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Old 12-11-2003, 09:33 PM
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Chevy 350 boring...

Hi. I have a 1972 350 high nickel(010) 4 bolt block im going to build shortly. ive desided im going to bore it instead of leaving it stock, but i dont know what i should bore it to. my uncle told me i should bore it .30 over so if i blow it up i can bore it another .30 over and use it again. i was thinking of going .20 over, but im not sure. what would be the best.

its gonna be a basically stock rebuild. just adding flat top hypers, 487x heads, and i think this comp cam.

Thanks.

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Old 12-11-2003, 09:36 PM
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have the bores checked at a machine shop, if it don`t need bored, don`t have it bored, having it bored will give little power gain, likely not enough that you`d notice. if it needs bored, have it bored to the mininum amount needed so in the future you know there`s enough meat left in the block to have it bored again.
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Old 12-11-2003, 09:42 PM
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Over boring will unsroud the valves and produce more power after 6800 with the right valve angles.
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Old 12-11-2003, 10:42 PM
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on your typical 1 HP per CI boring .030 is only gonna give 5 HP assuming the current bore is in good shape. for the most part, boring is just to make the cylinders round again, not a big perf. gain
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Old 12-11-2003, 10:43 PM
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Here's the deal there are 1000's ways of rebuilding an engine and only one correct way. The correct way is to bore out any engine so you will have a good base to build on. Let the machine shop check your engine and let them recommend the proper size. Don't be surprised if they tell you that you must bore it out to it's max. Remember if you rebuild it that one correct way chances are it will out last your car or you. So don't worry abought compromising on a good rebuild the 1st time.
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Old 12-16-2003, 11:22 AM
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Hey Johnsongrass, explain how an over bore will 'unshroud' the valves!
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Old 12-16-2003, 11:57 AM
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Re: Chevy 350 boring...

Quote:
Originally posted by Mike_82
Hi. I have a 1972 350 high nickel(010) 4 bolt block im going to build shortly. ive desided im going to bore it instead of leaving it stock, but i dont know what i should bore it to. my uncle told me i should bore it .30 over so if i blow it up i can bore it another .30 over and use it again. i was thinking of going .20 over, but im not sure. what would be the best.

its gonna be a basically stock rebuild. just adding flat top hypers, 487x heads, and i think this comp cam.

Thanks.
Take it to a machine shop, have them test the wall thickness and see how far it can be bored out. Personaly I would bore it out as far as it could go. for .030 over your talking 4-5hp gain. .060 is about double that. remember it's all about cubic inches here.
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Old 12-16-2003, 11:59 AM
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Increased bore sizes on a wedge head promotes better airflow by giving the air charge more area clear area before it hits the side of the cylinder. Chevy suffers from this with the std 23 degree angle. You note most of the best flowing heads are now around 12 degree. A way to fudge flow figures is to flow heads on a larger bore fixture on the bench.

Cstraub
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Old 12-16-2003, 07:13 PM
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I agree with that theory ctraub, my point was unshrouding the valves is a head porting procedure, a poor choice of words I supose to illustrate what you're saying.
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Old 12-16-2003, 07:42 PM
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I would recomend boring it the minum with out a doubt although boring it .06 over will increase the ci and unshowd the valves any gains you would see here will be lost on ring sealing as the block will flex more making it harder for the rings to seal well. Also it will leave some material for the next rebuild.
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Old 12-16-2003, 07:55 PM
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Just read the topic title and I agree, 350 Chevy's are boring
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Old 12-16-2003, 09:12 PM
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Perhaps I should fully explain. Maybe some guy's aren't worried about 5 horse power but my up bringing in circle track tells me that if YOU don't make that competitive edge than someone else will and that might get you "put on the trailer." Even considering that most circle track power goes up in wheel spin anyway but that one lap, turn or corner just might be the difference in 1st or the 1st loser.

Cstraub had it right, In the standard 23' chevy head the valves angles in relation the the theoretical bore center line will give the air charge a hard corner to negotiate into the bore. Tradition stock type heads with a swirl producing entry port shape will try to organize the incoming charge and steer the air fuel away the wall. The momentum of the charge traveling through the port will cause the charge to slide over the back of the valve and actually can in some extreme cases cause a lower pressure adjacent to the bore wall. This hinders total flow.

If you were to move the valve angles to 12-18' then the charge will hit the back of the valve on all sides equally. Given a generous radii of the valve margin this will increase the amount of air that will make it past the valve and into the bore. In layman's terms, the air/fuel won't hit the cylinder wall and result in stagnate air.

Almost all competitive engines are going to large bore, short stroke engines for this reason. Most late model engines are using a 4.185x3.55 which makes a 388ci. These engines also turn 9200 RPM every week for 6 months of the year and can be considered in a different class than street engines that cost 1/8 of a competitive circle track engine.

Last edited by johnsongrass1; 12-16-2003 at 09:19 PM.
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Old 12-16-2003, 11:16 PM
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John has obviously done his homework and has some good ideas...
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