So What Cranking Pressure Can We Expect To See Then?
For an average road engine we don't need to calculate the effect of all the above factors because the expected cranking pressure will be quoted by the manufacturer. Haynes manuals and other tuning guides also quote the figures. If the data is not available you can make a good estimate of the expected cranking pressure from a healthy engine just from the compression ratio. You can expect the cranking pressure in psi for a road engine with a standard cam to be about 17 to 20 times the value of the compression ratio. So our engine with a CR of 10:1 should generate between 170 and 200 psi. The better the condition of the engine the closer the figure will be towards the top end of this range. If the engine has been modified by fitting a different cam then you need to make some allowances. Fast road cams will reduce the expected figure by 10 psi or so. The best thing to do is take a set of compression readings as soon as the engine has been built so that you know what it generates in good condition. For competition engines it's even more important and I recommend to all my customers that they do a compression test after every race to make sure that nothing inside the engine is going wrong. The vast majority of them take no notice of course because it involves a bit of work. Finding a potential problem before it leads to an engine blow up can save a lot of time and money in the long run though.