mad dog
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Nov 24, 2020 at 11:24 AM
Dec 25, 2013
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mad dog

Well-Known Member, from ct

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Viewing latest content, Nov 24, 2020 at 11:24 AM
    1. AJ/FormS
      A typical street performance cam will be in the range of 112* to 108*.So let's shoot for something like 110*.
      You now have two cam parameters; a 110LDA and a 60* ICA. Let's go shopping.
      I like the Hughes fast-rates. They have short opening and closing ramps, designed for the Mopar lifters. This allows running a slightly bigger cam, without losing Dcr. Unfortunately, they only list the .050 numbers, so you have to call them.
      Now Comp does list their .006 numbers, which gets us pretty close. They have the XE268H, which IIRC has already been recommended to you. It has the 110LDA and a 60*ICA when installed per their 106 install. And the valve lifts will work on your heads without special work. And their ramps are not that slow.So that's a win/win/win.I would still call Hughes, because you may be able to upsize,without losing Dcr.Not losing Dcr is very important.This cam will perform much better with headers, and an exhaust cross-over.
      I hope I have been of some help to you.
    2. AJ/FormS
      Now the cam. To optimize the DYNAMIC compression ratio(Dcr) and cylinder pressure we need to determine the exact intake closing angle(ICA).But first we need to know the target Dcr. By listening carefully to success stories on this and other forums,I have come to believe that the Mopar LA cast iron non-quench designs will support about 8/1 Dcr on 89 to 91 gas. This is a best guess on my part.
      So plugging these numbers into the calculator spits out an ICA of 58 to 62 degrees.
      And that takes us to the cam spec of ICA = 60* plus/minus 2 degrees.
      The next parameter to then select is the operating range of the engine that you want to build. Do you want a high rpm screamer, a low rpm puller or something in between?
      A high rpm screamer will want a narrow Lobe Displacement Angle (LDA) of 106* or less. A torquey cam will want something like the stocker of 114*.
    3. AJ/FormS
      You cannot order tight-quench pistons until you know where your decks are at.And you cannot order a cam until the compression is calculated, which cannot be determined until the pistons are finalized.
      Well actually you can and you can, but the combo will not be optimized, and if it's not optimized, either it will not perform as expected, or you will be buying a lot of expensive gasoline and/or octane boosters.
      But let's say you don't care about optimization, cuz you're not a competitive kind of guy. Well then just throw any pistons in there that you can find, irregardless of tight-quench. At least now you can compute the compression ratio.So let's do that;
      So with a 30 over piston your swept volume is 705.68 ccs. Let's estimate .018 flat-top pop-ups(3.84cc), 8.9cc in the gasket, a 70cc chamber, and 6cc in the eyebrows. Doing the math; that comes to a total chamber volume of 81.06cc, and thus a STATIC compression ratio(Scr) of 9.7 to one.
    4. AJ/FormS
      Keep in mind that the original cam in that 340 was well matched to the non-quench, open-chamber-head design, and with it's mild compression ratio.It already had a go anywhere,anytime capability. And it was no performance dog in doing it.
      To improve on it, will take carefully balancing the cam,the compression ratio, and the quench, for the available gasoline.You have to stay within the available gas's octane rating, or suffer with those consequences.So let's say yo have 91 available as the first parameter. Okay, next parameter is the quench.Can you get a piston to fit into the open chamber head high enough to fall into the zone of acceptability of .035 to.055 or so? IDK. You will have to shop around. You will need a piston to come out of the holes far enough to compensate for the headgasket thickness (about .039), and to enter up into the chambers about.040, so that would be about a .080 pop-up, above your decks.So then the question is where are your decks?
    5. AJ/FormS
      So by this time, you may be thinking that I'm just another pro-360 idiot. Well you'd be right, but perhaps not for the reasons you might think.
      It all boils down to getting the most bang for your small-block dollar.
      For your stated goals in your opening remarks,of drive-anywhere/anytime the 360 will make more torque, and more power,on less cam, and through cast iron exhaust manifolds, while burning cheaper gas, and lasting longer, after having costed less to build, and not risking your very valuable, numbers-matching un-machined 340 shortblock.
      But let's say you still want to use your 340, and that you have found a capable machine shop. At this point, you just need help in choosing a cam and compression ratio.
      Okay, I can help with that.
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