Re: Who knows 5.7 Performance stuff? Cam Choice...
Okay, the stock Chevy 5.7 for the most part already is capable of drawing more CFM than you need. Simple math says a 5.7 at 5000 rpm is roughly 470 cfm. Most 4 bbl carburetors supply 625. Granted, the stock intake and head design along with the exhaust isn't the most efficient. The cam shaft geometry is for a smooth idle with a good solid torque curve starting at low rpm and peaking around 4800 which is where the HP and torque curve intersect. Keep in mind for most stock engines the HP curve has already started to fall at this speed and torque is flat. For our purposes this is, for the most part, perfect. So the propeller pitch and blade surface (diameter, number of blades) is adapted to this specific curve to allow the engine to reach that intersection but no further. To allow it to go further will cause a noticeable drop off in power as the stock engine simply begins to run out of CFM. It runs out not necessarily because of the carburetor limit but because of the cam, valve, and air runner designs. This is not a bad thing. These engines are not intended to go beyond 5000 rpm, and are not designed to sustain that rpm for an extended length of time. The stock small block Chevy will actually starve itself for oil eventually because the oil drain holes in the pushrod valley cant drain oil fast enough back into the oil pan. For those of use who like bursts of high rpms and for nearly all automotive use that is a non issue. The stock Chevy small block is balanced perfectly to produce maximum power and longevity.
However there's always folks who want more and hey, why not? The small block Chevy is renowned for high performance applications.
To get more "launch and pull" we need more low end torque. To get higher top speed, we need to turn a higher pitch prop at 5000 rpm. I am going to stick with 5000 rpm because that is what our drives are designed to withstand. So, we want to be able to use a higher pitch prop as our standard prop. For most 18-20 foot bowriders this may not be too difficult as those hulls generally aren't too deep and require less power to push through the water than say a 22 foot cuddy cabin with a deep V hull. A skiboat is not a good choice for a speed hull as they are designed to produce a certain wake at certain speeds and easily become unstable. The flatter the hull, the easier it becomes to make it go faster at top end.
Okay I've covered the basic starting points. Prop pitch and hull type. I'm not going into prop types as I can type all week on that. For all of us folks who just want a little more, most of you will stay with the prop you have and try to turn it faster. Do so at your own risk. I do not and never will recommend exceeding 5000 rpm with a stock bottom end and a stock drive, especially when they have god knows how many hours on them.
Okay, lets start with camshafts. The very basic tenets are: Lift gets you low end torque, duration gets you high rpm HP. If you change Lift only then low end torque theoretically improves. This is the basis of "RV" cams and supposedly "marine" cams. This is the reasoning: High lift allows more air/fuel mix into the cylinder all other things being equal. lets assume the duration has not increased. What now happens is more air and fuel is now allowed to work on the piston in the same amount of time as the original stock design. More air and fuel = more force driving down the piston in the same amount of time. This is effective to a point. At higher RPMs the time allowed for the air fuel mix to travel into, burn, and then exhaust becomes too short and inefficient resulting in a noticeable drop off in power at rpms above 4000. This is actually fine with most RVs and trucks used for towing. To keep power climbing above that and to smooth out the curve requires duration. Duration is the amount of time or "degrees" a valve is allowed to stay open. There are many other factors such as "ramp speed" and length of time at max lift, but I'll keep it simple. At higher RPM the engine starts needing help moving the intake and exhaust gasses in and out. The camshaft achieves this by keeping both the exhaust and intake valves open a certain length of time to assist in the pushing out of exhaust gasses and pulling in a fresh intake gasses. This is called cam overlap. All camshafts have it. Its a good thing as it cools cylinders and without it engines would self destruct. SO... at higher RPMs we need a good match up of duration, overlap, and lift to feed the engine efficiently. Therefore if you buy a camshaft with higher lift and a small increase in duration it almost cancels out the benefit at low rpm because the added duration does away with the higher lift benefit at low rpm which is where a marine engine needs it the most. Lets go back to what I wrote earlier about high lift only, add valves staying open longer... and that shortens the time the mixture is allowed to burn in the cylinder. This is why high duration cams in high performance engines produce the "lope" at idle and why those engine produce virtually no power below 3-4000 rpms. The point made here is.. Add lift.. add torque. Add duration, less torque but higher rpm power. Look at RPM ranges on cam spec sheets, I can guarantee in EVERY application the "range" will always be higher rpms than your current stock engine. If your current stock engine is designed for a range 1100 rpm to 4800 rpm and you change that with a camshaft with a range of 1300 rpm to 5500, all you have done is move low end power to the other end. That's all. If you can spin that prop a little faster, well then you've accomplished a little more high end speed at the cost of low end grunt. If that's what you want, Yay for you.
Remember, a boat sliding through water has a massive amount of friction to overcome, and a massive amount of weight to push aside. It's like climbing a 6 degree hill with a trailer on the back of your truck in a 30 mph headwind. Therefore a very small increase in top end HP does you no good because it requires so much more to overcome the external variables. This is why you cannot use automotive logic for small power increases that would be noticeable on an automobile rolling down the road. An extra 30 hp that has been gained at the cost of low end grunt is negligible and may result in unsatisfactory performance in a marine engine.
So how do we improve performance across the board and be able to still have a good smooth running engine for dockside maneuvering? The quickest most efficient way to achieve this is to increase compression, and increase the amount of time the mixture is allowed to work on the pistons. A camshaft change, valve changes, and smoother runners help achieve this but as you see, it's very very difficult to get a solid performer throughout the range with just valve timing and geometry. You can add a supercharger or a turbo and that will dramatically increase available power. These top end changes will affect bottom end life as most folks know.
So, how to increase compression without removing the engine? Find heads with a smaller CC chamber. Most stock truck head valve chambers otherwise called "Quench chamber" are about 72CCs. There are non vortec heads with 64 CC chambers and a few older versions at 54 CCs. Do not under any circumstance just swap to a smaller chamber without know exactly what your valve to piston clearance is. Most stock 350's nowadays have reliefs in the pistons and a lot have dish pistons with a lot of area concave in them. This is to assist in the swirling of the mixture to achieve more thorough mixing at the molecular level so we don't have larger drops simply falling out of the mixture and not burning and simply getting pushed out the exhaust which is what is known as HC on your car's emission reports by those pesky emission tests. The dished pistons along with the head chamber design is all engineered to work together to give you a good solid burn with the gasolines available. It works very well. If you change it with a cam or head change, you change that burn quality for better or, more likely, for worse. THIS is why you will always hear experienced engine builders tell you "It all has to work together"
Changing a 72 cc head to a 64 cc head will increase compression. Usually by .3-.5 points. You may or may not need to increase fuel octane depending on what you are starting with compression wise. Most small block Chevy's start around 8.5 - 9.5. Above 9.5 usually requires higher octane due to the increased temperature of the combustion taking place. Anyone that's ever pumped up a bike tire with a bike pump knows the pump gets hot compressing the air. This is what higher octane gas does, it "resists" igniting at higher temperatures by the heat of compression only. This is also why it is useless to burn high octane gas in an engine that doesn't need it. Back to heads. Lets say you stick with your original heads, you don't pull them and just change "rockers" to a different ratio. The stock ratio is 1.5. This means: The rocker is like a lever. Movement on one end via the pushrod is transferred to the valve at a specific ratio so the spring can be compressed easier with less wear on the camshaft train. Changing that to a 1.6 ratio will push the valve open substantially depending on the camshaft lift. If you increase camshaft lift and also rockers to 1.6 you must have reliefs in the pistons to accommodate the extra valve travel. DO NOT change rocker ratios without knowing for a fact what piston to valve clearance is and how long those valves will stay open. This is accomplished by removing the heads over and over to measure that clearance for each piston. It is the ONLY safe way to do this. Rotating the engine on a stand and making sure nothing hits is useless. At 5000 rpms everything moves much differently and impacts can happen. That is why there is a specification for valve to piston clearance. I personally use flat top pistons and right now am running 72cc heads. I have a set of Vortec 64cc heads I will install over the winter and I will have to change the camshaft. That's only logical as I am completely changing the way that engine will be moving air through it. So, the point here is changing a cam and deciding to change the rocker ratio without fully building for this can cause serious damage and at the very least, poor performance. Here the "It all has to work together" mantra is absolutely critical. DO NOT assume a voice from a camshaft manufacturing company knows what type of pistons or heads you are currently running. I have seen it many many times. Valves ever so lightly touching pistons wearing a slight groove in them, which produces a ridge, which exposes a sharp line which concentrates heat on those sharp lines and eventually melts a hole in the piston.
Changing springs is strictly for high rpm. This helps stop valve float at high rpm. Valve float means the valves no longer actually close at high speed because the spring cannot react quickly enough. Using high performance hi load springs on a marine engine is a waste of money and can actually damage your valves and heads. The valve seats and valves themselves, along with rotators under the springs must all be engineered correctly or head life will be much shorter and sometimes there can be instantaneous damage if there is already a very tiny crack in a valve seat which doesn't really affect normal operation but will split wide open with the added pressures. If you disregard all the other stuff I have written here, listen to me now. Don't mess with the stock Chevrolet valve configuration without an expert. This requires a machinist to engineer your heads and valves to do what you want them to do. Leave it alone, and it will work for you, even with most camshaft changes. A good machinist who knows what cam you are using along with the bottom end you are using can be invaluable. Grinding off head surface should only be done with all this information known to the machinist.
You can determine what head type you have, valve sizes, and chamber CCs by looking up the casting number on top of the head under the valve cover. Sometimes the bolt configuration on the end of the head will tell you also but the number is best. Get it, look it up online, and know what you have as a starting point.
Chevy cylinder head Identification - Small block casting numbers
I'm getting long winded here, but I should cover carburetor size increases. Do the math, Cylinder Bore X stroke X rpm will give you CFM required. If you install a 1000 CFM Holley double pumper on a stock engine, you kill bottom end torque because the primary butterflys are much bigger than stock and when they open engine vacuum drops more than it does with stock size and therefore the airflow slows WAY down, the gasoline liquefies and drops out of the air, and the engine literally starves and requires more time to increase RPM simply because there is suddenly way more air available than it needs. That slowdown is the "Bog" you feel and hear when a 4 barrel carb opens secondaries suddenly. Mechanical secondary carburetors are known for this. Rochester Quadrajets are mechanical secondaries. They require a restrictor flap to keep that bog to a minimum. Most stock Holley 4 bbls are vacuum secondaries. Most times the don't open and when they do, it's very little. This is because the engine doesnt require any more. Even at 5000 rpm it is possible your vacuum secondaries don't open simply because the engine is already being fed what it requires. Don't believe it? Stick a bolt in the secondary linkage and force it wide open at full throttle. I guarantee you there will be zero increase in speed. You can't feed it more than it can take. Funny thing that. Here you have a 625 CFM carb on a 350. Do the math and holy cow it should use 460 CFM! Well grab a 700 or 725 and lets get this puppy going! And nothing happens but you notice your low end grunt has diminished. Math doesn't always mean that's what it's going to use. It's just a baseline. 625 CFM Holley vacuum secondary 4bbls are widely regarded as the best choice for stock Chevy 350 engine use. Don't change it without a corresponding correct modification so the engine will actually use it. The more available air, the faster the engine has to rev to use it, and if the engine is limited by prop pitch you gain nothing. The engine simply cant reach out and use the extra CFM if it is unable to achieve the required RPM.
Use caution, and get all the information you can. Understand the basics before you change the inner workings of an engine. For the most part, changing cams and even heads on a boat usually isn't worth the expense especially if you have to pay someone to do it. If you want true performance, pull the engine and build it for what you want from the ground up.
IT ALL HAS TO WORK TOGETHER
Feel free to send any questions to
Walt@boatdoctor.com
My background in marine performance is dragboat racing. Pit crew for "Flat Hammered" blown gas flat drag boat, and 35 years building performance engines. I rarely build engines for sale anymore, usually just for friends and for free. Their parts of course.
There is a truly good feeling of satisfaction when its done right and you get out on the water at full throttle.
