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Automotive How performance car parts influence torque vs horsepower?

  1. Feb 22, 2012 #1
    I am currently majoring in mechanical engineering at Texas A&M after having completed my associates degree in automotive technology (4.0gpa) while i'v been fixing cars at Firestone Autocare. Its odd that i rarely find anyone who knows a thing about the physics of energy efficiency and the relationship between torque and horsepower in an every-day motor vehicle.

    Now, i know what your thinking! we could spend pages discussing horsepower vs torque proportions and how they vary depending on engine RPM SO ON AND SO FORTH! But thats what the weekend track racers and the gear heads want to know.

    ME, the engineer is curious about how parts/structure OF THE ENGINE (for example: camshaft lobe profile, valve timing/lift, stroke, bore, etc...) have an affect on power and torque values. It is interesting to think about especially when noticing the differences between output values of diesels vs gasoline powered engines seeing as the torque in diesels triumphs horsepower, and vise versa in modern gas engines.

    I have a very strong knowledge of the parts that come together in the right sequence to provide efficient combustion, but when it comes to physical factors that can give an engine a lot more low end torque as opposed to high end horsepower, it seems to be a bit more technical.

    PLEASE, ENLIGHTEN ME. i am interested to hear from anyone into this kind of stuff.
  2. jcsd
  3. Feb 22, 2012 #2
    Power = Torque * RPM

    It is as simple as that. I’ve worked power transmission systems on boats, ships, airplanes, locomotives, jets, heavy equipment, and many more systems. I’ve also spent thousands of hours in the shops with the mechanics. Every place I’ve been, the mechanics will get into strange discussions concerning the advantages of high or low toque, all based upon a fundamental ignorance of the physics involved. When that happens, just refer to the equation above and you will not be confused.

    The torque put out by the engine does not matter at all. What matters is how much power the engine puts out, and what percentage of that power you are able to transmit to your load. High torque at the engine compared to low torque only changes the gear ratios in between the engine and the load. If you have designed your power transmission correctly, the load won’t know the difference between a high torque engine and a low torque engine.

    In general, a low toque engine can come up to power more quickly than a high torque engine. That is not always true, but often is. In that case, you have a dynamic rate of change difference, but steady state will be exactly the same.

    As for the design of the engine, high torque increases the loads in all parts of the engine, so it is bigger and heavier. It will have a less favorable power to weight ratio. Consider a diesel, a gasoline engine, and a gas turbine. The only real difference for power transmission concerns is that we have progressed from a slow shaft speed (high torque) to a very high shaft speed (low torque). Assume that all three engines put out the same power. The diesel will be very heavy, the gasoline engine will be much lighter, and the gas turbine will be a small fraction of that. A 100 hp gas turbine at 100,000 rpm would be very small and light.
    Last edited: Feb 22, 2012
  4. Feb 23, 2012 #3

    Ranger Mike

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    Welcome..this forum is a gold mind for inquiring minds ...please look at these posts
    use your search function above...some real top notch people have posted great information

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    Torque vs RPM Mar28-09, 07:51 AM

    Why Does the Torque Curve Drop Off at Low RPM in a Typical Piston Engine? Jul11-08, 01:51 AM

    Engine size/type (ie. 2L inline-4) and fuel consumption Jun8-09, 08:01 AM

    Expansion ratio of burned Fuel Jul23-09, 02:50 PM

    piston size and proportionate forces Oct28-09, 07:17 PM

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    Pushing the piston. Nov22-09, 08:29 PM

    Internal Combustion Piston Lubrication Apr24-11, 09:56 PM

    gas pressure in internal combustion gasoline engine Nov22-09, 02:16 PM
  5. Feb 23, 2012 #4
    This is such a disgustingly gross over simplification, for people who have no part of the design or analysis.

    People who like a quck answer to what my engine can do use the left hand side of the power equation. Design requires the right hand side.

    An engine sticking out 1000Nm @ 300rpm.
    An engine stickign out 1Nm @ 300000rpm.

    Same ability to do work, but you can't use the same box for both.

    More air intake allowing
    More fuel to be burned making
    More power.

    For performance engines, everything is designed to get as much air fuel mix burned as possible.

    Less Simply:

    If you are going into engineering (re engines) and don't have this book, get it.
    It's as valuable as Roarks or Machinery's.

    OP you may want to post a more specific question, narrowing the scope for the thread.

    It may be worth starting multiple threads on different aspects of the things you want to know. Otherwise it'll quickly become a torque vs power thread and become a horrid mess.
    Last edited: Feb 23, 2012
  6. Feb 23, 2012 #5
    THANK YOU EVERYONE for the replies, i never get bored of hearing about these kinds of things.

    I think some might be confused as to what the context of my question was cause lol, no one wants to start an argument between the benefits of torque and horsepower depending on the application.

    I was more interested in how the mechanical structure, the ARCHITECTURE of engines and their internals, the way they are shaped, the durability them, TAKING IN ACCOUNT the amount of fuel/air/timing can be transmitted reasonably in modern applications of mass produced machines these days. It is understood that more air+fuel at 14.7/1 ratio you can get in and out of an engine, the more work will be done. I dont find electronics have that much of a say in this, because, with the right sensor placement, electronics sort of perform their function at an optimum level these days, thats more of another topic which will probably grow in the future (ie Variable cam timing/variable compression ratio etc.) It seems this thread is sort of jumping around cause its hard to decide what my perticular question is. I guess it really is my lack of knowledge of the physical characteristics of the combustion engine itself.

    Pkruse, you had a real good point about the variations of load and weight on the engine parts, so your saying diesels would require bigger and more durable parts just because of the mechanical load itself whereas turbine engines can use much smaller parts due to the smaller load (taking in account the way they perform the same amount of work is totally different. Never really thought about that.

    It looks almost obvious as the more displacement used to turn a crankshaft: the more torque an engine would have, so bigger cylinders is a torque affecting variable. I wasn't sure about compression, because I know compression is wanted as much as possible in order to burn ALL the fuel in the cylinder, and diesels can withstand more compression due to their durability. Port diameter/aerodynamics and inlet outlet size is a major variable of torque at low end vs power at high end, (usually changing these factors significantly sacrifice high end horsepower to benefit the low end torque, or silencing affect- depending on the application.)

    BTW thanks RANGER MIKE for listing all those references. this is seriously a gold mine, and i'm probably gunna link this to my local car enthusiast forum, those guys would love this stuff.
    Last edited: Feb 23, 2012
  7. Feb 23, 2012 #6
    High torque engines have high mechanical loads. But high revving engines have high inertia loads.

    If you imagine, a piston has to slow to zero at TDC. The faster the RPM, the faster it needs to decelerate from it's peak speed. high accelerations = high forces.

    At medium to high RPM, the inertia of the components acutally becomes the dominant loading and the gas force from the piston becomes less important.

    This is again all about getting air into the engine. You can tune a pump to work is a specific rev range.

    Small valves produce better flow at low RPM. But obvious have a limited capacity for flow.
    Large valves produce poor flow at low RPM but have higher capacity.

    This is why race engines sound and drive awfully at low rpm. They are designed to be at their peak at high RPM.
  8. Feb 23, 2012 #7
    Sorry, Chris, I need to step in here; you're describing how port size can affect the torque curve, not valve size. Generally for a given port size, the larger the valve the higher the flow (curtain area). Once the port is the main restriction the valve size doesn't matter.
  9. Feb 24, 2012 #8
    Good to see you spotted the deliberate mistake. I meant port.

    ive been using the wrong word a lot lately, its getting embarassing now.
  10. Feb 24, 2012 #9
    Chris: It really is as simple as I stated in my last note. If you want to increase the power output of your engine, then you have only two choices: increase torque and/or increase rpm.

    Increasing rpm has all the standard options: pay attention to the aerodynamics of your intake and exhaust flow paths, balance everything with as much precision as possible, replace stock parts with stronger parts, use light weight parts to minimize moving mass, stronger valve springs to prevent float, and all that sort of stuff. You also want to increase the mass flow rate of both your fuel and air, but I’ll talk more about that below.

    Increasing torque means increasing how much force the pistons can transmit to the crank. Some of your options are the same. Increase mass flow rate of fuel and air, increase compression ratio. That last one goes with increasing the thermodynamic efficiency of the engine, which has the same effect as increasing fuel and air flow.

    Adding a mechanical air pump to the intake will have the same effect as increasing compression ratio, though the two are not the same. I’ve seen diesel engines with compression ratios as low as 10:1, a ratio that no diesel can work at unless you blow in a whole lot of supplemental air.

    Remember that increasing the power output of the engine will also increase the waste heat output, so beef up your cooling system.

    Putting out much extra hp will increase the loads on everything. In the case of increasing torque, the source of the extra load is obvious. In the case of extra rpm, the source will be dynamic forces from spinning faster. So where ever possible, use stronger components and lubricate them better.

    A friend builds up Chevy small blocks to put out nearly 1400 hp. In his application, they only have to last a very few seconds, and sometimes they don’t even do that. He uses a Dodge crank that does two things for him: It is stronger, and it has a longer stroke. (He has to make room for it by grinding parts of the block away.) The longer stroke goes to increasing torque, but he is also spinning it very fast. He also tells me that the steel Dodge crank is much stronger than the original. His pistons and valves are Titanium, and the designs have been optimized to use as little of the material as possible. Very light weight and balanced with precision.

    Increasing the combustion pressures may defeat the stock head gasket design, so you may have to do something special to prevent a leak there.

    Another option is to increase the energy content of your fuel. Now you are talking about some special racing fuels. They may require changes in the air mixture, or you may decide to supplement the air with more oxygen. I guess the most common source of more oxygen is from nitrous oxide.

    Now that you have a more powerful engine, you need take a look at your power transmission. Do you need to change the gear ratios? Can it and its mounts tolerate the increased torque?

    How is that for a bunch of random thoughts, rapidly composed?
  11. Feb 24, 2012 #10
    BTW: for a really powerful strong engine, start with a spark ignition version of a diesel engine. That gives you a base that is very strong from a structual perspective.
  12. Feb 24, 2012 #11
    A great deal of science and physics goes into the design of a gas turbine. (Gas turbines that fly are most frequently called “jet engines.” Those that don’t fly are generally called simply, “gas turbines.” The design is the same, except that the flight version is always optimized for low weight and physical volume.) More sophisticated mechanical engineering goes into a gas turbine than probably any other design, with the possible exception of some space craft. I’ve never seen the reciprocating engine designers borrow science from the gas turbine industry in order to optimize their design, but let’s see if we can do a little bit of that here.

    A turbo charger is really just a small and greatly simplified gas turbine, with the reciprocating engine serving as the combustor. A gas turbine has a compressor up front, which feeds air into the combustor, which feeds into the turbine section. In a pure turbojet, the work done is by the mass flow rate of the exhaust gasses pushing the airplane forward. But in most jet engines flying today, the exhaust gas does little or no work. The airplane is propelled either by a conventional propeller (in the case of a turboprop), or by a ducted fan (in the case of a turbofan).

    Jet engine design engineers speak of “bypass ratio.” That is the ratio of the air entering the front of the engine divided by the amount of air passing through the gas turbine. The bypass air flows around the engine and blows out the back, providing most of the forward thrust. In the past, 15:1 was considered a high bypass ratio, but Pratt & Whitney is in the process of releasing a new engine with a 49:1 bypass ratio. The higher the number, the more efficient the engine and it is also much quieter. This promises to be a great “game changer” in the aviation world. Fewer parts, cheaper to build, cheaper to maintain, burns less fuel, and you can hold a normal conversation next to one running at full power. All this was made possible by a new gear box that enables the engine to turn a much larger fan at lower speeds. It has taken them 15 years to develop this gear box, after many have failed to do so in the last several decades. This is why I stand by my previous note, that it really is as simple as Power = Torque * RPM. All you have to do is install an appropriate gear box between the engine and the load. That statement was not always true in the past, before the development of much better gear design, but it is true today.

    So just like in your car, the power output of the turbine is normally a rotating shaft. In ground applications, that may turn a generator or some other equipment.

    About 75% of the power generated by the turbine section is used to turn the compressor. So if the turbine is putting out 1000 hp, only 250 hp is available for the output shaft. It is the same thing with a turbocharger. It is an error to believe that “free” waste energy in the exhaust drives a turbocharger. A TC might boost the power output of the engine by 100 hp, but then it might put a 40 hp load onto the engine. So the net power increase of the engine is only 60 hp. You never get something for nothing in the energy department, and I’m probably being overly optimistic to believe that this hypothetical TC would only consume 40 hp. It probably consumes more.

    You can find a great plethora of tutorials and videos of how people have used a standard TC to make their own jet engine, by adding an external combustor to it, normally powered by propane. But it could be powered by any other hydrocarbon fuel. When you see these videos, the first obvious question is of what practical value is it? How can I extract usable power from it? All I have is a very inefficient turbojet that is probably too heavy to fly and too bulky to mount into a real aircraft. So let’s conduct a thought experiment as to how we can use this to increase the power output of a reciprocating internal combustion engine.

    How about if we used the exhaust from this homemade jet engine to spin a second turbocharger? They have been doing that for decades with aero derivative engines for peaker units in power plants. A jet engine turns another turbine, which turns the generator. Could we not do the same thing here? What if we used this homemade jet engine to drive a second turbine to pump air into the engine without adding a parasitic load to the reciprocating engine? Now you could get the full 100 hp transmitted to the crankshaft.

    I know it would take much more engineering effort to work out the details of how to mount something like this into a real automobile, but does anyone see any holes in the basic concept?
  13. Feb 24, 2012 #12
    This thread is diverging rapidly from the point. With increasingly long posts.
    we need to break it up into several threads.

    Pressure driven turbos cause virtually no load. Back pressure causes losses on the order of 1-2% of engine output.
    The figures you are talking about are more like loads seen from superchargers.

    The reason why turbocharging is essentially free is they use energy that would otherwise be wasted. It's more energy recapture.

    Turbo compounding is being looked at on cars. It's also being looked at for the next lot of changes of F1 regs. It's interesting beucase the velocty tubines don't cause the backpressure that pressure turbines do.
  14. Feb 24, 2012 #13
    Why bother when cranks for SBCs come in every reasonable stroke and are a fraction of the cost of a Dodge crank?

    No, pistons for SBCs are not made from titanium.

    Better check with your friend, sounds like you've got a few facts mixed up.
  15. Feb 24, 2012 #14
    Oho, just seeing if I'm paying attention!

    Not that I've ever used the rong tirms or such ...:smile:
  16. Feb 24, 2012 #15


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    ??? The P&W website seems to be stating 9:1 or 12:1 bypass for their geared fans, not 49:1

    http://www.pw.utc.com/products/commercial/purepower-pw1000g.asp [Broken] (the "characteristics" tab).
    Last edited by a moderator: May 5, 2017
  17. Feb 24, 2012 #16
    AlephZero: They have been testing many versions of these engines for a long time now, and have bragged about being able to produce it in a 49:1 bypass ratio. From a purely engineering perspective, that is a reasonable number. They say that they plan to build the engine in many versions to fit many different aircraft, but that they are starting with the few you saw on their web page. All have under-the-wing space limitations that limit the bypass ratio. But note that the plane with the largest clearance is getting the highest ratio. What we need now is for the aircraft manufacturers to release a design that would accept such an engine. If these first few live up to expectations, I expect that will happen sooner rather than later.

    But all this is irrelevant to my main point that we have had great improvements in gear design technology, such that differences in shaft speeds at high power levels are now reasonably addressed in a power transmission design. Axioms concerning limitations that everyone used to assume as true must now all be questioned.
  18. Feb 24, 2012 #17

    I have no doubt that what you say is true. I also grant that if I were to collect the data required to run a proper engineering analysis, that it would show that the Dodge and Chevy cranes are essentially equal. But it would make no difference to him. His gut says that the Dodge is better. All he knows is that he and his friends have broken Chevy cranks, but they don’t tend to break the Dodge cranks. Purely anecdotal evidence not supported by science. But then I’ve already been told that folks who make cars go fast go more on the gut than on science.

    I know what Titanium looks and feels like. I know how a competent engineer would change the design if he were redesigning an aluminum part in Titanium. The price he paid for them was consistent with my experience for low production parts machined with precision out of Titanium. These pistons passed all those tests with me, but the real clincher was when I checked the literature that came with the parts, which said that they were indeed made out of Titanium. Titanium may be very rare for pistons in a SBC, but they are available.
  19. Feb 24, 2012 #18
    Chris: I think that perhaps you are more correct than me on the turbochargers. I checked with engineers who design turbo machinery, and I read a masters thesis on the design of turbochargers that I found in the MIT archives. And I evaluated the compressor and turbine maps for a typical automotive TC. They do indeed put a load on the engine, but the number is closer to what you say than my previous estimate of 40%, at least for stock automotive applications.

    But I learned something else interesting. The TC will always put a back pressure onto the engine, which is a load. But the trend when the system is correctly engineered is to do other things to reduce back pressure elsewhere. They try to use larger pipes, for example. The net result in a specific application may very well be that the engine has the same or less back pressure than it originally did.

    An automotive TC simply does not cost much energy to run because it really does not pump much air, compared to other applications of turbomachinery.
  20. Feb 24, 2012 #19
    its late and i've been out on the beerr.

    but we all have a differnet breadth of knowledge to bring to this foruim. and that what s makes it so great.
  21. Feb 24, 2012 #20

    jim hardy

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    Isnt this what led to the jet engine ?

    In WW2 turbochargers increased mass flow rate to point valves got in the way, and thrust from exhaust became significant? Since the recipcating engine was becoming almost a compressor/heater for the turbo they replaced it with centrifugal compressor and burner cans.....

    i've read that the air-racer guys run their P-51's at 4 atmospheres..... i dont know if thats a lot but sounds like it to me.

    old jim
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