Thread: Cam Theory and the Rover V8

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  1. #1 Cam Theory and the Rover V8 
    Young Brit
    Join Date
    Feb 2012
    I've been mulling over camshafts for my next Rover V8 and thought I'd share
    what I've learned. In "Be the Camshaft Expert" (July 2006 issue of Popular
    Hot Rodding) David Vizard presented some rules of thumb for selecting cam
    specs based upon what he's learned in developing a cam selection program.
    The program was reportedly 18 years in the making and uses data gathered
    from several thousand cam tests. The basic idea is that cam overlap sets
    the RPM range in which an engine will best operate and the total overlap
    is a function of the cam duration and lobe center angle (LCA). If you know
    the ideal LCA and the RPM range you wish to operate in, the duration will
    fall out. In the article, Vizard presents graphs of cubic inch displacement
    (CID) per cylinder per inch of valve diameter versus ideal LCA. The graphs
    are for inline valve heads with compression ratios between 9:1 and 11:1.
    If canted valve heads are used, the ideal LCA is 2 degrees less. The optimal
    LCA is also adjusted for compression ratios outside the assumed range. Once
    ideal LCA is known, you pick your desired overlap via application:

    1. Street towing 10 to 40 degrees
    2. Regular street 30 to 60 degrees
    3. Street Performance 50 to 75 degrees
    4. Street/Strip 70 to 90 degrees
    5. Amateur Race 85 to 100 degrees
    6. Professional Race 95 to 115 degrees

    Overlap is the period when both intake and exhaust valve are open and serves
    to set the RPM range over which the cam will be best suited. More overlap
    means a rougher idle and poorer low end response due to reversion of the
    exhaust charge into the intake plenum as well as loss of "effective"
    compression ratio (compression is literally blown out the exhaust port at
    low RPM). Where you fall in the overlap range is a function of valve size
    per cubic inch. Big valves on a 302 use the low end, a 350 with typical size
    valves use the mid-point, big inch small block or big block, use the right
    hand side. Given the small valve size of the Rover/Buick aluminum V8's,
    we'd typically use the larger overlap value (right hand side). Once LCA and
    overlap are known, duration falls out of the equation. Lift is determined
    by the intake valve diameter.

    In the article, Vizard presented the results of a test demonstrating the
    importance of picking the right LCA:

    Ideal LCA for that engine was 108 degrees. Narrowing to 105 degrees made
    similar power but had noticeably worse idle and low speed characteristics.
    Widening to 111 degrees lost power. Another LCA test was performed on a
    302 Small-Block Ford and repeated on a stroked (347 CID) version of the same

    "When used in the 302, the 276/280 roller hydraulic cam on a 110 LCA proved
    optimal, as even a 1 degree change either way produced worse results. Using
    a SCAT stroker kit, this engine was stretched to 347 inches and re-tested
    with the original 110 LCA cam. The stroker kit really helped both power and
    torque. When the 110 LCA was replaced with a more appropriate 108 LCA cam,
    the output made a further jump to the tune of some 20 lb-ft and 20 hp. The
    108 cam in the 347 gave as much in terms of idle and vacuum as did the 110
    LCA in the 302. Dozens of tests such as this show conclusively that the
    overlap and LCA--not the duration--are the first steps toward generating a
    cam spec."

    At low speed, lots of overlap is bad as it hurts low end but overlap helps
    as the RPM increases. To a degree, you can offset overlap with static
    compression. Another point raised in the article is that, for most V8's with
    reasonable heads, the ability to raise low speed torque with compression
    increase holds to around 285 to 290 degrees (at lash point) of cam duration.
    After that, drop off is faster than an increase in compression can recover.
    In another article ("Compression Comprehension") about running up to 12:1
    compression on pump gas, Vizard presented the results of a compression test:

    "When used in conjunction with a bigger cam, increased compression can
    work wonders for the entire curve. When a 265-degree cam (gray curve)
    was substituted for a 285-degree cam (blue curve), a substantial drop
    in low-speed output was seen. Raising the CR from 9:1 to 12:1 recovered
    almost all the lost low end and gave a further increase in top-end

    I wrote a little computer program based upon the article. The required
    inputs for the simplified program are:

    bore diameter (in inches)
    crankshaft stroke (in inches)
    intake valve diameter (in inches)
    static compression ratio
    canted or inline valve heads
    desired overlap (picked from the ranges listed above)

    Some results for various Rover/Buick combos are presented below. Vizard's
    cam selection program is not available to the general public but I know for
    a fact it takes much more into consideration than the simplified rules
    presented above. A friend has run his program and it uses actual head flow
    data, port size (length and cross-sectional areas), valve diameters, rocker
    ratios, desired idle vacuum, compression, bore, stroke, fuel octane,
    thermostat temperature, rod length and more. Basically, it attempts to feed
    the "air pump" in the most efficient manner, given the parameters entered.
    What it doesn't do is tell you what ramp rates you need. The recommendation
    is to use the most aggressive ramp you can tolerate for your application.
    However, more aggressive lobe shapes are noisier, wear more quickly and are
    harder on valvetrain parts so that neds to be taken into consideration as

    The simplified program assumes you'll use the same lobe profile for intake
    and exhaust. There's an implicit assumption that the exhaust flow is
    reasonable compared to the intake. Vizard also suggests the rocker ratio
    on exhaust is best kept 0.1 to 0.2 of a ratio lower than the intake ratio.
    The exhaust is under higher pressure and blows down early in the lift cycle
    plus the exhaust is less sensitive to valve acceleration than the intake but
    is sensitive to duration. I noticed in last year's Engine Masters Competition,
    most of the entries were using shorter rocker ratios. If an existing cam's
    LCA is too wide, higher ratio rockers may increase output. For rocker ratios
    between 1.5 to 1.9, each 0.1 ratio increase on the intake, the LCA needs to
    be spread by 0.75 to 1. Be aware there are cases where the ideal lobe center
    may be too narrow for acceptable street manners. For instance, when stroking
    an engine (keeping all other variables constant), Vizard suggests tightening
    the LCA up by a degree for something like every 16 cubic inches increase in
    displacement. That will lead to very tight lobe centers which may not be
    acceptable for certain applications. For instance, an engine equipped with
    mass-air EFI (which will measure reversion flow as if it were induction flow)
    might not be too happy with 104 degree lobe centers. Vizard's full-up
    program allows you to specify idle vacuum to get around problems like that
    but the simplified program doesn't have that sort of flexibility.

    Another thing to be aware of is that narrow lobe separation angles require
    an efficient exhaust with minimal back-pressure. If you have a bunch of
    back-pressure (from things like restrictive mufflers or headers that turn
    down sharply at the exhaust port exit) it will hurt a narrow lobe separation
    angle engine more than a wide one. A quote from the article drives home this
    point home:

    "Let's make one thing clear here: Big (but not excessive) overlap is a prime
    key to big power numbers, but only if your exhaust system sucks. Literally.
    If you have ever heard that an engine needs a little backpressure, you might
    want to ask yourself why an engine would want an exhaust system that literally
    pushes exhaust back into the combustion chamber rather than sucking it out.
    The simple answer is, it doesn't. If a big-overlap, big-cammed engine has an
    exhaust system with any measurable backpressure, the price paid is a big drop
    in output."

    If your exhaust system is restrictive, it may be wise to err on the side of
    a wider LCA.

    Since large valved heads tend to increase flow through high lift levels,
    the lift recommended by Vizard's rules may be excessive for heads equipped
    with large valves. The recommendation is to use a lift consistent with your
    reliability goal (higher lift wears valves, guides and seats more quickly).
    However, for small valve heads, like our little Rover/Buick aluminum V8's,
    the maximum lift is reasonable. In the range of 2 valve pushrod V8's, our
    Rovers are at the small valve diameter (per displacement) end of the spectrum
    but, based upon my limited experience, the simplified program seems to give
    pretty reasonable trends. As a starting point, you could do worse. Woody
    Cooper has had a lot of experience with custom cams in Rovers of various
    displacements and I believe his experience parallels Vizards predictions.

    More info on Vizard's cam rules can be found at:

    Dan Jones

    Cam specs for Rover/Buick street performance assuming 75 degrees overlap

    For 9 to 11 compression ratio

    CID Bore x Stroke Intake duration / LCA / Lift

    215 (3.50 x 2.8) 1.5 298 / 112 / 0.45 to 0.52
    219 (3.52 x 2.8) 1.5 298 / 111 / 0.45 to 0.52
    215 (3.50 x 2.8) 1.625 298 / 112 / 0.48 to 0.56
    266 (3.53 x 3.4) 1.5 288 / 107 / 0.45 to 0.52
    266 (3.53 x 3.4) 1.625 292 / 109 / 0.48 to 0.56
    292 (3.70 x 3.4) 1.625 288 / 107 / 0.48 to 0.56
    292 (3.70 x 3.4) 1.775 292 / 109 / 0.53 to 0.62
    297 (3.73 x 3.4) 1.775 292 / 108 / 0.53 to 0.62

    For Woody's 13:1 compression ratio:

    297 (3.73 x 3.4) 1.775 294 / 109 / 0.53 to 0.62

    Note that the durations used here are seat-to-seat duration (similar to
    advertised or SAE duration, not duration at 0.050").
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  2. #2  
    Young Brit
    Join Date
    Feb 2012
    Okay, so that's the theory. Here's some of the practice side from an
    engine builder friend of mine...

    If you set up a cam on blocks or centers and use a lifter jig, or plot
    it out on paper, it quickly becomes apparent that the range of actual
    lobe profiles is sharply limited. For flat tappet cams, the diameter of
    the lifter or the length and curvature of the finger follower defines
    the limits of possible profiles. Once your pressure line gets to the
    edge of the follower, there's no more to be had. For rollers, the
    diameter of the roller is your limiting factor. Eventually the pressure
    line reaches the height of the axle, and spits the roller off to the side.

    That's geometry. It does make a difference; that's why flat tappet
    racing lobes are specified by lifter diameter, and why rollers go to
    inverted flanks.

    Then you need ramps to take up the slack on opening, and to keep from
    pounding things on closing. That further restricts allowable profiles.

    Then you come to the big real-world part - wear. For a flat tappet,
    wear is pretty much a function of load at the pressure line. That's why
    Detroit cams are all so wimpy. They had to last 75,000 or more miles
    without wearing to the point where they had to be replaced. For
    hydraulic rollers, oil viscosity and pressure are your main limits,
    where leakdown starts to change the valve motion significantly from the
    lobe motion. And remember, that's assuming a 10 year old car that still
    has the oil it left the factory with, in a worst-case service condition,
    like pulling a camping trailer in New Mexico in the summer.

    For a "performance" cam, longevity is sacrificed first. You can load
    the cam more heavily, and shorten the ramps, and pick up lots of "free"
    power by kicking the valves open faster and closing them sooner.
    Competition Cams is infamous for this; some of their more aggressive
    street cams seldom saw more than 20,000 miles before rounding off lobes
    or hollowing lifter bases. They *did* perform as advertised. Note
    that Comp recommends armor-faced lifters for flat tappet lobes that
    require heavy spring rates.

    For "street" cams, you have to assume the stock valvesprings are being
    used, because a largish fraction of your customers *will* insist on
    running them. Getting more area under the curve with stock springs is
    a damned good trick; you can whack it off the seat, but you have to stop
    lifting early and carefully to keep the valvetrain together at max lift,
    and you have to set the valve down carefully to keep it from bouncing.

    Drag cams are quite specialized now, which is why you see the lobes
    separated from circle track stuff. Longevity on a cutting-edge drag cam
    is often less than a dozen runs for a flat tappet. For solid rollers,
    lifter failure happens first.

    Power is cheap. Longevity is what costs the big money.

    Crane and Isky like to talk about stepped lobes to handle harmonic
    motion of the valvetrain bits, and other crap like that. It's just
    marketing; on the dial indicator, it's all a smooth curve.

    What's creepy is to mike a brand new cam, and find .005" difference in
    lobe height from smallest to largest. Kind of makes me laugh at Crane,
    which claims accuracy down to *millionths* when tossing cams into the

    Extended exhaust duration only adds overlap, with its attendant idle
    and emissions problems. All-out racers use long exhaust durations for
    scavenging, but that ruins your fuel economy on the street. It's primarily
    a drag race trick to keep the power from dropping off quickly after power
    peak. On the street, a symmetrical profile or even a short exhaust will
    run just as well and with a lot more tractability. On the street, extended
    exhaust profiles date back to the 15-psi-backpressure old-style catalytic
    convertor days; they are obsolete.

    Overlap creates a lot of internal EGR during cruise which is great for fuel
    economy if the spark is advanced enough. In the BMW world, intake and
    exhaust cam profiles are either the same, or the intake gets the bigger cam.
    Independent runner intake manifolding can withstand a lot more overlap without
    trashing the low speed torque and tractability than a standard plenum carb

    Dan Jones
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  3. #3  
    Young Brit
    Join Date
    Feb 2012
    That is an awesome post. Great information, thanks Dan.
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  4. #4  
    Young Brit
    Join Date
    Feb 2012
    > That is an awesome post. Great information, thanks Dan.

    You're welcome. That reminds me, I still need to verify the results
    from the curve-fits in my program match that of Vizard's plots. That'll
    tell me if I got the program right or not.

    Dan Jones
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  5. #5  
    Great post Dan.

    When we get the BAD8 up running with this new cam grind, we will let you know how she runs!

    Keep up the good work though, awesome information here people.

    The Wedge Shop
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  6. #6  
    Ole Boy
    Join Date
    Feb 2012
    Mt Pleasant SC
    Wish I understood it!

    "Stick a Wedge In It"
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