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Old 04-26-2012, 09:09 PM
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How much HP gain bored 30 over

Hey guys does anyone know how much HP you gain with a chevy 350 when its bored 30 over, 60 over and so on with everything stock except for the pistons of course? Anyone have a rough guess?

Thanks

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Old 04-26-2012, 10:30 PM
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Not much if any at all. Power doesn't come from overboring a SBC. Keep in mind that efficiency (hp per ci) drops as an engine gets larger. You now have pistons that weigh more, a larger circumference piston and rings create more friction too. Someone also posted on here that a SBC350 would overheat if bored over +.080", so perhaps there are also heating issues that could impact power as bores grow too.

A hot rod in the true sense of the word will be 1 HP per cubic inch. So depending on your stock engine, if it made 350HP new, maybe it would make 355HP in +.030" form, or 360HP if +.060", but I wouldn't count on that.
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Old 04-26-2012, 11:01 PM
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Like he said just boring an engine does not increase the efficiency of the engine. Also the less horsepower the engine made before the over bore smaller the power gain is. Stroking a small block is the only efficient way for more ci and ultimately more power.
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Old 04-27-2012, 05:33 AM
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In theory.... 355/350 = 1.0143 which is around 1.4% more power assuming no other limiting factors (heads, exhaust, intake, ......)

So, a stock 250hp 350 would make 253.5 hp with a +0.030 bore. Better than losing 3.5 hp but not a noticeable gain. And if the bore is done poorly you could lose hp.

bigger bore is better, but the power is in the heads. Concentration on getting better heads first.
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Old 04-27-2012, 02:33 PM
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Quote:
Originally Posted by allansmith859
Hey guys does anyone know how much HP you gain with a chevy 350 when its bored 30 over, 60 over and so on with everything stock except for the pistons of course? Anyone have a rough guess?

Thanks
Modern boring is a method to restore the cylinder wall to serviceable dimensions and quality. Gone are the days of having enough material (wall thickness) that you can truly add enough displacement to feel the power difference. That went when thin wall castings began to appear in the 1960's and 70's.

Today the problem is a loss of power when boring. Getting beyond .030 it's possible that the remaining material is so thin that the cylinder distorts under combustion pressure and piston side loads which then allows pressure to be lost as excessive blow-by taking torque and power with it. These days many engines are not recommended to be bored beyond .015 inch.

You need to check, some heavy duty truck or performance blocks have extra thicknesses in them and will tolerate boring beyond .030. But when contemplating .060 most any modern block needs to be sonic tested for wall thickness before dialing the boring bar way out.

Bogie
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Old 04-27-2012, 04:06 PM
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Thanks guys that def helped me out.
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