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Tractor Pulling Discussion Forum

Engine power question

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Curious

04-26-2006 16:47:25

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Ok my question is if you have a 600 cubic inch 6 cylinder and it makes 150hp. Then you build the same exact thing but only in a 4 cyl at 400 cubes. Will the 4 cylinder make 2/3rds the power of the 6 or will it make less?

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Crutch

04-27-2006 12:31:47

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Re: Engine power question in reply to Curious, 04-26-2006 16:47:25

150 /600 = .25 hp per Cubic Inch.
Must be at a very low rpm. Sounds like a restricted induction system also.

So if all things are equal except Cubic inches then the 400 ci engine will have access to greater intake flow if the same carb is used and the same size intake plenium and runners which could allow a higher volumetric effeciency. Of course if you are build with the same bore and stroke design and a smaller induction system the results should be very similar. IMO. There are really to many variables that would be hard to match to make this question easy to answer.

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D boll

04-27-2006 05:11:48

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Re: Engine power question in reply to Curious, 04-26-2006 16:47:25

A six has three intake cycles per 360 degrees whereas the 4 has 2, theoretically a 6 should make more power at a given displacement. This is what hurts big inch 2 cyls., can't get the cyls. full. Alot of variables come into play so none of this is gospel but if a 970inch G could utilize those inches 100% the output would be incredible!

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uh, yeah

04-28-2006 06:04:30

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Re: Engine power question in reply to D boll, 04-27-2006 05:11:48

D Boll, so you're saying a 20HP twin cylinder lawn mower has more power than a 20HP single?

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Lamont

04-27-2006 18:30:40

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Re: Engine power question in reply to D boll, 04-27-2006 05:11:48

D boll, Could you expound on that a little more? I'm not quite understanding the 3 intake cycles per 360 degrees part. By the way, I'm glad those big inch Deeres can't use all tha volume effectively!

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shinnery

04-27-2006 23:11:52

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Re: Engine power question in reply to Lamont, 04-27-2006 18:30:40

Lamont
What I understood him to mean was a four stroke engine takes two revolutions of the crankshaft to fire all the cylinders. So in one revolution of a 6 cyl. engne you would only get 3 intake cycles and three Compression/power cycles. Is this clearer? Bryce

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Warren Jerome

04-26-2006 20:04:21

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Re: Engine power question in reply to Curious, 04-26-2006 16:47:25

Less I'd say. Those remaining 4 still have to pull the belts, timing gears and chains, etc. that don't drop any of their required HP. Back in the early seventies, the govt made auto manufacturers start using net HP, and the ratings dropped like a rock because those things use quite a bit of power.

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stroker 88

04-26-2006 18:36:28

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Re: Engine power question in reply to Curious, 04-26-2006 16:47:25

if everything is the same. in theory it should make more than 2/3. cause it will have less internal friction.

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Lamont

04-27-2006 18:26:06

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Re: Engine power question in reply to stroker 88, 04-26-2006 18:36:28

It will have the same internal friction IN PROPORTION. You would lose 1/3 of the power and with that you would lose 1/3 of the friction. However, the posts above address MANY variables that would make it all theoretical.

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stroker 88

04-27-2006 19:46:07

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Re: Engine power question in reply to Lamont, 04-27-2006 18:26:06

it will not have the same internal friction, cause it will have less bearing surface. also skirt area of piston will be less.

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Lamont

04-27-2006 20:33:59

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Re: Engine power question in reply to stroker 88, 04-27-2006 19:46:07

Actually, proportionally the 6 cyl. has less bearing surface. I'm assuming 7 mains for the 6 cylinder and 5 mains for the 4 cylinder. 7 to 6 is less than 5 to 4. The piston skirt is an even proportion - 4 skirts to 4 cylinders equals 6 skirts to 6 cylinders since each cylinder will theoretically produce the same power. I understand the terms MORE and LESS but you have to consider everything in PROPORTION.

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Rahlo

04-29-2006 14:57:31

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Re: Engine power question in reply to Lamont, 04-27-2006 20:33:59

I think what Dboll is trying to say is the filling of cylinders is less. Now don't freek out, let me explain what he is, I think trying to say. If you have a 2 cylinder engine with 500 cid and a six cylinder with the same. Once the rpms have slowed the 2 cyl will have a less full air charge than the 6 cyl because of the larger cyl displacement. Now this is assuming that the engines are naturally aspirated. This will cause a less leathal charge of fuel and air thus the power stroke is weekened by less compaction. This is why a 6 cyl has more power than a 2 cyl if the engines are built comparably ie: compression, duration of the cam, etc.

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Lamont

04-29-2006 17:55:12

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Re: Engine power question in reply to Rahlo, 04-29-2006 14:57:31

Yeah. I understand the efficiency of filling the larger cylinder is hard to match to filling a smaller cylinder. I just didn't understand what he meant by intake "cycles." That word cycle threw me off. I think of a cycle as going through all four strokes. He must have been meaning there are 3 cylinders being filled with air. Seems like we talked about that last night. Anyway, this business about less drag on a 4 cyl. vs. a 6 cyl. has to be dealt with from a porportion standpoint. Truth be told there are so many variables to consider that would be proportional issues it would be difficult to build a 4 cyl. engine with the same bore and stroke as a 6 cyl. engine and come up with exactly 2/3 of the power. (Unless it was accomplished by manipulating the RPM) It could be done but not just by simply "chopping off" 2 cylinders.

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