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stroking vs bigbore, interms of air intake

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Now the cylinder has a constant intake bore (smaller then stock). There is an apparatus in the intake track that has a volume greater or equal to the volume of the cylinder in the down stroke; this is supposed to compensate for the intake bore restriction... How does increasing the stroke compare to increasing the bore.

I assume that rule of thumb will still hold true, that the larger bore will make more top end power due to the mean piston remaining the same (ei: higher RPM as opposed to the stroker) at the same increased torque value.

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As far as I know, big bore will gain hp, and a stroker will gain tq

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Now the cylinder has a constant intake bore (smaller then stock). (If I understand you correctly you are talking about two different things here. Do you want to talk about the intake track (runner) or the cylinder bore? What does smaller than stock have have to do with it and what is stock?) There is an apparatus in the intake track that has a volume greater or equal to the volume of the cylinder in the down stroke; this is supposed to compensate for the intake bore restriction... (What is the apparatus you are speaking of?)How does increasing the stroke compare to increasing the bore. (Now there's a great question. I would like to study it more but I believe in theory engines are just air pumps. If you take two identical engines, CID included, on one you increase the bore and on the other you increase the stroke to arrive at a specific CID and staying within the guidelines of angular forces, in theory you should end up with the same air flow.)

I assume that rule of thumb will still hold true, that the larger bore will make more top end power due to the mean piston remaining the same (How do you mean the piston will stay the same?) (ei: higher RPM as opposed to the stroker) at the same increased torque value.

There are many variables that effect this. I believe you need to be more specific in your question.

I just read your question again. I think I understand you are telling us that a given engine/cylinder now has a restricted intake track, True? Now the volume of the intake track is identical to the CID and how will the power be affected and what is the difference between increasing the bore as opposed to increasing the stroke and leaving the intake track alone?

This is definitely the riddle of the day.

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This is for a project. The rules of the competition state that the intake bore can be only 30mm, very small for a 450 engine. To try to compensate a bulge was place in the intake track (after the throttle valve) that has a volume the size of the cylinder volume. The theory being that the budge will contain enough fuel to prevent the intake from being restricted.

Now I know for a fact the the stroker will not be able to rev as high; which in turn means that max air flow will be less then the big bore. I think this would be more beneficial to use a stroker because it would be less subject to intake restriction as the big bore.

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I assume this is a 4-stroke?

You have not said what the goal of the project is.

The rule of thumb will apply that in general, engines of any size will produce more peak power and more power at a higher speed if they have a larger bore and a shorter stroke. This, however, is more related to the lower piston speed at a given RPM with the shorter stroke than it is regarding air flow. It IS true that the smaller bore size ends up placing a practical limit on the size of the valves that can be placed in the head, and that can limit air flow, but the issue really centers around the piston speed vs. the combustion speed of the fuel.

Apart from that, there are certain offsets. A smaller combustion chamber is easier to fill efficiently, for instance.

Since the running engine is a dynamic entity, your intake chamber will likely not achieve the desired effect. It must fill itself at a rate high enough to be effective, as well as empty itself into the chamber through the aforementioned smaller valves, and these will become the bottleneck regardless of the size of any internal plenum you might use. There would also be problems of fuel separation using a carb or fuel injected upstream from the plenum.

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Rule of thumb is, if you have a restrictor you must make the engine as fuel efficient as possible to get the most power. In theory this means low rpm engine which can convert as much fuel and air in into motion as possible. But the airbox between restrictor and cylinder must be a lot bigger than volume of the cylinder.

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without the goal of the competition it's hard to say what is your best approach? if the motor your given is the correct now I wouldn't bother with bore and stroke, not really much there, even stroke to rod ratios won't be enough to mess with on a motor that small. I'd take a hint from nascar, when they went to restrictor plate they went to huge compression ratio numbers and late intake valve closing times. The motors were made to survive the huge compression by careful throttling by the driver. Put your foot in it coming off pit row and you'd likely blow the motor, ease into the throttle and the motors would survive. Same thing was done with normally aspirated high altitude aircraft, throttling was key. So if your "test" allows you to do this, that and cam timing will give you the most power.

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30 mm isn't too tiny of a restriction if shaped right.

30mm was for a good long time the standard carb size on all standard model British bikes, including 500 and 600cc singles. Certain high performance models used 38's.

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This is for a project. The rules of the competition state that the intake bore can be only 30mm, very small for a 450 engine. (does this tell us you are using a modern 450 cc race engine?)To try to compensate a bulge was place in the intake track (after the throttle valve) that has a volume the size of the cylinder volume. The theory being that the budge will contain enough fuel to prevent the intake from being restricted. (If it is the theory only that you are testing and it is a naturally aspirated internal combustion engine, then think about what happens after a restriction. Sure there are many things you can do to compensate for the restriction but if I understand you correctly you are testing the restriction and increased volume after it verses all things being equal except a decreased volume after the restriction and before the combustion chamber. The air pump will only flow as much volume as the restriction will allow given that the pressure doesn't change on either side of the restriction from one test to the next.)

Now I know for a fact the the stroker will not be able to rev as high;(as high as what?) which in turn means that max air flow will be less then the big bore. (is CC the same? If so I don't believe this to be true unless you are only speaking of theory and an rpm level only obtainable in FI) I think this would be more beneficial to use a stroker because it would be less subject to intake restriction as the big bore.

There are still a lot of areas we are guessing at. We need more info. and/or more specific questions in order to help. What are the rules and regulations of the project?

I just read the original question again. How does increased bore compare to increased stroke? With all other variables being equal, volume of air flow I believe will be identical. If what you are trying to do is increase HP via additional CC's to overcome a restricted intake track and want to know if it would be more beneficial to increase bore or stroke, that depends on many variables. I don't see how that would be beneficial in testing the original theory. I guess I have too many questions.

Edited by kx450f63
additional info.

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