Re: 5 bangers galore

[bbell@csn.org (Bruce Bell)]

>from Carl DeSousa
>I think it was brilliant of Audi to use another cylinder to balance
>the motor instead of adding a balance shaft. 

Is this true? Don't I remember something from the 70's about the 5 cyl being
more dificult to balance from a design point of view? You know fireing order
crank design etc. Remember in a 4cyl. you get two bangs each revolution. A
5cyl alternates between 2 and 3 firings each revolution. So just think half
the time were driving a 4cyl and the other have weve got a 6cyl. 

General rule: The more cylinders, the smoother the motor. 

Audi did not use a "Balance shaft in its 4 cyl motors. Audi used an
"intermediate shaft which supplied power to the distributor and oil pump.
Porsche *did* use a "balance" shaft in the 944. Porsche, licensed this
patented technology from one of the Japanese automakers (Mitsubisi?) Anyone
remember the details? counter-rotating I think.

>That 5th cylinder "out torques" all 4
>cylinder using balanced shafts.   Part of the reason why the S4 can
>obtain max torque at 1900 RPM?  Now horse power is another matter.
>With only 2.2L the 4 valves/cyl and turbo take care of that.  

Keep in mind that 5 cyl. engines have a 20% increase in displacement over a
comparable 4 cyl. e.g. compare the 85-86 Coupe GT with the 85 VW Jetta GL
engine. Both were 0.445 liters per cyl. had 8.5:CR used the same valve and
head configuration. Both made about 22hp and 24.4lbs of torque from each
cyl. (minor differences due to exhaust setups, etc. I think the 944 porsche
makes more torque than the audi 5 cyl engines, but it has more displacement
too. (i'm talking normally aspired here since turbocharging is (almost) like
having a variable displacement engine.

The reason the S4 engine gets maximum torque so low in the RPM range has
nothing to do with 4/5cyl design. Audi uses a *small* turbin that spins up
faster, a high (for turbos) compression ratio (exhaust going to turbo has
more energy) and a cam profile designed to move the torque curve to the low
RPM range (reduced or no overlap).


Bruce

bbell@csn.org (Bruce Bell)