HOW MUCH INDICATED COMPRESSION IS IDEAL?
The more accurate question is, "What operating temperature is ideal". Each particular engine arrangement has a different ideal indicated compression reading. This happens because compression is only one of the factors that determines the engine's operating temperature. The factors are : peak rpm ability, ignition advance, compression, and gasoline octane level. Because of all these variables, few engine builders will agree on any one specification, however almost all engine builders will agree on certain perimeters. The following scenario will help you to understand why many engine builders are so coy to make a specific recommendation.
JOE'S STORY - Joe is the proud owner of a stock 1992 Kawasaki 650 stand up that he bought brand new. After two years of reliable performance, Joe decides the boat needs more power. Joe's buddy down the street says that increasing the compression on his 650 X2 made a world of difference. Joe is the kind of guy (like many of us) that likes to get the maximum he can with the minimum cost and...without compromising reliability.
Joe goes to a local shop that builds race boats and asks the race mechanic, "What is the most compression I can run in my stock 650...while running on pump gas and not losing any reliability ?" The mechanic says "Oh, about 175 psi". Joe says "You don't understand...I want to get the max out of this thing". The mechanic thinks for a moment, and then tells him " On a totally stock 650, with a good fresh top end, and 92 octane, you could run 190 -195...max!"
With this piece of free, and very reliable advice, Joe takes his boat over to his machinist friend to do the milling work. With a Snap-on gauge in hand, they figure they'll just keep cutting the head surface and squish bands until they reach 195 psi. A few hours later, it's done.
Joe goes riding for the weekend with his pals. Right away, he notices a giant increase in overall acceleration and speed. In the early going, he drives away from his buddy that's riding a brand new 650...cool. But later on in the day, they repeat the drag race. Joe and his pal are dead even. In fact, it seemed like the longer they held it wide open, the slower Joe's 650 became. The next morning, when all the guys first got on the water again, Joe had the fast boat again. But like the day before, as the engine got hotter, it got noticeably slower. Joe and his pals figured that the milled head was pushing the two year old pistons and rings past their limit.
Back home, Joe disassembled the engine and took his cylinder and pistons to his machinist buddy. After some quick measurements they discovered that the bores had .007" of clearance and the pistons were collapsed .003"...obviously his riding buddies were right. With a fresh top end and a .003" clearance bore job, Joe went out for an afternoon of break in riding. Right away the boat had even more bottom end than before, not to mention better throttle response. After two tanks of careful break in, he met up with the guys. It was time to hang this baby wide open and blow his pal's doors off. At the start, he ran off. But about 80yards out, his pal's stock 650 caught up and rode away. As Joe was riding along wide open, noticing how much slower the top end speed seemed, the rear piston seized.
This time, Joe took his entire boat to the race shop to be fixed once and for all. The mechanic called Joe the next day and said, "We found the problem...Your front cylinder had 225 psi compression. You milled the head way too much". Joe responded "I cut it to 195 psi just like you said." The mechanic then reminded Joe that he said 195 psi on a stock boat with a fresh top end. He said "Joe, nobody sets up the compression on a worn top end. Boring a worn top end can easily increase the indicated compression 20 - 30 psi. That's why engine builders always cut heads to a particular cc volume They know which volume will give ideal indicated readings on a fresh cylinder. That same volume will give slightly lower readings on a worn top end, but it won't cause a seizure when the cylinder is bored. When you originally cut your head, the mechanical compression ratio was way too high, but the top end was so worn out it couldn't create enough compression to kill itself. The excessive mechanical compression ratio only caused it to over heat and slow down a little when it got hot. But after the cylinder was bored to the correct clearance, the indicated compression became high enough to cause an instant meltdown instead of simple over heating."
He added, "Joe, the same thing might have happened if you had put on a pipe. Remember you originally asked me what was the max compression you could run on your otherwise stock 650...and I said 195 psi, which is right. But the added rpm's of a bolt on pipe will also cause a lot of additional heat. To keep that heat from killing that engine set up, you'll need to back off the compression to about 180 psi." Joe responded, "Well, that's kind of ridiculous. How is anybody supposed to know what the ideal compression is for their particular engine and bolt on parts etc."
The mechanic replied, "Well, you either do the meltdown point testing on your own, or buy the head modification from someone who has already done it. Our shop only sells a few different engine set-ups or kits for your engine. We know the ideal head volume for those few kits that we sell...and we really don't worry about the rest."
Joe's story is not an uncommon one. Stories like this one is why engine builders often shy away from offering compression information. It has nothing to do with compression ratios being some kind of top secret. It has allot more to do with the engine builder being unsure about your measuring procedure, your bolt on parts, and the amount of wear your engine has. The engine builder understands that even with the best of intentions, his free advice can result in an expensive engine failure. For him it makes better business sense to give no recommendations rather than potentially costly recommendations.
