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bore size

 
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birkey
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PostPosted: Mon Nov 12, 2012 1:26 am    Post subject: bore size Reply with quote

I have a 283 and was looking into going. 125 over. Will the thin cylinder wall create problems with heat?
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Big Dave
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PostPosted: Mon Nov 12, 2012 12:36 pm    Post subject: Reply with quote

Yes it will. Assuming you do not hit water first.

Cast iron, like all metals is a conductor. That is to say it conducts heat as well as electricity. We choose copper over cast iron to conduct electricity because copper is a better conductor. Copper is better than aluminum (which is used in the power lines in front of your house) but not as good as gold used in MIL Spec equipment, or silver used in soldering. We do not generally use iron to conduct electricity except in cars, where the steel sheet metal is often uses as the ground path.

There is a measurable difference in horse power just from the use of aluminum heads versus cast iron heads. The cast iron holds the heat in better than the aluminum that allows it to bleed off to the coolant faster than cast iron. This is why with two identical heads the one cast out of iron makes more power.

As you reduce the thickness of the iron cylinders the distance the heat has to travel falls off and the amount of heat shed increases allowing more of the heat to escape to the coolant reducing power and heating the coolant up. This is because the rate of conduction speed is a known quantity and changes with thickness and temperature.

You already shed 32% of the heat of combustion to the coolant normally with a stock motor. As you over bore it more will escape leaving less heat in the engine to do work.

Big Dave
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birkey
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PostPosted: Mon Nov 12, 2012 1:43 pm    Post subject: Reply with quote

Someone told me when you bore a cylinder it will run hotter. And your saying it will run too cool. Correct?
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SMOKEmUP
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PostPosted: Mon Nov 12, 2012 1:46 pm    Post subject: Reply with quote

Why the 283? Why not get a 350 block if you want a 4" bore?
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birkey
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PostPosted: Mon Nov 12, 2012 1:46 pm    Post subject: Reply with quote

I didn't really know how that was possible or why they said that.
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birkey
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PostPosted: Mon Nov 12, 2012 4:06 pm    Post subject: Reply with quote

well for one i hate that the 350 is so overrated. i want to build something that is unusual (eventhough it has been done many times). the 350 is such a typical build. I want to take a motor and extensivly modify it, and i like the challenge of getting max hp out of a very small engine. i dont want to stroke it though.
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birkey
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PostPosted: Mon Nov 12, 2012 4:30 pm    Post subject: Reply with quote

big dave, if i were to run a high compression (10:1 - 10.5:1) would i get the necessary cylinder temp on pump gas without detonating?
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Big Dave
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PostPosted: Mon Nov 12, 2012 8:32 pm    Post subject: Reply with quote

birkey wrote:
Someone told me when you bore a cylinder it will run hotter. And your saying it will run too cool. Correct?


Wrong the engine will run hotter, because more of the heat energy that should be driving the rear wheels is seeping into the coolant driving up the temperature.

As to running higher compression you stand a better chance of staying out of detonation with a short stroke motor than you do with a long stroke motor. You can run 10.25 on current pump gas with a 283 because all of the old 1961-'66 full size Chevy's you see at car shows are still running the original 293 motors with that static compression ratio.

If you want an unusual motor build a 350 made up of a 327 crank in a 0.030" over bored SBC 400 block. 352.4 cubic inches that is different from every one else as it should easily spin to 8,000 RPM with a solid roller cam and some good heads.

Big Dave
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birkey
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PostPosted: Tue Nov 13, 2012 7:38 pm    Post subject: Reply with quote

im looking at a 400 now. just might happen
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