eLTwerker

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!

Adrenaline_Z

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.

eLTwerker

So with good tuning and an efficient set up can almost any cam be emissions legal?

Adrenaline_Z

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.

Ferocity02

iTrader: (60)

IMO, overlap is the specification of a cam that relates the most to the level of emmisions. Overlap is affected by duration and lobe seperation angle.

hammertime

iTrader: (1)

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.

Adrenaline_Z

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.

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.

SScott236

But isnt choppy idle just a result of overlap, so wouldnt any cam with overlap have problems?

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?

thehammer69

iTrader: (2)

Not only does "Exhaust gas takes the place of air" in the cylinder at low rpms, but also fresh air charge with unburnt hydrocarbons escape out the exhaust valve. It's the escaped unburnt hydrocarbons that really kill you. You need to have a cam with close to negative six degrees of overlap at .050" lift to pass emmisions.

Hammer

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.)

hammertime

iTrader: (1)

Adrenaline_Z

There's more to this than duration at the lobe. Rocker ratio, pre-load/lash,
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.

Not just a result of overlap, but one of the major parameters that can be
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.

Exhaust gas that remains , or reverts back to the chamber can't burn again.
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.

Quick Double Nickel

iTrader: (5)

So would an active EGR system have increasingly negative affects on emissions output as the cam duration (overlap) increases?

eLTwerker

Good question. If exhaust gases can't burn again in the cylinder, what is the point of EGR?

RednGold86Z

That gas is relatively cool compared to peak combustion temperatures, and it has no O2, thus is considered inert. This inert and cool dilution will lower the peak temperature of the combustion, thus reduces NOx formation (the primary goal of EGR). The reduced power from EGR is offset by a larger throttle angle to get the desired power. This larger throttle angle and higher manifold pressure will reduce pumping losses and can result in increased (or at least balanced) economy, as long as the spark and fuel are optimized, and the dilution is not extreme.

eLTwerker

Thanks. Now I understand a little better.