Discussion
grahambell said:
pawsmcgraw said: how come the wrc subaru has 300 bhp and 690ibls/ft of torque?
That doesn't sound right. Think you need to check your source of info.
www.prodrive.co.uk/display1.asp?topArea=motorsport&displaySubArea=wrc&thirdID=239&noFlash=0
Its 580 nm.whats that in old moneies?
Still torquy mind!
Okay here we go ..... BHP is, as a number of people have already said, a calculated figure and therefore of little importance. The importance of torque is greater and clearer. In terms of increasing torque within a combustion engine there are few options .... you may gain a few ft/lb here and there by modifying the heads, exhausts and inlet tract but when it comes down to it there is no substitute for capacity. The greater the volume of gas that you explode each time engine fires the cylinder the greater torque you will produce (all things being equal!). The most important issue to discuss is the way in which that torque produces motion in the vehicle ... it's no good having 400ft/lb of torque at 800rpm if it is all gone by 1500rpm as you would need about 200 gears to reach any speed! What we are intersted in is the maintenance of torque throughout the rev range .... the longer we can hang on to it the longer we can maintain positive drive to the wheels and therefore acceleration. Ideally we want 400ft/lb of torque at 1500rpm and still 400ft/lb at 6500rpm ( that makes 495BHP by the way).
In short ... get as much torque as you can afford and then get someone who understands cams and port flow to hold onto that torque for as long as possible!!
In short ... get as much torque as you can afford and then get someone who understands cams and port flow to hold onto that torque for as long as possible!!
I agree with this principle and have often thought it would be nice to plot a graph of roadwheel torque against speed in the best use gears. For this I would need mph per 1000rpm in each gear and an engine torque graph, then mapping one with the other I have my answer. This must be the ultimate way of comparing cars performance potential?
It's to do with harmonics and hydrodynamics, two areas which I would normally hand over to Dr Mouse snr. on, but he's out on a wild turkey chase, so.....
To increase peak power, in theory all you need to do is make the intake and exhaust as free breathing as possible, so over any given period of time a larger volume of gas can pass through the system.
..but as we all know, peak power only comes at the top of the rev' range, and that is impractical, if not to say downright painful, both to engine and ear to use all the time. The greater the torque of the engine, the greater it's ability to accelerate the car by increasing the rev's from lower in the rev range (the characteristic many refer to as laziness).
...finally he get's to the point...
The tourque of an engine (as taken by me from the writings of David "The Wizard" Visard Re; Minis), is dependant on the momentum of the gass flow at low revs. This produces/increase torque by sucking the charge air along a long narrow tube, thus incresing it's speed, Mr Venturi will see to that. It's unwillingness (the old man would call this inertia I think) then to stop creates positive pressure forcing the gas in to the cylinder as the valves open. Exactly the same happens in reverse at the exhaust side to.
The harmonics come into play at this point. As the gass flow is produced by a non uniform source (the opening and closing valves) this sends sock waves down the intake and exheust tracts. If the lengths can be adjusted such that the time it takes for the wave travel the length of the pipe and back are syncronised with the next time the valve opens, the pressure throws just that little extra bit of charge into the hot place....phew!
Sometimes bigger is not better, esecially with exhausts. get the harmonics wrong and you can end up with backfiring through the intake, which is very intersting when your head is under the bonnet too
, or exhaust being sucked back into the cylinders (not clever). Either way, Bobs aren't usually doing anything with their bean cans than demonstraing their ineptitude. 
So as you can see, the balance between increased tourque and peak power is a trickey one, and favouring one can degrade the other. If you want more of both you need a different fuel, a bigger mill or +1Atm.
Mouse
...oh, Dad's just back... dad......
To increase peak power, in theory all you need to do is make the intake and exhaust as free breathing as possible, so over any given period of time a larger volume of gas can pass through the system.
..but as we all know, peak power only comes at the top of the rev' range, and that is impractical, if not to say downright painful, both to engine and ear to use all the time. The greater the torque of the engine, the greater it's ability to accelerate the car by increasing the rev's from lower in the rev range (the characteristic many refer to as laziness).
...finally he get's to the point...
The tourque of an engine (as taken by me from the writings of David "The Wizard" Visard Re; Minis), is dependant on the momentum of the gass flow at low revs. This produces/increase torque by sucking the charge air along a long narrow tube, thus incresing it's speed, Mr Venturi will see to that. It's unwillingness (the old man would call this inertia I think) then to stop creates positive pressure forcing the gas in to the cylinder as the valves open. Exactly the same happens in reverse at the exhaust side to.
The harmonics come into play at this point. As the gass flow is produced by a non uniform source (the opening and closing valves) this sends sock waves down the intake and exheust tracts. If the lengths can be adjusted such that the time it takes for the wave travel the length of the pipe and back are syncronised with the next time the valve opens, the pressure throws just that little extra bit of charge into the hot place....phew!
Sometimes bigger is not better, esecially with exhausts. get the harmonics wrong and you can end up with backfiring through the intake, which is very intersting when your head is under the bonnet too
, or exhaust being sucked back into the cylinders (not clever). Either way, Bobs aren't usually doing anything with their bean cans than demonstraing their ineptitude. So as you can see, the balance between increased tourque and peak power is a trickey one, and favouring one can degrade the other. If you want more of both you need a different fuel, a bigger mill or +1Atm.
Mouse
...oh, Dad's just back... dad......
danger mouse said: It's to do with harmonics and hydrodynamics, two areas which I would normally hand over to Dr Mouse snr. on, but he's out on a wild turkey chase, so.....
To increase peak power, in theory all you need to do is make the intake and exhaust as free breathing as possible, so over any given period of time a larger volume of gas can pass through the system.
..but as we all know, peak power only comes at the top of the rev' range, and that is impractical, if not to say downright painful, both to engine and ear to use all the time. The greater the torque of the engine, the greater it's ability to accelerate the car by increasing the rev's from lower in the rev range (the characteristic many refer to as laziness).
...finally he get's to the point...
The tourque of an engine (as taken by me from the writings of David "The Wizard" Visard Re; Minis), is dependant on the momentum of the gass flow at low revs. This produces/increase torque by sucking the charge air along a long narrow tube, thus incresing it's speed, Mr Venturi will see to that. It's unwillingness (the old man would call this inertia I think) then to stop creates positive pressure forcing the gas in to the cylinder as the valves open. Exactly the same happens in reverse at the exhaust side to.
The harmonics come into play at this point. As the gass flow is produced by a non uniform source (the opening and closing valves) this sends sock waves down the intake and exheust tracts. If the lengths can be adjusted such that the time it takes for the wave travel the length of the pipe and back are syncronised with the next time the valve opens, the pressure throws just that little extra bit of charge into the hot place....phew!
Sometimes bigger is not better, esecially with exhausts. get the harmonics wrong and you can end up with backfiring through the intake, which is very intersting when your head is under the bonnet too
So as you can see, the balance between increased tourque and peak power is a trickey one, and favouring one can degrade the other. If you want more of both you need a different fuel, a bigger mill or +1Atm.
Mouse
...oh, Dad's just back... dad......
Think about pushing someone on a swing. To get the maximum results with the least effort on your part, you want to synchronize your pushes with the movement of the person on the swing. Ideally, you want to push them down at the moment they have hit the peak on their backswing.
If you synchonize carefully, you can keep them moving with minimal effort on your part and minimal stress on theirs. If you push them either early or late, you're working against their movement rather than with it.
The trick in extracting maximum power from an engine with the minimum stress on components is to get everything working in sync.
That's why I can't understand the use of poppet valves (i.e. reciprocating movement) to control flow. The pistons reciprocate because they must. The valves shouldn't have to. Forcing them to do so not only robs the engine of power but durability as well.
I suggest a wankle is the solution to reciprocating pistons, but what would we call this site then? 
What would you suggest as a better valve?
I forgot to say that due to the nature of all things harmonic, they only really work within very narrow frequency phases. So you the greater the adavantage you seek, the peakier the engine will become, robbing you of flexibility.
What would you suggest as a better valve?
I forgot to say that due to the nature of all things harmonic, they only really work within very narrow frequency phases. So you the greater the adavantage you seek, the peakier the engine will become, robbing you of flexibility.
danger mouse said: ...I forgot to say that due to the nature of all things harmonic, they only really work within very narrow frequency phases. So you the greater the adavantage you seek, the peakier the engine will become, robbing you of flexibility.
Hence variable intake manifolds. Long tracts for harmonics at low engine speed, short for high speed.
Right, so how on earth would you tune intake lengths - trial and error? (computers sound cool and all that but how likely outside of an F1 lunatic asylum?)
Also without changing anything else are you saying that for example enlarging a TB over stock can lower the venturi effect and result in wallowing air mass (and lower torque). I understand this with reference to valve throats and the like but if a throttle body isn't larger than the manifold or the the intake pipe it interconnects would this have any detrimental effect?
Also without changing anything else are you saying that for example enlarging a TB over stock can lower the venturi effect and result in wallowing air mass (and lower torque). I understand this with reference to valve throats and the like but if a throttle body isn't larger than the manifold or the the intake pipe it interconnects would this have any detrimental effect?
...try putting "Varioram" into google, Porsche have an answer for everthing...
The throttle bodies do have an effect, the air has to be able to get though them at the higher speed without creating a heap of extra tubulance, which will stangle the engine. That is why I went at the TB on Twink with a grind stone, along with the butterfly, when I installed the ram-air duct.
...the Wizard did recomend trial and error, but I believed he refered to it as informed guess work....
>> Edited by danger mouse on Monday 23 December 16:21
The throttle bodies do have an effect, the air has to be able to get though them at the higher speed without creating a heap of extra tubulance, which will stangle the engine. That is why I went at the TB on Twink with a grind stone, along with the butterfly, when I installed the ram-air duct.
...the Wizard did recomend trial and error, but I believed he refered to it as informed guess work....
>> Edited by danger mouse on Monday 23 December 16:21
Captain Muppet said:
JonGwynne said: ...That's why I can't understand the use of poppet valves (i.e. reciprocating movement) to control flow...
Cheap, low wear, good sealing, simple lubrication. What are you proposing as a better solution?
BTW Good answer DM!
Cheap? Surely not. They are mechanically complex and therefore expensive to design and built.
Low wear? OK, I'll give you that. Under normal operating conditions, they don't wear a great deal. However, if your cam-belt breaks...
Good sealing? Compared to what?
Simple lubrication? Really? Again, what are you using as a comparison?
Options? How about ball valves and sleeve valves to name just two?
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