Thanks for all of the prompt responses...I have been looking over cam, lifter and timing kits. How do duration, lift and RPM range all come together? Those seem to be the differing factors in my choices here...I'm not a racer, but I do want my little Cuda' to get up and go when its all said and done...Probably more off the start than high speeds.
Lift and duration go hand in hand to some degree; to get more lift, you need more duration. The more you get of those, the more fuel-air mix you can get into the engine and the more power... a simple concept. There ARE variations in cam lobe designs that can get more lift without excessive duration (more on that later). As you increase the durations, the 'usable' RPM range (the RPM range over which you have decent torque and HP from the engine) moves higher and higher. So for a street engine, where you spend most of your time at low and mid RPM's, that is one reason to not go too big on duration.
The other reason is that if the durations get large, then the closing of the intake valve, gets later and later, and the later that happens, the less compression build-up in the cylinders you get during the compression stroke, and the more intake charge reverses back out of the cylinder. These things hurt low RPM torque in particular, and, again, for a street/cruise engine, that is another important reason to not go too big on duration. And that problem is exacerbated in a lower CR engine like your 318; it will make low RPM torque
lower than what you have now, and make it like a /6 at low RPM's! (The last line of your post, about good performance starting off, says you want to maintain low RPM torque so that is why to go on and on about durations.)
The term 'too large' in this case means the advertised duration numbers. What you would ideally like is to get not much bigger advertised duration on a new cam versus the stock cam's duration, but have the cam open the valves faster (faster ramp rates) to achieve higher (more lift) to get more air into the cylinders and take advantage of the headers and 4 BBL carb in the mid and higher RPM ranges.
But this gets into the matter of fast versus slow ramp rates; i.e., how quickly the valves are opened and closed. Slow ramp rate cams (like suggested in post #13) are easy on the valvetrain but that 272 duration will indeed reduce your low RPM torque. Fast ramp rate cams like in post #11 are better to keep the low RPM torque up while being harsher on the valvetrain. (This may be what BBM means by 'aggressive' cams in post #13.)
So which do you do? With the lower stock 318 compression ratios, the low RPM is torque and performance is already soft, so I personally would prefer to go to the high ramp rate cams and work with the challenges there. That means changing the valve springs like suggested in post #13. High rate cams are like the Lunati Voodoo's (PN's 10200700 or 10200701) or the Howard's 711381-10. (Kits with cam and lifter are available.)
With those, you want to buy the springs or some like suggested above, to help keep the valvetrain from 'float' at high RPM's. If you go with these, do your homework on the springs offered/recommended and see if any require mods to the springs seats in the heads; if so, to keep life simple, you may want to ask here about alternative springs.
When installing, for your low CR engine, I would personally install the cam with a few degrees extra advance. That gets into cam timing, which you should do, but is a different discussion. That extra cam advance is the reason to buy a timing set like suggested, that has an settable crank sprocket.
With any cam upgrade, slow or fast ramp, you NEED to learn about ZDDP content in motor oils and how to manage that. It is not rocket science, but is important, because if your oil does not have the right ZDDP levels, then the new cam likely faces terminal damage. There are also correct cam break-in procedures to follow. All of these things are found on this forum in various discussions. Search and ask away!
Finally, some other aids for low RPM torque:
- There are a number of ignition system and ignition timing changes that will make marked improvements in low RPM torque. Those are relatively easy to do, and so you should learn about those and plan them in now, regardless of whether you change cams or not.
- If you changed pistons and got the compression ratio up, that would REALLY make it get up and got from a dead stop, and then the world of cam selections opens up quite bit larger. But, that is likely a bridge to far for you, and that is totally understood... learn it a bit at a time.
Sorry to get so long winded....hope that helped!
