Camshaft tuning - Turbo and N/A
TLDR;
"Advancing the intake and retarding the exhaust increases overlap and should move the power up in the RPM range, usually at the sacrifice of bottom end power" - valve to valve more likely to happen.
"Retarding the intake and advancing the exhaust decreases overlap and should result in a wider power band at the sacrifice of some top end power." -valve to valve less likely, more low end
"usually it is the intake that is moved to change power characteristics since small changes here seem to have a greater effect."
Something else I'd like to add is that retarding the exhaust cam brings the exhaust valves closer to the pistons and advancing the intake also brings the intake valves closer to the pistons.
On four
stroke engines, it is important to realize that the cam rotates once for every
two rotations of the crankshaft.
Volumetric
efficiency is based on cylinder fill. If a 2.0L engine is filled with 2.0L of
an air/fuel mixture, we say its volumetric efficiency is 100%. If a 2.0L engine
fills with 3.0L of an air/fuel mixture, we say its volumetric efficiency is
150%. A forced induction engine will have a larger than 100% volumetric efficiency
since the intake charge and combustion chamber are being pressurized. A
naturally aspirated engine can also have a slightly larger than 100% volumetric
efficiency, but it will only happen for a short duration, and is usually only
in the peak of the powerband.
The art
of designing camshaft profiles is meant to increase the volumetric efficiency
in the RPM range that the customer requires. Camshafts don’t make magical
horsepower from nowhere, they simply move the powerband around by changing the
volumetric efficiency to attain the desired results.
The four
strokes of the engine are:
Exhaust
Intake
Compression
Combustion
**The
“start” is not important because it’s a CYCLE, meaning it repeats**
Looking
at a camshaft, the sequence would be as follows:
The
exhaust lobe pushes open the exhaust valve and the piston comes up to push the
exhaust out, then starts to close. The intake starts to open, just as the
exhaust is closing, piston goes down, and the intake valve closes. Then both
valves stay closed for the compression and combustion strokes. This means that
the first lobe to come through the rotation will be the exhaust lobe,
immediately followed by the intake lobe.
Overlap
is the point where the exhaust valve is closing, and the intake valve is just
opening.
To
increase overlap, you have to RETARD the EXHAUST, and/or ADVANCE the INTAKE.
To reduce
overlap, you have to ADVANCE the EXHAUST, and/or RETARD the INTAKE.
Simple
cam tuning rules for NATURALLY ASPIRATED engines:
Advancing
both cams => more low-RPM power, less high-RPM power
Retarding
both cams => more high-RPM power, less low-RPM power
Less
overlap => more low-RPM power, less high-RPM power
More
overlap => more high-RPM power, less low-RPM power
In a
naturally aspirated engine, the extra overlap is called "scavenging".
Scavenging is using the out-flowing exhaust to help draw in the next intake
charge (partially causing lumpy idle).
Simple
cam tuning rules for BOOSTED engines:
Advance
intake and exhaust => more low-RPM power, less high-RPM power
Retard
intake and exhaust => more high-RPM power, less low-RPM powerRetard intake
and exhaust => more high-RPM power, less low-RPM power
Less
overlap => lower EGTs, faster turbo spool, less fuel
More
overlap => higher EGTs, slower turbo spool, more fuel
Boosted
engines don’t like overlap. The incoming cold air and fuel cools down the
outgoing exhaust charge, condensing the exhaust gasses. This is VERY
counter-productive in a turbo application since the engine needs no help from
scavenging to fill the cylinder. I've heard this being called "turbo
chill".
Cool,
condensed gasses in the same space push less hard on the turbo, causing lag.
HOT gasses are better at spooling the turbo, thus the advanced exhaust timing
to open the valve sooner in the power stroke. This steals some of those hot,
expanding exhaust gasses to help spin the turbo a little faster. When the
piston is near the bottom of the bore, hardly any energy is going into rotating
the crank anyway, so stealing expanding gasses won’t hurt anything. The
retarded intake just helps cut down the overlap further.
Retarding
overall cam timing:
Retarding
overall cam timing is better for high-RPM power. This is because the valves are
closing later. The intake valve is closing AFTER the piston has started to
travel back up the bore for the start of compression stroke. This is terrible
at low RPM because the intake air velocity is low, and air that was once in the
cylinder is now being pushed back into the intake manifold and causing
turbulence.
At
high-RPM, the rules change. Air has weight, and thanks to Sir Issac Newton, we
know that once it is moving, it doesn’t want to stop moving. This means that
the air can continue to flow into and fill the cylinder, EVEN AFTER the piston
has begun to travel UP the cylinder bore. This can allow an engine to exceed
100% volumetric efficiency, if even by a small amount.
Advancing
overall cam timing:
Advancing
overall cam timing is better for low-RPM power. This is because the valves are
closing a little sooner. The intake valve is closing AT or NEAR when the piston
is at the bottom of the bore for the start of the compression stroke. This is
great at low RPM because the intake air velocity is low and easily affected by changes
in the direction of piston movement in the engine. Almost as soon as the piston
gets to the bottom of the bore on the intake stroke, the valve gets slammed
shut so no air can escape as the piston begins to travel back up the cylinder
on the compression cycle.
At
high-RPM, this may become a restriction since the air has inertia and responds
a little slower to pressure changes, potentially choking the air flow to the
engine a little.
Conclusion:
This
information is aimed at allowing tuners to understand what happens when cam
timing is altered. When a larger duration camshaft is being installed, unless
the lobe centerlines have been changed, the overlap will be increased. If
installing larger camshafts in a turbo application, advancing the exhaust and
retarding the intake will reduce the inherent increase in overlap caused by
upgrading to a larger profile. Most cam grinders, especially regrinders, put
the new profile in the same position as the old profile because it is easier,
or the only way possible. This has to be changed when the cams are installed in
an engine to attain the desired result.
A
forced-induction engine should idle smooth when properly tuned, and a naturally
aspirated engine should be “lumpy” and have a lope if it is tuned aggressively
towards the high-RPM range. If a forced induction engine is loping at idle,
fuel is being wasted, turbo spool time is being increased, and power is being
lost.
-Dave
Atchison
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