Honda Car Forum

"JXStern" <JXSternChangeX2R@gte.net> wrote in message
news:emv7j0lugov97bth8hos898g9kuu8s209c@4ax.com...
>
> Mine sort of rolls, actually.

True <g>.

I think what you are getting at is this: The more air & fuel you can cram
into the cylinder, the stronger the combustion. However, due to the laws of
physics, there is a limit as to how much air and fuel you can cram in there
until the compression ratio gets too high and the fuel spontaneously
combusts on the compression stroke before the spark plug fires. If you want
more power, you'll have to increase displacement by increasing the bore
inside the cylinders, or just get a bigger engine all together.

Jonathan

"GusTheCat" <GusTheCat.1brooq@realcaraudio.com> wrote in message
news:GusTheCat.1brooq@realcaraudio.com...
>
> Chris Aseltine wrote:
>>
>> LOL, Dude, your car does not run on air, your car run on gas.
>> However, with more air you can burn more gas...
>
>
> --
> GusTheCat
> ------------------------------------------------------------------------
> Posted via RealCarAudio.com - The checkmate of the caraudio community.
> http://www.RealCarAudio.com
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> http://www.realcaraudio.com/forums/m...fo&userid=3684
> View this thread:
> http://www.realcaraudio.com/forums/s...threadid=44098
>

K-town wrote:
>
> I think what you are getting at is this: The more air & fuel you can cram
> into the cylinder, the stronger the combustion. However, due to the laws of
> physics, there is a limit as to how much air and fuel you can cram in there
> until the compression ratio gets too high <snip>

You probably mean "until the pressure gets too high", Forcing more air
into the engine does not change the compression ratio.

Randolph <trash@junkmail.com> wrote in
news:4134C5B2.3C9F8D72@junkmail.com:

>
> K-town wrote:
>>
>> I think what you are getting at is this: The more air & fuel you can
>> cram into the cylinder, the stronger the combustion. However, due to
>> the laws of physics, there is a limit as to how much air and fuel you
>> can cram in there until the compression ratio gets too high <snip>
>
> You probably mean "until the pressure gets too high", Forcing more air
> into the engine does not change the compression ratio.
>

Sure it does.Compression ratio is the ratio of the volume of the cylinder
at BDC(bottom dead center) to volume at TDC. When you FORCE more air/fuel
mix into the cylinder,you effectively enlarge the volume at BDC(it acts
like a more voluminous cylinder),compressing down to the unchanged original
volume at TDC,thus increasing the compression ratio.

That's why turbos and superchargers use higher octane gasoline,because of
the effective higher compression ratio.
--
Jim Yanik
jyanik-at-kua.net

Jim Yanik wrote:
>
> Randolph <trash@junkmail.com> wrote in
> news:4134C5B2.3C9F8D72@junkmail.com:
>
> Sure it does.Compression ratio is the ratio of the volume of the cylinder
> at BDC(bottom dead center) to volume at TDC. When you FORCE more air/fuel
> mix into the cylinder,you effectively enlarge the volume at BDC(it acts
> like a more voluminous cylinder),compressing down to the unchanged original
> volume at TDC,thus increasing the compression ratio.
>
> That's why turbos and superchargers use higher octane gasoline,because of
> the effective higher compression ratio.

Nope, the compression ratio is, as you said, the ratio of the volume of
the cylinder
at BDC(bottom dead center) to volume at TDC. Absolute pressure, mass of
air, etc. makes no difference. If I keep my foot off the throttle, I
draw a moderate vacuum in the cylinder on the intake stroke, and get a
relatively low pressure on the compression stroke, but the compression
ratio is the same no matter the position of the throttle. The
compression ratio is purely a function of the engine geometry. When
turbo charged engine typically require premium fuel, it is because the
*pressure* is high, not because the compression ratio is high.

Randolph,

After reading your post and thinking a little, I am inclined to agree
with your logic. However, based on that logic, HOW would someone increase
the actual compression ratio, since the volume of air/fuel inside the
cylinder does not change the ratio? Enquiring minds would like to know! ;-)

Jonathan

P.S. for Jim Yanik: The reason why I am inclined to agree with Randolph is
this: Say your compression ratio is 10:1, you put ten "units" of air/fuel
mixture in the cylinder. It will be compressed to 1/10 of it's original
"size" (volume) upon combustion. If you put in 20 "units", it will be
compressed to a 20:2 ratio, which reduces back to 10:1. 30:3 reduces to
10:1, and so forth. More "units" just causes a more powerful combustion,
creating more pressure to push the piston down more forcefully.

"Randolph" <trash@junkmail.com> wrote in message
news:4134EC5B.36FE2C33@junkmail.com...
>
> Jim Yanik wrote:
>>
>> Randolph <trash@junkmail.com> wrote in
>> news:4134C5B2.3C9F8D72@junkmail.com:
>>
>> Sure it does.Compression ratio is the ratio of the volume of the cylinder
>> at BDC(bottom dead center) to volume at TDC. When you FORCE more air/fuel
>> mix into the cylinder,you effectively enlarge the volume at BDC(it acts
>> like a more voluminous cylinder),compressing down to the unchanged
>> original
>> volume at TDC,thus increasing the compression ratio.
>>
>> That's why turbos and superchargers use higher octane gasoline,because of
>> the effective higher compression ratio.
>
> Nope, the compression ratio is, as you said, the ratio of the volume of
> the cylinder
> at BDC(bottom dead center) to volume at TDC. Absolute pressure, mass of
> air, etc. makes no difference. If I keep my foot off the throttle, I
> draw a moderate vacuum in the cylinder on the intake stroke, and get a
> relatively low pressure on the compression stroke, but the compression
> ratio is the same no matter the position of the throttle. The
> compression ratio is purely a function of the engine geometry. When
> turbo charged engine typically require premium fuel, it is because the
> *pressure* is high, not because the compression ratio is high.

"Randolph" <trash@junkmail.com> wrote in message
news:4134EC5B.36FE2C33@junkmail.com...
>
> Jim Yanik wrote:
>>
>> Randolph <trash@junkmail.com> wrote in
>> news:4134C5B2.3C9F8D72@junkmail.com:
>>
>> Sure it does.Compression ratio is the ratio of the volume of the cylinder
>> at BDC(bottom dead center) to volume at TDC. When you FORCE more air/fuel
>> mix into the cylinder,you effectively enlarge the volume at BDC(it acts
>> like a more voluminous cylinder),compressing down to the unchanged
>> original
>> volume at TDC,thus increasing the compression ratio.
>>
>> That's why turbos and superchargers use higher octane gasoline,because of
>> the effective higher compression ratio.
>
> Nope, the compression ratio is, as you said, the ratio of the volume of
> the cylinder
> at BDC(bottom dead center) to volume at TDC. Absolute pressure, mass of
> air, etc. makes no difference. If I keep my foot off the throttle, I
> draw a moderate vacuum in the cylinder on the intake stroke, and get a
> relatively low pressure on the compression stroke, but the compression
> ratio is the same no matter the position of the throttle. The
> compression ratio is purely a function of the engine geometry. When
> turbo charged engine typically require premium fuel, it is because the
> *pressure* is high, not because the compression ratio is high.

Or cams with lots of overlap....high BMEP.

K-town wrote:
>
> Randolph,
>
> After reading your post and thinking a little, I am inclined to agree
> with your logic. However, based on that logic, HOW would someone increase
> the actual compression ratio, since the volume of air/fuel inside the
> cylinder does not change the ratio? Enquiring minds would like to know! ;-)

To put some numbers on it, let's say you have a 1.6 liter, 4 cylinder
engine with a 9:1 compression ratio. Let's assume the bore to be about
80 mm and the stroke to be about 80 mm. That would give you a 0.4 l
displacement pr. cylinder. At bottom dead center, the total volume above
the piston would be 0.45 liter. At top dead center the volume above the
piston would be 0.05 liter. Now, let's say you grind down the bottom
side of the head 2 millimeters and stick it back on. This does not
change the displacement. It does decrease the volume above the cylinder
by about 0.01 liter, so at top dead center there would be 0.04 liter,
and at bottom dead center you would get 0.44 liter. Now the compression
ratio has increased to 0.44 / .0.4 = 11.

If you somehow could move the crankshaft up or down with respect to the
cylinders, you would also vary the compression ratio. This is just what
Saab did a few years back. See
http://www.edmunds.com/news/innovati...7/article.html

"K-town" <jdu52580@carolina.rr.com> wrote in
news:7n7Zc.3680$683.455808@twister.southeast.rr.co m:

> Randolph,
>
> After reading your post and thinking a little, I am inclined to
> agree
> with your logic. However, based on that logic, HOW would someone
> increase the actual compression ratio, since the volume of air/fuel
> inside the cylinder does not change the ratio? Enquiring minds would
> like to know! ;-)
>
> Jonathan
>
> P.S. for Jim Yanik: The reason why I am inclined to agree with
> Randolph is this: Say your compression ratio is 10:1, you put ten
> "units" of air/fuel mixture in the cylinder. It will be compressed to
> 1/10 of it's original "size" (volume) upon combustion. If you put in
> 20 "units", it will be compressed to a 20:2 ratio, which reduces back
> to 10:1. 30:3 reduces to 10:1, and so forth. More "units" just
> causes a more powerful combustion, creating more pressure to push the
> piston down more forcefully.
>
> "Randolph" <trash@junkmail.com> wrote in message
> news:4134EC5B.36FE2C33@junkmail.com...
>>
>> Jim Yanik wrote:
>>>
>>> Randolph <trash@junkmail.com> wrote in
>>> news:4134C5B2.3C9F8D72@junkmail.com:
>>>
>>> Sure it does.Compression ratio is the ratio of the volume of the
>>> cylinder at BDC(bottom dead center) to volume at TDC. When you FORCE
>>> more air/fuel mix into the cylinder,you effectively enlarge the
>>> volume at BDC(it acts like a more voluminous cylinder),compressing
>>> down to the unchanged original
>>> volume at TDC,thus increasing the compression ratio.
>>>
>>> That's why turbos and superchargers use higher octane
>>> gasoline,because of the effective higher compression ratio.
>>
>> Nope, the compression ratio is, as you said, the ratio of the volume
>> of the cylinder
>> at BDC(bottom dead center) to volume at TDC. Absolute pressure, mass
>> of air, etc. makes no difference. If I keep my foot off the throttle,
>> I draw a moderate vacuum in the cylinder on the intake stroke, and
>> get a relatively low pressure on the compression stroke, but the
>> compression ratio is the same no matter the position of the throttle.
>> The compression ratio is purely a function of the engine geometry.
>> When turbo charged engine typically require premium fuel, it is
>> because the *pressure* is high, not because the compression ratio is
>> high.
>
>
>
>

Intake compression by turbo or supercharger is just raising the comp ratio
by external means,a variable compression ratio.(that's what I meant by
"effective".)
Just milling the head(s) of a natually aspirated motor of the same
displacement gives more HP and torque.All that changes is the comp ratio.
Otherwise,why mill heads? All it would do is require a more-
expensive,higher octane fuel.
--
Jim Yanik
jyanik-at-kua.net

Jim Yanik wrote:
> Intake compression by turbo or supercharger is just raising the comp ratio
> by external means,a variable compression ratio.(that's what I meant by
> "effective".)

NO! Turbos do not increase the *compression ratio*, turbos increase the
pressure! Variable pressure, yes. Variable compression ratio, no. The
term "compression ratio" is well defined, and reaaly is not open for
interpretation.