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Some questions on Torque

cheekychimp

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I read a thread a few days ago about exhausts and ended up looking at a number of manufacturers who produce exhausts with valves that can be opened and closed (primarily to allow the user to make the exhaust quieter when the vehicle is at idle or cruising).

All interesting stuff but Barry then put me onto a site about Active Exhaust Systems. Basically the same stuff but they had an option to install a black box that measures back pressure in the system and automatically adjusts the degree by which the valve is opened in accordance with the amount of back pressure that is detected. I can't really see that being of any benefit in a forced induction engine but it was supposed to help low end and mid-range power in normally aspirated vehicles.

So that got me thinking. I realized that I really don't understand torque. I know long intake runners generate it, but I have absolutely no idea why. I know NA engines need back pressure to generate it, but again I have no idea why. I also remember being told that a lot of rally cars had engines capable of producing over 500 hp but most only made about 300 hp because the rules stated they had to run 34 mm restrictor plates. The throw off from this of course was that they made barrel loads of torque. Again I realized I didn't understand why.

So what creates torque exactly? And does a restriction in the exhaust or at the turbo inherently create torque or is the engine simply tuned to produce torque (rather than power) because airflow has been restricted?

If a valve in the exhaust can affect the torque characteristics of a normally aspirated vehicle, then couldn't a 'variable restrictor' in a forced induction engine create torque at low rpms and allow power to develop at higher rpms?

Or does the engine have to be retuned everytime the size of the restrictor is changed?
 
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Dan D

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Torque is Force – Rotational to be specific. The rotational force comes from a translated linear force. The rods and crankshaft translate the linear force applied to the pistons. So, let’s ignore the rotational part and focus only on the linear force since they are proportional.

The linear force applied to the piston comes from pressure. The pressure comes from a combustion reaction which is approximately governed by the ideal gas law PV = nRT. We’ll ignore any of the complicated thermal or fluid dynamics and focus on the easy stuff. Just remember an increase in pressure happens with an increase in temperature and vice versa.

Combustion is exothermic – it releases heat.

Combustion in a gasoline engine = 2C8H18 + 25O2 ~> 16CO2 + 18H2O

Oxygen is 21% of the air going into the engine. The rest of the gases are not involved in the reaction.

The piston comes up and compresses the air/fuel charge – this change in pressure is fixed based on your engine’s compression ratio so we’ll ignore that as well.

The combustion happens which gives off heat. The other gasses inside the cylinder go up in temperature causing the cylinder pressure to go up. That pressure places a force inside the combustion chamber and work is done on the piston (because it moves and the rest of the cylinder doesn’t). Now we have our linear force.

What affects the amount of force? Pressure. More pressure = More Force = More Torque. How do we get more pressure (ignoring the compression ratio)? We get more of two things – 1. Overall volume of air so the pre-combustion cylinder pressure is higher. 2. oxygen & fuel so more combustion reactions occur and more heat is given off. If we don’t have Nitrous, achieving #1 and #2 are one and the same.

How do we get more air volume in the cylinder? Artificially increase the pressure of the air in the cylinder (turbo or supercharger) so there are more molecules per unit of volume and/or ensure that as much of the available volume in the cylinder is empty to make room for the incoming air.

When the piston comes back up after the power stroke, the exhaust valves open and the piston pushes the combustion byproducts and air out of the cylinder and into the exhaust stream. The more efficiently the cylinder and combustion chamber empty out, the more room there is for incoming air. Not only does as much volume as possible need to get out, but it has to do it as fast as possible because the exhaust stroke event is only so long in duration.

Simplified physics in place, exhaust pressure is bad for generating linear force and thus torque. What is important is the velocity and volume of exhaust exiting the cylinder. No single exhaust (or intake for that matter) has exactly the right design to maximize exhaust velocity for a given volume in all conditions. Systems like variable intake runner length & variable exhaust help maximize the range in which near ideal conditions can be achieved by providing a proportional change in geometries to the change in gas-flow condition.

The backpressure = torque mistake really comes from old school carbureted engines in which control over the air/fuel ratio was not directly achievable. A livable air/fuel ratio across a wider operating range was achieved by limiting air flow using backpressure to keep a more forceful combustion event occurring.
 

cheekychimp

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This reinforces my current understanding that a forced induction engine really benefits from the least restrictive exhaust available albeit that you reach a point of diminishing returns where more noise nets negligible amounts of additional power. But are you saying that a fuel injected normally aspirated engine would benefit just as much from a free flowing exhaust but that my 1.8 carbureted GLS might not enjoy the 3" straight through catback treatment so much?

That surprises me because a lot of NA car owners I know have admitted experiencing a loss of low rpm and midrange power following the fitment of a larger diameter exhaust BUT that top end power was increased significantly. Is that imaginary or other forces at play here?

And how does the use of a restrictor come into play here?
 

Dan D

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Maintaining exhaust velocity for a given volume is important. You'll hear the term scavenge. Basically a high speed fluid stream leaves a low pressure area behind it (think boat wake). This low pressure helps pull remaining fluid into the stream which in the case of an engine helps to empty the cylinder. The larger the diameter orifice (exhaust pipe), the lower the velocity, the less “scavenging”.

Also keep in mind that each set of revolutions in the operation cycle moves a given volume of air through the engine. Let’s say we have a 1 cylinder engine with a 1 liter displacement and all things are ideal so each time the engine goes through a cycle (intake, compress, combust, exhaust), 1 liter of air moves through. Now think about a small oil change funnel. If you slowly pour a quart of oil into the funnel, one quart of oil will make its way through the funnel and into the motor. If you dump one quart quickly into the funnel, some overflows and spills because it can’t flow through the funnel fast enough. What’s the difference in the two situations? – Time.

Go back to our 1 cylinder engine. At 1000 RPMs we are pouring exhaust, 1 liter at a time slowly through the exhaust piping. At 7000 RPMs we are pouring exhaust 1 liter at a time very quickly through the exhaust system. If the exhaust is small, the gas will flow easily at 1000 RPM, but be restricted at 7000 RPM just like the oil in the funnel. If it is restricted, then less than 1 liter can flow each cycle. The exhaust gas velocity will be high because the orifice size is small so at low RPM, we empty out the cylinder. But at high RPM, we have backpressure and can’t empty the cylinder despite the high velocity. If the exhaust if really big, there is no restriction at 1000 rpm or 7000 rpm, but the velocity is slow so the cylinder doesn’t empty as well. The trick is to find the best balance of restriction vs velocity for the application.

Restrictor plates are actually a similar story. Typically someone interested in going fast will tune the car to maximize torque in the upper RPM range (giving high horsepower). Why? Power is the amount of work done in a given amount of time. Work is Force X Distance. You want to get from the starting line to the finish line in as little time as possible. I like to think about the relationship of power and force (torque in a car) like this:

Picture two big heavy boxes on one side of the room. Two guys each need to get one of the boxes to the other side of the room, 100ft away by pushing them. One guy is 150lbs bean pole, the other is 350lbs lineman. Each guy takes a running start and hits their shoulder into the box. The little guys box moves 1ft and the big guy’s box goes 5ft. The big guy applies more force so more work is done. Now the little guy starts into a fury of repeatedly running into his box once every 10 seconds, each time moving it 1ft. The big guy can only manage once every minute, moving 5ft each time. After 10 minutes, the big guy has moved his box 50ft. In the same amount of time, the little guy moved his box 60ft. Even though each application of force was significantly greater from the big guy, there was less power from the big guy in the ten minute interval because the little guy applied his force so much more often which let him go a greater distance.

Switching back to cars, the little guy’s furious pace is high rpms, the big guys slow lumbering pace is low rpm. At high RPM, the engine is applying force more often than at low RPM. If the engine and car are tuned such that applied force is maximized at high RPM, you will do more work which means you’ll cover more distance in a given period of time (starting line to finish line).

Thinking back to the funnel, air going into the engine is subject to the same sort of restriction vs velocity as exhaust going out. A restrictor plate is like putting the funnel into the throttle body. At low rpm, air flows unrestricted into the engine. As RPMs rise, the funnel becomes a restriction because we’re moving more air in a given period of time. Past a certain point, there is enough restriction that more air can’t go through. If you can’t increase the volume of air, you can’t increase the force applied during combustion. Because there is always a compromise between volume and speed of intake and exhaust, restrictor plate cars choose to optimize at lower RPMs to increase the applied force to compensate for the fact that they can’t apply it as often. That is why they often appear as low end torque monsters with little horsepower.
 

broxma

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On the subject of scavenging, the header, as scavenging in a turbo application is not really applicable except in odd turbo location applications, would have to be very precisely fabricated in order to set the correct runner length. And correct me if I am wrong but the beneficial nature of a tuned length header drops with RPM as the exhaust pulses are no longer spaced far enough apart to make use of the reduction in pressure behind the previous wave front. The idea is very similar to driving behind a semi on the highway. However at some point, the exhaust pulses would eventually be so close it would be an equivalent to having the nose of the car in the trailer of the truck.

An old school hot rodder, dear friend and automotive engineer, Kurt Klein, once discussed with me the ideal intake air speed for combustion. I do not recall that specific number. The number he gave me was in ft/sec and his assertion was that given a static air speed, one could determine rough increase in HP or torque based on increasing or decreasing the static speed. Once the ideal speed was reached, the car could not increase in power due to lack of overall flow capability and an increase in turbulence which would in effect slow down the air speed. That is to say, for a given motor, intake air has a particular speed which is ideal and that number is relatively constant in terms of HP for most combustion applications. Torque could be increased by speeding up the flow rate but only to a certain point at which time, it would again start to decline.

I most closely associate this discussion with the difference between the 1G and 2G intake manifolds on the DSM's. Clearly the absolute flow limit of the 1G manifold is higher, but the smaller runner 2G manifold creates more torque at a given HP due to increased airspeed. Honestly, I just sort of accept that as a rule and don't question it. I know there is an upper limit to the division although I am not sure at what point HP begins to suffer due to absolute flow restriction.

/brox
 

curtis

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^ This is why big boy top fuel dragsters aren't getting any faster There limited to cubic inches, wheel base, tire sizes etc and they can only get x amount of air in the engine. Something else to ponder on those is fuel consumption and making a pass. There down in the 4 second range now so they start the engine and it idles in the pits they then do a burn out then stage and bam there gone. They leave at max rpms and that doesn't change that much during the pass. If you figure a 4 second flat pass at 8K the big old nasty motor only spins around 533 times to over 300mph. Holy sh*t thats some serious torque with not a whole lot of effort
 

cheekychimp

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Quoting Dan D:
A restrictor plate is like putting the funnel into the throttle body. At low rpm, air flows unrestricted into the engine. As RPMs rise, the funnel becomes a restriction because we’re moving more air in a given period of time. Past a certain point, there is enough restriction that more air can’t go through. If you can’t increase the volume of air, you can’t increase the force applied during combustion. Because there is always a compromise between volume and speed of intake and exhaust, restrictor plate cars choose to optimize at lower RPMs to increase the applied force to compensate for the fact that they can’t apply it as often. That is why they often appear as low end torque monsters with little horsepower.



Thanks for the answers so far guys, I'm beginning to get an idea of how things are working here and what mods I want to do on a much milder scale with the 'other' car (can't really call it 'stock' anymore).

Dan regarding the paragraph above, I am assuming when you say the cars "choose to optimize at lower rpms" you do indeed mean the car is retuned with the restrictor plate fitted and removing the funnel would create a 'lean' condition that would have to be tuned for again?

Could you theoretically have a valve before the throttle body that opened in conjunction with a custom fuel map to optimize torque at low rpms and then allow power at high rpms? I am not interested so much in how difficult or complicated it would be, rather just confirming that my understanding of the 'theory' is correct.
 

DR1665

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Wow. This thread got physical. /ubbthreads/images/graemlins/hsugh.gif

Seems to me the issue - when it comes to either intake or exhaust - is velocity, which is a function of volume in this case.

NA cars lose bottom end with larger diameter exhausts (and excessively ported heads) because they don't flow sufficient volume to achieve ideal exhaust gas velocities at lower engine speeds. Once the engine is operating at higher RPM, the volume of air being moved through the combustion chamber is increased, resulting in better efficiency. Turbo cars enjoy larger exhausts (though they are not immune to the same issues) because the larger volumes of air pumped into the combustion chamber translates into larger volumes of exhaust, resulting in higher exhaust velocities.

This is the principle behind the Cyclone manifold. The longer runners are tuned to promote increased velocities of the smaller volumes of air being drawn into a lower compression engine off-boost. Prior to the turbo pressurizing the system above atmospheric, the engine is naturally aspirated, and so the longer runners are smaller in diameter to keep the velocity up. Once the turbo pressurizes the system, the smaller runners present a restriction, thus the butterflies opening to allow a greater volume of air to be forced into the combustion chambers. It's a compromise, and a damn smart one at that.

I'll be honest, I have no idea what you mean by adding a valve before the throttle body, Paul, but if you were thinking about doing something similar for the exhaust, you might want to consider ways to replicate Cyclone intake functionality on the hot side. Randomly, could one of those electric exhaust cutouts (or a spare WG) be adapted to route exhaust between 1.5" and 2.0" parallel lines in similar fashion? How might a 1.5" exhaust affect spool-up? With both opened, would this be the same as a 3.5" exhaust?

Interesting topic, this. And we haven't even touched on how temperature/density plays into the equation yet!
 

broxma

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Speaking on this topic, I saw a 4G63 head down at Kendrick today which had been "Ported". It appeared the owner had done it himself or paid very little for the work. The intake and exhaust were so large there was literally about 2mm flat surface for the gasket. It did not appear that the bowls had been touched and the short side radius was almost straight. Great for flow maybe, but I doubt it had been benched, but surely a massive loss of torque.

/brox
 

DR1665

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Quoting /brox:
Speaking on this topic, I saw a 4G63 head down at Kendrick today which had been "Ported". It appeared the owner had done it himself or paid very little for the work. The intake and exhaust were so large there was literally about 2mm flat surface for the gasket. It did not appear that the bowls had been touched and the short side radius was almost straight. Great for flow maybe, but I doubt it had been benched, but surely a massive loss of torque.



Are you suggesting people not try performing headwork for themselves? Heads for these cars can be found all over the place - maybe even under beds and couch cushions. If you don't have a couple lying around, OMG, $50-$100 locally. Why not give it a go yourself and see what happens?

Considering we get the term "porting" from modifications made to the intake and exhaust ports to affect gas velocities with respect to desired power delivery, it makes my skin crawl when people refer to combustion chamber or bowl work as "porting."

And while we're at it, how is the term "flow" useful to anyone at all? Flow is like PSI. Either, by themselves, is useless. They require some sort of numeric accompaniment in order to provide any relevance. It is useful to suggest a GT28RS might flow 15-30lb/min at peak efficiency, compared to a GT35R at 30-50lb/min, but without additional information, it's akin to saying Shep's car goes.

Finally, what's this about "massive loss of torque?" Headwork affects the powerband. How do we know the owner of this head didn't have custom intake manifold bolted to the head with short, straight runners firing the intake charge directly at the valves? Unless this head was severely ham-fisted by some mad scientist with OCD, gigantic, straight intake ports simply shift the torque curve aggressively to the right, possibly beyond the capabilities of the rest of the engine, and unless the head is tested on a flowbench, we're all just talking out our asses, speculating.

Although, it would be funny to tell someone, "Congratulations! When we do the math, your head will make peak power somewhere between sixteen and seventeen thousand RPM."

C'mon, gents. Let's bring the A-game, now.

Note: Nothing personal. Pet peeves, these. It was this or start stabbing people around the office. That never ends well.
 
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cheekychimp

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Quoting DR1665:
I'll be honest, I have no idea what you mean by adding a valve before the throttle body, Paul, but if you were thinking about doing something similar for the exhaust, you might want to consider ways to replicate Cyclone intake functionality on the hot side. Randomly, could one of those electric exhaust cutouts (or a spare WG) be adapted to route exhaust between 1.5" and 2.0" parallel lines in similar fashion? How might a 1.5" exhaust affect spool-up? With both opened, would this be the same as a 3.5" exhaust?



Brian,

Sorry, I probably wasn't expressing myself well, it was just that Dan said a restrictor was a bit like sticking that funnel in the throttle body. I am still struggling with this concept that restrictor fitted cars are usually torque monsters and I am trying to understand if they are simply tuned to produce as much torque low down because the airflow simply isn't sufficient at higher rpms or if the restrictor itself influences flow creating additional torque inherently as with a smaller diameter exhaust.

I was thinking about something like the MAFT-Pro that can 'manage' fueling in a car to reach a target AFR and wandering if a variable sized restrictor that progressively opened from say 34mm to 70mm as the vehicle increased rpms would allow the generation of high torque low down and high horsepower at the top end.

I am 'feeling' that there would need to be variable cam timing, a cyclone type manifold system and custom timing maps to make it work. I am also struggling with the 'math' here and wondering if there was no restrictor, if the car instead of producing 300 horsepower and 300 ft/lbs would simply produce 500 horsepower and 450 ft/lbs making the whole variable restrictor idea pointless.

I do however like the idea of the hot side cyclone setup. I just wonder how you might do that? If you used a waste gate channel wouldn't you be reducing the amount of exhaust gases available to spool the turbo effectively creating more lag? I suppose this depends very much on the setup and waste gate location. I was just looking at the HKS manifold.

Edit: Forget that, I see what you mean now /ubbthreads/images/graemlins/banghead.gif you mean using a waste gate further down the exhaust path.
 
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broxma

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Quoting DR1665:
Are you suggesting people not try.... stabbing people around the office. That never ends well.



You would have had to be there.

/brox
 

Rausch

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Quote:

I also remember being told that a lot of rally cars had engines capable of producing over 500 hp but most only made about 300 hp because the rules stated they had to run 34 mm restrictor plates. The throw off from this of course was that they made barrel loads of torque.



As you mentioned, those cars are built and tuned around that restriction. They usually run a much less aggressive cam profile (in terms of duration) than a 'race' car. The reason being: They need to generate max power below the 'choke point' that the restrictor has on airflow volume. The shorter duration allows for less valve overlap, or no overlap at all. Think of it as artificially changing the stroke to some degree.

The longer the force generated by the exploding air/fuel mix can act on the piston, the more work it can do in one cycle. Same idea as a longer stroke. Closing the intake valve sooner and opening the exhaust valve later than you would on a high revving engine allows that mix to burn longer in the cylinder, generating more torque. This works in a relatively narrow band, much like high hp producing engines, but on the other end.

The shorter duration cams used won't work well at all in the higher rpm ranges, as the valves aren't open long enough to allow enough air/fuel or exhaust gases to pass the valves when those events are happening faster and faster. The reason they work well down low is the same reason they don't work up top.

There is a ton more as to how those rally cars make massive torque, but hopefully that helps to make some sense of things. /ubbthreads/images/graemlins/smile.gif
 
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cheekychimp

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Yes it does, thanks Aaron. I completely forgot about cams /ubbthreads/images/graemlins/banghead.gif

Back to the drawing board on that then. Something else that came up though in regards to the Cyclone manifolds. Because of the cast design restrictions, the difference in the lengths of the primary and secondary runners is minimal. Suppose you built a tubular runner setup similar to an equal length tubular exhaust manifold. If you substantially increased the length of the primaries and perhaps even shortened the secondaries, how would that Cyclone work? Or is it like gearing in a transmission where you don't want a huge jump from one to the other?
 

DR1665

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Hey Paul, just to add to what Rausch explained, the restrictors increase torque because they reduce the diameter of the turbine inlet, resulting in higher intake charge velocities at lower engine speeds. After a certain point, the restrictor becomes just that - a restriction - because it denies the turbo/engine the volume of intake charge sought. A prepped PGT GVR4 with a 32mm restricted 14b can't even maintain speed in 5th gear, and makes nothing but hot air and noise above about 5000rpm.

This is the *only* reason why you see me participate in any discussions on RWD conversions. 2WD turbo cars are exempt from restrictors.
Guess a 500whp RX7 would *never* be dangerously fast like a 300hp Evo might.

Anyway, rally car builders do a number of things to tune for this bottle neck. The high-lift/short-duration cams Raush mentioned, for instance. Additionally, "bang-bang," or anti-lag systems (ALS) keep the boost up between gears and off-throttle. Given both of these are pricey, I'll be seeing what I can do with intercooler plumbing design as a means of keeping charge volume up post-restrictor.

The key thing to keep in mind is not just how much torque, but the torque curve. Given a rally car which does not benefit from revving out beyond 4500-5000rpm and tends to see a lot of corners, shifting peak torque lower in the RPM range while maintaining as flat a torque curve as possible makes for a car capable of accelerating as fast as possible under multiple conditions.

A final option to consider, rather than engineer complex intake/exhaust systems, just twincharge the car. Youtube "twincharged Talon" for examples. Ray's GT35R feeds a 2.3L stroker by way of an Eaton M90 and an air-to-water box on DSMap/Jackal. Last I heard, he was putting 650awhp down, but had something like 22psi and 450lb-ft from 2000rpm to redline.
 

cheekychimp

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I've always liked the theory of twin charged cars but heard they were very hard to maintain. That is an awesome set of figures though. Where is he running the supercharger from though? The obvious choice is the A/C belt but I just cannot afford to lose my A/C here unfortunately. I did see something a while ago on electrically powered superchargers. It was whilst I was looking for an electrically driven A/C compressor. I wonder if one of those would produce enough boost?
 
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RedTwo

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There's not a lot of room when you think about it /ubbthreads/images/graemlins/jawdrop.gif

If you can have an electric water pump, surely you can run an electric super charger. It would have lower pressures/forces (compared to pushing relatively dense water around) involved - wouldn't it?
I seem to remember reading that the twin charged Lancia Delta S4 'turned off' the supercharger at turbo speeds. The Nissan March Super Turbo (circa 1990) had, given the name, twin charge and apparently has a magnetic clutch driving the S/C.

How about a compound turbo set up with a really small initial turbo? The twin turbo Galants (7/8G) spool very quickly (to 10psi too!) due to having compressor and exhaust wheels about the same diameter of a shot glass!
7G (manual): TD025L-7G-4
8G (all) TD03-7T
GTO US: TD04-9B-6
GTO JP: TD04-13G-6
 
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