Piston Compression Heigth - Ford Muscle Forums : Ford Muscle Cars Tech Forum
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post #1 of 3 (permalink) Old 01-22-2014, 07:23 AM Thread Starter
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Piston Compression Heigth

I'm am trying to build a semi-budget motor with some parts I already have. The crank is a Eagle 3.1 stroke ESP 4340, the rods are Eagle 5.155 ESP H-beam. I don't have pistons and that is where I'm looking for direction. I know I want to be around 10 to 1 compression ratio. The heads are GT40 cast iron with bigger valves and some bowl work, I know there are better heads but I have these and for now I'm going to try to work with what I have. Anyways, I'm going to use a stock 5.0 block so as I understand it is 8.2" deck height and I have it figured like this:
289 302 3??
1.435 1.500 1.550 Stroke divided by 2
5.155 5.090 5.155 rod length
1.605 1.605 1.490 Piston compression heigth
8.195 8.195 8.195
The 8.195 is close to the 8.2 so I'm guessing I have that figured out right. So now I just have to figure out the CC of my heads to get the right compression ratio in a piston with a compression height of the 1.490? Just trying to educate myself before trying to order pistons, the most expensive part of this build, in money that I have to put out now anyways. For what I'm doing I think hyperutectics(spell) would be fine.

Last edited by nytling92; 01-22-2014 at 07:28 AM.
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post #2 of 3 (permalink) Old 01-22-2014, 09:09 AM
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Re: Piston Compression Heigth

You're on the right track for your 316 build calculations. Pistons with exactly 1.490" CH are going to be scarce. One advertised for a 408 with a 6" rod would work. Good luck. What I would suggest here is to create a range of piston choices based on deck height and rod differences. This will greatly expand your piston options.

For example, stock deck height is nominally 8.206. Start working your numbers from there. This number can be altered by decking shorter, or raising with thicker gaskets. So, reasonably a milled short deck of 8.190, and a deck height clearance of .010 could use 1.475 CH. Likewise, an 8.206 block and .060" gaskets (+.020") would give .035 quench with a 1.526 CH piston. So, your shopping range expanded.

You can expand it even farther if you used stock 5.09" rod length, or the most common 5.4" stroker rods. Just the stock rods increase your overlapping piston CH options to a range of 1.475" to 1.591".

Finally, consider less expensive machining to also expand your choices. For example, you could use common Chev 350 pistons if you re-bushed your rods for .927 pins. Or, turning .010 or 020 off the top of the pistons can make other options feasible (now 1.475 to 1.611 or more - hey that's a 289/302 piston!). Creative options like this are why some builders can make stuff happen when other can't. Do your thing and have fun!


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post #3 of 3 (permalink) Old 01-22-2014, 10:23 PM Thread Starter
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Re: Piston Compression Heigth

Thank you for the insight. I'm going to stick with the rods I have, hopefully less money to pay out now. You brought up some interesting ideas to look into. I appreciate your response. Ward
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