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Moderator, l337 M0d3r4t0r
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Discussion Starter #1
Can anyone explain to me why if I have a more agressive cam (have a 220 duration .370 lift) cam and a 218 duration .347 lift cam. Why do I get higher compression on the other cam?
 

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Toyota Powertrain Guru
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more duration = more overlap and therefore reduced low speed cylinder filling. At higher speeds the added duration increases cylinder filling ability as time for the intake stroke gets faster (high RPM). Shorter time compensated for by longer opening time.
 

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Also known as the miller or atkinson cycle. Leaving the intake open when you start to compress reduces pumping losses of the engine having to suck air and compress it (compressing air saps power from the crank that is later re-gained in the expansion). When you have really good high RPM breathing, the reduced pumping losses are not really a factor and the duration is more a matter of evacuating the cylinder, however, you definately lose some low-end torque because you're not compressing the full volume.

Essentially, what he ^ said plus some jargon.

~Tim
 

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Moderator, l337 M0d3r4t0r
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Discussion Starter #4
sorry what i mean was why is my smaller cam having lower compression then my larger cam!

I would have though it was oppsite.
 

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wastegate hose is pulled
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ZaZZn said:
sorry what i mean was why is my smaller cam having lower compression then my larger cam!

I would have though it was oppsite.
Maybe the valve lash was too tight before or something, maybe the cams are degreed differently too. Compression test results can also change based on a host of variables like cranking rpm etc... How much of a difference are we talking about?
 

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Moderator, l337 M0d3r4t0r
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Discussion Starter #6
we are only talking a little diffrence in lift and a few degrees less like 5-6 on the intake.
 

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Carma Performance
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765 Posts
No one has brought up LSA or lobe seperation angle. It is possible that your lager cam could have a wider lobe seperation which in turn would yeild higher cranking compression numbers due to less overlap. Did your new cams come with a cam card?

Erin
 
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