At this point, the “short block” is done – the block, rods, crank, and pistons are all installed and ready to go. From a cost and weight perspective, the engine is *easily* more than half done…but from a time / importance / part count perspective, we’re just getting started.
The camshaft is arguably the most important part of an engine, in terms of dictating how the engine will behave when it’s done – it affects every single other aspect, from dynamic compression ratio, valve train choices, the works. An engine is an air pump, and the cam dictates when the valves open and close to pump that air. From a physical perspective, the cam lobes push up on the lifters, which themselves push up on the pushrods, which rotate the rockers, which open and close the valves. When the cam isn’t pushing everything to open, the valve springs keep things closed. All told there are 6 unique parts responsible for opening and closing the valves when needed (not including “misc” parts like retainers, clips, etc) – but the cam is the one that dictates when it all happens, and to what extent. Each and every one of those parts has to work together as well – the weight, geometry, and design must all work together. Very roughly though, the whole operation can be split into two halves… the valve train, and the cam. For the purposes of this article I’m going to group items 1 and 2 (cam and lifters) as part of the “cam selection”, and items 3-6 (pushrods, rockers, valve springs, and valves) as part of the valve train…because to me, that’s the cleanest way to split them up from a theory perspective.
Breathe in, breathe out, breathe in, breathe out…
Let’s start with the valve train, because it’s a bit easier. In reverse order from how they were originally listed, my valves are 2.190″ intake and 1.88″ exhaust valves in a “standard” big block configuration (angles, etc). The intake are stainless, and the exhaust are inconel to handle the excessive heat generated by continuous high load / high RPM operation inherent to a performance marine engine. The retainers and clips / locks are just standard Comp cams parts – so generic I didn’t even record their spec, because there’s nothing special about them. This engine isn’t going to rev all that high, so specialized alloys to shave the last few grams of weight just weren’t worth it.
Next up are the springs. They’re shockingly important from a performance perspective because they have to ensure the valves close in time to avoid getting smashed by the piston as it travels back towards top dead center. If they’re too loose, “valve float” will occur at higher RPM, and they won’t be able to close the valves quickly enough…which is really bad. Like, “I need a new engine” bad. On the flip side, if they’re too stiff, they can rapidly wear every other aspect of your valve train if the rest of your components aren’t up to the task – which is almost just as bad. Thankfully, in this case, we’re not looking to hit any RPM records, and our valve lift isn’t that aggressive. So while we can’t use run of the mill OEM springs, we don’t need all out race effort springs either. Talk to your cam company or engine builder when spec’ing valve springs – they need to work with every other part of your build. I spent a huge amount of time on the phone with Comp Cams ensuring my springs would work well, and they were happy to help! I ended up with Pacaloy dual springs that had ~150lb of seat pressure (the “resting” pressure when the valve is fully closed), and ~400lb of open pressure (the max pressure you’ll see when the valve is fully open). Like everything else in the valve train, I cleaned them meticulously before install. However, unlike almost anything else in the engine…this is easier said than done. Springs are *highly* engineered from a metallurgy perspective, and even the tiniest imperfection can cause catastrophic failure. They come pre-oiled from the factory, because even a hint of rust will spell disaster..and they’ll rust almost instantly. So I had to be very quick and clean them one by one in a “clean, oil, clean, oil” procedure to ensure they never sat un-oiled for more than a few seconds.
The “Nervous System”
If the camshaft is the “brain” and the valves are the “lungs”, something has to tell those lungs to breathe. Enter the rockers and pushrods – for which the “pushrod” V8 is named after. Many, if not most, engines have the camshafts mounted directly on top of the valves – with multiple camshafts for each valve bank. Overhead cam, dual overhead cam, etc are all examples of this type of engine…the cam acts directly on the valves. The benefit to this is that there is no valvetrain between the cam and the valves…meaning much less mass, much more stiffness, and an all around more “RPM happy” engine. The alternative is to have just one camshaft hidden between the “V” of cylinders, which is then connected to the valves via the lifters, pushrods, and rockers. While this adds complexity and mass to the valve train, making it much more difficult to acheive high RPM stability, it’s crucially much more compact – so a given size and weight of engine can have much greater displacement. Since a boat motor requires moderate RPM at best, but gobs of torque, the pushrod is a natural choice.
After the valves and valve springs come the rockers. They’re mounted to the heads and are basically a lever that pivots on their mounting studs…as the pushrod pushes up on one side, they push down on the valve on the other side. In my case, I used comp magnum full roller rockers – they’re steel, very strong, and have both a roller tip and a roller base. Is this complete overkill? Probably. Will it last forever? Definitely. And that peace of mind was worth it for this build! The full roller rockers mean that there’s no friction points on the rocker – they roll on bearings both where they pivot, and where they contact the valve. This reduces wear somewhat significantly and is more able to handle the higher stresses of prolonged high RPM operation, with stiffer valve springs.
Last but not least, the pushrods. These rods, well, push on the rockers. Their only job is to bring the motion of the cam – hidden deep in the V of the engine – all the way up to the top of the heads. However, they need to be both extremely stiff and extremely light while they accomplish this. Any lack of stiffness can cause the valve train to stop functioning (and can lead to failure), and any increased mass makes it harder for the springs to close the valves. In my case, I went with Comp Hi Tech Chromoly 0.080″ 3/8″ pushrods. Like the valve springs, they’re a few steps up from stock, but not anywhere near a full race effort. The length of my pushrods are custom ordered to exactly fit the geometry of the engine. The length needs to be ideal for your rockers to sweep across the middle of the valve, so they don’t try to bend the valve stem or push unevenly on the valve body. If the rods are too short, or too long, the geometry won’t work. In my case, I used a pair of “checker springs”, and the comp hi-tech adjustable pushrod, to ensure my pushrod lengths would have optimal geometry. The checker springs also served double duty – I was able to mock up one cylinder with the cam, and rotate the engine through a couple of turns to check piston-to-valve clearance using some playdoh. It’s very, very important that there’s sufficient clearance between your valves and your pistons at all stages of the engines rotation!
At this stage, we have the right valves for my engine (they fit the engine and the heads I have, and they’re the right metals to withstand the operating conditions of a marine engine). We also have the right springs (the have the right travel length and right stiffness for my valves and cam), and good solid rockers and pushrods to support the rest of the valve train (strong, stiff, and light – and the correct length to achieve optimal geometry). All that’s left is something to control it all…which is coming up in part two!
