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01-29-2020 #1Registered User
- Join Date
- Mar 2009
- Location
- Southern Maryland
- Posts
- 402
Compression height and rod length
Help me gain a better understanding of compression height. I understand what it is and how it's calculated. Let's say the application is a Ford 4.0sohc v6. The bore is 3.953, the stroke is 3.31. Rod length 5.748. Deck height is 8.858. Rod journal is 2.125. Compression height is 1.44
So I'm think about strapping an m90 (or m112) blower on this motor. It has a crummy aftermarket. So I started digging and found that 3.95 and 3.97 are common Mopar 318 bore sizes. 318 stroker pistons are available at 1.46 compression height. Chevy rods 5.7. that's pretty close. As close as I can get without custom parts which would send me in a differemt direction. Digging a further, there are other 318 pistons 1.71-1.78 in compression height.
My question is, that if I add rod length and piston compression height and keep that very close to stock, can i use shorter rods and taller pistons as long as I don't deviate much from that stock measurement? I am trying to gain forged rotating assembly and lower compression without expensive custom pistons
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05-05-2020 #2Registered User
- Join Date
- May 2020
- Posts
- 19
My general take on it. A taller compression height piston will be stronger and support the bore more, though heavier to move.
There is only short term plus’s from cheap skating.
Plan, save and spend on what you really want.
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11-07-2020 #3
The short answer is yes you can. It's Hot Rodding at its core. We've built Pontiac engines with long BBC rods, and Slant Six engines with Honda rods and pistons.
The devil is in the details like piston pin bore diameters, rod bearing sizes, crankshaft journal width, valve clearances and such. You can also offset grind the crankshaft with the right rod to stroke or destoke the motor. Don't forget to look at import motors for components.
In this modern age of the interweb it is much easier to imagineer engines than back in the day when you would have to buy parts to mock it up to see if it would work. Just remember to keep a good record of what pieces you use in the completed engine.
I've found this site useful from time to time.
https://www.vcalc.com/wiki/KurtHeckm...e+stroke+ratio
Good luck and happy engineering!If it breaks I didn't want it in the first place.
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