Selecting SBC Cylinder Heads
Selecting SBC cylinder heads for any combination is a key element for producing more power, simply put, horsepower is derived from the amount of air that fills the cylinder and is trapped by the intake valve closing at just the right time inorder to pack the cylinder full with as much air as possible,
whether the air is pulled into the cylinder by N/Aspirated applications using just good ring seal or more air being forced into the cylinder by turbo or blower applications (more air = more hp), it's easy to start to see how the cam and cylinder head selection work together to create and trap power, but happens to be just two of the necessary components that will have to work hand-in-hand to produce a combination that will have enough torque to move the weight of the vehicle and still supply us with enough power and rpm to pull thru the other end to produce mph and be able to run on 93 octane, so let's take a look at some of the components that can have the biggest impact on your cylinder head selection and driving requirements..
Type of camshaft and RPM: Your choice of camshaft, whether, a hydraulic flat tappet or hydraulic roller will dictate a maximum operating rpm range: 6000-6500 (because of valve float), and 7000-7500 for the solids and since airflow is measured in cfm (cfm is both a volume and a speed, and can be calculated) this will help determine how much airflow is necessary to fill your cylinders to that desired rpm for a certain size cylinder and also the cam type and cam lift that you intend to use will tell us what valve spring combination we need: 1.250 for hyd. flat tappets up to .525 lift, 1.470 for hyd. rollers and solid flat tappets up to .575 lift and certain Brodix SBC heads will use a larger spring to handle more lift. Camshafts also have the ability to move the power around in any combination, on one hand, a shorter duration camshaft will give you more power downstairs, but at the same time sacrifice some power upstairs-- and on the other hand, a larger camshaft sacrifices power downstairs to give you more power upstairs but usually requires the help of a higher stall speed converter to slip thru the dead low rpm band. (any time that you are going to continually run to the higher rpm, titanium retainers would be a good option to protect your investment)
Compression: makes heat in the cylinder, heat makes power, the more compression the more heat and power, but to keep the heat from growing to an un-controllable cylinder temperature (heat: is combustion chamber heat, not water temperature), it requires adequate octane to cool the heat. When 106 octane was available, carbureted engines with 11.5 or 12 to 1 compression didn't have a problem because the octane could keep the cylinder temperature under control, but carbureted at that compression with todays 93 octane, our cylinder temperature will just keep rising until the point of detonation. (most of our Brodix Cylinder Head selections have head options for a variety of combustion chamber sizes to help with your desired compression)
Camshaft selection and Compression: can play an important role in the overall combination along with the compression, longer duration cams, (such as the comp cams custom hyd. roller 230° -- 236°, .541 lift that we use in our N/A 383 street/strip engine), not only gives us the sound that we all like to hear and access to upper rpm power, but also help us relieve some of the compression (10.6 to 1 carbureted--see note 1 below) to operate on 93 octane and at the same time, along with the proper head selection (IK-200 w/64cc and optional bowl work), help us achieve the rpm zone (6200rpm with the proper stall speed and gearing even with the 7501 air-gap dual plane manifold) that we need for that strong pull upstairs (10.95 et/122.50mph), on the other hand, choosing a little shorter duration cam that produces power earlier (not requiring as much converter), but at that compression would create too much cylinder temperature, so it may be suggested that the compression be lowered to 9.5 to 10 to 1, depending on your exact cam specs. The above 383 S/Strip combination we used in a 3270 lb car shows some of the boundaries we used and how certain components worked together to accomplish his intended driving usage--everyone has different goals, whether you want to build shear torque to move a heavy car w/o the use of a stall converter, or a killer street/strip car, or find that happy medium, selecting the right components to fit your needs can work in your favor--let's look at how airflow fits into the plan--
NOTE 1: if you're considering a tuneable injection system like the FAST XFI Fuel Injection System or similar, then you can increase the compression to 11.0 to 1 or more and still be able to tune the air/fuel ratios through the entire rpm zone to keep the cylinder temperature in check and pick up some more power from the compression and tuning ability of the FAST XFI System and for anyone using forced induction, I definitely suggest such a system--see custom kits in our FAST Injection section.
Airflow--Cubic Inches--RPM--Runner Size
Airflow, Cubic Inches, RPM and Runner Size have to work together, and this is very basic: if you have a 355ci, and you divide that ci by 8 (to determine the cubic inches of 1 cylinder because flow numbers are for 1 cylinder) you have 44.37 ci for each cyl of that engine, and taking this a step further, let's say one port of a cylinder head flows 250cfm (see note 2) at .550 lift (assuming that's your actual valve lift), we know that particular amount of air will continuously fill that size cylinder and make power (see note 3) to 6700rpm. Now, if you were building a 383 ci, one cylinder would be 47.87ci, not much different than the 355, but that same volume of air would only fill the 47.87ci cylinder of a 383 and make power to 6246rpm, so, basically the 383ci will require more airflow (270cfm) to make power to the same rpm as the 355ci. On the other hand, if you're working with a solid cam in a 383ci that has an rpm band up to 7500, you would need at least an honest 300cfm to make power to that RPM!!! Now that we know more cubic inches needs more airflow to fill the cylinders to that particular rpm where your cam type (hyd. or solid) requires to operate, let's take a look at how to add a little more airflow:
NOTE 2: the cfm that we're using in the above examples would be with a manifold bolted onto the cylinder head and then tested, and from our testing, the average out of the box manifold will usually depreciate the cylinder head airflow cfm enough to create a 300 to 500 rpm loss (depending). Some of the flow charts we display will have a test showing a manifold bolted on, sometimes with work done and sometimes w/o work.
NOTE 3: "making power" refers to pulling to a specific rpm under load, not just freewheeling rpm.
Filling the Cylinder with More Air can be accomplished by matching the intake ports to allow a smoother transition of airflow coming from the intake manifold and entering the intake runner which will help fill the cylinder with more air and also some professional bowl blending (pictured to the right) which recontours the bowl area to create a venturi effect helping the air to speed up as it passes thru the smaller valve seat area and flow results will increase thruout the entire lift range promoting power across the whole power band --see optional purchases in any of the SBC cylinder head sections for both of these operations--
CNC chambers option is a relatively low cost option for promoting increased low lift flow that helps the airflow get started earlier which is often overlooked--The intake charge, largely due to a slower speed as compared to the exhaust speed, is slow to get moving, even as the piston passes TDC the charge is still stumbling around in the runner waiting for a wakeup call and a direction, it's not until the piston really picks up some speed travelling down the bore that the suction will start to affect this volume of air and fuel and begin to fill the cylinder. Anytime you help the flow start earlier, especially in the .250 thru .400 lift range (especially important when you only have a max lift of .550) will always create more power by filling the cylinder a little better and earlier.
Runner Volume usually coordinates with the flow numbers, the larger runner will be capable of flowing more air and the smaller runner, a little less air but at the same time, the smaller runner, because of less volume will have the advantage of getting the air moving sooner to fill the cylinder a little earlier to make the power sooner, but the disadvantage of moving too fast thru the runner causes increased friction and the air can become hot thinning the mixture and losing some of the explosivness. We generally use the 180 runner on 327-350ci that are mostly cruisers that want to develope quick throttle response to move heavy vehicles, the 200 runner is generally used on 355-383ci for any street/strip action, the 406ci generally requires a little more air to make power to the 6000-6500rpm range and therefore will use an IK 210 full cnc or a set of track 1 cylinder heads.
Forced Induction explains the real meaning of more air = more hp by forcing all the available air into the cylinder sooner, talking about air speed and friction in the runner, whew!!-smokinnng for sure!! For an example, if you took a set of heads from a N/A 355ci that had enough airflow to make power to 6200rpm, and you installed them on another 355ci, set up for forced induction, you'll make a lot more hp because all the available air is forced into the cylinder sooner, but not easily understood, because it used all the available air sooner, and the rpm that you make power up to is now lowered, usually by 800 to 1200rpm, so a larger runner and more airflow is definitely needed. Also, because there is often more cylinder temperature involved with forced induction and nitrous, the valve material is usually upgraded to a titanium or inconel, sometimes beryllium copper seats, and silicone guides. We also list these in optional purchases, in the RaceRite, Track1 and 18° cylinder head sections.