Marquis
Automotive
- May 29, 2002
- 162
Here are a collection of "myths" and misguided practices I've come across. Some of them speak for themselves and others I've attempted to disprove. Comments are invited, but I reserve the right in not responding if I judge the comment to be too "misguided"! [Please feel free to add your own misguided experences you've stumbled across!]
1) Long stroke engines are more torquey! Some people claim that the longer strokes gives more leverage to the crank. This is simply not true. If there was an engine that was savagely over square and another one that was undersquare , but both had the same valve sizes and the cylinders were the same size there would be very little difference in the cylinder charging per engine speed. The gas velocities through the valves are identical . Infact there is a case that says the for a given cylinder size, and valve size, the bigger bore engine would charge more efficiently because it has less bore shrouding hence the ports can potentially flow better. There are other factors at work and there is no doubt an optimum point exists, for you must take into account combustion and the effect of surface to volume ratios of the combustion chamber itself (going too oversquare can give a unfavourable chamber shape prone to heat loss and poor combustion efficiency). Origination of the myth: It is true that for a given engine cylinder size, if the valve sizes are increased for a given valve timing the torque curve will tend to “pivot” around the peak torque region favouring top end. It’s also usually true that when manufacturers make a larger bore engine they will usually fit the largest valves possible-which will ten to give the engine better top end performance characteristics-however this I quite different from saying a longer stroke alone is better for low speed torque.
2)· Siamesed ports on the Mini A series make the engine more torquey. This theory for this overrated engine is ridiculous. Siamesed ports are not only bad for outright flow but also dampen out if not totally kill any tuning pulses. There can be nothing functionally good about Siamesed ports except packaging and potentially cost.
3)· Deleting twin exhausts and simplifying to a single under the floor pipe from a Y piece on a straight six/v6/Boxer six. Often done on the original straight six BMW 3 series where the original twin pipe system is replaced for a big bore single pipe that joins the twin 3 cylinder pairs very close to the engine. Regardless of the new back pressure level this is a SURE way to kill torque ( at low speed on an engine with Variable am phasing and some low speed over lap and all the way through the rev range on a more conventional fixed cam timing -esp 2 valve- engine). This also alters the noise characteristics of the engine a lot giving less 1 and half order.
4)· Con-rod to stroke ratio-exploding the myth. It’s not only the amateurs that come up with misguided opinions. A supposed “expert” in the field, who has written many books and used to post articles in engine performance magazines makes huge claims for increasing the con-rod to stroke ratio of an engine but typically fails to back it up with any quantified examples. It’s been claimed that doing this delays peak piston velocity (although also making it lower) and moving it in line with when the the cam shaft is at peak lift. Well let’s shed some light on the matter-an engine with a 81 mm stroke and a con rod length of 130 mm running at 6800 rpm will have an instantaneous piston velocity of 30.1 m/s (around peak) at 75 deg ATDC. Now increasing the con rod length to 140mm moves this peak piston velocity down to 29.94 m/s but will only delay it by barely over a crank degree! A similar neglible gain argument can be laid down over the favourable geometric angularity –he piston being driven less into the bore (thrust). If you’re an clean sheet engine designer, as long as you stay within sensible norms, I would therefore recommend that other considerations such a block height and engine weight predominate. If you’re trying to modify an existing engine I would say you’re better off spending the money else where to get the performance gain, such as the cylinder head, rather then getting custom pistons/rods.
5)· Ludicrous claims by “chip” companies. Now let’s totally forget about Turbocharged engines for the moment-it complicates things. There are several chip companies who claim ludicrous improvements, such as 10-15% increase in torque/power on a standard unmodified engine. In terms of fuel mixture with the flexibility afforded by modern engine management systems most manufacturers will tune their engines at full load to get maximum brake torque ( known as LBT)-this is the best torque for a given engine speed by varying the mixture. The number for Air fuel ratio for LBT varies depending on engine speed, it tends to be almost 13 :1 at low engine speeds and closer to 12.5:1 at higher speed. Cars with close coupled catalysts may use the fuel to cool the catalysts at high engine speeds (perhaps going o 11.8 :1).At part load all modern engines run 14.7:1 or stoichiometric. In terms of ignition timing, in order to achieve both best torque AND fuel economy you should aim to get MBT ignition timing-which is to get the best torque figures at given engine speed when sweeping the ignition. At low speeds if you calibrate higher you’ll often run into knock and there is absolutely no reason at full load to retard even more. With this in mind these chip companies must be getting these claimed output increases due to the fact that engine fall into a band of various tolerances. Also they often remap the car to run on higher octane fuel. This is all well and good, but the argument still falls over, due to the will and huge numbers that are often claimed, the fact that most engines ae not knock limited at high engine speeds, thus will not benefit in being calibrated for a higher octane fuel. Add to this that modern engine management systems will actively run into and seek up to the knock limit and run very close to this for optimum efficiency and you may see why I am very sceptical of a lot of these claims
5)· Thinking a four cylinder engine can have an exhaust note like a V8 (or any config you want) just by changing the exhaust system. Utter uninformed rubbish! The engine order derived noise are driven by the engine pulsing of various cylinders and there various interactions.
6) · Doing a capacity increase/conversion, but not understanding the fundamentals (BMW 2.5 litre- 2.7 litre) of dynamic engine architechture, using an inadvisable combination of con-rods/pistons and ending up with a CR of less then 8:1 rather then the intended 10:1 and finding the engine doesn’t produce much more torque then the standard 2.5. Yes I’ve seen this done quite recently
7)· Trying to rectify the above situation by skimming the block but not taking into account the fact that the cam timing has now changed
8)· Altering ignition timing of an engine “by ear” – no strobe, only to find later when checked that the engine was now timed at 50 deg BTDC at idle speed!
1) Long stroke engines are more torquey! Some people claim that the longer strokes gives more leverage to the crank. This is simply not true. If there was an engine that was savagely over square and another one that was undersquare , but both had the same valve sizes and the cylinders were the same size there would be very little difference in the cylinder charging per engine speed. The gas velocities through the valves are identical . Infact there is a case that says the for a given cylinder size, and valve size, the bigger bore engine would charge more efficiently because it has less bore shrouding hence the ports can potentially flow better. There are other factors at work and there is no doubt an optimum point exists, for you must take into account combustion and the effect of surface to volume ratios of the combustion chamber itself (going too oversquare can give a unfavourable chamber shape prone to heat loss and poor combustion efficiency). Origination of the myth: It is true that for a given engine cylinder size, if the valve sizes are increased for a given valve timing the torque curve will tend to “pivot” around the peak torque region favouring top end. It’s also usually true that when manufacturers make a larger bore engine they will usually fit the largest valves possible-which will ten to give the engine better top end performance characteristics-however this I quite different from saying a longer stroke alone is better for low speed torque.
2)· Siamesed ports on the Mini A series make the engine more torquey. This theory for this overrated engine is ridiculous. Siamesed ports are not only bad for outright flow but also dampen out if not totally kill any tuning pulses. There can be nothing functionally good about Siamesed ports except packaging and potentially cost.
3)· Deleting twin exhausts and simplifying to a single under the floor pipe from a Y piece on a straight six/v6/Boxer six. Often done on the original straight six BMW 3 series where the original twin pipe system is replaced for a big bore single pipe that joins the twin 3 cylinder pairs very close to the engine. Regardless of the new back pressure level this is a SURE way to kill torque ( at low speed on an engine with Variable am phasing and some low speed over lap and all the way through the rev range on a more conventional fixed cam timing -esp 2 valve- engine). This also alters the noise characteristics of the engine a lot giving less 1 and half order.
4)· Con-rod to stroke ratio-exploding the myth. It’s not only the amateurs that come up with misguided opinions. A supposed “expert” in the field, who has written many books and used to post articles in engine performance magazines makes huge claims for increasing the con-rod to stroke ratio of an engine but typically fails to back it up with any quantified examples. It’s been claimed that doing this delays peak piston velocity (although also making it lower) and moving it in line with when the the cam shaft is at peak lift. Well let’s shed some light on the matter-an engine with a 81 mm stroke and a con rod length of 130 mm running at 6800 rpm will have an instantaneous piston velocity of 30.1 m/s (around peak) at 75 deg ATDC. Now increasing the con rod length to 140mm moves this peak piston velocity down to 29.94 m/s but will only delay it by barely over a crank degree! A similar neglible gain argument can be laid down over the favourable geometric angularity –he piston being driven less into the bore (thrust). If you’re an clean sheet engine designer, as long as you stay within sensible norms, I would therefore recommend that other considerations such a block height and engine weight predominate. If you’re trying to modify an existing engine I would say you’re better off spending the money else where to get the performance gain, such as the cylinder head, rather then getting custom pistons/rods.
5)· Ludicrous claims by “chip” companies. Now let’s totally forget about Turbocharged engines for the moment-it complicates things. There are several chip companies who claim ludicrous improvements, such as 10-15% increase in torque/power on a standard unmodified engine. In terms of fuel mixture with the flexibility afforded by modern engine management systems most manufacturers will tune their engines at full load to get maximum brake torque ( known as LBT)-this is the best torque for a given engine speed by varying the mixture. The number for Air fuel ratio for LBT varies depending on engine speed, it tends to be almost 13 :1 at low engine speeds and closer to 12.5:1 at higher speed. Cars with close coupled catalysts may use the fuel to cool the catalysts at high engine speeds (perhaps going o 11.8 :1).At part load all modern engines run 14.7:1 or stoichiometric. In terms of ignition timing, in order to achieve both best torque AND fuel economy you should aim to get MBT ignition timing-which is to get the best torque figures at given engine speed when sweeping the ignition. At low speeds if you calibrate higher you’ll often run into knock and there is absolutely no reason at full load to retard even more. With this in mind these chip companies must be getting these claimed output increases due to the fact that engine fall into a band of various tolerances. Also they often remap the car to run on higher octane fuel. This is all well and good, but the argument still falls over, due to the will and huge numbers that are often claimed, the fact that most engines ae not knock limited at high engine speeds, thus will not benefit in being calibrated for a higher octane fuel. Add to this that modern engine management systems will actively run into and seek up to the knock limit and run very close to this for optimum efficiency and you may see why I am very sceptical of a lot of these claims
5)· Thinking a four cylinder engine can have an exhaust note like a V8 (or any config you want) just by changing the exhaust system. Utter uninformed rubbish! The engine order derived noise are driven by the engine pulsing of various cylinders and there various interactions.
6) · Doing a capacity increase/conversion, but not understanding the fundamentals (BMW 2.5 litre- 2.7 litre) of dynamic engine architechture, using an inadvisable combination of con-rods/pistons and ending up with a CR of less then 8:1 rather then the intended 10:1 and finding the engine doesn’t produce much more torque then the standard 2.5. Yes I’ve seen this done quite recently
7)· Trying to rectify the above situation by skimming the block but not taking into account the fact that the cam timing has now changed
8)· Altering ignition timing of an engine “by ear” – no strobe, only to find later when checked that the engine was now timed at 50 deg BTDC at idle speed!
