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COMPRESSION RATIO CHOICES


mhawkins

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I am rebuilding a friends P15 218, he wants to keep it stock but I am really tempted to up the compression ratio. Terrill has stock type pistons which are 6.6 :1, But sealed power has #37p 7.8:1

We have to bore it anyway so Drawbacks or advantages?? also the head will probably be surfaced also. max ratio with regular pump gas ??? Also if its already been covered please direct me, I must have missed it. Thanks in advance guys. By the way he engines all standard but 2 of the cylinders have trauma, looks like the cylinders will clean up at .030.probably had rings and gaskets late to mid 60s. 

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Mine was decked and shaved and ran great at 8.4:1.

 

It was balky to start at times, but I believe that was a starter issue.

 

I'll bet 8.5:1 would be no problem with modern ignition.

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I am running a 56 230 Plymouth engine in my car.  It started off with 7.5 to 1 CR.  I took .040 off the head and.010 off the block.  Pistons are .030 over. Machinist estimated new CR at about 8.4 to one.  Stock 230's went to 8 to 1 in 58.  with more squeeze a bit more initial timing is called for.  I run about 5 btdc.  Still run regular with no problems.  Just did an extended road trip, got 18/19 mpg with a 4.11 rear. 

Edited by greg g
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My head was badly warped, so they took about eighty thousands out of it. The po overbored it .040 and I'm running electronic ignition. I believe someone on here said that meant I was at a little above 8-1?

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Ok, point well taken, owner will be in full agreement with whatever is done. Just getting as much info as possible before spending any money. and by the way thank you all for responses. This reinforces my thoughts on the project, Will keep you posted as we make progress.

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I think the hopped 218 will wind up faster (if it's in a car that won't bog it.) The 230 has more bottom end torque & won't bog as easily under load, but winds up much slower.

 

At least this is my experience, driving a Club Coupe with hopped 218, and 4dr P15 sedan with stock 230.

 

So the hopped 218 would be better in a lighter car than the stock 230, but the 230 probably better in the heavier car.

 

 

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