HOW TO SELECT YOUR NEW CAMSHAFT



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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.

Getting The Right Cam - Engine RPM Calculations - Basic Parts Selection Guide
Which Specs Are Better?


Getting the camshaft that is right for YOU

The 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:
  • Block the front wheels
  • If your vehicle doesn't have posi:
    • Raise one of your rear wheels off of the ground and properly support your vehicle
    • Put a chalk mark on the raised tire and on the driveshaft
    • With the transmission in neutral and the emergency brake off rotate the tire two full turns (to compensate for the action of the differential) while counting the number of times the drive shaft turns.
  • If your vehicle has posi:
    • Raise both of your rear wheels off of the ground and properly support your vehicle
    • Put a chalk mark on the same position of both tires and on the driveshaft
    • With the transmission in neutral and the emergency brake off rotate both tires in the same direction one full turn while counting the number of times the drive shaft turns.
    • Check to make sure the lines on the tires still match position. If they aren't in the same positions as at the start something is wrong.
  • The number of driveshaft turns is your Rear Axle Ratio. The common gear ratios are 3.08, 3.23, 3.55, 3.73, 3.91, 4.10, 4.56, and 4.88.
To get your correct tire diameter measure from the ground to the center of the wheel and multiply by 2. Measuring from any other point on the tire will give you a larger diameter than your vehicle actually uses.

Here are your Vehicle Speed To Engine RPM calculations

KILOMETERS:

    KMH
1   --- = KM per minute
     60

2       KM per minute  X  100,000 = cm per minute

                                  cm per minute
3                          --------------------------- = Tire RPM
                           (cm Tire O.D.cm  X  3.1416)

4              Tire RPM  X  Rear Axle Ratio = Engine RPM
MILES:

    MPH
1   --- = Miles per minute
     60

2       Miles per minute  X  63,360 = inches per minute

                                    inches per minute
3                           ------------------------------ = Tire RPM
                            (inches Tire O.D.  X  3.1416)

4              Tire RPM  X  Rear Axle Ratio = Engine RPM

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 Guide

This 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.

TORQUE AND CITY

(idle to 3600 rpm, max. HP @ 4600 rpm)
Good fuel economy, light to medium towing and improved low-end for city use. No internal engine modifications are required.
ENGINE          CAM RANGE*
151  4 cyl.    up to 19477
181  4 cyl.    up to 19767
189  6 cyl.    up to 19393
250  6 cyl.    up to 19686
300  6 cyl.    up to 20220
302  V8        up to 19567
350  V8        up to 19827
400  V8        up to 19830
454  V8        up to 20190
500  V8        up to 20617
*The cam range refers to the 2nd half of the Accelerated Motion part number.
Compression:   Gasoline 9.0:1 or less, Propane 10.5:1 or less
Exhaust:       Stock or dual exhaust
Fuel Inj.:     Should work with factory EFI computers
Gear Ratio:    3.7:1 and lower (numerically)
Idle:          600 rpm or less
Ignition:      Recurved distributor and electronic ignition


RV AND COMMUTER

(1000 rpm to 4000 rpm, max. HP @ 5000 rpm)
Good fuel economy, medium towing and improved mid-range performance for every day use. No internal engine modifications are required.
ENGINE          CAM RANGE*            AIR/FUEL RANGE      HORSEPOWER
151  4 cyl.    19477 to 20277         222 to  278 cfm     Max. 121 HP
181  4 cyl.    19767 to 20567         266 to  333 cfm     Max. 145 HP
189  6 cyl.    19393 to 20193         278 to  348 cfm     Max. 151 HP
250  6 cyl.    19686 to 20486         368 to  460 cfm     Max. 200 HP
300  6 cyl.    20220 to 21020         442 to  552 cfm     Max. 240 HP
302  V8        19567 to 20367         445 to  556 cfm     Max. 242 HP
350  V8        19827 to 20627         515 to  644 cfm     Max. 280 HP
400  V8        19830 to 20630         589 to  736 cfm     Max. 320 HP
454  V8        20190 to 20990         668 to  836 cfm     Max. 363 HP
500  V8        20617 to 21417         736 to  920 cfm     Max. 400 HP
*The cam range refers to the 2nd half of the Accelerated Motion part number.
Compression:   Gasoline 9.5:1 or less, Propane 11.0:1 or less
Exhaust:       Stock, dual exhaust, or small tube headers
Fuel Inj.:     Factory EFI computers might require modification
Gear Ratio:    3.9:1 or lower (numerically)
Idle:          650 rpm or less
Ignition:      Recurved distributor and electronic ignition


MILD PERFORMANCE

(1400 rpm to 4400 rpm, max. HP @ 5400 rpm)
Fair idle, moderate fuel economy, improved mid-range performance and enough torque for medium/heavy towing. Nice for ski boat.
ENGINE          CAM RANGE*            AIR/FUEL RANGE      HORSEPOWER
151  4 cyl.    20277 to 21077         250 to  309 cfm     Max. 136 HP
181  4 cyl.    20567 to 21367         300 to  371 cfm     Max. 163 HP
189  6 cyl.    20193 to 20993         313 to  387 cfm     Max. 170 HP
250  6 cyl.    20486 to 21286         414 to  512 cfm     Max. 225 HP
300  6 cyl.    21020 to 21820         497 to  615 cfm     Max. 270 HP
302  V8        20367 to 21167         500 to  619 cfm     Max. 272 HP
350  V8        20627 to 21427         579 to  717 cfm     Max. 315 HP
400  V8        20630 to 21430         662 to  819 cfm     Max. 360 HP
454  V8        20990 to 21790         752 to  930 cfm     Max. 409 HP
500  V8        21417 to 22217         828 to 1024 cfm     Max. 450 HP
*The cam range refers to the 2nd half of the Accelerated Motion part number.
Compression:   Gasoline 8.5:1 to 10.0:1, Propane 10.0:1 to 11.5:1
Exhaust:       Stock dual exhaust or small tube headers
Fuel Inj.:     Factory EFI computers will require modification
Gear Ratio:    3.1:1 to 4.1:1
Idle:          700 rpm or less, slight lope
Ignition:      Recurved distributor and electronic ignition


PERFORMANCE

(1800 rpm to 4800 rpm, max. HP @ 5800 rpm)
Good upper mid-range performance. Works well for medium and heavy towing with low (high numerially) gears.
Good for jet boat and skiing. Biggest for I/O boat.
ENGINE          CAM RANGE*            AIR/FUEL RANGE      HORSEPOWER
151  4 cyl.    21077 to 21877         279 to  341 cfm     Max. 151 HP
181  4 cyl.    21367 to 22167         334 to  409 cfm     Max. 181 HP
189  6 cyl.    20993 to 21793         349 to  427 cfm     Max. 189 HP
250  6 cyl.    21286 to 22086         461 to  565 cfm     Max. 250 HP
300  6 cyl.    21820 to 22620         554 to  678 cfm     Max. 300 HP
302  V8        21167 to 21967         557 to  682 cfm     Max. 302 HP
350  V8        21427 to 22227         646 to  791 cfm     Max. 350 HP
400  V8        21430 to 22230         738 to  903 cfm     Max. 400 HP
454  V8        21790 to 22590         838 to 1025 cfm     Max. 454 HP
500  V8        22217 to 23017         923 to 1129 cfm     Max. 500 HP
*The cam range refers to the 2nd half of the Accelerated Motion part number.
Compression:   Gasoline 9.0:1 to 10.5:1, Propane 10.5:1 to 12.0:1
Exhaust:       Dual exhaust, headers are recommended
Fuel Inj.:     Performance EFI system recommended
Gear Ratio:    3.3:1 to 4.3:1
Idle:          750 rpm or less, small lope
Ignition:      Recurved or perf. distributor, electronic ignition


HOT PERFORMANCE

(2200 rpm to 5200 rpm, max. HP @ 6200 rpm)
Strong upper rpm performance. Not for towing. Good in jet boat with A impeller. Biggest for skiing. Multi-angle valve grind is recommended.
ENGINE          CAM RANGE*            AIR/FUEL RANGE      HORSEPOWER
151  4 cyl.    21877 to 22677         308 to  373 cfm     Max. 166 HP
181  4 cyl.    22167 to 22967         369 to  447 cfm     Max. 199 HP
189  6 cyl.    21793 to 22593         386 to  467 cfm     Max. 208 HP
250  6 cyl.    22086 to 22886         510 to  617 cfm     Max. 275 HP
300  6 cyl.    22620 to 23420         612 to  741 cfm     Max. 330 HP
302  V8        21967 to 22767         616 to  746 cfm     Max. 332 HP
350  V8        22227 to 23027         714 to  864 cfm     Max. 385 HP
400  V8        22230 to 23030         816 to  988 cfm     Max. 440 HP
454  V8        22590 to 23390         926 to 1121 cfm     Max. 499 HP
500  V8        23017 to 23817        1020 to 1235 cfm     Max. 550 HP
*The cam range refers to the 2nd half of the Accelerated Motion part number.
Compression:   Gasoline 9.5:1 to 11.0:1, Propane 11.0:1 to 12.5:1
Exhaust:       Dual exhaust with headers
Fuel Inj.:     Performance EFI system required
Gear Ratio:    3.5:1 to 4.5:1
Idle:          800 rpm or less, has a lope
Ignition:      Recurved or perf. distributor, electronic ignition


STREET/STRIP

(2600 rpm to 5600 rpm, max. HP @ 6600 rpm)
Strong performance above 2500 rpm. Expect low manifold vacuum. Mild port work and multi-angle valve grind are recommended. Automatic transmissions may require a high stall torque converter.
ENGINE          CAM RANGE*            AIR/FUEL RANGE      HORSEPOWER
151  4 cyl.    22677 to 23477         338 to  405 cfm     Max. 181 HP
181  4 cyl.    22967 to 23767         406 to  485 cfm     Max. 217 HP
189  6 cyl.    22593 to 23393         423 to  507 cfm     Max. 227 HP
250  6 cyl.    22886 to 23686         560 to  670 cfm     Max. 300 HP
300  6 cyl.    23420 to 24220         672 to  804 cfm     Max. 360 HP
302  V8        22767 to 23567         677 to  810 cfm     Max. 362 HP
350  V8        23027 to 23827         784 to  938 cfm     Max. 420 HP
400  V8        23030 to 23830         896 to 1072 cfm     Max. 480 HP
454  V8        23390 to 24190        1017 to 1217 cfm     Max. 545 HP
500  V8        23817 to 24617        1120 to 1340 cfm     Max. 600 HP
*The cam range refers to the 2nd half of the Accelerated Motion part number.
Compression:   Gasoline 10.0:1 to 11.5:1, Propane 11.5:1 to 13.0:1
Exhaust:       Dual exhaust with headers
Fuel Inj.:     Performance EFI system required
Gear Ratio:    3.7:1 to 4.7:1
Idle:          850 or less, has a lope
Ignition:      Performance distributor and electronic ignition


PRO STREET AND RACE

(3000 rpm and up)
Super top-end performance. Manifold vacuum might be too low for power brakes and automatic transmission modulators. NOT for daily driving. Mild port work and multi-angle valve grind are recommended. Automatic transmissions will require a high stall torque converter.
ENGINE          CAM RANGE*            AIR/FUEL RANGE      HORSEPOWER
151  4 cyl.    23477 and up           369 cfm and up      181 HP and up
181  4 cyl.    23767 and up           443 cfm and up      217 HP and up
189  6 cyl.    23393 and up           462 cfm and up      227 HP and up
250  6 cyl.    23686 and up           611 cfm and up      300 HP and up
300  6 cyl.    24220 and up           734 cfm and up      360 HP and up
302  V8        23567 and up           738 cfm and up      362 HP and up
350  V8        23827 and up           856 cfm and up      420 HP and up
400  V8        22830 and up           978 cfm and up      480 HP and up
454  V8        24190 and up          1110 cfm and up      545 HP and up
500  V8        24617 and up          1223 cfm and up      600 HP and up
*The cam range refers to the 2nd half of the Accelerated Motion part number.
Compression:   Gasoline 10.5:1 and up, Propane 12.0:1 and up
Exhaust:       Dual exhaust with headers
Fuel Inj.:     Race EFI system required
Gear Ratio:    3.9:1 and higher (numerically)
Idle:          850 rpm or more, will be rough
Ignition:      Performance distributor and electronic ignition


Minimum chassis horsepower required to reach a 1/4 mile speed:
HPq = (0.00426 x MPH) x (0.00426 x MPH) x (0.00426 x MPH) x WEIGHT


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:
  • Large cars: Use short or medium duration, exhaust can be longer. Wide lobe seperation is better.
  • Small Cars: Use long intake duration, exhaust can be longer. Lobe center seperation can be short.
  • Propane and Natural Gas: Use short or medium duration single pattern. Wide lobe seperation is required.
  • Trucks: Use short duration. Wide lobe seperation is better.
  • Nitrous or Supercharger: Use long duration. Wide lobe seperation is required. Supercharger compression must be lower than shown in the guide.
  • Turbos: Use short or medium duration, exhaust can be shorter. Wide lobe seperation is required. Compression for turbos must be lower than shown in the guide.



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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.