Originally posted by John Maher on the Cal-look Lounge, November 2008:
Can we identify these heads?
It’s possible your brand new cylinder heads may not the best choice for good low and mid-range performance. You can’t mask a head deficiency with cam, rockers, carbs etc. Huge ports can flow a lot of air but the cfm number should be judged on cfm per square inch of flow area i.e. EFFICIENCY of the port. Big ports and impressive cfm numbers may work well on a high rpm drag race motor but when fitted to an engine intended for street use could be a pain to live with on a daily basis
With regard to extracting good performance from conservative ports and valve sizes, check this graph (strong low and mid-range torque took top priority).
2110cc bus motor, 151bhp and 157ftlb torque
With a slight change in jetting, peak figures are now slightly higher than those in the graph… 153bhp @ 6000rpm and 160ftlb @ 4500rpm. The engine is all based on fairly conservative stuff….
- Engle 120 camshaft
- 1.25 rockers
- 40mm x 35.5mm valves (JMR modified heads)
- Weber 40IDFs.
The key ingredient is the small, fast efficient port design.
I could achieve the same cfm numbers with a lot less work by fitting larger valved heads but port velocity would fall, the engine would make less torque in the rpm range where it’ll spend most of its time and throttle response would be lazy.
With flow data you have a good idea of maximum bhp potential
Once you know lift at which flow increase tails off, you have a target peak lift figure to aim for.
If you have a duration figure in mind this probably helps narrow cam choice down to 2 or 3 candidates.
JMR flowbench
With the bus example shown in the graph, budget didn’t stretch to a set of 1.4 rockers. Based on flow data I knew I wanted close to .500″ total lift but didn’t want to run excessive duration to achieve it. The E120 w/1.25 rocker combination delivers an average of .470″ lift at the valve
That takes pretty good advantage of the flow characteristics of the heads (172cfm @ .500″ – tested at 28″ H2O) …. more than some of the ‘custom ported’ 44mm valved heads I’ve tested!