cams and emissions
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cams and emissions What characteristics of a cam lead to producing more or less emissions? Is overlap the main factor? I'm sure head efficiency has a part to play. Can someone just give me a run down of how duration, lift, LSA, advance/retard and overlap effect emissions. Thank you!
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The easy answer is all of the parameters will affect emissions.
Breaking it down in point form:
IVC with SCR and VE% will determine the effective compression at any RPM.
As the engine moves away from the torque peak (above, or below), combustion
become less efficient due to lack required heat and comrpession.
An early opening EVO can increase CO and HC as the mixture has less time
to combust.
EVC and IVO (Overlap) can act for and against emissions depending on the
RPM and intake/exhaust tuning. If exhaust gas is allowed to revert, it can
act like EGR and displace incoming charge. If the intake charge is allowed
to escape into the exhaust before EVC, then an increase in HC will occur.
Breaking it down in point form:
IVC with SCR and VE% will determine the effective compression at any RPM.
As the engine moves away from the torque peak (above, or below), combustion
become less efficient due to lack required heat and comrpession.
An early opening EVO can increase CO and HC as the mixture has less time
to combust.
EVC and IVO (Overlap) can act for and against emissions depending on the
RPM and intake/exhaust tuning. If exhaust gas is allowed to revert, it can
act like EGR and displace incoming charge. If the intake charge is allowed
to escape into the exhaust before EVC, then an increase in HC will occur.
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Don't go running to the bank with this information; there's too much to
consider besides the valve timing.
I'll throw out some cam specs as a baseline we'll see what sort of debate
comes of it. Keep in mind, the same cam used in a motor with slightly
different exhaust and intake components, heads, rocker ratio, CID, etc.
will differ.
I would say 240 degrees of duration @ 0.050" with about 50 degrees of
overlap will be the breaking point of getting an emissions pass without fighting
with the tuning.
consider besides the valve timing.
I'll throw out some cam specs as a baseline we'll see what sort of debate
comes of it. Keep in mind, the same cam used in a motor with slightly
different exhaust and intake components, heads, rocker ratio, CID, etc.
will differ.
I would say 240 degrees of duration @ 0.050" with about 50 degrees of
overlap will be the breaking point of getting an emissions pass without fighting
with the tuning.
Originally Posted by Adrenaline_Z
I would say 240 degrees of duration @ 0.050" with about 50 degrees of
overlap will be the breaking point of getting an emissions pass without fighting
with the tuning.
overlap will be the breaking point of getting an emissions pass without fighting
with the tuning.
I've seen 224/224 with -5 overlap in other discussions, which is about 114* lsa, and your numbers would be a major departure.
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Originally Posted by hammertime
I'm not saying you are wrong, but the math implies that is a cam something like 240/240 and a 95* lsa, unless there was a typo. Sounds like the MS4 at 239/242 111* LSA (18.5* overlap) should pass fairly easily. Can you explain?
I've seen 224/224 with -5 overlap in other discussions, which is about 114* lsa, and your numbers would be a major departure.
I've seen 224/224 with -5 overlap in other discussions, which is about 114* lsa, and your numbers would be a major departure.
Yeah, you're right. I crossed my 0.006" timing with 0.050" timing for overlap.
That would have put me at about 292 degrees @ 0.006" on a 120 LCA...or something
close to that.
Good thing somebody is awake.
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As a mechanic that has done emission repairs for years I can say this. If it has a choppy idle that is a sign that your going to have problems. The idle is rough because of incomplete combustion which will peg the hydrocarbons. Back when Ohio had the IM240 test my car barely squeaked by with a 214-224 cam (with cats) and it idles pretty smooth.
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From: Lexington, South Carolina
Originally Posted by 2001WS6Vert
As a mechanic that has done emission repairs for years I can say this. If it has a choppy idle that is a sign that your going to have problems. The idle is rough because of incomplete combustion which will peg the hydrocarbons. Back when Ohio had the IM240 test my car barely squeaked by with a 214-224 cam (with cats) and it idles pretty smooth.
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That choppy idle is mostly due to the overlap at low rpms. Exhaust gas takes the place of air. Not as complete combustion, but the trapped exhaust would be getting burned twice. Wouldn't a good tune be able to adjust air/fuel to help out?
Originally Posted by eLTwerker
That choppy idle is mostly due to the overlap at low rpms. Exhaust gas takes the place of air. Not as complete combustion, but the trapped exhaust would be getting burned twice. Wouldn't a good tune be able to adjust air/fuel to help out?
Hammer
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What I'm wondering is, do any more unburnt HCs escape during overlap then would be pushed out of the exhaust valve if no overlap were to occur? Wouldn't at least SOME of the HCs be reverted back into the cylinder and be reburnt as compared to ALL of the HC being expelled if there was no overlap? (Not particularly good for low end power, though.)
Originally Posted by eLTwerker
What I'm wondering is, do any more unburnt HCs escape during overlap then would be pushed out of the exhaust valve if no overlap were to occur? Wouldn't at least SOME of the HCs be reverted back into the cylinder and be reburnt as compared to ALL of the HC being expelled if there was no overlap? (Not particularly good for low end power, though.)
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Back when Ohio had the IM240 test my car barely squeaked by with a 214-224 cam (with cats) and it idles pretty smooth.
tuning, compression and more can play a factor in the emissions.
It's also the events at the valve that matter, not necessarily the cam specs
alone. I would say, if that was a cam only modifcation, the LSA would have
to be sub 112 degrees to produce enough overlap to be an issue. 214-224 @
0.050" is a baby cam in the big picture.
But isnt choppy idle just a result of overlap, so wouldnt any cam with overlap have problems?
attributed to the choppy ilde. Every motor requires some amount of overlap
to be effective. Whether the cam specs state a negative value at 0.050", there
is still overlap occuring at the opening and closing points of EVC and IVO
through rocker ratio.
Also note from above, it's the timing at the valve that makes the difference.
Not as complete combustion, but the trapped exhaust would be getting burned twice. Wouldn't a good tune be able to adjust air/fuel to help out?
It's "inert" and therefore acts as a "cooling agent" in the chamber, and will
also displace clean incoming charge.
Tuning in/out fuel may help reduce some of the emissions, but it's a band-aid
fix.
It's very difficult to get the best of both worlds with static valve timing.
What makes power up top, hinders clean emissions and torque in the lower
register.
There's absolutely no gain with having a negative/zero overlap at the valve, especially in a naturally aspirated engine.
This actually prevents the exhaust gas from leaving, as well as getting
"scavenged out" within a tuned RPM range.
I have yet to see a performance, 2 valve/cyl pushrod engine that has
zero overlap at the valve.
Last edited by Adrenaline_Z; Mar 7, 2007 at 08:30 PM.
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Originally Posted by eLTwerker
Good question. If exhaust gases can't burn again in the cylinder, what is the point of EGR?
