mhawkins Posted August 27, 2015 Report Posted August 27, 2015 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. Quote
martybose Posted August 27, 2015 Report Posted August 27, 2015 Max is probably about 8 to 1. Mine started out a 9 to 1, and gets real grumpy if I don't feed it 91 octane. Marty Quote
Ulu Posted August 27, 2015 Report Posted August 27, 2015 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. Quote
mhawkins Posted August 27, 2015 Author Report Posted August 27, 2015 Thanks guys I did find some more info in the technical archives but the specifics you give without over analysis is refreshing. thanks again I think this is going to be fun!!! Quote
greg g Posted August 27, 2015 Report Posted August 27, 2015 (edited) 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 August 27, 2015 by greg g Quote
Don Coatney Posted August 27, 2015 Report Posted August 27, 2015 Compression test will give an indication of compression ratio as shown. Quote
ledfootslim Posted August 27, 2015 Report Posted August 27, 2015 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? Quote
Don Coatney Posted August 27, 2015 Report Posted August 27, 2015 Electronic ignition has absolutely nothing to do with compression ratio. Quote
TodFitch Posted August 27, 2015 Report Posted August 27, 2015 Hmmm. Owner wants it built stock, builder is doing his own thing . . . Doesn't sound like a good situation to me. 1 Quote
Plymouthy Adams Posted August 27, 2015 Report Posted August 27, 2015 thought the same thing myself...hope the builder is providing the coin out of his pocket for this... 1 Quote
mhawkins Posted August 27, 2015 Author Report Posted August 27, 2015 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. Quote
greg g Posted August 28, 2015 Report Posted August 28, 2015 MY experience, hot rodding a 218 will just about equal the performance of a stock 230. Quote
Ulu Posted August 28, 2015 Report Posted August 28, 2015 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. Quote
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