Producing More Power
HorsepowerFor a complete explanation of what horsepower is and what horsepower means, check out How Horsepower Works!|
Using all of this information, you can begin to see that there are lots
of different ways to make an engine perform better. Car manufacturers are constantly playing with all of the following variables
to make an engine more powerful and/or more fuel efficient.
Increase displacement - More displacement means more power because
you can burn more gas during each revolution of the engine. You can increase displacement by making the cylinders bigger or
by adding more cylinders. Twelve cylinders seems to be the practical limit.
Increase the compression ratio - Higher compression ratios produce
more power, up to a point. The more you compress the air/fuel mixture, however, the more likely it is to spontaneously burst
into flame (before the spark plug ignites it). Higher-octane gasolines prevent this sort of early combustion. That is why high-performance
cars generally need high-octane gasoline -- their engines are using higher compression ratios to get more power.
Stuff more into each cylinder - If you can cram more air (and
therefore fuel) into a cylinder of a given size, you can get more power from the cylinder (in the same way that you would
by increasing the size of the cylinder). Turbochargers and superchargers pressurize the incoming air to effectively cram more
air into a cylinder. See How Turbochargers Work for details.
Cool the incoming air - Compressing air raises its temperature.
However, you would like to have the coolest air possible in the cylinder because the hotter the air is, the less it will expand
when combustion takes place. Therefore, many turbocharged and supercharged cars have an intercooler. An intercooler
is a special radiator through which the compressed air passes to cool it off before it enters the cylinder. See How Car Cooling Systems Work for details.
Let air come in more easily - As a piston moves down in the intake
stroke, air resistance can rob power from the engine. Air resistance can be lessened dramatically by putting two intake valves
in each cylinder. Some newer cars are also using polished intake manifolds to eliminate air resistance there. Bigger air filters
can also improve air flow.
Let exhaust exit more easily - If air resistance makes it hard
for exhaust to exit a cylinder, it robs the engine of power. Air resistance can be lessened by adding a second exhaust valve
to each cylinder (a car with two intake and two exhaust valves has four valves per cylinder, which improves performance --
when you hear a car ad tell you the car has four cylinders and 16 valves, what the ad is saying is that the engine has four
valves per cylinder). If the exhaust pipe is too small or the muffler has a lot of air resistance, this can cause back-pressure,
which has the same effect. High-performance exhaust systems use headers, big tail pipes and free-flowing mufflers to eliminate
back-pressure in the exhaust system. When you hear that a car has "dual exhaust," the goal is to improve the flow of exhaust
by having two exhaust pipes instead of one.
Make everything lighter - Lightweight parts help the engine perform
better. Each time a piston changes direction, it uses up energy to stop the travel in one direction and start it in another.
The lighter the piston, the less energy it takes.
Inject the fuel - Fuel injection allows very precise metering
of fuel to each cylinder. This improves performance and fuel economy. See How Fuel Injection Systems Work for details.
Q and A
Here is a set of questions from readers:
- What is the difference between a gasoline engine and a diesel engine?
In a diesel engine, there is no spark plug. Instead, diesel fuel is injected into the cylinder, and the heat and pressure
of the compression stroke cause the fuel to ignite. Diesel fuel has a higher energy density than gasoline, so a diesel engine
gets better mileage. See How Diesel Engines Work for more information.
- What is the difference between a two-stroke and a four-stroke engine?
Most chain saws and boat motors use two-stroke engines. A two-stroke engine has no moving valves,
and the spark plug fires each time the piston hits the top of its cycle. A hole in the lower part of the cylinder wall lets
in gas and air. As the piston moves up it is compressed, the spark plug ignites combustion, and exhaust exits through another
hole in the cylinder. You have to mix oil into the gas in a two-stroke engine because the holes in the cylinder wall prevent
the use of rings to seal the combustion chamber. Generally, a two-stroke engine produces a lot of power for its size because
there are twice as many combustion cycles occurring per rotation. However, a two-stroke engine uses more gasoline and burns
lots of oil, so it is far more polluting. See How Two-stroke Engines Work for more information.
- You mentioned steam engines in this article -- are there any advantages
to steam engines and other external combustion engines? The main advantage of a steam engine is that you can use anything
that burns as the fuel. For example, a steam engine can use coal, newspaper or wood for the fuel, while an internal combustion
engine needs pure, high-quality liquid or gaseous fuel. See How Steam Engines Work for more information.
- Are there any other cycles besides the Otto cycle used in car engines?
The two-stroke engine cycle is different, as is the diesel cycle described above. The engine in the Mazda Millennia uses a
modification of the Otto cycle called the Miller cycle. Gas turbine engines use the Brayton cycle. Wankle rotary engines use the Otto cycle, but they do it in a very different way than four-stroke
- Why have eight cylinders in an engine? Why not have one big cylinder
of the same displacement of the eight cylinders instead? There are a couple of reasons why a big 4.0-liter engine has
eight half-liter cylinders rather than one big 4-liter cylinder. The main reason is smoothness. A V-8 engine is much smoother
because it has eight evenly spaced explosions instead of one big explosion. Another reason is starting torque. When you start a V-8 engine, you are only driving two cylinders (1 liter) through
their compression strokes, but with one big cylinder you would have to compress 4 liters instead.
Guide to Happy Ignition Timing.
Ignition timing plays a huge
role in how an engine runs, and not just from a power standpoint. Timing also effects fuel economy, ease of starting, engine
life and can effect the temperature at which the engine runs.
The Method: How is timing set? The best way to set the timing is by using a timing light. You
can get close by ear or by using the timing tab for the harmonic balancer in conjunction with a test light, but a Stroboscopic
Timing Light is the best way.
The Setup: First, unhook and plug the vacuum advance line, which lets you see just the initial
advance without the effects of the vacuum advance. Connect the inductive pickup for the timing light to the No.1 cylinder
plug wire. Be sure that it is fully clipped around the wire, then connect the power leads to the positive and negative posts
of the battery.
Checking the Timing: With the engine idling (in gear with the parking brake set in an
auto tranny car (Be Careful!)), aim the light at the timing tab on the front of the motor (down behind the crank pulley) and
pull the trigger. The light will pulse in time with the firing of the #1 cylinder and let you see the timing mark on the harmonic
balancer. The numbers on the timing tab tell you what the ignition advance or retard is (you need advance) depending on which
number the mark lines up with.
Adjusting the Timing: The initial advance is adjusted by loosening the lock bolt on the
distributor hold down tab. Rotating the distributor clockwise retards the timing and rotating it counterclockwise gives you
more initial advance. Most cars will have the stock timing setting on a sticker somewhere in the engine compartment. Be careful
when turning the distributor with the engine running, you can get quite a shock through deteriorated plug wires. Once you
have set the timing, lock down the distributor, recheck the timing to make sure you didn't change it while tightening down
the distributor clamp, and plug the vacuum advance back in.
Vacuum Advance: Vacuum advance is controlled by the canister that's on the side
of the distributor. Its function is to provide extra advance at high vacuum to increase fuel economy. The amount of advance
can be adjusted by swapping on a different vacuum canister or by putting on an aftermarket adjustable canister. Also, where
you have the vacuum line hooked up affects the vacuum advance. Ported vacuum (above the throttle blades, usually on the side
of the carburetor) gives you no advance at idle and increasing advance (to a point) as the throttle opens. As the throttle
approaches wide open, the vacuum goes away and so does the vacuum advance. Full manifold vacuum takes its signal from below
the throttle blades and is highest at idle and gradually drops to zero at WOT. Full manifold vacuum generally isn't used except
for some factory stock applications, and even then it's often not a good idea.
Centrifugal Advance: Centrifugal advance is produced by weights in the distributor that
are under the rotor. As rpm increases, the weights swing out and change the relation between the rotor and distributor cap.
The amount of centrifugal advance is affected by the shape of the weights and a bushing that sits in a slot. The fatter the
bushing and/or weights, the less advance possible. The rate of advance is effected by the weights and the springs that are
attached to them. Heavy weights and light springs give you the advance sooner (at a lower rpm). Lighter weights and heavier
springs delay the centrifugal advance. The purpose of centrifugal advance is to light off the air/fuel mixture sooner as rpm
increases so that there is sufficient time for proper combustion. You can buy kits to tailor your centrifugal advance to your
Timing Recommendations: How do you know where to set your timing? On a drag car its fairly
simple. You don't need vacuum advance and you typically want all the centrifugal advance in by your torque converter stall
speed and the initial is set to whatever yields the lowest ETs without detonation. On most street cars you want some vacuum
advance to help your mileage, but you don't want so much that you get part throttle detonation (often called pinging or rattling
valves). Most stock centrifugal advance mechanisms come in too late for a performance car. 2500-3000 rpm is a pretty good
starting point for having all the centrifugal advance in. If you are racing a car with a 2000 stall converter, it can be difficult
to get all your advance in my the stall speed without having detonation, so some compromise is necessary. The amount of centrifugal
advance is determined by what total advance (initial + centrifugal) is necessary to have the best power without causing hard
starting. Too much initial advance makes the engine hard to start, so you can have the same total advance by reducing the
initial and allowing more centrifugal. More initial advance can improve off-idle throttle response, but as mentioned above
there's a point at which the engine becomes difficult to start because of too much initial advance.
Setting Ignition Timing Curves
Tick-Tock Timing The Right Way
By Scott Crouse
Photography: Bob Mehlhoff, Scott
Having a car with outrageous low-end throttle response and top-end horsepower is something
every gearhead strives for. The correct combination of parts will get you close, but its the tune-up that truly makes the
difference. This month we are taking a look at ignition timing and how it can be used to improve an engines midrange torque,
throttle response, fuel mileage, and temperament.
Ignition timing is crucial to every engine combination. Remember that its average power that
accelerates your car, not the peak. Optimizing the ignition timing curve is a cheap and easy way to gain power across the
entire rpm band.
Unfortunately, like anything else, there is only so much gain that can be achieved before
detrimental effects begin to occur. Advancing the timing of an engine causes the ignition system to ignite the compressed
air/fuel mixture as the piston attempts to squeeze the mixture into the chamber of the cylinder head. Starting the combustion
process before the piston has reached top dead center (TDC) may seem hazardous to the engine because the piston has to work
against negative force. However, because the combustion process takes time to occur, this advance improves power at that particular
rpm. The amount of advanced ignition timing is relative to an engines bore/stroke compression ratio, fuel octane, and a dozen
other variables. Because you cannot run excessive amounts of ignition timing at all times, distributors are designed to employ
timing curves that progressively induce timing into the cylinders.
A distributor is capable of setting a timing point that is advanced by two independent methods.
First, a Chevrolet V-8s initial timing >> must be set at idle by rotating the distributor clockwise for retard or counter-clockwise
to advance. This initial timing point acts as a base point for the engines ignition system. Once initial timing is set, there
are two ways of adding additional timing to an engine. One method uses a vacuum canister designed to work off a ported vacuum
source (Holley carburetors locate this on the metering block), which is typically referenced from just above the closed throttle
blades. As the throttle begins to open, the engine displays its highest level of vacuum and causes the distributors vacuum
canister to advance the timing.
Mechanical advance is the second method of ignition timing advance. As the distributor spins
fast enough to activate the mechanical-advance weights, the engine receives initial timing, mechanical timing, and vacuum
timing under part-throttle conditions. As the engine accelerates to wide-open throttle (WOT), the vacuum drops, eliminating
the vacuum canisters timing. For example, part-throttle total timing would look something like this: 10 degrees initial +
10 degrees vacuum + 20 degrees mechanical = 40 degrees of total timing. At WOT, there is no vacuum present and the canister
timing is eliminated, giving your engine a total of 30 degrees timing. The reason your engine is able to sustain more timing
at part-throttle is because only a limited amount of air and fuel make it into the cylinder at part-throttle. Lower cylinder
pressures enable the combustion process to start sooner and help improve part-throttle response by increasing torque. This
additional part-throttle timing improves efficiency and torque.
Ideal ignition settings will allow your engine to run the maximum amount of timing at all
engine speeds without detonation. Now that you know how distributor timing works, you can manipulate it to improve your torque
and horsepower curves.
Time to Vacuum
Vacuum canisters control part-throttle timing. By igniting the spark sooner during part-throttle
operation, the combustion process is aided. When vacuum canisters first appeared on distributors, the factory designed them
to employ a nonadjustable amount of predetermined advance at maximum engine vacuum. As an engine accelerates and vacuum decreases,
the canister slowly pulls timing from the engine until it reaches zero vacuum advance. The factory vacuum canisters were designed
to work with individual engine combinations. HEI systems were designed to employ less mechanical advance to help control emissions,
while point-type distributors featured high amounts of mechanical advance. When engines are altered and modified, their timing
demands also change, which is why Crane Cams and Moroso designed adjustable vacuum-advance kits. By simply inserting a 3/32-inch
Allen wrench into the end of the canister, the internal vacuum-advance springs can be adjusted to control the engines rate
of vacuum advance. The system is also designed to work with a vacuum-timing limiter plate. This plate allows its user to preset
the total amount of vacuum timing at maximum engine vacuum.
The mechanical-advance system inside a distributor uses springs to determine an engines rate
of advance. There are a total of two springs, two weights, and four pins. Each spring is attached to one weight pin and one
timing pin. As the engine accelerates, centrifugal forces attempt to pull the weights from the timing plate. The rate at which
the springs let the advance weights move determines the timing curve. Using higher- or lower-load springs will bring timing
in at a lower rpm. For example, two light-tension springs would allow the mechanical timing curve to start at a low engine
speed such as 1,000 rpm. By 2,500 rpm, the maximum amount of mechanical advance would be employed and provide the engine with
an extremely quick advance curve.
A timing slot in the mechanical-advance mechanism limits total engine timing advance by using
a pin inside a slot. In order to increase an engines total timing, this slot must be elongated with a carbide cutter. If the
total timing needs to be limited, the slot will have to be welded shut. Many aftermarket distributors (such as MSD) supply
offset bushings that allow the total timing to be increased or decreased without any cutting or welding.
Standard flash timing lights like
this self-powered MSD unit are designed to work with marked dampers. If your damper does not display timing marks, you can
wrap MSDs timing tape around it.
This factory HEI distributor reveals
the location of the vacuum canister advance arm (A), the mechanical-advance weights (B), and mechanical-advance springs (C).
The square, black box (D) on the bottom right is the electronic module that triggers the spark.
With the weights and springs removed,
we can better understand the mechanical-advance pin locations. Pins (A) locate the weights while pins (B) locate the advancing
timing plate. As the distributor spins, the weights are drawn outward and pull on the springs to advance the timing curve
based on the length of the slot (C).
The elongated slot on the bottom
of an advance plate determines the total amount of mechanical advance. While many aftermarket companies have developed offset
bushings to adjust total timing, the factory units require carbide cutting or welding to lengthen or shorten the slot.
The distributor cap on the left features
an external coil design used through 1974. In 1975, GM switched over to an HEI ignition system with an in-cap coil that lasted
through 1986. In 1987, GM converted the HEI to an external-coil design.
When vacuum-advance canisters were
first introduced, they were calibrated to the engine. This GM canister reads 357/20the 357 is the last three digits of the
units part number, and the 20 refers to the total crankshaft degrees of vacuum advance. Todays replacement canisters all feature
nearly the same advance total and do not display this numbering system.
Because dialing in vacuum advance
can play such an important role in part-throttle response, mileage, and engine temperature, Crane Cams and Moroso have designed
fully adjustable vacuum canisters. The Crane unit also includes mechanical-advance springs.
Vacuum canisters advance according
to engine vacuum, which is why they must be connected to a ported vacuum source. Ported vacuum is drawn from just above the
throttle blades to make sure the vacuum canister does not advance at idle.