How do you design/build an engine for low end torque?

roccaas

roccaas

Well-known member
I've always been curious about engine design for low end torque. If we fix the number of cylinders and the displacement, how does the engineer bring torque to the low end?

I've always understood that a long throw crank/long connecting rods would bring torque to the low end, and that smaller engines/fewer cylinders must rev higher to get torque (BMW's 4 cylinder M engine) that a large, lazy V8 gets by the bucketful at low revs (Chevy Small block).

How does forced induction, better breathing, piston size, etc impact the low end torque design?

How did Henry and Edsel get torque from a very large (3.3L) inline 4 that runs to 2,500 rpm, when the engine is "square".
 
I don't believe quantity of cylinders effects RPM power band.

For example look at a 1950s Ferrari V-12.

My understanding which isn't much, agrees with azmodela. Cam and head design. How well the engine breaths at any given RPM.

I think the main thing that hinders an under-square engine is valve area. Smaller cylinder diameter = smaller valves. What you gain from it I have no idea. Am interested in hearing other responses.
 
Long stroke, undersquare bore, and lots of duration on the cam. Low RPM is to be expected. A heavier Flywheel is also a plus for super torque on a big engine. This makes it less apt to slow down under a quick load. Compare a model A four cylinder to a similar size cubic inch flathead V8. Big bore, big stroke 4 cylinders on the A, versus basically half size pistons and a nominal stroke but long rods on the Flathead.

The best comparison though is a Diesel from a tractor which will turn all day long at 1500-2000 RPMs to make alot less horsepower compared to a higher RPM engine. The difference is the torque on the diesel is far greater versus the overall Horsepower on the high revving gas engine.

Torque gets you out of the hole and RPMs continue your speed increase down the track. IT's all a trade off, go For Horsepower in a car and torque in a truck or tractor.
 
roccaas said:
I've always been curious about engine design for low end torque. If we fix the number of cylinders and the displacement, how does the engineer bring torque to the low end?

I've always understood that a long throw crank/long connecting rods would bring torque to the low end, and that smaller engines/fewer cylinders must rev higher to get torque (BMW's 4 cylinder M engine) that a large, lazy V8 gets by the bucketful at low revs (Chevy Small block).

How does forced induction, better breathing, piston size, etc impact the low end torque design?

How did Henry and Edsel get torque from a very large (3.3L) inline 4 that runs to 2,500 rpm, when the engine is "square".
Click to expand...


The Model A engine may be nearly "square", but it is a comparatively long-stroke engine, at least by modern standards.

My understanding was that if you wanted torque, you used a long stroke. This increases the amount of leverage achieved by the "throws" on teh crankshaft. The trade-off is, the longer the stroke, the faster the piston travels in Feet-per-second ( at a given RPM), and the lower your [safe] red-line is going to be.

I would say the Model A engine was already well designed for low-end torque...
 
Cam duration and better flow through the heads and intake do increase torque but they mostly increase horsepower. Increasing bore and stroke increase mostly torque. Torque gets you moving and to me is a bit more useful than horsepower.

The tricky part with increasing stroke is geting the right rod length to crank stroke. If this is off you will wear the cylinder walls unevenly taking years off your block.
 
Tim said:
Long stroke, undersquare bore, and lots of duration on the cam. Low RPM is to be expected.
Click to expand...

Actually, the greater the duration, the further the low end torque is DECREASED assuming the valve lift remains the same. Additionally the torque curve becomes narrower, i.e., over a smaller RPM range as the increased duration moves it up the chart. With that said, opting for the limited duration cam with broad a low end torque curve WILL limit (or eliminate) engine speeds at the higher end of the spectrum.

Unfortunately there is no "one size fits all" optimal duration or lift vs. duration (which is what it boils down to). Increasing compression and improving breathing matched with fuel delivery will increase the optimal minimum duration somewhat.

When you opt for a very high duration cam you have the potential for very high (comparatively) engine speeds. To make best use of the torque primarily at the higher end, lower gearing is essential. That's when you hear of guys looking for 4.11 gears! The last step is to incorporate a close ratio gear box so you can keep the engine speed inside the narrower torque curve you have with this configuration.

 
Anson said:
Cam duration and better flow through the heads and intake do increase torque but they mostly increase horsepower. Increasing bore and stroke increase mostly torque. Torque gets you moving and to me is a bit more useful than horsepower.

The tricky part with increasing stroke is geting the right rod length to crank stroke. If this is off you will wear the cylinder walls unevenly taking years off your block.
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Horsepower is power or watts. Torque is simply a moment force and is meaningless alone.

I often see people try to separate torque and horsepower usually saying horsepower is less useful. A broad power band gets you off the line better, not a different torque spec.

Horsepower = 746 watts or 745 watts depending on who you ask and the time of day. Either way it is POWER.

It is calculated in the auto world by multiplying torque by RPM and dividing by 5252.

For example, if an engine produces 200ft\lb at 2000rpm you will have 76hp. If you produce the same amount of torque at 4000rpm you will now have 152hp.

An engine producing 50ft\lb at 5000rpm will produce the same power as one producing 100ft\lb at 2500rpm and can be compensated for using gearing and your torque at the wheels will be identical.

And in the case of increasing bottom end torque you are also increasing bottom end horsepower.

Sorry,

I'm loaded with useless info like this :)
 
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Mr Tube,

I think that's probably accurate as a clinical definition.

In the case of automotive applications, torque might better be defined as "useful power".

In the case of most older engines ( pre-1960-ish), we find they achieve their peak torque at about 1/3 to 1/2 the engine RPMs that they achieve their peak HP...

So, a stock Model A produces peak HP at 2200 RPM, and peak torque around 800-1000 RPM (going from memory here).

The best powerplants for producing peak torque at 0 to low RPMs, and being consistent through their operating range, are electric motors and steam engines. But both bring their own complications to the practicality aspect.

That engine producing 50 ft lbs at 5000 rpm is going to need so many gears to get its load moving as to be pretty useless...

Another old saw is that " Horsepower is for "show" and Torque is for "go". "

( It's true too... you can get a Model A moving in 2nd or even high-gear. Try that with Honda Civic ( starting in 3rd or 4th ) (or other torqueless, hi-revving modern four-banger of your choice)... )
 
Special Coupe Frank said:
Mr Tube,

I think that's probably accurate as a clinical definition.

In the case of automotive applications, torque might better be defined as "useful power".

In the case of most older engines ( pre-1960-ish), we find they achieve their peak torque at about 1/3 to 1/2 the engine RPMs that they achieve their peak HP...

So, a stock Model A produces peak HP at 2200 RPM, and peak torque around 800-1000 RPM (going from memory here).

The best powerplants for producing peak torque at 0 to low RPMs, and being consistent through their operating range, are electric motors and steam engines. But both bring their own complications to the practicality aspect.

That engine producing 50 ft lbs at 5000 rpm is going to need so many gears to get its load moving as to be pretty useless...

Another old saw is that " Horsepower is for "show" and Torque is for "go". "

( It's true too... you can get a Model A moving in 2nd or even high-gear. Try that with Honda Civic ( starting in 3rd or 4th ) (or other torqueless, hi-revving modern four-banger of your choice)... )
Click to expand...

Frank,

You are correct. I simply wanted to bring my thoughts to the table.

Keep in mind something nice many modern engines have is multiple cam lobes with different profiles for the same valve. This way your torque remains peaked over a much broader range. The 250HP 3.5L Honda engine in our Saturn doesn't switch cam lobes until 4500rpm.

This means idle-4400rpm is one cam profile and 4500-6300 is another profile.

I believe the torque is around 220-240ft\lbs throughout most of the curve from 2000rpm to 6300 which at least to me, was impressive for a 3.5L stock engine.


The only Honda engine I know anything about is the J-35 and its only because I own a GM vehicle with one in it. ;)
 
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The only reason small-displacement / high-revving Euro / Asian engines have been successful has been due to their early embracement ( is this really a word?) of "multi-speed" ( read: more than 3 forward speeds ) gear-boxes.

For whatever reasons, the American Motorist was unwilling to accept a manual transmission with more than 3 speeds until the 1960's... Chrysler tried offering a four-speed syncro-mesh with a "double-high gear" in 1930-31; third gear was a "city high", and fourth-gear was direct drive for the open road. They didn't sell, Chrysler dropped it.

And with the limited speed potential of our roads prior to the 1950's, you could get away with a 4.1 rear, 85-100 HP long-stroke engine and a 3-speed ******. Heavy cars like the senior Buicks and Packards had 4.3 and 4.6 rears to help them get under way.

You really need a five or six-speed ****** with top gear being an Overdrive to make a small displacement / short stroke / high-revving engine work for modern driving.

My experience with modern no-torque / hi-rev engines has been with my wifes' Hondas and her present 2000 New-Beetle. Yeah, they red-line around 7,000 RPM, but they don't generate any usable torque below 1500-2000 RPM... makes life hard for the clutch !
 
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An easy way to see torque's usefulness is in a diesel powered semi. It has many times more torque than horsepower.
 
Hello everyone and thank you for the membership. I would think that in order to build a high torque engine, the most important factor would be cylinder efficiency. To get the best possible combustion requires the ability to fill the cylinder with a charge of air and fuel at a ratio of around 17:1 very lean or 14:1 street driven. After the cylinder is filled you will want to close the intake valve very quickly so that the compression cycle can start as soon as possible. This requires a fast opening and closing intake valve profile with enough duration to completely fill the cylinder. Cylinder space should be large volume, and the combustion camber small for a good quench of the mixture. Deck height should be minimal. A good high energy ignition system to ignite the mixture. Now here is part of the camshaft profile difference. The exhaust valve should slowly start opening before bottom dead center and slowly close so it completely closes just after top dead center. This way the gasses going out of the cylinder causes a higher flow rate in the intake port helping to recharge the cylinder. For this to work you must use tuned exhaust headers. Bigger is not always better so stay on the smaller diameter for primary tubes. Same goes for cylinder head ports. You want to increase the speed of the flow going into and out of the cylinder. hope this helps. PS. Any comments are welcomed.
 
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Since the thread has been revived, I will also welcome Eclipes and thanks for the detailed comment. I noticed that the thread mentioned stroke, which increases mechanical advantage (torque), bit I didn't see mention of increasing compression, which we all know is a major contributor to torque. Ron W
 
as Marco says, consideration in the trans and rear are certainly where the torque would best occur.

is this A engine of 4 years ago being used in a tractor pull???
 
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