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  • It is important that one understands that there is no written rule to measure power directly. It has a lot of dynamometers which can measure the torque and allow the power to be calculated. This is a basic equation which allows one to develop design and development work. There are two main methods to find the right power which is used in the automotive industry.  

    Puma Race Engines

    There are two small valves like the cast-iron block and head, pushrod valve train. It is hardly a state of the art which is a multivalve aluminium engine which has 50% more power and can weigh two times as much. There is plenty of cash flows which will allow you to have a very reliable service. The crossflow allows you to get some kind of modification. This allows one to have some of the best ad smooth flow. This over a period of time can allow you to get the right speed for the operation on rolling roads. The crossflow was the successor to the pre crossflow and was simply a modular engine. This allows them to stay the same, allowing you to have the right capacity and can get progressively better with longer strokes and cranks. They also have a small engine which is oversized like a bike engine. You can exactly have the same big valve which will allow you to build the real screamer.

    Bottom end

    The crossflow can easily be bored out to ensure that there is a long way to go when it comes to gaining capacity. There is an 83.5 mm which gives 77.6 mm stroke crank although it might seem like it is pushing limits. It is still oversized fro the features that it comes in. There is pressure testing which can be blocked. The capacity does very less to give it the right peak of power and can allow you to have the right torque that can increase the rpm and cab generate drops in the right proportion. The standard piston can be oversized up to 0.090, which can give 83.25mm. This is supplied in an unfinished flat which can be expensive for most.

    Cylinder Head

    This is a port design which is actually pretty good and very straight to the point. It has a very reasonable downdraft and can have a flow per square inch of value can be pretty high. The engine size of the valves is quite smaller. Most engines have bigger valves, and one of the first is the 41.3 mm inlet valve which is available off-shelf which can be found for a lower price. The engine might have its very own problem, but with a bit of variation, one can easily find the right tool allowing the right drill....

  • We've seen in the previous article how torque and power are defined and calculated - now let's look more closely at how they relate to engine design. The concept of an engine's torque output seems to be confusing to many people judging by newsgroup threads but it needs to be clearly understood if one is to design the best ways to improve power output. Torque can be thought of as the instantaneous turning force generated at the crankshaft. As such it is a measure of the amount of energy being developed in the engine during EACH operating cycle - in other words a function of the amount of air/fuel mixture being burned per cycle. Copyright David Baker and Puma Race Engines Power can be thought of as a measure of the amount of energy being developed in the engine per minute - in other words a function of the amount of air/fuel mixture being burned per cycle multiplied by the number of cycles per minute. So power is torque times speed as we have already seen. To increase torque we need to either process more air/fuel mixture per cycle or extract more energy from the air/fuel that is processed. We can do the latter in a variety of ways including: 1) Improving mechanical efficiency with attention to design of such things as bearings, piston rings etc. 2) Increasing compression ratio which extracts more energy from the mixture being burned. 3) Optimising fueling and ignition timing. We'll look at the above another time - for now lets concentrate on getting more air/fuel mixture into the engine. We can simplify even further by leaving out the fuel part of "air/fuel" mixture as this is really a calibration issue and falls under 3) above. It is increasing the air consumption that is the real problem and in fact it is not a bad idea to think of an engine as an air pump. The better we can make this pump work the more torque and power we can generate. Our problem of increasing torque output has now ended up as a problem of getting more air into the engine each cycle. There are only 2 ways to do this: 1) To increase the engine size. This is not always an option or at least not always a cost effective option. We may be running in a racing class where the engine size is limited or we may own an engine where parts such as longer stroke crankshafts or bigger pistons are expensive. As a general rule though, a bigger cylinder will process more air per cycle than a smaller one unless limited by other factors. 2) To increase the filling efficiency of the cylinders - i.e. to increase "Volumetric Efficiency". If a cylinder is 500cc in volume but processes only 400cc of air each cycle we can say that the volumetric efficiency is 80%. In fact to be absolutely correct it is normal to express VE in terms of mass of air not volume but that is getting more complicated than is needed for now. To get into the cylinder, the air has to pass through the carb or injection system, the inlet manifold and finally through the port and valve. The more restrictive to flow each of these components is, the harder it is for the air to get through them. By testing each of these items on a flow bench and modifying them to increase their flow capacity we can allow the air an easier passage into the cylinder and this will increase not only VE and therefore torque but also allow the engine to run at higher speeds and increase peak horsepower. Copyright David Baker and Puma Race Engines In fact the ultimate horsepower potential of any engine is really a function of the flow capacity of the induction system. By just increasing engine size, say with a longer stroke crank, we will increase torque at low rpm but not necessarily increase peak horsepower by much at all. The flow capacity of the induction system imposes the ultimate limit on the amount of air that the engine can process per minute and whether we have a small engine running at high speed or a big engine running at low speed, it is total airflow per minute that matters. The only real difference between a 3 litre car engine producing 200 bhp and a 3 litre Formula 1 engine producing 800 bhp is the flow capacity of the cylinder head. We can also increase airflow per cycle by opening the valves for longer or to a higher lift. This has its downside though because long duration camshafts don't work well at low engine speeds and while this might be ok for a race engine it is not what we want for a road engine. Increasing the airflow capacity of the induction system has very little downside although there can still be minor adverse effects on low speed performance. As a general rule it is much better to have a high flow induction system and be able to use a short duration camshaft to achieve the desired horsepower than vice versa. The most restrictive part of the induction system and therefore the part that often shows the greatest benefits from being improved is the cylinder head. In fact the flow efficiency of the cylinder head is the key to good engine design and is the reason why modern engines are increasingly being designed with 4 or more valves per cylinder rather than 2. More valves mean more valve area and it is valve area that limits flow. Cylinder head design merits its own section and we'll discuss it in detail in other articles. Although both power and torque per litre are higher than for 2 valve engines we see a similar story with a much greater spread of power outputs than torque outputs. In fact only the BMW stands out for its high torque output (perhaps even a tad suspiciously so) although there is a 52% spread of power per litre figures. We ought by now to be realising that increasing torque per litre is much harder to do than increasing power. In fact torque per litre figures can be used as a very good guide to the truth or otherwise of quoted power claims. It is hard to get even a race 2 valve engine to produce much more than 75 to 78 ft lbs per litre and for a 4 valve engine more than 85 to 88 ft lbs per litre. For big budget engines where a lot of time and money has been spent on dyno testing of inlet and exhaust manifold lengths and diameters then of course it is possible to push the limits higher. With well developed cylinder heads, good inductions systems (i.e. sidedraft carbs or even better, multi butterfly throttle body systems) and efficient full race camshafts it is possible to modify small bore road car engines into race ones producing around 80 ft lbs per litre for 2 valve designs and low 90s ft lbs per litre for 4 valve designs. You'll very rarely see figures that high though unless serious development work has been done. Obviously it's much easier to get high torque and power outputs if the starting point is a custom designed big bore small stroke racing engine with lots of valve area rather than a small bore long stroke road car one. The general tuning article looks at power and torque targets for modified road car engines in more detail. Copyright David Baker and Puma Race Engines It is possible to increase peak torque even further by selecting the intake and exhaust lengths to "pulse tune" the engine most efficiently at peak torque rpm. This will reduce peak power though and as maximising power is the primary goal for a competition engine this strategy is not normally of any use. Occasionally there are race series where the engines have to abide by an rpm limit which is lower than that at which they could otherwise produce best power. In such cases the engines will be tuned to maximize output at the limited rpm which can lead to torque/litre figures approaching 100 ft/lbs per litre. The reduction in peak power this creates is of no consequence if the engine is not allowed to rev that high. Such torque figures should not be used as a guide to what is possible from conventional best tuning on a non rev limited engine though. I have still to come across reliable data for any engine producing more than about 93 to 94 ft/lbs per litre where ultimate power was the aim - except of course for unreliable estimated "flywheel" power and torque figures derived from rolling road wheel bhp measurements in which case the sky is the limit. I once saw a rolling road power curve where peak torque was supposedly 120 ft/lbs per litre from a 4 valve engine of fairly uninspiring design. Even the operator finally admitted something didn't look right when we went through the maths together. The conclusion was that there had been massive wheelspin during that power run and none of the figures generated were of any use at all. When you see power claims that look suspicious, calculate the torque values using the formulae in the previous article. If you see peak torque values higher than those suggested above then I suggest you start to get, if not suspicious, then at least very analytical. Modern motorbike engines are quite similar to custom race car engines in terms of them being short stroke, 4 valve etc and although I have no data to hand I think it would be interesting to see the sort of torque per litre figures being claimed for them given that they achieve well over 100 bhp per litre. If anyone wants to summarize some power specs for me I would be grateful. Copyright David Baker and Puma Race Engines You might think that it is only possible to get 100% Volumetric Efficiency from an engine - after all when a cylinder is full of air at atmospheric pressure surely that is the end of the story. What this fails to take into account though is what is called "Pulse Tuning" which is taking advantage of the pressure waves which exist in the induction and exhaust system. These pressure pulses can actually ram air into the cylinder to achieve up to 130% VE although it takes very carefully designed pipe lengths and diameters to achieve this and the effect only works over fairly narrow rpm bands - usually with a corresponding adverse effect somewhere else in the rpm range. We can see by now that there is a close relationship between VE and torque per litre and it might be reasonable to ask if it is possible to calculate one from the other. Well the full answer is no because the torque achieved also depends on burn efficiency, mechanical efficiency and other things. A rough guide though is that if you multiply the torque per litre by 1.4 you get a close approximation of the VE as a percentage. So the 4 valve engines running at 72 ft lbs per litre are perhaps achieving about 100% VE in road tune. 130% VE would equate to 93 ft lbs per litre which also ties together the maximum figures I have seen from different sources for both of these measures quite nicely. ...

  • The first two articles have covered the main items that need to be considered when trying to evaluate the power potential of an engine. All that needs to be done now is to look at the equations that turn valve area into potential bhp. We will assume that all ancilliary parts of the engine design such as induction system, exhaust system, compression ratio etc can be modified such as to impose no further restriction on the power potential. What we are going to calculate is the potential peak power of a fully modified engine in race tune with excellent port work and "perfect" induction and exhaust system design. This analysis applies mainly to engines designed originally for normal road use but we will also consider briefly how a custom designed race engine like an F1 engine might fit into this scenario. Copyright David Baker and Puma Race Engines

    Step 1 - calculate the valve area

    The area we need here is the total area of all of the inlet valves in square millimetres. Hopefully everyone reading these technical articles will know how to do this but I suppose for the sake of completeness... Valve Area of each valve = diameter squared x pi ÷ 4 - then multiply by the number of inlet valves in the engine. For a fully modified engine we must consider not the standard valve sizes but the maximum valve size that might be fitted. As a rule of thumb, most road engines have enough space to enable valves 7% bigger in diameter to be fitted into the combustion chamber but of course this varies from engine to engine.

    Step 2 - Adjustment for the type of engine design

    We have seen that different engine designs have different power potentials for a given valve area. A basic adjustment to "weight" the valve area for these considerations needs to be done. Copyright David Baker and Puma Race Engines 2 valve per cylinder, parallel valve - reduce area by 10% 2 valve per cylinder, inclined valve - leave area as is 4 valve per cylinder - increase area by 10% Custom designed, money no object 4 valve per cylinder race engine - increase area by 25% This rather rough and ready "weighting" gives us a broad brush approach to refining the power prediction based on just engine type and valve area. We make no consideration as yet of engine size or the effect of the camshaft design and valve train type. Some common sense is going to have to be applied if it can readily be seen that a particular engine has a severe design limitation in some particular area such as valve lifter diameter. The venerable MGB engine for example is so limited by its siamese port design and pushrod valve train that it doesn't get anywhere near the power potential predicted here. Some of the parallel valve engines with overhead cams can rival the inclined valve engines in power output though. With enough development work on port design and cam profile the bhp targets can eventually often be beaten. In the USA the Chevrolet V8 engine has been refined over the years by dozens of engine tuners spending thousand of man hours on research. Its power output, given the age of its initial pushrod design, now beggars belief and rivals that of some OHC 4 valve engines. Copyright David Baker and Puma Race Engines

    Step 3 - Predict the flywheel bhp

    Take the adjusted valve area from step 2 and divide by 30

    This now gives us our predicted flywheel bhp in full race tune. In other words, high compression ratio, top notch carburation or throttle bodies, good exhaust system, race camshafts and fully ported, flow bench developed cylinder head. For fast road tune a good target would be 75% of the above figure and for rally tune about 85% to 90% of it. Note that engine size has never entered this analysis at all. In fact engine size does play a role in potential power output but nothing like as much as is commonly believed. That's a story for another article though. Copyright David Baker and Puma Race Engines

    Examples

    1) - The venerable 2 litre Ford Pinto engine has been tuned by so many people that its power potential is pretty well known. A really good one can just beat the 200 bhp mark and David Vizard in his excellent book on the engine achieved 212 bhp after years of development work. Lets see how the numbers stack up. Std inlet valve size is 42mm but the normal big valve used in race engines is 44.5mm. Engine type is SOHC with inclined valves so no adjustment to the base valve area is required. Valve area is 44.5 x 44.5 x 3.1416 ÷ 4 = 1,555.3 sq mm per valve x 4 valves per engine = 6,221 sq mm Power potential = 6,221 ÷ 30 = 207 bhp. So not a million miles out then. Copyright David Baker and Puma Race Engines 2) - Let's try a little comparison between two different engine types - the 3.5/3.9 litre Rover V8 and the 1905 cc Peugeot 405 M16. At first glance you might think there would be no contest. The Rover has twice the number of cylinders and twice the engine capacity but does it work out that way? The Rover V8 has 8 cylinders and 40mm inlet valves - total valve area 10,053 sq mm. Being a 2 valve parallel valve pushrod engine we need to reduce this by 10% though and end up with 9,048 sq mm for a power target of 302 bhp. The Peugeot is a 4 valve per cylinder engine with 34.6mm inlet valves. Total valve area 7,522 sq mm. We need to add 10% though for the 4 valve engine type to arrive at 8,274 sq mm and a power target of 276 bhp. Rather less in it than might otherwise have been thought. Of course the 1.9 litre 4 cylinder engine is going to need to turn some pretty serious rpm to develop the power its cylinder head is capable of supplying though. 3) - and finally for a bit of fun. See if you can guess whose engine this is. 3 litre, all aluminium V10, 4 valve per cylinder and 35mm inlet valves - I think we can also safely say that money was no object here 🙂 Total valve area is 19,242 sq mm and we need to bump this up by 25% for the engine type to arrive at 24,053 sq mm Power target is therefore 24,053 ÷ 30 = 802 bhp. Hmm, not too shabby for a 3 litre engine. Vorsprung durch technik as they say in Germany just before sliding into the tyre wall and breaking both legs. Copyright David Baker and Puma Race Engines

    Conclusion

    Please don't get the idea that from one simple measure like inlet valve area we can arrive at a definitive power target for an engine. What this analysis should have done is give you the basic tools for understanding how to evaluate an engine and arrive at sensible power targets which will be at least "in the ball park". It hopefully shows though just how important valve area is compared to other factors like engine size or number of cylinders. Actually achieving the power targets depends primarily on the skill of the cylinder head modifier though and that's why most engines never get anywhere near their full potential. The difference between a well modified head and a poor one can be 20% of the engine's power potential. Copyright David Baker and Puma Race Engines...

  • In the first article we reached the following conclusion:

    The single biggest factor that determines an engine's ultimate power potential is the total inlet valve area

    Not all cylinder head designs have the same flow efficiency for a given valve area though - and it is the flow potential rather than the valve area itself that really determines the power potential - but valve area is much easier to measure and provides an ideal starting point for further analysis. There is no point however in having big valves if the port shape or other factors restrict the flow. To discuss this further it is best to consider engines with 2 valves per cylinder separately from 4 valve valve engines (or even 5 valve engines which are gradually appearing in road cars). Copyright David Baker and Puma Race Engines

    2 Valve Per Cylinder Engines

    Engines with only one inlet and one exhaust valve can be further split into two main categories.

    Parallel Valve Engines

    In this type of design the valve stems are parallel to each other and usually, but not always, parallel to the cylinder bore axis. Examples might be the Mini, MGB, VW Golf, Peugeot 205 and Ford Crossflow engines. The total valve diameter is directly limited by the bore size because the valves open into the bore. There obviously needs to be some clearance between the two valves and also between each valve and the adjacent bore wall simply to prevent contact. Production engines might have 3mm or 4mm for each of these clearances although this can be reduced on a race engine to around 1.5mm between the valves and 1mm to 1.5 mm between each valve and the bore wall to allow the largest possible valves to be fitted. So at best there is a limit on total valve diameter of about 3.5mm to 4.5mm less than the bore diameter. This remaining space would normally be allocated as about 55% to 57% for the inlet valve and 43% to 45% for the exhaust valve diameter. In other words the exhaust valve would be around 80% of the diameter of the inlet valve for best power output - perhaps even a little less in some cases. Copyright David Baker and Puma Race Engines Combustion chambers can either be of the "bathtub" type where the volume is contained mainly in the cylinder head or the "Heron" design where the head face is flat and the volume is in the piston dish and between the top of the piston and the top of the bore. Regardless of the exact design chosen there is always going to be some loss of flow potential because of shrouding between the valves and the closely adjacent bore wall or combustion chamber walls. In simple terms there is just not enough space for the airflow to get past the valve head into the cylinder cleanly. The bigger the valves and the closer they end up to an adjacent wall the greater the shrouding effect becomes and a law of diminishing returns sets in. In some cases a smaller valve ends up producing more flow than a larger one if the required clearance space around the valve head can't be achieved. The effect of shrouding can be to reduce the flow and power potential by around 10% compared to the same sizes valves with zero shrouding. Early designs of this type of engine had pushrod type valve trains and the Mini and MGB engines were limited even further by their siamese port design. Some of the more modern single overhead cam engines can rival the inclined valve type design in their power output though. A little common sense needs to be applied when evaluating these types of engine.

    Inclined Valve Engines

    In these designs the valves are angled both relative to each other and to the bore axis. Examples include the Ford CVH and Twin Cam engines which have large angles between the valves and the Ford Pinto engine which has a fairly small included angle. This design has two main advantages. Because the valves open away from the bore wall in towards the centre of the cylinder there is little or no shrouding of the flow. As the valve opens further and the airflow increases, so the necessary space around the valve head increases at the same time. Secondly it enables larger valves to be fitted in a given bore diameter than the parallel valve head design. Within limits, the greater the angle between the valves the larger they can become although if the included angle is too large the inlet valve can hit the exhaust valve when they are both open during the overlap period. Copyright David Baker and Puma Race Engines Disadvantages of this design is that the combustion chamber has to be something like a hemisphere, or at least fairly domed, and this shape isn't very compact and doesn't burn well. The advantages of extra valve area and lack of shrouding outweigh this consideration by a large margin though.

    4 Valve Per Cylinder Engines

    The constraints of fitting 4 valves and their related valve trains into a cylinder head means that all 4 valve designs end up being fairly similar - at least in flow terms. The inlet valves are angled away from the exhaust valves, the spark plug ends up central in the chamber and usually twin overhead cams are used - although a few designs manage with a single cam and rockers. There is little or no shrouding with most 4 valve engines and in effect they are like multi valve versions of the inclined design of 2 valve engine. There is a significant difference though between 4 valve and 2 valve engines in terms of flow and power potential for a given total valve area. This is because the ratio of total valve area to total valve circumference is not the same. To understand this better let's look at an example. Copyright David Baker and Puma Race Engines Compare an engine with two small inlet valves of 25mm diameter with a similar sized engine with one large valve of 35.36mm diameter. The total valve area is the same in both cases - about 982 square mm. So the total peak flow when the valves are fully open should be very similar. The total circumference is very different though. The two small valves have a total circumference of 157mm. The single large valve has a circumference of only 111mm. The ratio is 1.41 to 1 - or in other words the square root of 2. This has a big effect on flow at low valve lifts. If all three valves are open by the same small amount - say 1mm - the two small valves have a flow area which is 41% bigger and consequently flow more air. As the valves open fully and the valve area becomes the limiting factor, this effect diminishes and ultimately disappears. The effect of this improved low lift flow is to give the two small inlet valves a power advantage over a single valve of the same area. The effect depends on the cam profile used on each engine but is in the region of 10% to 15%....

  • Ask a selection of people this question and one of the first things that will spring to mind is engine size. After all, big engines tend to produce more power than small ones. The second article on Power and Torque lists some data for standard road engines. It should be apparent that there is quite a big spread in the amount of power per litre that the engines on the lists produce. Even allowing for them having 2 valves per cylinder or 4 valves per cylinder doesn't help much. The spread of outputs is from only 45 bhp per litre for low tuned 2 valve engines up to 200 bhp per litre for motorbike engines. Taking these engines, or any other sample, and plotting a graph of power against engine size won't produce anything like a straight line. Copyright David Baker and Puma Race Engines To answer the question we need to understand more about how engines produce power in the first place. All the power comes from the fuel burned - the more fuel the engine can burn per minute the more power it can produce. But to burn fuel, or anything else, we need oxygen and that comes, at least in the case of most engines, straight from the air. To burn petrol efficiently in an engine it needs to be mixed with about 14 times its own weight of air. For best power about 12.6 times as much air and for cruise conditions and good economy about 15 times as much. A gallon of petrol (UK gallon) weighs about 7.5 pounds - so every gallon of fuel needs about 100 pounds of air. Air weighs just over 2 pounds per cubic yard so that translates into about 50 cubic yards of air per gallon of fuel. To put it into perspective, that's about the same volume of air as in a room 12 feet by 14 feet. So the problem of power potential becomes one of processing as much air as possible per minute. The fuel is not a problem - we can squirt in as much or as little of that as we need just by calibrating the carb or fuel injection system. So what limits the amount of air the engine can process? Well the ultimate limit comes from the flow capability of the cylinder head. Even a small engine can potentially pump a lot of air if it spins fast enough but if the cylinder head can't flow that air then the cylinders can't fill. The flow capability of the cylinder head is in turn dictated by the size and number of the valves because these are what create the biggest restriction to flow. The inlet valves are the critical ones here because all they have going for them to allow air through them is the force of atmospheric pressure. The exhaust valves have the piston acting as a direct pump so they create less of a problem. So we can summarize all of this in the following rule:

    The single biggest factor that determines an engine's ultimate power potential is the total inlet valve area

    So does engine size play any part at all here? Well to an extent yes - because the bigger we make the engine the more valve area we can cram into it. But it isn't the engine size by itself that produces the power. There are other ways of getting more valve area into a given engine capacity. We can use 4 valves per cylinder instead of just 2 - that makes better use of the available bore area because less space is wasted in the gaps between the valves. We can also build the engine with more cylinders which also leads to more piston area and therefore more potential valve area. The final thing we can do is to alter the bore/stroke ratio. The bigger the bore and the smaller the stroke, the greater the bore area for a given capacity. There are limits of course to how far we can take all these things. Engines have been built with more than 4 valves per cylinder but it gets increasingly complicated to design the camshaft mechanisms. The more cylinders the engine has, the bigger and heavier it gets and also frictional losses increase. If the bore is made too big compared to the stroke then the piston doesn't travel very far and gets in the way of the valves opening - also the combustion chamber shape gets further away from the ideal compact design and ends up wide and flat which doesn't burn the fuel mixture very efficiently. Copyright David Baker and Puma Race Engines So like everything else, these matters are all compromises. Road engines tend to have longer strokes and fewer cylinders. Race engines have bigger bores, shorter strokes and more cylinders. But at the root of any engine design, it is the total inlet valve area that determines the power potential. The smaller the engine, the faster it needs to rev to make use of that airflow potential but other than that, the engine size doesn't really make too much difference to the power capability. In the next article we'll look in more detail at the other factors that need to be considered along with the valve area to determine the power potential. ...