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How much CID does a .030 bore job add to a flathead 6�?


MarcDeSoto

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I was wondering what the new cid of my 48 DeSoto engine is?  Stock cid is 236.7 cid.  I bored out the cylinders .030".  So using the formula for the volume of a cylinder, Pi R squared times the height, times 6, I got my new cid of 240.8.  I wonder if that adds any HP?  The Chrysler 6 in contrast has a stock cid of 251.  

 

According to a formula I found, each cubic inch of displacement increases HP by 1.1 to 1.2 HP.  So, since I increased the displacement by about 4.5 cubic inches, I figure I increased the horsepower from 109 to 113.5.  Marc.

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The increase in cu. In. By it self should have a small hp gain but it would be so small you wouldn't feel it in the seat of the pants. You are probably close but it wouldn't be a 1:1 or a linear scale. As Chrysler 1941 stated, there will be a compression ratio change but to confirm the CR change, you'd also need to figure in the difference in head gaskets from the old gasket to the new and if the head was milled, even enough to clean it up. Then you'd need to figure in the new piston compression height if it's different from the old pistons. 

Edited by Bbdakota
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given 218 as baseline

 

.020=220.4

.030=221.8

.040=223.1

.060=225.87

.080=228.6

.100-231.3

.110=232.7

 

hardly worth the CI gain folks think they getting going full bore for the heck of it  instead of just cleaning the wear damage and building it as needed and save the block for their lifetime plus an heir.

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What is cool though, is my 25” long 228 ci Canadian engine, bored 0.060 over will become a standard bore 237 engine.  Looking forward to it. 

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My 218 had to be taken 60 over. I did not know there was any headroom after that so I feel less worried having taken it that far (but the bores were already 30 thou over at the outset and looked pretty crappy). Surprisingly the engine ran really well given the circumstances. Flat heads are truly amazing! M

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I can't remember if the head was shaved.  I wasn't under the impression that because I bored the cylinders .030 over, that I was now going to enter it in the Indianapolis 500!  I was just cleaning up damage from broken rings and general wear.  I did put in .030 over pistons and rings.  Can't wait to get it running.  

Edited by MarcDeSoto
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