HOW TO SELECT YOUR NEW CAMSHAFT
This is a basic guide for proper camshaft selection.
There are always exceptions, and lots of people ignore the guidelines,
but if you stay close to them you will have a combination that works well.
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Getting the camshaft that is right for YOUThe most important thing to remember when designing your engine is how it will be used most often. All of the components that you choose for your engine must match your primary use (and each other) or you will be disappointed with the results. This is especially true for internal engine parts such as pistons, heads, and cams. If you make a mistake choosing one of these items you will have a lot of expense and work ahead of you to make the corrections.Over-camming the engine is the most common mistake made when choosing a cam. A big cam will not give better bottom end power and big horsepower numbers also won't happen at low rpms without big cubic inches or a supercharger. Another common mistake is building a high compression engine and then choosing a Torque/City or RV/Commuter camshaft to match the type of driving the vehicle will be used for. Unfortunately this will produce cylinder pressures that are too high for pump gas to handle. This is the way to go only if you plan to run on propane (107 octane) or natural gas (130 octane). So how DO you choose your camshaft?First, you must decide how the vehicle is going to be used most often.If you are using it daily to go back and forth to work, how much of that time is in city traffic? And how much of your traffic time is spent sitting in line at the light? These questions might seem unnecessary, but if you spend 25% of your time at idle and another 50% of your time in traffic between 0 and 50 KMH (30 MPH), this information becomes important. This would be the speed that determines the low end of your RPM range. Next you will have to determine your usual cruising speed and/or top speed. If you have a tach it is easy to get your normal operating range by checking your engine speed as you drive. If you don't have a tach in your vehicle some calculations will be required to determine what RPM you are running at. For this you will need to know your Tire Outside Diameter (O.D.) and your Rear Axle Ratio. If you do not know your Rear Axle Ratio there is often a tag on one of the differential cover bolts that will tell you. Or you can find out the hard way:
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Here are your Vehicle Speed To Engine RPM calculations
Keeping your idle requirements in mind, work out the low end of your RPM range. For a vehicle that is driven daily in the city it will be your idle rpm. For a drag vehicle it will be the launch rpm. For an oval track vehicle it will be your rpm at re-start. Now repeat the formula for your top speed. For a street vehicle the highway cruising speed (90 KMH or 60 MPH) will be the top of your torque range. For a drag vehicle peak horsepower and rpm will be at the fast end of the strip. For an oval track vehicle peak horsepower and rpm will be at the fast end of the straights. Now you know what RPM range you require for your driving. The optimum RPM range of a cam is really only about 3000 RPM from the beginning of the torque range to the end, with another 1000 rpm to peak horsepower. They will operate above and below this but not to the best performance. If you require an RPM range that is wider than this you will have to make a compromise. Which RPM range is the most important to you? For hydraulic lifter cams, Rhoads variable lifters will add about 1000 rpm to to lower end of the camshaft's normal operating range. |
Basic Parts Selection GuideThis guide uses 8 typical performance levels from mild to wild. If you are not sure which range to use, be conservative. If you choose too high an rpm range it will be less reliable, harder on parts, and rarely be used. An rpm range that is lower than you need will still be used even if your top end is slightly limited.Each performance level suggests the range of Accelerated Motion camshaft, air/fuel requirements, compression ratio, exhaust type, gear ratios, and ignition, that will work together the best for that performance level. Maximum engine HP (not including nitrous or supercharging) and expected idle speed are also shown. The RPM range that a cam works best in will change with the engine size and head design. Recomendations shown in this guide are for average engines. Please see the parts catalogue for specific engine recomendations.
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What makes one cam in the range better than another?There are many myths and legends about which cam is better, single pattern (intake and exhaust the same) or dual pattern (intake and exhaust different). The fact is that unless your exhaust ports are very restricted there is no way to tell. On the dyno you would always be comparing apples to oranges.Lobe Center Separation is as big a consideration as duration. Lobe center separation plays a role in determining how much valve overlap (the amount of time the intake and exhaust valves are both open) your engine will have and what your vacuum and idle quality will be. Street cams with wide lobe center separation (114) generally will have a good idle, high vacuum, and a nice wide RPM range. Separations closer to 108 (less separation means more valve overlap) can create problems for some computer engine controls due to their rougher idle and lower manifold vacuum. They have a shorter RPM range but produce much stronger mid-range power with some improvement to the top end. Now we get to valve lift. Many customers believe that the cam with the highest lift will perform much better than a moderate lift cam. While it is true that a high lift cam will provide better flow by getting the valve further out of the way, there are limits to this as well as other considerations. The high rate of lift required to achieve high lift on a short duration cam is very hard on the valve train and causes valve to piston interference problems (especially with narrow lobe separations). High lift also causes several other problems including valve spring retainer to valve guide/seal interference, rocker arm to stud interference, and valve spring coil bind. Adjustable rocker arms are often required to take up any clearance created in the valve train when the valve is closed. All of this must be checked and corrected before a high lift cam can operate properly. If you are using a cam with enough duration to make a high lift effective, the usual limit for noticable improvement is reached when the lift equals 25% of the valve diameter. This means that if your valve diameter is only 1.84 you will get the best flow at only .460 lift. Lifts higher than 25% of the valve diameter will add more duration at the maximum flow point but excessive lifts will cause more problems than they are worth in an engine that is not fully race prepared. The bottom line on cam lift is that it is pointless to go overboard. All newer designs have adequate lift for the operating range of the cam. The minimal gains from an extra high lift cam are not worth the extra work in a street vehicle. Here are some basics for choosing a cam from your range:
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Accelerated Motion
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Last updated August 2012.
(C) Copyright 1996-2012 by Doug Friesen, doug@amotion.com. All rights reserved. The Accelerated Motion name and logo is a registered trademark of Doug Friesen. All other marks, names and part numbers are the property of their respective owners and are used for the sole purpose of promoting sales and proper use of their products. The information contained at this site is accurate to the best of my abilities and is subject to change without notice. 
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