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Discussion Starter #1 (Edited)
I think my machinist might have taken more off the old block than what he was suppose to giving me a bit less deck height than I probably should have. I am posting this here as most wont see this in my build thread and as always, I appreciate the help!


Well, I took all of my measurements for my final compression ratio and it came out like this:

Bore 88mm / 3.465 in
Stroke 94mm / 3.7 in
Head 43 cc's (I am going to triple check this tomorrow after work)
Deck Height 0.014 in
Piston Valve pocket -2cc
Stock Felpro head gasket 0.016 in

When I enter all of that into CFM's Cometic calculator and zealautowerks calculator I get 12.51:1

However Summit Racing outputs 13.5:1 and when I calculated it out by hand I got 13.5:1.

Why am I getting lower numbers using their calculators?

If the 12.51:1 compression is the correct one then I should go with a .045" Cometic gasket to get 11.56:1.

How do I lower the compression enough if 13.5:1 is the right compression?
- Messing with summit's calculator, it looks like if I machined the top of the pistons to have 0.032 deck clearance and used a 0.050 Cometic I would be at 11.64:1 but at that point it is more than doubling the piston to deck height and using one of the thickest gaskets I can find. Perhaps increasing the combustion chamber volume would do the trick?


My goal is a safe engine with useable power on 93 octane
 

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Discussion Starter #3
Supertech 88mm 11.6:1

P4-DU88-N2x4&GNH-8800x4 - P/N at CFM


I just plugged in the number from the specs that came with the pistons.

Here are the specs on the pistons:

Bore 88.00mm
Comp Height 1.122" (28.50mm)
Comp Ratio 11.6:1 (10.5:1)
Dome/Dish Vol -2cc
Gram Weight 303g
Head CCs 42
Oversized 0.5mm+
Pin Dia 21mm (.827")
Skirt Slipper
 

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Discussion Starter #4
So it looks like the 13.5:1 CR is correct. I am going to buy a better dial guage to double check my piston/deck clearance. In addition I reached out to tb1999, and he is helping me find ways to increase the volume in the combustion chamber. I will keep this thread updated just in case anyone else runs into this issue while building their first motor


It looks like unshrouding the valves is a way to increase the volume but I wonder if it will increase it enough.
 

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Your wasting your time by trying to lower the compression with removing material from the combustion chamber , you wont be able to remove enough

This is a NA build then ? if so just make a thicker HG or buy one from cometic they will make you one any thickness call and talk to them

Tom
 

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Discussion Starter #7
Wont that put my quench height too high increasing detonation as I would need a head gasket of 0.065 in to lower it to 11.61:1. I am not sure what is optimal but a quench height of 0.079 seems high. Perhaps if it is high it can get mostly worked out through tuning?
 

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HG is your only choice or shorten the rods , yes they can be shortened some , HG would be your easiest way , you will never remove enough from the head to lower it to your compression goals

Tom
 
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