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Choosing a "Real" Chrysler Cam | 10/24/2009 |
Choosing a camshaft is probably the most important part of designing an engine combination, yet it can be the most confusing and frustrating part of your engine building experience. One of the problems is the confusing and contradictory way that cams are advertised and the way the specifications are listed.
There are some basic principles that are universal, apply to all brands of camshafts and must be understood.
- Camshaft size: Bigger or smaller is based on duration, not lift, more specifically duration @ 0.050" tappet lift. Advertised duration figures are not reliable numbers when trying to compare one brand to another for these reasons: first, advertised duration numbers include clearance ramps, which have no positive effect on performance. Second, advertised duration figures are not always checked at the same point with every brand, so the same cam can have several advertised duration figures depending on who or how it is checked. Third, using the duration @ 0.050" is a reliable way to compare cam size from one brand to the next, a good apples to apples comparison.
When selecting any camshaft for your engine/chassis combination, the size must be determined by the duration at .050" tappet (lifter) rise. Using a 1.5:1 rocker ratio this equals .075" valve lift. Any duration less than this has no positive effect on power.
- Understanding the duration at 0.050" may seem confusing but the important thing that must be remembered is: as the duration at 0.050" gets larger the camshaft gets larger and vise versa.
- The lower (or shorter) the duration at .050", the lower the RPM use, such as RVs, towing, stock engines, etc. As the duration at .050" increases, the power increases, however, the power band also moves up in the RPM range. This requires additional engine and chassis modifications to work best. One cam will not do everything well, you may have to make "tradeoffs". In other words, if you want to drag race with a cam that pulls at 6800 rpm, don't expect the engine to lug a trailer around at 2000 rpm or vice versa.
- Cams have power bands, "sweet spots" or RPM ranges that they work best in. This power band or "sweet spot" does not mean that the cam will not work above or below this range. If, for example, the sweet spot is rated at 2000-5700 RPM, the engine will still produce power above 5700 RPM (at least ours will), but above 5700 RPM, the next larger cam will produce more power. By the same token, cams will produce power below their sweet spot, but the smaller cam will have more power there (see power curve graph).
- The cam size (duration at 0.050") determines where the "sweet spot" will be. All brands of cams of the same size will produce sweet spots in approximately the same rpm range.
- The "sweet spot" will be determined for the most part by the size (duration @ 0.050") of the intake lobe. Other factors such as the lobe separation angle, center line and exhaust lobes have some effect, but these are usually for custom cam installations.
- A change of approximately 5 duration @ 0.050" is considered one (1) size or step.
- The power the engine can develop in the "sweet spot" is determined for the most part by the rate-of-lift or how quickly the valve is kicked open. The quicker the better (ours are the quickest, similar to a roller cam).
- The best engine combination is when the sweet spots of all the components (camshaft, cylinder heads, intake manifold, carburetor size, compression ratio, headers and exhaust size) are in the same RPM range. For example, an RV torque cam with a large, single plane intake is a very poor combination.
- If you collect a very poor combination of "sweet spots" (also know as parts) and then try to pick a camshaft, you leave the cam supplier with an impossible job. We don't sell cams to people with really screwed up combinations unless they are willing to change some components.
- The first component you should pick when you build your engine is the camshaft. Add the other parts to compliment it, not fight it. Check our list of questions on choosing a camshaft before you call.
- Our Basic Camshaft Guidelines list general guide lines for engine/chassis combinations. There are exceptions, but "tradeoffs" are involved. The actual power ranges will vary somewhat with the engine size, compression ratio (cylinder pressure) and the intake/heads ability to flow air. If you need to call for camshaft information, we will want to know these facts:
- Engine size (cubic inch displacement)
- Weight of vehicle, with driver (if a driver is used)
- Use of vehicle (What are you ging to do with the vehicle)?)
- Altitude where vehicle will be run.
- Desired ET or MPH
- Present camshaft (duration @ .050", Lobe seperation angle, lift etc...)
- Final gear ratio
- Stall speed of torque converter
- Tire height and width
- Fuel type
- Static or Mechanical compression ratio
- Iron or aluminum heads
- Cranking cylinder pressure - checked with a compression gauge
- Intake manifold
- Header type and size
- Carburetor size
- Rocker arm type and ratio
- Will power adders be used? (N.O.S., turbo, supercharger etc...)
- Cylinder head air flow numbers
Note: Please try to use numbers such as: 1600 RPM stall, not "stock stall" because Chrysler provided many varieties of stock parts.
Remember, the more information you provide, the more accurate the recommendation. We can move the lobe separation angles and mix/match lobes for very special applications. If you follow these instructions, in most cases, you will choose the correct camshaft.
Cylinder pressure note:
The cylinder pressure is a result of 5 things, the static compression ratio, cylinder head material (iron or aluminum), the closing point of the intake lobe, the altitude of operation of the engine and the final displacement of the engine. When using 93 octane gasoline (good stuff, no ethanol or oxygenated junk), build your engine for 165psi cylinder pressure with iron heads. Whenusing aluminum heads aim for 195 psi cylinder pressure.This is generally the maximum safe pressure and power without detonation or odd spark curves. As the camshaft gets larger (duration at .050" tappet lift), the static compression ratio must be raised to maintain the cylinder pressure. For maximum power in your engine, request our computer generated cylinder pressures/static compression ratio numbers when ordering your Real Chrysler Camshaft.
Mopar Performance camshaft note:
When replacing a Mopar Performance camshaft with a Hughes Engines Real Chrysler camshaft, use one that is 5 to 7 smaller at .050" tappet lift for increased power in all RPM ranges.
| The Power Curve |
The graph below describes what you should expect when you build an engine for a specific power range. If the engine breathing combination (cam, carb, intake, heads, headers and compression ratio) are properly matched, the duration at .050" on the intake lobe will generally dictate where the power curve will occur. This way you can compare the potential results with cams from various sources. Note that as the engine power increases, the power curve will become narrower and steeper. It will also move up in the RPM scale.
If your combination develops power in the "A" curve and you choose a smaller (duration @ .050") camshaft, you can expect to see your power curve move to the left, the "B" curve. Notice that the "B" curve is wider, but has a lower peak power. If you choose a larger (duration @ .050") camshaft, your power curve will move to the right, the "C" curve. This curve is narrower, but has higher peak power. If you pick a camshaft with the same or very close (2 to 3 duration @ .050") your power curve will be approximately the same. HOWEVER, when installing a HUGHES ENGINES "REAL" CHRYSLER CAMSHAFT you can expect to see the maximum power increase and the power curve widen. With a Hughes Engines camshaft (curve "D"), the power will start sooner and pull longer, resulting in more peak power. The RPM location of the peak power will remain about the same as long as the duration at .050" on the intake lobe remains the same (curve "D" vs. curve "E")
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| Advancing and Retarding the Camshaft |
| Advancing the camshaft, (a lower intake lobe center line number, 104 vs. 108 for example) will generally move the power band down in the RPM range and reduce maximum power somewhat. Retarding the camshaft ( a higher intake lobe center line number 108 vs. 104) will generally move the power band up in the RPM range and may increase maximum power slightly. However, these rules are not absolute. Depending on the engine combination, the opposite results can occure. Call for details with your specific combination. |
| A WARNING on Computer Engine Designing and Testing |
| We have many people call us requesting timing figures to enter in their computer dyno programs. We are happy to supply these figures. However, we will only supply them at .050" tappet lift because we do not consider the advertised duration figures to be relevant. Advertised duration is not checked at the same point with every manufacturer. A lot of lower priced computer dyno programs will not accept duration at .050" tappet lift figures. Most of these simulation programs are not designed with Mopar engine features, such as quench heights, longer rods, lifter diameter, etc. Therefore, the results from these types of programs are misleading. The better programs (and there are very good ones) naturally cost considerably more. If the program you are using doesnt ask much (such as flow figures, quench heights, rod lengths, cam timing at .050", etc.), do not expect good indications of the power potential.. |
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