Piston tech/geometry question
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Joined: Aug 2004
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From: Park City Utah
Piston tech/geometry question
I read somewhere (and now can't remember where) fairly recently, that the wrist pin has to be centered (vertically) in the piston. So, a longer rod requires a shorter piston, not just a wrist pin higher up in the piston. Likewise, I shorter rod, requires a taller piston, to keep the pin centered in the piston. I had never heard of such a thing until I read that. I had always thought that piston height (how tall it is) was a compromise that the builder settles on that balances wear (life) for friction and weight. But I read the above and thought, "O.K."
Now, I'm reading Engine Masters and this guy (Jon Kaase) built the outright most powerful engine; a 537 CID, BBC, that made 1133hp at 7500-7600 RPM. This guy used custom made Diamond pistons that positioned the wrist pin at the very bottom of the piston, with very little below the pin, in the skirt area. Looks like the head of the piston would bang back and forth in the cylinder, b/c the pin is so low. What gives?
Now, I'm reading Engine Masters and this guy (Jon Kaase) built the outright most powerful engine; a 537 CID, BBC, that made 1133hp at 7500-7600 RPM. This guy used custom made Diamond pistons that positioned the wrist pin at the very bottom of the piston, with very little below the pin, in the skirt area. Looks like the head of the piston would bang back and forth in the cylinder, b/c the pin is so low. What gives?
Last edited by Tom400CFI; Jul 31, 2013 at 05:57 PM.
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Joined: Apr 2005
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From: Dale City VA
Kasse did this before in EM with a BBF.
His thinking was that the taller piston is a better heat sink taking longer to get hot and help with detonation considering the high compression and pump gas they run. He uses a pretty short stroke to rod ratio to get the piston on and off TDC faster further helping with preignition control, also giving a stronger intake tract signal.... this stuff comes from his knowlege of building IHRA Pro Stock engines.
Really as far as how much piston hangs below the wrist pin... that is a product of how much space/area is availiable before the piston will hit the crank counterweight. Of course the amount on top of the wrist pin (compression height) is a product of block deck height, rod lenght and stroke....
A longer piston will be more stable in the bore but weigh more and stability is controled alot by skirt design as well. So those are all trade offs one has to balance when designing a piston.
Will
His thinking was that the taller piston is a better heat sink taking longer to get hot and help with detonation considering the high compression and pump gas they run. He uses a pretty short stroke to rod ratio to get the piston on and off TDC faster further helping with preignition control, also giving a stronger intake tract signal.... this stuff comes from his knowlege of building IHRA Pro Stock engines.
Really as far as how much piston hangs below the wrist pin... that is a product of how much space/area is availiable before the piston will hit the crank counterweight. Of course the amount on top of the wrist pin (compression height) is a product of block deck height, rod lenght and stroke....
A longer piston will be more stable in the bore but weigh more and stability is controled alot by skirt design as well. So those are all trade offs one has to balance when designing a piston.
Will
