Discussion
May be a stupid question, I'm no mechanic so excuse me if it is!
Why do normal, everyday cars only rev to between 6-8 thousand revs? Most mid-sized motorbikes have 500-750cc engines, so are about half/third the size of an everyday car, yet rev to (in some cases) over twice a car and get much better acceleration out of each gear. Just watching Fifth Gear and there was an old Merc racer on there that had a 2.4 v8 and rev'd to 19,000 rpm.
I know it's not always necessary to rev that high on a road car, but is it feasibly possible to do so when designing a sports car? And if so, why don't manufacturers do it?
Why do normal, everyday cars only rev to between 6-8 thousand revs? Most mid-sized motorbikes have 500-750cc engines, so are about half/third the size of an everyday car, yet rev to (in some cases) over twice a car and get much better acceleration out of each gear. Just watching Fifth Gear and there was an old Merc racer on there that had a 2.4 v8 and rev'd to 19,000 rpm.
I know it's not always necessary to rev that high on a road car, but is it feasibly possible to do so when designing a sports car? And if so, why don't manufacturers do it?
It's a big list of things, but the main one is reliability. I'm not sure, but over a lifetime, a car probably does many more miles then a motorbike. If you were able to rev up to 10 grand or 12 grand every time you pulled away, you'd wear out bearings quicker, and the bores. An F1 cars engine which revs to 18k only lasts for a couple of races.
There's some other things aswell, to stop the engine running lean at high revs, which will overheat it, you'd either have to have more fuel being pumped in, which at low revs would make the engine horrible and lumpy, or run if on an open loop ECU which would be useless for fuel economy at low revs. Valves can't close fast enough at those speeds. I believe it's ducati who have cams which close the valve aswell as open it. The stroke on most engines is too long, so the piston has to move the same distance, but much faster.
If you look at the pistons and the bores on a 500cc engine compared to a 2000cc, or even a 1000cc engine, the size difference is huge.
There's some other things aswell, to stop the engine running lean at high revs, which will overheat it, you'd either have to have more fuel being pumped in, which at low revs would make the engine horrible and lumpy, or run if on an open loop ECU which would be useless for fuel economy at low revs. Valves can't close fast enough at those speeds. I believe it's ducati who have cams which close the valve aswell as open it. The stroke on most engines is too long, so the piston has to move the same distance, but much faster.
If you look at the pistons and the bores on a 500cc engine compared to a 2000cc, or even a 1000cc engine, the size difference is huge.
Bike engines only rev high because they HAVE to
Small engine = Low torque
Power = Torque x RPM
So, a 150bhp 1000cc bike and a 150bhp 2000cc car have massively different characteristics.
A car engine, designed to rev to 20,000 rpm would be a pig to live with every day, as it would have very little power low down in the rev range and require thrashing very severely to make any kind of progress. Imagine a VTEC engine, but having to rev it twice as high!
Small engine = Low torque
Power = Torque x RPM
So, a 150bhp 1000cc bike and a 150bhp 2000cc car have massively different characteristics.
A car engine, designed to rev to 20,000 rpm would be a pig to live with every day, as it would have very little power low down in the rev range and require thrashing very severely to make any kind of progress. Imagine a VTEC engine, but having to rev it twice as high!
'tis all down to piston force. Steel (which they make crankshafts out of) has a certain strength, and it's the strength of the crank that's the limitation. Imagine a crankshaft that has a stroke of 50mm on it. That means that from the top of the compression stroke to the bottom of the inlet stroke the piston has to move 50mm for each explosion. There is one explosion per rev, so the piston has to move down 50mm 6,000 times per minute. so each explosion makes the piston move 50mm in 1/100 of a second, at an average speed of about 675mph.
Double the stroke, and the piston speed doubles, as does the force. So at 6,000 rpm on
the engine with the bigger stroke the crank will probably snap.
Motorbike engines have very small pistons, both in terms of stroke and bore, which means that normal steel can be used to make a high revving crank.
On racecar engines cranks are made out of much stronger metal.
Double the stroke, and the piston speed doubles, as does the force. So at 6,000 rpm on
the engine with the bigger stroke the crank will probably snap.
Motorbike engines have very small pistons, both in terms of stroke and bore, which means that normal steel can be used to make a high revving crank.
On racecar engines cranks are made out of much stronger metal.
I asked the same question of a McLaren F1 engineer, and his simple answer was an F1 engine revving much higher is only expected to last for maybe two races, which of course would be useless in a road car. Road car engines are designed for durability, not a short life of very high performance like a race car engine.
davepoth said:
'tis all down to piston force. Steel (which they make crankshafts out of) has a certain strength, and it's the strength of the crank that's the limitation. Imagine a crankshaft that has a stroke of 50mm on it. That means that from the top of the compression stroke to the bottom of the inlet stroke the piston has to move 50mm for each explosion. There is one explosion per rev, so the piston has to move down 50mm 6,000 times per minute. so each explosion makes the piston move 50mm in 1/100 of a second, at an average speed of about 675mph.
Double the stroke, and the piston speed doubles, as does the force. So at 6,000 rpm on
the engine with the bigger stroke the crank will probably snap.
Motorbike engines have very small pistons, both in terms of stroke and bore, which means that normal steel can be used to make a high revving crank.
On racecar engines cranks are made out of much stronger metal.
Exactly as he said.Double the stroke, and the piston speed doubles, as does the force. So at 6,000 rpm on
the engine with the bigger stroke the crank will probably snap.
Motorbike engines have very small pistons, both in terms of stroke and bore, which means that normal steel can be used to make a high revving crank.
On racecar engines cranks are made out of much stronger metal.
Basic rule of thumb for an everyday engine, the bigger the CC the lower the maximum RPM. Best thing to compare is the slow burble of a large capacity V8 to the deafening buzz of a tiny two stroke (say model plane for instance)
You can increase the maximum RPM an engine can do by fitting stronger forged conrods, pistons, head bolts etc. but its not worth it for a basic mass produced production engine.
petrolveins said:
I asked the same question of a McLaren F1 engineer, and his simple answer was an F1 engine revving much higher is only expected to last for maybe two races, which of course would be useless in a road car. Road car engines are designed for durability, not a short life of very high performance like a race car engine.
I think it's worth mentioning that one of the reasons an F1 engine can rev so highly is that they are massively oversquare (bore is often more than twice the stroke) which doesn't make for very good efficiency. In comparison, most "normal" car engines are square (bore = stroke) or pretty near to.Thinking back a bit I think it was the S2000 (or some similar Jap roadster) that has a very high mean piston speed which is actually greater than some of the sports bikes.
As others have said, it's not particularly desireable to have a very highly revving road engine because wear on everything (bearings, piston rings, valvegear etc...) would be much greater and most drivers don't want to be cruising on the motorway with the car screaming its arse off.
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