401 V8 Camshaft Selection

(401 pic) 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.

Cam Basic RPM Duration
@ .050"
int/exh
Advertised
Duration
int/exh
LCA Valve lift
int/exh
Description
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
'74-79 360
'74-78 401
Lobe lift .266" .286"
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
Valve overlap 41.50* 68.32*
Intake duration (adv) 263.50* 296.32*
Exhaust duration (adv) 263.50* 303.55*
 
Lobe separation angle
(computed by me, error-prone)
111* 115.8*

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Back to Obi-Wan's Wagoneer engine rebuild page
last updated 29 Jun 2001
Obi-Wan (obiwan@jedi.com)

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