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  1. #1
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    Default Stroke/Bore RELATION TO Torque/Horsepower

    I was just wonder, if you happen to build/rebuild/modify your own block and you change the combustion changes and the specs of bore and stroke, how does this affect the torque and horsepower?

    What I'm asking is what is the relation to different combinations of stroke and bore for different torque and horsepower?

    Thanks!

    TR

  2. #2
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    Anybody?

  3. #3
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    Default I've got no clue...

    ...but I think that is a great question.

    Perhaps the information is available somewhere with graphs showing the power and torque curves of differently built engines?
    Heidi

    Do a search of Grumpyvette's (Grumpy Vette?) posts as he explains things in great detail and I think he may have discussed this previously...

  4. #4
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    Default Re: Stroke/Bore RELATION TO Torque/Horsepower

    Originally posted by Stallion
    I was just wonder, if you happen to build/rebuild/modify your own block and you change the combustion changes and the specs of bore and stroke, how does this affect the torque and horsepower?

    What I'm asking is what is the relation to different combinations of stroke and bore for different torque and horsepower?

    Thanks!

    TR
    bore and stroke is a direct relation to engine size and it's assumed that the bigger the motor, the more power you're going to make.

    What kind of pistons you will use also change the compression ratio for different power.
    Edmond.

    Contact me for the best prices on Amsoil.

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    Default Re: Re: Stroke/Bore RELATION TO Torque/Horsepower

    Originally posted by Edmond
    bore and stroke is a direct relation to engine size and it's assumed that the bigger the motor, the more power you're going to make.

    What kind of pistons you will use also change the compression ratio for different power.
    Yes I know...there is no replacement for displacement!!!

    I just figured what Heidi was talking about, maybe an analysis on different bores and strokes resulting in different hp and torque outputs. I'll look for his posts and see if he has anything to say on the subject.

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    Default Re: Re: Re: Stroke/Bore RELATION TO Torque/Horsepower

    Originally posted by Stallion
    Yes I know...there is no replacement for displacement!!!

    I just figured what Heidi was talking about, maybe an analysis on different bores and strokes resulting in different hp and torque outputs. I'll look for his posts and see if he has anything to say on the subject.
    Yes, there are a lot of articles out there in regards to different bores and strokes for the small block Chevy. I think the most popular is the 383 stroker. You see most guys go with that combo. It's only a 33 cubic inch gain in terms of number and if you were to just bore out the block and not replace the crank, I believe the displacement would be around 355.

    But when you put that stroker crank in, you get more torque. I believe that more stroke equals more torque.

    Ask the guys and they'll ask you what you want to do with the motor and how much money you want to spend. Maybe this should be moved to the C3 mods forum?
    Edmond.

    Contact me for the best prices on Amsoil.

  7. #7
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    Rob has some good formulas:

    http://corvetteactioncenter.com/tech.../formulas.html

    Torque is directly proportional to displacement:

    Torque = MEP x Displacement / 150.8

    (MEP = mean effective pressure)

    MEP = HP x 792,000 / Displacement x RPM

    Displacement is proportional to the cylinder volume, the length of the stroke and the number of cylinders.

  8. #8
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    Thanks for those equations!! What units are the displacement in? CI? Liters? But I get the general idea now. Thanks!!

    TR

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    Depends on what you want to use. English or metric.

  10. #10
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    Okay, I see. We'd probably want to use cubic inches so we get results in ft. lb. because if we use Liters then we would get Newtons per Meter I think.

    Thanks again!

    TR

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    Actually, the newton/meter is an S.I. (International System) unit. Metric would be kgf/m (Kilogram Force/ Meter) or derivitive thereof. But you can use whichever you are comfortable with as long as you don't try to cross systems.

  12. #12
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    I'd watch just plugging numbers into those equations. The "150.8" and "792000" are probably conversion factors, for a particular system of units (I would assume that cubic inches, horsepower, ft-lbs, etc. would be the ones they use).

    Alright, stroke vs. bore... engines come in three flavors:

    Under-square: stroke longer than bore
    Square: stroke and bore equal
    Over-square: bore larger than stroke

    Really, only two, since under-square engines are extremely uncommon, and impractical for modern use. Any of them can make the same torque, and the same power (within the same operating range). The more over-square the engine is, however, the more the engine will like high rpms. The power band will be shifted upwards, as you go further and further over-square.

    The limiting factor is the result of piston speed. While the rotating components operate in rpms, the pistons and connecting rods are moving up-and-down, at some number of feet per minute. The longer the stroke, the greater the distance the piston moves per revolution. So, at any given number of rpms, the longer-stroke engine will have greater piston speed. Get too high, and you'll lose more and more power to increased friction, not to mention increased wear, and eventually having a mechanical failure.

    So, that puts a practical limit on the operating rpm range of the engine. And, since horsepower relies heavily on rpms, an engine which is limited will not produce as much peak power as an engine that can turn more revs.

    On the other hand, torque is basically force exerted on a lever. In this case, the pressure in each cylinder pushes on the piston's surface area (pounds-per-square-inch, multiplied by the square inches of the piston, results in pounds), which is dependant upon the bore, and that force is multiplied by the lever length (stroke) and the number of cylinders, to get the torque. Increasing either the bore (which increases the area of the piston top) or the stroke (which increases the lever length) will increase the torque. However, the practical limitations of piston speed apply, so the long-stroke, small-bore engine must make its power down low, before it runs out of revs, while the short-stroke, large-bore engine can move its powerband upwards, and make more ultimate power, with more rpms.

    It all depends on what you want. Some people actually under-stroke engines. Replace the 3.5"-stroke crank you have in your 350 with a 3.25" crank, and you have 327 cubic inches. This engine will be farther over-square than yours. You have a 1.14 bore/stroke ratio. That engine will have a 1.23 bore/stroke ratio. So, it will be happier in the upper-rpm range. A 383 typically has a 1.07 bore/stroke ratio (4.030" bore, 3.75" stroke), but GM's HT383 has a 1.05 (4" bore, 3.8" stroke). Both of these engines are very close to square, so they have limited upper-rpm power, but lots of low-end grunt.

    Other odd engine combos: put a 3.25" crank in a 400-block (4.125" bore), and you get a 347 (close enough to 350), with a 1.27 bore/stroke ratio. That's even further over-square than the under-stroked 327. Put a 3.5" crank in the same engine, and you have a 374, and a 1.18 bore/stroke ratio. That's about halfway in-between the stock 350's 1.14, and the under-stroked 327's 1.23, with a large gain in displacement. The stock 400's ratio is 1.1, putting it between the 383 and the stock 350, again with a large gain in dispacement. Personally, that's what I've decided on (406 = 4.155" bore, 3.75" stroke = 1.11 bore/stroke ratio). Sort of halfway between the 350 and the 383, with a big increase in displacement.

    Hope this helps a bit...

    Joe

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    That's exactly what I was looking for, Joe!! Absolutely perfect explanation and easy to understand. Thanks a lot!!!

    TR

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    Originally posted by MaineShark
    I'd watch just plugging numbers into those equations. The "150.8" and "792000" are probably conversion factors, for a particular system of units (I would assume that cubic inches, horsepower, ft-lbs, etc. would be the ones they use).
    Great explanation Joe! The formulas and conversion factors were given only for representative purposes only, since factor like iMEP cannot be determined without an indicator, not normally available the average fellow.

  15. #15
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    Not a problem... I used to teach, and I can't quite get it out of my system

    Joe

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