Welcome to this group of tinkerers, Neil. I agree with your dim view of unwarranted enthusiasm for ever higher compression ratios as though the law of diminishing returns does not apply to this facet of the internal combustion gem. You, Tone, Steve, Cody, etc. have an intuitive understanding of motion which most do not. It’s a gift from our Creator, like being able to play a musical instrument. You can close your eyes and see the engine running. Pick up a steam engineering textbook which was state-of-the-art 110 years ago. You will discover the “indicator card”, which mechanically recorded the instantaneous cylinder pressure through the cylinder’s stroke as a graph on a card. Like an engine EKG. We used to instrument diesel engine fuel injection lines at both ends with pressure transducers and trigger an oscilloscope from the crank to observe fuel pressure dynamically 40 years ago. If there is a transducer that can survive the conditions, a cylinder head could be instrumented this way. Those caps on Tone’s diesel would be the easy way to gain access to the combustion chamber. Anecdotal evidence shows that woodgas likes to run with plenty of advance. Obviously the ideal is to build pressure rapidly just after TDC without detonating the last pocket of the charge via spontaneous combustion ignited by heat, pressure, and hot spots. You have heard of BMEP, right? Brake (Prony brake, the old-fasioned dyno) Mean (average) Effective (the collective result of many factors in the always-changing volume) Pressure (confined gasses in the chamber). Now freeze-frame the high-speed camera’s film in your mind. See Instantaneous Effective Pressure. You can see that a very slow-burning mixture would just chase the moving piston on the power stroke, not imposing much force on it, giving poor torque. A steam engine is different - pre-pressurized gas crosses the slide valve into the chamber until it shuts, a point called steam cutoff, partially down the power stroke. Internal combustion has a batch of mixture to work with in the chamber for each cycle. The lopey idle that hot-rodders hear as BIG POWER is actually inconsistent batches caused by inconsistent charge dilution during a long valve overlap period at the exh/intake interchange at TDC. Wave action in both the intake and exhaust are only “on the pipe”, as 2-stroke tuners say, at one RPM, and this is dependent on the speed of sound, which varies with density, which varies with temperature. So the whole thing is a randomized mess. The power you feel as vehicle acceleration is the sum of thousands of batches processed. Tuning an engine to only run right at one very high RPM is only good for fun, not practical usage. Side note: We see the ram horns emblem on Dodge trucks. This evolved as a trademark. The original meaning was the ramcharger(R), long intake runners on a V-8 which had carburetters located over the opposite bank’s valve covers! These, tunnel rams, headers … all are efforts to take advantage of pipe organ theory to cause a reflected high-pressure wave (think of a Slinky(R) toy) to arrive at the intake valve just before the overlap period, giving a chamber-packing effect without paying for a supercharger. But, of course it only works at one RPM and at multiples of that RPM. So why is there an overlap period at all? To a large extent, there isn’t any in modern engines because it causes unburned hydrocarbons, …pollution. But the reason for it was to increase duration, to give more (area times time), in turn to deal with the reality of inertia in a spring-closed cam system operating the valve train at high RPM. You have to keep the follower on the cam’s return slope so the valve does not hammer the seat at closure. Ever-stronger springs cause other problems. The other end of ‘duration’ has an Atkinson effect on the intake stroke and a negative effect on BMEP on the tail end of the power stroke. Add it all up and at low RPM the cammed-up Rat motor makes less low-rpm torque than the showroom floor mouse motor. So the rodder puts 4:11 or higher gears out back and burns gasoline as if he owns the oil well. And the engine is only running right when the rig is going fast enough to get a big speeding ticket. So now the MFG’s hot-rod with variable valve timing and get the strong, flat torque curve we could only dream of 40 years ago. But the price is complexity and the potential for breakdown with age, and sensitivity to proper maintenance. It all boils down to this, and I think Steve would echo this, You Can’t Have Everything. A steam engine makes mountains of torque right from zero RPM and does not even need a starter, but then there is that huge, dangerous boiler that feeds it. About compression ratio: The John Deere 2-cylinders with their 5 1/2 to one compression ratio consistently held the fuel economy title year after year in the Nebraska tractor tests back in the 30s & 40s. Compression ratio is one of many factors, it is not the sole determinant of performance. Scavenging ratio is never spoken of but is more important. Charge dilution is important. EGR is intentional charge dilution. Unavoidable nitrogen in woodgas is charge dilution. Gas mixing during valve overlap is charge dilution. The proportion of chamber volume at TDC to that at BDC is held out as a magic number OMG it’s got a 13:1 CR - what a powerful engine - hear, hear!! How simple-minded. Put a compression tester on it and give me the PSI reading. Factor in your local barometric pressure to compute the pressure in absolute PSI or megapascals. That is what counts. Of course it changes with engine speed and charge density, and that’s the point of swapping cam profiles, header and intake lengths, superchargers, intercoolers etc. That’s all harmless fun, but there is also a place for practicality, and I vote for Tone’s Fergie to be second in the future DOW Hall of Fame, right after Wayne’s V-10 hauling that long trailer full of cattle. -RedOak from the “States” Edit: I meant 4.11:1 gears. Edit#2: The Atkinson effect occurs at the beginning of the compression stroke, defeating some of the prior intake stroke at low RPM. At high RPM, moving intake air column inertial begins to counter this. Organ pipe effect can also counter this at one narrow portion of RPM range. Hello peaky torque curve! Edit 3: Sorry, I should have proof-read this, it’s NOT ‘to arrive at the intake valve just before the overlap period’, but '‘to arrive at the intake valve at the end of intake valve duration’.
