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Discussion Starter · #1 ·
Hi All,

I regularly see engine ratings for the same or similar displacement engines with varying horse power and torque ratings depending on their intended use.

Some engines are used in towing/hauling applications, thus having lots of torque (trucks, SUVs, mini vans). Others are used in passenger or performance cars, and tend to have less torque, but perhaps higher horsepower ratings. But they are the same or very close in displacement.

My newbie knowledge level understands that an engine's horsepower is affected at the very least by compression, air flow efficiency, cooling efficiency, gasoline used, and cam profile.

What is it exactly about the way an engine is configured that allows the torque to vary?

Thanks in advance,
 

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HP vs TQ

Most engines have an effective operating range of about 3000 rpm. Later model EFI engines, in heavy cars, operate in the 1500-4500 rpm range. With the advent of variable cam timing the effective rpm range has been widened to enhance TQ and HP.

Typical older design 2 valve engines (example SB Chevy) built for TQ generally had a 180 degree (divided) intake manifold, short duration cam, small intake ports, and valves, in the head. All these enhance low rpm TQ.

Same SB Chevy modified to produce more HP would require everything be bigger and would sacrifice low rpm TQ for high rpm HP. Now the engine may have an effective rpm range from 3000-6000.

Same basic engine but designed for a different use.

Ron
 

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Discussion Starter · #3 ·
Hey Ron,
Ron Golden said:
Most engines have an effective operating range of about 3000 rpm. Later model EFI engines, in heavy cars, operate in the 1500-4500 rpm range. With the advent of variable cam timing the effective rpm range has been widened to enhance TQ and HP.
That's another thing that mystifies me: Why is an engine effective in a certain rpm range only. I understand that, due to breathing efficiency and capability, an engine can run out of air at a higher range. Or the lack of enough fuel can cause an engine to stumble as well. But those can't be the only factors?




Ron Golden said:
Typical older design 2 valve engines (example SB Chevy) built for TQ generally had a 180 degree (divided) intake manifold, short duration cam, small intake ports, and valves, in the head. All these enhance low rpm TQ.
So, if I understand this correctly, it is the entire top end that plays a crucial role in determining operating range as well as horse power and torque. Does displacement have any effect?




Ron Golden said:
Same SB Chevy modified to produce more HP would require everything be bigger and would sacrifice low rpm TQ for high rpm HP. Now the engine may have an effective rpm range from 3000-6000.

Same basic engine but designed for a different use.
I guess this would be going my first question, but what is going on between 0-2999 rpm that would keep the engine from being efficient?




Thanks for the helpful info Ron! Learned a few things today :D
 

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The maximum amount of torque an engine can produce is related to its breathing ability.
How much air can each cylinder of the engine, inhale. The more air, the more fuel, the more torque the engine will produce. Every engine has a speed (RPM) at which it can inhale the most amount of air. Run the engine slower than that RPM, and air actually flows back out of each cylinder, before the intake valve closes. Run the engine above that speed, and there is simply not enough time for the cylinders to fill completely.

You can shift the RPM that an engine produces peak torque by changing the camshaft, (or variable valve timing) the length of the headers, and intake manifold runners, but the maximum amount of torque really does not change that much. In fact, a high RPM engine may actually have a lower peak torque than a low RPM engine of the same displacement, and basic type.

Horsepower is a product of torque multiplied by RPM. So, if you gain more RPM than you lose in torque, by changing the RPM of the torque peak higher, you will gain horsepower.

Lets just think about what the intake valve does. At TDC, starting the intake stroke, the intake valve has already started to open. The piston is moving at its maximum speed down in the cylinder about 75 degrees past TDC. Ideally, you want maximum intake valve opening then. As the piston continues to move down, toward BDC you want the intake valve to stay open, as long as possible. In fact, you still want the intake valve to be partially open at BDC, (but closing) because the piston does not really move much when the crankpin is around BDC. Also, the inertia of the moving air in the intake runner will keep filling the cylinder when the piston is at BDC, and even when the piston has started to move back up. But if you close the intake valve too late, some air in the cylinder is pushed back out. This is what happens with a long duration high RPM camshaft, if you are running the engine below it's torque peak. Speed the engine up, and the air rushes in the cylinder and cannot change direction, and be pushed back out before the intake valve closes, and more air gets trapped in the cylinder. This increases the amount of torque the engine produced compared to running it too slow.
I hope that clears it up a little.

Here, read this. It was written for Datsun engines, but the theory applies to all engines. You can gloss over the parts about the overhead camshaft, but the rest of the information is good. Really good.
http://www.datsport.com/racer-brown.html
 

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HP vs TQ

DanielC,
Excellent info. Unfortunately my typing skills aren't anywhere as good as your and I would have been here all night typing what you did.

Ron
 

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Discussion Starter · #7 ·
Daniel, your descriptions are extremely helpful. The one fact I've never come across in my research is the one you mentioned in the quote below:
DanielC said:
The maximum amount of torque an engine can produce is related to its breathing ability.
How much air can each cylinder of the engine, inhale. The more air, the more fuel, the more torque the engine will produce. Every engine has a speed (RPM) at which it can inhale the most amount of air. Run the engine slower than that RPM, and air actually flows back out of each cylinder, before the intake valve closes. Run the engine above that speed, and there is simply not enough time for the cylinders to fill completely.
This description makes it sooo much easier to visualize what is taking place inside an engine depending on its speed. I never even thought about the air being pushed back out due to lack of speed. Not to go off-topic, but would this then also explain why turbo lag exists in any amount at low rpms?


I briefly looked over the article you linked and it is a very long read. I will be getting on it asap though as it looks quite informative. Thanks for posting that.




Ron, your response actually helped set the stage for Daniel's response, because it helped me expand on my initial question.




Thanks again Ron and Daniel. Your replies have helped me tremendously. :thumbup: I can always count on the members of this forum to school me :D
 

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My thought on turbocharger lag. I am no expert.
The turbocharger feeds itself. The energy to compress the air on the intake side of the turbocharger comes from heat energy in the exhaust. The more exhaust gases, and the hotter they are, the faster the turbocharger spins, and the more air gets pushed into the engine, and this creates more exhaust to drive the turbo.
But it takes a little time for the wheel in the turbo to spin up. It also takes time for a change in pressure in the intake to go through all four strokes of the engine cycle, and to make the exhaust hotter, that makes the turbocharger spin faster.

A common misconception it that the back pressure between the exhaust valve and turbocharger drives the turbocharger turbine (exhaust) wheel. It does to a small degree, but the energy to spin the turbocharger is captured from the exhaust gas cooling as it goes through the turbine itself. The exhaust has to heat up first.
 

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DanielC said:
It does to a small degree, but the energy to spin the turbocharger is captured from the exhaust gas cooling as it goes through the turbine itself. The exhaust has to heat up first.

WTH? You couldn't be further from the truth. In fact you want the exhaust to cool as little as possible through the turbocharger. Whoever gave you an idea like that probably was an internet expert who has never even touched a turbo.
 

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May be I did not express it quite in an understandable way. You do not want to lose heat in the exhaust gas, in the manifold and piping up to the turbine wheel, but once there, you do want the heat to be "used up" The heat loss represents the energy that makes the turbine wheel spin. That same spinning motion is used by the compressor wheel to compress the intake charge, and also unfortunately, heat it up.
 

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Discussion Starter · #11 ·
DanielC said:
May be I did not express it quite in an understandable way. You do not want to lose heat in the exhaust gas, in the manifold and piping up to the turbine wheel, but once there, you do want the heat to be "used up" The heat loss represents the energy that makes the turbine wheel spin. That same spinning motion is used by the compressor wheel to compress the intake charge, and also unfortunately, heat it up.
I understood what you meant, but thank you for the clarification. :thumbup:
 
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