Things you always wanted to know the answer to [Vol. 2]
Discussion
glazbagun said:
This is probably common knowledge to many, but why do engines in high states of tune seem to idle in barps, or even worse, sound like spanners going round in a washing machine?
It's down to fixed cam timing. Most competition engines don't have variable valve timing. The timing is optimised for the peak power band, normally much higher up the range. That means the ignition timing is far from good at idle/low rpm, hence it sounds rough. An extreme case is the engine of a dragster. As well as optimising the timing for the power band, you have to take into account that at the speeds and torques experienced by the engine, the camshafts actually twist, anywhere up to 10-15 degrees end to end. The best engine builders machine the cams taking this twist into account; so at low engine speeds/loads the cams are essentially wrong, so again the ignition timing is far from ideal and you get the rough running. Soon as you pile on the power, everything harmonises and the engine runs smooth and gives you the best performance where you need it.Ah.. a good question, and one that is going to have a long-ish answer.
It's usually down to the cams, and more specifically the duration of them.
A highly tuned race type engine might have cams in the range of 280-290degs or higher, which means they are open for most of their rotation (which is half the speed of the engine as it'll be a 4 stroke engine...)
This means the valve are open when the piston is coming up on the compression stroke. At high revs, these long durations are great, you get high gas speeds, the air is rocketing in, and you can even get fancy effects of the air bundling itself in like people crowding onto the tube, even though the piston is coming up just from the momentum of the air.
These high gas speeds are great for packing the air in, but you'd only tend to get them at high rpms.
There is ways to get this boosting effect at lower engine speeds, but it means things like narrow intake pipes to get the air rushing, and the end uo being too narrow to get lots of air in at high speeds.
One of the most common examples is the BMW E36 328i, the narrow thin intake pipes boost the air speed and pack the cylinders at low and mid RPMs, boosting midrange torque, but they kill the top end power as eventually they can't fit all the air the end wants down the pipe.
Back to these "cammed" engines, they use the long durations as they'll still pack some air in even when the piston is coming up quite quickly, the air hasn't got a chance to get in, bounce back and disappear as it doesn't have the time, but at low engine RPMs, it does.
So at idle, the air goes into the cylinders, sits around, and then can get pushed out of them before the valve has shut, and that means air goes in, turns around and leaves again, and it's pretty awkward to get them running right.
So, longer duration cams are better for high RPM performance, as the air goes into the cylinder, and doesn't have the time to bounce out again, so it gets plenty of air and power, but at low RPMs, they run like a bag of nails.
Most things about engines tend to be a compromise somewhere, so you'll have things like Vtec which used two sets of cam lobes for high and low engine speeds, variable intakes so they can have a thin torque boosting pipe for low speeds, and bigger pipes for high speeds etc, but these race cars pick one cam that's set up for the best high speed perfomance at the cost of the low RPMs.
It's usually down to the cams, and more specifically the duration of them.
A highly tuned race type engine might have cams in the range of 280-290degs or higher, which means they are open for most of their rotation (which is half the speed of the engine as it'll be a 4 stroke engine...)
This means the valve are open when the piston is coming up on the compression stroke. At high revs, these long durations are great, you get high gas speeds, the air is rocketing in, and you can even get fancy effects of the air bundling itself in like people crowding onto the tube, even though the piston is coming up just from the momentum of the air.
These high gas speeds are great for packing the air in, but you'd only tend to get them at high rpms.
There is ways to get this boosting effect at lower engine speeds, but it means things like narrow intake pipes to get the air rushing, and the end uo being too narrow to get lots of air in at high speeds.
One of the most common examples is the BMW E36 328i, the narrow thin intake pipes boost the air speed and pack the cylinders at low and mid RPMs, boosting midrange torque, but they kill the top end power as eventually they can't fit all the air the end wants down the pipe.
Back to these "cammed" engines, they use the long durations as they'll still pack some air in even when the piston is coming up quite quickly, the air hasn't got a chance to get in, bounce back and disappear as it doesn't have the time, but at low engine RPMs, it does.
So at idle, the air goes into the cylinders, sits around, and then can get pushed out of them before the valve has shut, and that means air goes in, turns around and leaves again, and it's pretty awkward to get them running right.
So, longer duration cams are better for high RPM performance, as the air goes into the cylinder, and doesn't have the time to bounce out again, so it gets plenty of air and power, but at low RPMs, they run like a bag of nails.
Most things about engines tend to be a compromise somewhere, so you'll have things like Vtec which used two sets of cam lobes for high and low engine speeds, variable intakes so they can have a thin torque boosting pipe for low speeds, and bigger pipes for high speeds etc, but these race cars pick one cam that's set up for the best high speed perfomance at the cost of the low RPMs.
t lovers go at night to send some Zzzz's up ?RenesisEvo said:
It's down to fixed cam timing. Most competition engines don't have variable valve timing. The timing is optimised for the peak power band, normally much higher up the range. That means the ignition timing is far from good at idle/low rpm, hence it sounds rough. An extreme case is the engine of a dragster. As well as optimising the timing for the power band, you have to take into account that at the speeds and torques experienced by the engine, the camshafts actually twist, anywhere up to 10-15 degrees end to end. The best engine builders machine the cams taking this twist into account; so at low engine speeds/loads the cams are essentially wrong, so again the ignition timing is far from ideal and you get the rough running. Soon as you pile on the power, everything harmonises and the engine runs smooth and gives you the best performance where you need it.
Superb answer.TD
TheEnd said:
Ah.. a good question, and one that is going to have a long-ish answer.
It's usually down to the cams, and more specifically the duration of them.
A highly tuned race type engine might have cams in the range of 280-290degs or higher, which means they are open for most of their rotation (which is half the speed of the engine as it'll be a 4 stroke engine...)
This means the valve are open when the piston is coming up on the compression stroke. At high revs, these long durations are great, you get high gas speeds, the air is rocketing in, and you can even get fancy effects of the air bundling itself in like people crowding onto the tube, even though the piston is coming up just from the momentum of the air.
These high gas speeds are great for packing the air in, but you'd only tend to get them at high rpms.
There is ways to get this boosting effect at lower engine speeds, but it means things like narrow intake pipes to get the air rushing, and the end uo being too narrow to get lots of air in at high speeds.
One of the most common examples is the BMW E36 328i, the narrow thin intake pipes boost the air speed and pack the cylinders at low and mid RPMs, boosting midrange torque, but they kill the top end power as eventually they can't fit all the air the end wants down the pipe.
Back to these "cammed" engines, they use the long durations as they'll still pack some air in even when the piston is coming up quite quickly, the air hasn't got a chance to get in, bounce back and disappear as it doesn't have the time, but at low engine RPMs, it does.
So at idle, the air goes into the cylinders, sits around, and then can get pushed out of them before the valve has shut, and that means air goes in, turns around and leaves again, and it's pretty awkward to get them running right.
So, longer duration cams are better for high RPM performance, as the air goes into the cylinder, and doesn't have the time to bounce out again, so it gets plenty of air and power, but at low RPMs, they run like a bag of nails.
Most things about engines tend to be a compromise somewhere, so you'll have things like Vtec which used two sets of cam lobes for high and low engine speeds, variable intakes so they can have a thin torque boosting pipe for low speeds, and bigger pipes for high speeds etc, but these race cars pick one cam that's set up for the best high speed perfomance at the cost of the low RPMs.
For a minute there I thought I had it nailed....It's usually down to the cams, and more specifically the duration of them.
A highly tuned race type engine might have cams in the range of 280-290degs or higher, which means they are open for most of their rotation (which is half the speed of the engine as it'll be a 4 stroke engine...)
This means the valve are open when the piston is coming up on the compression stroke. At high revs, these long durations are great, you get high gas speeds, the air is rocketing in, and you can even get fancy effects of the air bundling itself in like people crowding onto the tube, even though the piston is coming up just from the momentum of the air.
These high gas speeds are great for packing the air in, but you'd only tend to get them at high rpms.
There is ways to get this boosting effect at lower engine speeds, but it means things like narrow intake pipes to get the air rushing, and the end uo being too narrow to get lots of air in at high speeds.
One of the most common examples is the BMW E36 328i, the narrow thin intake pipes boost the air speed and pack the cylinders at low and mid RPMs, boosting midrange torque, but they kill the top end power as eventually they can't fit all the air the end wants down the pipe.
Back to these "cammed" engines, they use the long durations as they'll still pack some air in even when the piston is coming up quite quickly, the air hasn't got a chance to get in, bounce back and disappear as it doesn't have the time, but at low engine RPMs, it does.
So at idle, the air goes into the cylinders, sits around, and then can get pushed out of them before the valve has shut, and that means air goes in, turns around and leaves again, and it's pretty awkward to get them running right.
So, longer duration cams are better for high RPM performance, as the air goes into the cylinder, and doesn't have the time to bounce out again, so it gets plenty of air and power, but at low RPMs, they run like a bag of nails.
Most things about engines tend to be a compromise somewhere, so you'll have things like Vtec which used two sets of cam lobes for high and low engine speeds, variable intakes so they can have a thin torque boosting pipe for low speeds, and bigger pipes for high speeds etc, but these race cars pick one cam that's set up for the best high speed perfomance at the cost of the low RPMs.
Smartarse.
TD
TheEnd said:
They'll burn up the middle part of the wire, and form into two blobs of metal at each side.
You can see a little of the gravity bending things down, but you can also see there's a large bit of wire missing.
There's another one, blobs in different directions as the melting metal pulls itself into a ball from surface tension.
Thank you Sir.You can see a little of the gravity bending things down, but you can also see there's a large bit of wire missing.
There's another one, blobs in different directions as the melting metal pulls itself into a ball from surface tension.
Had myself a bit confuddled with that thinking the metal vaporised but then reasoning that if it vaporised, surely it'd reach melting point before vaporising point, unless the current rise was huuuge. Surface tension bit is neat, although I suppose there is some element of vaporisation and/or it just goes BANG, looking at that one with bits of burnt metal residue coating the inside of the glass
When you thaw out frozen fruit juice, and you drink some of the liquid that has melted off before the whole lot has thawed, why do you get concentrated juice , so that if you keep doing it you end up with a lump of ice that is mostly just water?
If you do it with milk it is the opposite, most of the melt is very watery and the ice is concentrated milk.
Why, why, why?
If you do it with milk it is the opposite, most of the melt is very watery and the ice is concentrated milk.
Why, why, why?
Ayahuasca said:
When you thaw out frozen fruit juice, and you drink some of the liquid that has melted off before the whole lot has thawed, why do you get concentrated juice , so that if you keep doing it you end up with a lump of ice that is mostly just water?
If you do it with milk it is the opposite, most of the melt is very watery and the ice is concentrated milk.
Why, why, why?
It'll be due to the different thawing temps of the parts of the juice - squash does exactly the same - I foolishly made up a camelback of orange squash back in July when it was blazing hot for mountain biking the next day and left it in the freezer - spent 4 hours drinking dribbles of neat squash and returned home still with a block of virtually clear ice - at least it kept my back cool. If you do it with milk it is the opposite, most of the melt is very watery and the ice is concentrated milk.
Why, why, why?
Feirny said:
Why is it always sunny on Goolge StreetView? Not once I have ever seen a hint of rain or bad weather on there.
Just think about that for a moment. What would rain on the camera lens do to the picture quality?There are lots of pics with snow on the ground and quite amusing when two areas meet photographed on different days.
Tyre Tread said:
Just think about that for a moment. What would rain on the camera lens do to the picture quality?
There are lots of pics with snow on the ground and quite amusing when two areas meet photographed on different days.
Yeah I understand that, but the logistics of it all must be ridiculous. Getting out on a sunny day without any dark clouds, or danger of bad weather.There are lots of pics with snow on the ground and quite amusing when two areas meet photographed on different days.
I'm going to spend all afternoon looking for one of those stitched parts.
I just had a look around a couple of areas and found plenty with overcast skies. I must admit I use google streetview a lot but had never "looked up at the sky" and taken note of it.
The cameras will be set to make the most of the light availaable I suppose and unless you "look up" you may not notice the weather.
The cameras will be set to make the most of the light availaable I suppose and unless you "look up" you may not notice the weather.
P-Jay said:
Ayahuasca said:
When you thaw out frozen fruit juice, and you drink some of the liquid that has melted off before the whole lot has thawed, why do you get concentrated juice , so that if you keep doing it you end up with a lump of ice that is mostly just water?
If you do it with milk it is the opposite, most of the melt is very watery and the ice is concentrated milk.
Why, why, why?
It'll be due to the different thawing temps of the parts of the juice - squash does exactly the same - I foolishly made up a camelback of orange squash back in July when it was blazing hot for mountain biking the next day and left it in the freezer - spent 4 hours drinking dribbles of neat squash and returned home still with a block of virtually clear ice - at least it kept my back cool. If you do it with milk it is the opposite, most of the melt is very watery and the ice is concentrated milk.
Why, why, why?
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