Y2K2LS1

What characteristics of a cam are needed to bleed off cylinder pressure? Are you compromising power for longevity of the motor? How important is this?

NoGo

iTrader: (2)

Bleeding of cylinder pressure is associated with an overlap between the exhaust and intake durations of the cam.

Performance cams try to get the intake valve open as early as possible and hold it open as long as possible. This, naturally, will admit the most amount of air. Unfortunately, extending the intake duration will cause some interference with the exhaust duration. Extending the intake duration into the exhaust duration causes for excessive blow-by (bleeding the cylinder pressure).

During the scavenging process (getting rid of the exhaust gas) both the intake and exhaust valves are open at the same time. With the intake and exhaust open at the same time, the fresh incoming air can escape into the exhaust. This is referred to bleeding off the cylinder pressure, or blow by.

This is a necessary evil of high lift, high duration cams.

Blow-by, and exhaust gas reversion (another side effect of high lift, high duration) are most obvious at low engine RPM. This is due to the very low air velocities associated with the cylinder. This is why cammed cars rock and sputter at low engine rpm, but run like a beast once you climb on the throttle.

Blow-by is most important when considering forced induction, and usually isn't refered to as "bleeding the cylinder", outside of the FI circle.
With forced induction, a long overlap between the intake and exhaust duration is going to allow an excessive amount of the charge air (pressurized inlet air) to slip right out the exhaust.
This will lower your charge pressure from excessive flow, lowering your boost, as well as increase your BSFC (brake specific fuel consumption) meaning less fuel economy, and larger injectors to do the same work.

Good Luck,
Kevin

LS1derfull

Great explanation, the term is blow down, not blow by. It should also be mentioned that on N/A car with high compression, using cam specs to bleed cylinder pressure, is only effective below peak torque, because cam will trap more pressure at peak and above. This can still cause detonation there, just a heads up. <img border="0" title="" alt="[Wink]" src="gr_images/icons/wink.gif" />

Y2K2LS1

wow some great info!

Can anyone give some examples, like for NA XXXXXX cam and if you wanted to bleed some pressure off for FI or N20 something more like XXXXXX cam will do the trick.

Maybe some guys who have picked a cam for FI/n20/high comp and considered "blow down" could chime in and tell us about their situation?


<img border="0" title="" alt="[Smile]" src="gr_stretch.gif" />

racer7088

NOGO,

Bleeding off of cylinder pressure is not accomplished whatsoever at overlap. Overlap just helps scavenge the chamber of exhaust and jump starts the intake port and tract into motion. It actually increases cylinder pressure when done right always as well as cleaning the chamber of exhaust.

A later intake closing will drop your cylinder pressure at lower rpms because air will reverses direction when it has little velocity and goes back up and out the intake. This is different from intake reversion during the overlap phase which is very bad where exhaust actually backs up into the intake. If you spread the lobe seperation, increase the intake duration or retard the cam all will result in later intake closing and reduced lower rpm cylinder pressure. This will also increase higher rpm cylinder filling and pressure and also allow a little more compression before incurring lower rpm detonation than with an earlier intake closing point.

Conversely with a tighter LSA or smaller intake duration or more advanced cam position you will gain lower rpm cylinder pressure from the earlier intake closing point and the resulting higher effective cylinder pressure you will now have and you also can't run as much static compression before detonating again than you could with the bigger, wider or more retarded cam with the later intake closing point!

Orange Krush

WOW ! <img border="0" title="" alt="[Eek!]" src="gr_eek2.gif" />

I love this place ! <img border="0" title="" alt="[Wink]" src="gr_images/icons/wink.gif" />

There is always something new to learn at this place ! <img border="0" alt="[worship]" title="" src="graemlins/gr_hail.gif" />

x-pipe

</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by NoGo:
<strong> Bleeding of cylinder pressure is associated with an overlap between the exhaust and intake durations of the cam. No it's not.

Performance cams try to get the intake valve open as early as possible and hold it open as long as possible. This, naturally, will admit the most amount of air. Unfortunately, extending the intake duration will cause some interference with the exhaust duration. Extending the intake duration into the exhaust duration causes for excessive blow-by (bleeding the cylinder pressure). No offense, ,but this is completely incorrect.
During the scavenging process (getting rid of the exhaust gas)(not right either) both the intake and exhaust valves are open at the same time. With the intake and exhaust open at the same time, the fresh incoming air can escape into the exhaust. This is referred to bleeding off the cylinder pressure, or blow by. Blowby is pressure escaping into the crancase from the combustion chambers!

This is a necessary evil of high lift, high duration cams.
Blow-by, and exhaust gas reversion (another side effect of high lift, high duration) are most obvious at low engine RPM. This is due to the very low air velocities associated with the cylinder. This is why cammed cars rock and sputter at low engine rpm, but run like a beast once you climb on the throttle.

Blow-by is most important when considering forced induction, and usually isn't refered to as "bleeding the cylinder", outside of the FI circle.
With forced induction, a long overlap between the intake and exhaust duration is going to allow an excessive amount of the charge air (pressurized inlet air) to slip right out the exhaust.
This will lower your charge pressure from excessive flow, lowering your boost, as well as increase your BSFC (brake specific fuel consumption) meaning less fuel economy, and larger injectors to do the same work. You should come to school here and learn this stuff.

Good Luck,
Kevin </strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">

Z-Rated

</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by Orange Krush:
<strong> WOW ! <img border="0" title="" alt="[Eek!]" src="gr_eek2.gif" />

I love this place ! <img border="0" title="" alt="[Wink]" src="gr_images/icons/wink.gif" />

There is always something new to learn at this place ! <img border="0" alt="[worship]" title="" src="graemlins/gr_hail.gif" /> </strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">No joke!! I love topics like this w/ GREAT explanations

Drew