Cam swap

Discussion in 'Engine & Performance' started by noah_t, Oct 31, 2018.

  1. MikeB

    MikeB Full Access Member

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    I like that cam for higher compression, and also its revised version (#24502476) with less intake duration for lower compression engines. https://paceperformance.com/i-62555...aft-gm-350-330-h-p-crate-engine-camshaft.html

    Both have lobes that are easy on the valve train, and quiet, too. Even GM roller cams aren't over the top aggressive like aftermarket cams.

    [/QUOTE]
    That's always the best answer to the question, "Which camshaft should I use?"

    If folks paid more attention to compression ratio, heads, low restriction exhaust, and optimal ignition timing, they would be amazed at how much power their engine would make with even a stock or entry level performance cam. This whole idea about simply installing a big cam and expecting more power is just nuts, especially in a 4WD pickup.
     
    Last edited: Nov 7, 2018
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  2. shiftpro

    shiftpro Full Access Member

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    Thanks for your comments Ruster. I'm kinda with ya on the single pattern cams, at least with modified old heads or new aftermarket heads that actually flow. I'm thinking the dual pattern cams were an answer to smogger heads beginning to show up in the early 70's. (?) One thing in common with all these heads, is the restricted exhaust flow (and everything else wrong to design into heads like low compression, thin castings, guide bosses too big or too small and more I imagine).
    When I was auto machining and doing lots of heads, we modded many heads with the first step, opening the exhaust. Larger exhaust valve at the very least and open the bowl underneath. These heads always delivered impressive results, and it really wasn't much more work added to a proper valve job and installing seats. Kinda like stepping up to a stroker crank if your original needs gringing and rods need sizing. Seemed to be the best bang for the buck, stick a bigger exhaust valve in it.
    As I remember, two engines that benefited the most from just a larger exhaust valve.... BBC with 781 or 049s. And the VERY other end of the scale, SBfords. The 302/289 heads... seat of the pants felt like the car doubled it's torque and hp. And slightly higher compression pistons, headers, dual planes to complement everything... daily drivers late 70's. The shop I worked at more often than not, talked customers into raising compression and a little more cam. Almost nothing left the shop 'stock'. Times were different back then..
     
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  3. shiftpro

    shiftpro Full Access Member

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    Ok cam this...
    496 with 781 heads. The heads have bigger exhaust valves. Intake flows 85 cfm over stock, air port speed is 350 ft/sec over the short turn and 380 ft/sec in the corners. I don't have the chamber volume numbers handy, my machinist measured this and with the Probe domed pistons he calculated my compression ratio at just over 10.5 /1.
    Block was decked almost .040 to zero out the pistons at deck height.
    Fuel will be propane (octane 110), twin Impco 425s.
    Engine is going in to a 1 ton crew cab, 4L80. 488 gears and 35" tires.
    Need a wide LSA. Propane don't like overlap. It expands too much.
     
  4. Rusty Nail

    Rusty Nail Build the wall - deport them all!

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    “Knowledge gained is as useless as pride if filed away and never applied.”


    Especially.
    Most assuredly the weight distribution and the tire sizes (sidewall) play a HUGE role.

    I don't know the first thing about propane OR big blocks, but thanks.

    However I echo many sentiments regarding cylinder heads, and anyone paying attention has watched me build TWO PAIRS in the last year, on this MSB.
    Both got bigger exhaust valves.

    Worthy of note in this subject is that the exhaust valve is smaller because the spent gas is mechanically evacuated from the cylinder in the combustion cycle.
    Compared the intake charge which is finite and limited - especially regarding an N/A motor.

    It can be calculated, and the exhaust is mechanically evacuated. That said, only so much can be done with any desireable effect regarding exhaust valve size or the ports even.

    Nobody makes mention of what I feel is an equally important of an OVERALL combination - the exhaust collector length.
    That MUST be considered to properly tune a "powerband".
    We're talking about making or breaking , now - if you ask me.
    What?! You say your collectors are "the ones that came with the headers" ?
    When was the last time in your life that three inches was enough for ANYBODY? :D

    Unacceptable.
     
    Last edited: Nov 8, 2018
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  5. MikeB

    MikeB Full Access Member

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    One of these days when I get tired of sinking money in my old truck, I'm gonna pull the L31 Vortec heads (new in 2015) and learn how to do some mild pocket porting. Seems like I have a book or two that devote a few pages to the subject.

    Conventional wisdom says exhaust valve diameter should be around 75% of intake valve diameter. For my heads with 1.94/1.50 valves, that number is 77%. And my camshaft has split duration, so I'm not sure a larger exhaust valve would make more torque or horsepower. On a related subject, Ed Iskenderian famously said if you have a split duration camshaft, that means the intake duration is too short. :) Of course that assumes you have enough static compression to handle the longer duration (later intake closing angle).
     
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  6. Rusty Nail

    Rusty Nail Build the wall - deport them all!

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    What happened to OP?
     

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