a trick?

I agree with that. Just look at the girl who does 30,000 a month (or whateer the number was) and the one behind her who did 20,000. Teen-age girls. Who wants to be one of those. (Other than Norminn maybe, but we were talking aobut "real men".
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Sounds fairly reasonable to me. My 1995 Oldsmobile Delta 88 LSS has the "3800 series II" engine. My owner's manual says the compresiion ratio is 9.4:1 and specifies 87 octane gasoline.
- Don Klipstein ( snipped-for-privacy@misty.com)
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snipped-for-privacy@manx.misty.com (Don Klipstein) wrote in

often,the spark advance is retarded to prevent knocking. many modern engines have knock sensors that allow the engine electronics to alter spark timing to stop knock,and permit operation of lower octane fuels without damaging the engine,although at reduced power.
Today's Chrysler Hemi's are no comparison to the Hemi's that are so reknown.Today's hemi's have low compression ratios so owners do not have to spend more on high octane premium.
Racing fuel is 105 octane.
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Don Klipstein wrote:

Aluminum heads, along with thoughtful chamber design, help a lot. I'm guessing that said 3800 has aluminum heads. I forget the CR on my 944, but I think it's over 10:1, and it runs happily on pump premium. That engine is *all* aluminum, however...
nate
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wrote:

Thanks, you guys.
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Hemi engines can incorporate larger valves than other combustion chamber shapes. This means that they breathe better and have a higher volumetric efficiency (a measure of how much air (and hence fuel) the engine can swallow).
Because of the hemi shape the distance from the spark plug tip to the furthermost point in the combustion chamber is shorter in a hemi, than in any other chamber shape of the same volume. What this means practically is the hemi engine can carry more power without knocking than other combustion chamber shape.
HTH,
EJ in NJ
mm wrote:

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