Camshafts 101

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In its simplest form, the Camshaft in an LT1 is responsible for the the operation and timing of the opening and closing of the valves. The purpose of this article is to assist with the understanding of how a camshaft works, and help with the selection of an after-market camshaft to increase the performance of the LT1 engine.

To sum it up, this quote from Street Racers Online hits the nail on the head:

"If cylinder heads are the heart of an engine, then the camshaft and valve train have to be the brains of the operation.
Timing the opening, closing, lift, and duration of each valve event is central to increasing power and torque."
Fig. 1 An OHV Valvetrain Simplified
Fig. 1 An OHV Valvetrain Simplified


Contents

[edit] How does the camshaft work?

The camshaft in an OHV (Over Head Valve) engine is located in the block above the crankshaft. It is rotated by being connected to the crankshaft through the timing chain. The purpose of the camshaft is to open the valves in the series according to the firing order of the engine in order to make power.

For the following simplified explanation, refer to the image (Fig. 1) to the right
The camshaft has a series of lobes on it that rotate according to the rotation of the timing chain. The camshaft rotates (red) and pushes on the lifter (yellow). In turn the lifter is pumped up and pushes the pushrod (yellow) upwards on the back end of the rocker (blue). Because the back end of the rocker has been pushed up, the opposite end pushes down. The opposite end pushing down is in contact with the valve stem (green), so in reaction with the valve stem being pushed down against the valve spring, the valve is opened. As the lobe of the particular valve rotates away, the valve spring pushes back up, closing the valve again.

[edit] Why replace the factory camshaft?

Fig. 2 A Camshaft.
Fig. 2 A Camshaft.

The phrase: "If its not broken, don't fix it" doesn't always apply. The factory camshaft in the LT1 does work, but its setup is geared more towards driveability and gas mileage than pure performance. It is very possible (and easily attainable) to have an after-market camshaft that makes much better power than the factory, but is still very well mannered for the street.

A good way to think of it as well is to imagine the piston when it starts its intake stroke and the intake valve opens, the air/fuel mixture in the intake runner starts to accelerate into the cylinder. By the time the piston reaches the bottom of its intake stroke, the air/fuel mixture is moving at a pretty high speed. If you were to slam the intake valve shut, all of that air/fuel mixture would come to a halt and will not enter the cylinder. By leaving the intake valve open a little longer, the momentum of the fast-moving air/fuel mixture continues to force air and fuel into the cylinder as the compression stroke is started by the piston. In theory the faster the engine goes, the faster the air/fuel mixture flows and the longer we would want the intake valve to stay open. We would also want the valve to open wider at higher speeds.
Also affecting the cams performance is lift, the duration, overlap and timing.


[edit] What do the numbers on a camshaft mean?

The specs for a typical factory camshaft found in a LT1 are: 205/207 Duration @ 50*, .447"/.459" Lift, 117 Lobe Separation Angle
To many people this seems like an entirely different language.

[edit] Duration

Duration is the angle in crankshaft degrees that the valve stays off its seat during the lifting cycle of the cam lobe. Increasing duration keeps the valve open longer, and can increase high-rpm power. Doing so increases the RPM range that the engine produces power. By increasing duration without a change in lobe separation angle will result in increased valve overlap.

[edit] Lift

Lift refers to maximum valve lift. This is how much the valve is "lifted" off its seat at the cam lobe's highest point. The intake and exhaust valves need to be open to let air/fuel in and exhaust out of the cylinders. Generally, opening the valves quicker and further will increase engine output. Increasing valve lift, without increasing duration, can yield more power without much change to the nature of the power curve. However, an increase in valve lift almost always is accompanied by an increase in duration. This is because ramps are limited in their shape which is directly related to the type of lifters being used, such as hydraulic, flat, or roller.

[edit] Lobe Separation Angle

Lobe Separation Angle (Also known as LSA and Overlap) is the angle in crankshaft degrees that both the intake and exhaust valves are open. This occurs at the end of the exhaust stroke and the beginning of the intake stroke. Increasing lift duration and/or decreasing lobe separation increases overlap. At high engine speeds, overlap allows the rush of exhaust gasses out the exhaust valve to help pull the fresh air/fuel mixture into the cylinder through the intake valve. Increased engine speed enhances the effect. Therefore increasing overlap, increases top-end power and reduces low-speed power and idle quality.


[edit] Aftermarket Camshafts: Where do I start?

Just like any situation when you are deciding to purchase parts to increase the performance of your LT-Powered Vehicle, you have to decide the ultimate goals for the project.

  • Is the car a daily driver?
  • What is your personal definition of streetable?
  • Is the car an automatic or a manual?
  • How fast do you want to spend?

[edit] My Car is a Daily Driver

So your LT-Powered Vehicle is your daily driver. It is your primary (and usually only) mode of transportation to get to work, school, the movies, and the grocery store. You are looking to take the next step, but don't want to sacrifice daily driver reliability or gas mileage. And most importantly, you don't want to break the bank with this project. In this situation, there are quite a few options as far as camshafts are concerned to bump up the power, but not sacrifice too much for your A to B Vehicle.

[edit] Common 'Off-The-Shelf' Cams

Some of the most common cam-only setups can be easily acquired through your local speed parts shop, national level mail order company, or directly from the cam manufacturer. They are readily produced, so they are less expensive than a 'one-off' or 'custom' camshaft grind. They have been used, and are tried and true to produce results. All individuals have their own personal preferences and experiences. The goal of this article is to show all of the options, and the user can decide which best fits their project goals and budget.

  • GM Performance Parts LT4 Hot Cam - 218/228 Duration @ 50*, .525"/.525" Lift, 112 LSA
Common GM Production Cam. Considered hot by GM because it is a step up from the factory cam found in GM LT4 Production Engines. Has a slightly choppy idle. Produces good midrange, and is streetable with decent vacuum. Typically produces 310-340 rwhp depending on vehicle platform and supporting modifications. Requires tuning for best performance and streetability.
GM Part Number: 24502586 (Just Cam) or 12480002 (Hot Cam Kit)
Common Comp Production Cam. Considered to be the most mild of the popular Comp Off-The-Shelf Cams. Has a slight chop at idle. Emissions friendly and can run on factory tuning. Produces decent midrange and is very streetable. Typically produces 300-330 rwhp depending on vehicle platform and supporting modifications. Requires minimal tuning for best performance and streetability.
Comp Part Number: CCA-07-304-8
  • Comp Cams CC 305 - 220/230 Duration @ 50*, .510"/.510" Lift, 114 or 112 LSA
Common Comp Production Cam. Considered to be comparable to the GM LT4 Hot Cam. Has a slightly choppy idle. Produces good midrange, and is streetable with decent vacuum. Typically produces 310-340 rwhp depending on vehicle platform and supporting modifications. Requires tuning for best performance and streetability.
Comp Part Number: CCA-07-305-8 (114 LSA) or CCA-07-305-8-112 (112 LSA)
Common Comp Production Cam. Considered to be somewhat comparable to the GM LT4 Hot Cam. Has a slightly choppy idle. Produces good midrange, and is streetable with decent vacuum. Is considered to have an edge on the CC305 and LT4 Hotcam because it features the Comp XE (Extreme Energy) Lobe Profile which results in a more aggressive low and mid-range. Typically produces 320-350 rwhp depending on vehicle platform and supporting modifications. Requires tuning for best performance and streetability.
Comp Part Number: CCA-07-503-8
Common Comp Production Cam. Considered to be extremely aggressive for a street cam. Has a choppy idle. Produces good midrange, but shines most in the topend. Considered by some to be streetable, but is definitely more on the extreme side, produces okay vacuum. Typically produces 330-350 rwhp depending on vehicle platform and supporting modifications. Requires tuning for proper function and streetability. Cam flows well in very high RPMs, ported or aftermarket cylinder heads are recommended.
Comp Part Number: CCA-07-306-8


:Vehicle platform variables depend on drivetrain (whether the car has a T-56 or 4L60E transmission). Typically an automatic transmission will cause more parasitic powerloss and will dyno with lower rear wheel horsepower. Supporting modifications are considered long tube headers, low restriction exhaust system, 52mm or larger throttle body, and "hotter" ignition to help with combustion of the increased ammounts of air/fuel in the cylinder. These numbers are not the standard, and also depend heavily on the mechanical condition of the engine and its other essential accessories.

[edit] 'One-Off' and 'Custom Grind' Cams

Many camshaft manufacturers are able to produce custom grind camshafts to fully fit its application. There is no replacement for a custom grind cam in terms of meeting project goals of streetability and power. Many of the popular custom camshaft producers are:

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