RHS™ Tech Info. & FAQs

RHS™ offers technical assistance simply by filling out our online cylinder head technical assistance form, or by calling us at 1-877-776-4323 M-F from 8am to 5pm CST. We also offer tech assistance in the form of an extensive archive of informative magazine articles, and our latest offering—The RACING HEAD HELP™ racer’s FAQ. Got a question or answer that you think should appear in the FAQ? Email us, and we’ll see about getting it posted.

I’ve picked out a set of heads, any ideas on an intake manifold?

Look at velocity for low rpm torque, and flow for high rpm horsepower. Most carbureted performance intake manifolds are divided into two categories – dual plane and single plane. Dual plane manifolds are better for low and mid-range torque, and single plane manifolds (e.g., open plenum intake manifolds) emphasize mid and upper-rpm horsepower. In general, dual plane manifolds are better for street use; single plane manifolds are better for track use. Check with your intake manifold’s manufacturer for specific information. EFI manifolds differ in design characteristics, with improved midrange and top-end horsepower being common advantages of aftermarket EFI intakes.

Any ideas on what type of what type of aftermarket headers I can run with RHS® heads?

In most cases, aftermarket headers are more than acceptable – they are encouraged. They will typically improve the performance potential of any cylinder head (or engine, for that matter).However, RHS® heads feature spark plug locations that are optimized for better atomization to ensure maximum performance. Therefore, it’s important to check for fitment issues prior to ordering aftermarket headers – especially in unique, less common engine applications.

Based on my cylinder head choice, what is the right pushrod size for my engine?

Some variables influencing pushrod length are engine block deck height, cylinder head dimensions, camshaft specifications, and many other characteristics that vary between individual engines. We recommend you measure using a simple, inexpensive pushrod length-checking to tool verify geometry, and contact us so we can set you up with the correct pushrods the first time around.

What valve springs should I run?

Valve springs should be chosen based on compatibility with the camshaft you plan to use. Figure 1 lists general guidelines to help you decide which spring to run based on pressure requirements for different types of cams. Your valve springs must also have coil bind clearance of at least .060” over the camshaft’s peak lift. All fully assembled cylinder heads offered by RHS® display maximum valve lift in their tech sheets. Beehive springs typically have lighter open and seat pressures than conventional springs. The best way to choose a set of valve springs is to go with your cam manufacturer’s recommendation after first verifying that the valve springs will satisfy your cylinder head’s spring pocket diameter and installed height requirements.

What kind of camshaft should I run with my new cylinder heads?

Figure 2 lists a “ballpark” range of camshaft durations to consider based on the intake runner volume of various RHS® cylinder heads. Unless you run a high valve lift (above .550”) camshaft with 220 or 235cc heads, you may not realize any performance advantage over cylinder heads with smaller intake runners. We do not make recommendations based on advertised duration because various rating methods used by cam manufacturers, which makes for poor camshaft comparison. Remember – this is a general guideline. Your intended application or the advice of your engine builder should always take precedence when it comes to cam selection on an individual basis.

Can you tell me if I have enough piston-to-valve clearance based on my engine data? And where do rocker arm ratios figure into all of this?

Piston-to-valve clearance varies between individual engines. Not checking it could lead to your camshaft having more lift than clearance, which can bend your valves, or worse, punch holes in your pistons with them – necessitating engine teardown or even a ground-up engine replacement. Write down your clearance once you’ve measured it, so if you decide to swap cams, some easy math can tell you if you have enough clearance. Measurement needs to be performed again if engine modification occurs, such as swapping blocks or heads, machining deck surfaces or changing head gaskets. Refer to your existing measurement to verify fit if you swap to a higher lift cam or change to a higher rocker arm ratio.

The following equations help explain the effect rocker arm ratio has on valve lift. As an example, we look at a common rocker swap: Small Block Chevy – switching from the factory 1.5-1.52 ratio to a 1.6 ratio. Assuming a .500" lift camshaft (based on 1.5 ratio); we first need to calculate lobe lift. The equation is as follows:

Valve lift / Rocker Ratio = Lobe Lift

Plugging in the numbers gives us: .500/1.5=.333" lobe lift. Rearranging the equation gives us our gross valve lift with the new rocker ratio:

Lobe Lift x New Rocker Ratio = Valve Lift

Inserting the cam and rocker specs gives us .333 X 1.6=.534" – a substantial difference in lift. Many people say "switching to a 1.6 from a 1.5 gives .030" more lift!" This is true in some cases, but this number is a ratio – not a constant. Do not assume lift increase will be any set number. Do the math, record the numbers and verify that you have clearance. You'll never have to worry about putting a valve through your brand new forged racing pistons!

* If running a split pattern cam with a different intake and exhaust lobe lifts, do the equation twice (once for each lobe) to accurately calculate your gross valve lift.

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