401 V8 Camshaft Selection
While rebuilding the 401 V8 from my '77 Wagoneer, I found that the camshaft it had (which appeared to be aftermarket, since the measured lobe lift was higher than stock on many lobes) was quite worn. Since I had the engine completely torn apart anyway, now was the time to get a new cam.
Choosing a cam can be one of the most critical aspects of engine performance. A cam that's too "small" won't allow your engine to reach its full power potential. A cam that's too "big" won't idle well (or at all) and won't accelerate at low RPM. An excellent description on cam theory and how to choose a good cam can be found in David Vizard's book How to Build Horsepower, Vol I. COMP Cams has provided a good, basic tutorial on valve timing.
The torque converter on my TH400 tranny (non-lockup) stalls at about 1200 RPM, according to the shop that rebuilt my tranny. A stronger engine will increase the stall speed somewhat. Doing a little math told me that my tranny will be spinning 2400 RPM at 60 mph, or 3200 RPM at 80 mph (interstate speed limit is 75 around here). When you figure in roughly 10% torque converter slippage, the engine will be spinning 2640 or 3520 RPM, respectively, when cruising. When accelerating to pass somebody or when towing my Wrangler up a steep hill (8000-lb total vehicle weight), the RPM could be even higher. I therefore decided that a good power range for my cam would be 1500-4500 RPM. Conveniently, this is the exact RPM range provided for most RV/towing cams. FYI, the red line for the rotating assembly in most AMC V8's is just over 6000 RPM. This cam should cause the valves to float and my engine to lose power long before I'm in danger of throwing a rod.
Stock-ish AMC engines are generally safe using valve lifts of up to .500" before you have to start worrying about the valves kissing the pistons. The stock 401 cam has .458" of lift for both intake and exhaust.
The lobe center angle (LCA) determines how much overlap there is between the exhaust and intake openings. A wider LCA provides a wider and flatter (but also somewhat lower) torque curve than a narrower LCA, so I shot for LCA's of 110 or, preferably, 112 degrees. My calculations show the stock 401 cam as having about 115.8 degrees LCA and the 304/360 having 111 degrees, but that could be wrong (I assumed the lobes were symetric about the centerline).
The final, most critical, and also most advertised specification on a camshaft is the valve-open duration. Based on the RPM range I need and the cam usage descriptions provided by the cam manufacturers, the ideal cam for my application will have around 205 to 215 degrees of duration at .050" of lobe lift. This translates to about 260-275 degrees of advertised duration. Since there is no standard way of measuring advertised duration, duration @ .050" lift is the best way to compare cams from different manufacturers. The stock 401 cam has advertised durations of 296 (intake) and 303 (exhaust). Dual-pattern grinds like this, which give more duration and often more lift to the exhaust valves, are not uncommon. I'm unclear as to what their real benefit is, though.
Here, now, are links to my short list of cams to install. I'm leaning heavily toward the Crane H-260-2.
|LCA||Valve lift |
|Crane Cams PowerMax H-260-2||1500-4500||204/216||260/272||112||.456 / .484||Smooth idle, daily usage, off road, towing, economy, also mild turbocharged, 2200-2800 cruise RPM, 8.0 to 9.5 compression ratio advised.|
|COMP Cams 260H||1200-5200||212/212||260/260||110||.447 / .447||Good low end torque. Strong mid-range power. Excellent for towing or performance. Smooth idle in 304 and up.|
|Crower Baja Beast / Level 2 (258H)||1500-4500||204/210||258/264||112||.445 / .448||Excellent low end and mid-range power. Redline 5500+|
|Crower Power Compu-Pro / Level 2 (276HDP)||1500-4000||212/220||276/281||112||.464 / .488||These cams provide excellent low end and mid-range power and extended RPM range for spirited street and off-road driving. A perfect combination of mileage and power. Modifications should include small diameter tube headers, low restriction dual exhaust, aftermarket manifold, increased cfm carburetor and reworked or performance ignition. Increase in compression ratio to 9.5:1 is recommended for maximum output. Works well with automatic transmission or 4-speed. Redline 5500+|
|Edelbrock Performer-Plus||idle-5500||204/214||278/288||110||.448 / .472|| This Performer-Plus camshaft is designed to produce optimum torque
from the low-end to the mid-range in AMC V-8s. It is a smooth idling
cam for daily drivers, trucks, vans, RVs and 4x4s. It also offers
improved throttle response and torque.
Ed Francis reports that this cam actually has good mid-high torque, but pitiful low-end torque in a stock-ish 401.
|Isky Cams 135125||1500-4800||202/202||256/256||112||.450 / .450||Best torque & economy in passenger cars & trucks. Max compr., 9.5:1, smooth idle, std axle ratio, good vacuum, computer compatible. Std. to 600 CFM Carb.|
|Summit Racing SUM-8600||1500-4500||214/224||272/282||112||.472 / .496||Good idle quality. (rumor has it Summit's mfg quality is poor)|
For comparison, here are the stock AMC V8 cam specs taken from my 1977 technical service manual:
|'74-78 304 |
|Valve lift (1.6x)||.426"||.458"|
|Intake valve opens||14.75* BTDC||25.57* BTDC|
|Intake valve closes||68.75* ABDC||90.75* ABDC|
|Exhaust valve opens||56.75* BBDC||80.80* BBDC|
|Exhaust valve closes||26.75* ATDC||42.75* ATDC|
|Intake duration (adv)||263.50*||296.32*|
|Exhaust duration (adv)||263.50*||303.55*|
|Lobe separation angle
(computed by me, error-prone)
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last updated 29 Jun 2001 Obi-Wan (firstname.lastname@example.org)
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