This article by Bill Hyatt has already appeared in the April issue of The Sacred Octagon. Bill sent me his copy back in February, but it has only been possible to publish it in TTT 2 now.
“It is very easy to get discombobulated when chasing power, it is best to start from basics.
When chasing greater performance for safer touring in modern traffic, 25% greater torque will be immediately noticed, whereas 25% greater HP will barely be noticed when staying out of the redline area. Since very little time is spent at WOT (wide open throttle), more grunt for passing, climbing hills is advantageous w/o shifting down. HP via greater rpm means a narrow max performance RPM range which would indicate need for 5-6 speed tranny.
The methodology to achieve greater vintage power is based on the premise that one has to understand basic concepts involved in achieving the end game, before selecting from the proper of many false trails available. Missing building elements based on First Principles can lead one down a wide selection of erroneous paths, not conducive to the best results sought. Hence the trilogy of discussion put together so far: Thoughts on Torque & Tightening for Critical Fasteners, then Valve Train Dynamics, & finally Chasing Vintage Power.
For performance, the primary design parameter is obviously to achieve the best power to weight ratio, tempered by various outside constraints such as the capabilities of the chassis, tranny, diff., brakes, etc. and these components being able to accommodate any extra loads from the added power and the adverse effect they have on the service life of the engine.
To achieve more power there must be more fuel. To burn more fuel, there must be more air (O₂). More air is difficult to achieve with NA (naturally aspirated) engines, but abundant air (O₂) is available with FI (forced induction) engines via classic supercharging, turbo charging, or adding oxygenators such as nitro methane, or on demand nitrous oxide. The down side is that the larger the explosive combustion event generated, while maximizing BMEP & torque, also generates extra thermal spikes that can exceed the temperature limits of stock, non-specialized engine components, resulting in burned valves & pistons etc. (Bronze alloy valve guides and seats are becoming common, acting as heat sinks, drawing heat away from valves, also copper bnz. alloy top piston rings are being tested as heat sinks to better transfer excess piston crown heat to cylinder walls). Also, there are the extra induced mechanical stress loads that may affect the rotating assemblies and engine structure, possibly making them incapable of sustaining additional loads long term. As high horsepower requires high RPM, the stability of the valve train assembly becomes a weak point.
It is bad form to be passed by a Trabant, a Yugo, a Goggomobil, a Crosley or a Cyclops as long time readers of Road & Track may recall. One has to be able to say ta-ta, and show them who is real boss.
- Torque & RPM are measurable quantities of engine output.
- Power (HP), rate of doing work, is calculated and dependent on torque & RPM.
- HP = Torque x RPM/5252: E.g. 50 (lb-ft) torque x 4K rpm = 38 HP, therefore 50 lb-ft. of torque x 7K rpm = 66 HP and so on.
- Torque = HP x 5252/RPM.
- BMEP (PSI) = 150.8 x torque (lb-ft)/ displacement in c.i. (cubic inch)
Max HP: This occurs at max available engine intake airflow and max fuel burn, so there is no leftover residual unburned air/fuel mix remaining after the combustion event. This is the Stoichiometric ratio, in lay terms, where X amount of a reactant (fuel, gasoline, e.g.) completely burns or reacts with another reactant (air) so that there is no leftover residual unburned potential air fuel mixture remaining after the combustion event. i.e. complete combustion.
Max Torque: This occurs in NA (Naturally Aspirated) engines when there are max units of air (O₂) charge to fill the cylinder(s), i.e. the best valve timing that induces the max unit of air charge to optimally mix with the fuel units available in the cylinder at the combustion event. Bigger combustion explosion = greater BMEP = greater torque & HP. Under fixed atmospheric pressure it is hard to get air introduced into the cylinder(s) to maximize the combustion event, whereas extra fuel can be easily introduced and maximized with bigger jets to match the available air for the best air: fuel ratio.
Without fitting at least a temporary O₂ sensor, the problem is getting the ideal average (Stoichiometric) 14.7:1 air: fuel ratio to be spot on for complete combustion and max “bang”. Therefore, it is easy to see that maximizing torque (via more O₂) is a very productive means of generating power w/o going crazy chasing HP via possibly undesirable uber RPM in vintage engines. For non racers seeking more power, chase torque, not HP. A little richer air:fuel mix, say 14:1 gives more power, and a little leaner, say 16:1 gives better fuel mileage. Today’s wideband lambda O₂ sensors provide a digital readout allowing one to optimize air:fuel tuning ratios throughout the RPM range with vintage carbs by testing different needle and jet combinations. Gas engines will putter along nicely at air:fuel ratios ranging from 10:1 to 20:1 compared to the more ideal 14.7:1 Stoichiometric ratio that results in the greatest BMEP.
HP via higher RPM destabilizes vintage valve train dynamics, stresses vintage engines, etc., See link above.
BMEP: (Brake Mean Effective Pressure), is the more important figure. In lay terms, it is the average pressure imposed uniformly on the piston from top to bottom of the power stroke, producing a measurable (brake) output amount, i.e. how big the explosive event is on combustion. The bigger the bang slamming the piston down, the more torque is induced into the crankshaft producing power. BMEP is a derived theoretical figure, not associated with cylinder pressures. It is just a comparative measurement of the efficiency of any given engine to produce torque for a particular displacement size. The more available air (O₂) inhaled to mix with fuel during the intake charge cycle, the greater the explosive event = greater BMEP = greater Power. Spinning the engine to greater RPM = greater HP for any given torque, BUT GREATER RPM WILL IMPACT SERVICE LIFE OF VINTAGE ENGINES. Solution is to chase torque, aiming at maximizing a high, flat torque curve to the desired red line. Getting more air (O₂) mixed with the most fuel to achieve the ideal Stoichiometric mix is the goal. All kinds of engine mods can increase engine efficiency which can be measured by greater BMEP, e.g. larger cylinder displacement will allow more fuel: air mix to be ingested for greater bang on ignition. Higher compression ratio is the simplest means of greater BMEP via bigger explosion at combustion.
With NA (Naturally Aspirated) engines, greater HP means chasing RPM, not necessarily a good thing in a vintage engine, as a way to increase BMEP. It is better to chase torque by maximizing available air (O₂) to grow explosive combustion event. However, it is easy to maximize BMEP with FI (Forced Induction) via blower or turbo charging (introducing more air O₂). Nitro methane or other on demand O₂ enhancers such as nitrous oxide can be added into the cylinder on the fuel side of the equation. These all make an enhanced explosive charge by mixing with more O₂ at the combustion event. With FI “x” greater amounts of combustible air/fuel mix will be introduced into the cylinder to create a bigger bang (maximizing BMEP) and forcing the piston down with more power thus increasing the torque loads introduced into the crank. While the FI path will easily generate 1.5-2 times or more HP, it really generates lb-ft torque via more air/O₂, (the real goal) to the point of closing in on HP; thus there is no need for excessive RPM in vintage engines that would be necessary on a NA engine. Of course, the downside of FI, as with a NA engine looking for HP via greater RPM, is that the bottom end, pistons, valve train, etc in the engine need to be bullet proof due to increased, loads, stresses, dated engineering, & thermal issues.
I doubt if the service life of a FI engine would be adversely affected by its extra power, compared to a new OEM XPAG engine. It may be possible to get 30-50K miles between rebuilds for the FI engine compared to semi-annual or annual rebuilds for a NA engine with similar HP. A few engine mods and technology such as thermal coatings, anti-wear coatings, modern metallurgy, tolerances, etc, could greatly increase engine reliability and valve train service life to the current daily driver levels of 100K + miles as compared to 30-40K miles back in the 40s, 50s, & even 60s.
Just adding a cam w/o a lot of other work to improve air/fuel intake efficiency is unlikely to add significant performance. An engine with FI would require different cam/valves than one for a NA engine. Higher comp. ratio is the simplest means of greater BMEP via a bigger explosion at combustion.
Enlarging ports to XL size may actually decrease HP as there is not enough velocity to transit atmospheric air/fuel mix to fill the combustion chamber during the intake charge phase of the cycle.
Principles of Wave Form theory: Air/fuel mixes in and out of a 4-stroke engine are a very complicated transient, resonant pressure wave study (but not as complicated as on a NA 2-cycle engine). The end game with either 2 or 4 cycle IC (internal combustion) is to maximize the air/fuel charge into the cylinder to combust the charge with greatest efficiency to achieve the biggest explosive bang resulting in the greatest BMEP.
As an aside: See Walter Kaaden, who eventually achieved 200 HP/Cu. Inch out of a NA 2-cycle engine some 55-60 years ago, a figure unmatched in 4-cycle NA engines until recently.
https://en.wikipedia.org/wiki/Expansion_chamber Kaaden’s research into pulse jet resonate pressure wave concepts, incorporated in powering the notorious WWII German V1 Buzz Bombs, led to his later 2 cycle pulse gas expansion experiments. These experiments were aimed at using exhaust gas pulses to help evacuate combustion cycle gasses, while further scavenging intake gas flow during intake cycle, to maximize O₂ available beyond what is available via atmospheric pressure alone. Vintage readers may recall the bulbous expansion chambers incorporated in 2-cycle exhaust systems. Without FI, (forced induction) the objective was to enable the 2-cycle engines of the same displacement to compete against 4-cycle engines, by ingesting greater volumes of air/fuel mix than theoretically possible in N.A. 4-cycle engines; these being limited to 100% max gas/air fill of swept volume of cylinders i.e. 100% efficiency, (almost never possible, except possibly with modern direct fuel injection into the cylinder.) In a 2-cycle engine, anything above 100% efficiency is money in the bank, maximizing torque via greater BMEP.
Airflows are not steady state, but rather RPM related gas pulses (one pulse per rpm in 4-cycle, engines and two in 2-cycle engines.) stopping, starting, echoing back and forth, and overlapping each other. Air momentum, velocity, valve timing, and valve overlap all have to complement each other to maximize pulse units of atmospheric air charge available throughout or at desired RPM range. (This is not unlike sea waves hitting a restriction like a seawall or beach and refracting back as undertow or as reverse/negative wave forms enhancing the next incoming positive wave form.)
By using Wave Form theory, Kaaden was able to increase air (O₂)/fuel mix into the cylinder(s) beyond the theoretical 100% efficiency of atmospheric pressure fill alone. This functionally provided a supercharging effect in a naturally aspirated 2-cycle engine. Ultimately, he was able to achieve a volumetric efficiency equivalency of 140% compared to a 4-cycle NA engine with the theoretical potential of 100% fill efficiency. This resulted in a much greater BMEP using 2 stroke technology, than with 4 stroke engines of the same displacement, (essentially putting 4-cycle Moto GP at all levels out of business for a time). That is, until Honda & other 4-cycle stalwarts succeeded in getting 2-cycle bike engines banned on environmental emissions grounds. Today, with emissions issues being resolved, 2-cycle engines are making a comeback. At least one major mfgr of 4-cycle outboard engines is dropping its entire line of 4-cycle engines and is returning to building 2-cycle engines.
The trick was a succession of various evolutionary experiments in bulbous expansion chamber design shapes, to refract/echo the expanding high pressure exhaust gas pulses back into the combustion chamber. The next positive pressure wave(s), maximized the exhaust gas suction, backflow pulses at the end of the exhaust cycle. As the intake cycle starts, these suction pulses provide a scavenging effect, accelerating the intake air/fuel inflow charge to exceed the potential of atmospheric pressure to fill the cylinder with air fuel charge, sort of a ping pong or yoyo effect.
Smaller ports can generate higher airflow velocity and more momentum to respond to pulses sucking the air/fuel mix into and out of the combustion chamber during each intake/exhaust cycle at low rpm. Large area port passageways can diminish air pulse velocity, therefore the airflow has less momentum to optimally fill or evacuate any given cylinder displacement, unless very high RPMs are needed to generate HP, as opposed to torque at low RPM. Then there is the issue of optimizing and coalescing the fuel air swirl/tumble mixture in the combustion event to enhance the air/fuel mix. Ideally there should be no isolated, non-atomized fuel concentrations in the combustion chamber, igniting or auto-igniting isolated flame fronts. These contribute to knock and extended combustion, thus diminishing one cohesive bang on the ignition event, and not maximizing the BMEP punch. Less fuel/air mix entering the combustion chamber = less BMEP explosive punch to generate torque.
The last thing to contemplate is the trade-off between added performance vs. the service life of the engine. The service life will be reduced as the performance of the engine is increased. ‘Tis a delicate balance on a vintage engine!
Greater service life with uber performance as a goal would lead one down the path to an engine swap, as there are plenty of modern, tiny light engines with 150 hp+ that offer a service life of over 100K+ miles with virtually no maintenance. The question is, will the chassis handle new power? Beware of O. P. (Originality Police) though!”
For Further information see:
Re Kaaden: see Mat Oxley, Stealing speed; Jan Leek, MZ The Racers: The Birth of The Modern Two-Stroke. (An out of print collector’s item.)
Re. Valve Train/Cam dynamics: see Blair, G.P. et al, Valve Train Design.
WSH TC 4926 02-23-17.