Compression Ratio Calculation
I need some help in calculating the compression ratio on my engine i acquired after my uncle passed with minimal info. It has been working fine on Sunoco 94 but I want to now figure out exactly what its compression ratio is.
This is a stock Chevy 350 bottom end with 416 heads unshrouded with large valves (not really sure what unshrouded with large valves means?).
Cam is Comp Cams 450 lift, 224 duration, 206 lobe centre
I did some hunting around and here is the data I have collected sofar:
Cylinder Bore Size = 4" (stock chevy engine spec)
Piston Stroke Length = 3.48" (stock chevy engine spec)
Head Gasket Bore Diameter = 4.1" (stock chevy engine spec)
Compressed Head Gasket Thickness = 0.038" (stock chevy engine spec but unsure on whether a different head gasket was used as the heads are not stock)
Combustion Chamber Volume In CCs = 58 cc (stock 416 heads specs, unsure if large valves will affect that?)
The last 2 requirements are confusing to me so I need some help figuring those out.
Piston Dome Volume In CCs Negative For Dished Pistons (Use '-') = ??
Enter Piston Deck Clearance Negative If ABOVE Deck (Use '-') = ??
I entered in 4.5 for the first one and 0.016 for the second just to give me something and came up with a compression of 12:1.
Any help is appreciated!
Thank You :D
Here just use a calculator:
Expert Advice - Expert Advice - SummitRacing.com
You divide the volume of the cylinder at BDC by the volume at TDC.
*There are flattops with 2 valve reliefs, approx +2 to +4cc
*There are flattops with 4 valve reliefs, approx +5 to +7cc (this would be used in a typical performance rebuild)
*There are dished pistons (concave on top). Dished pistons would be like +9cc to +18cc. (this would be a typical untouched factory engine).
*There are also D-dished pistons, which are flat on one side and deep dished on the combustion chamber side. These would be like +9 or more cc's.
*There domed pistons, which are convex on the top, thus taking up space out of the cylinder, so these are NEGATIVE cc's. Typical values would be like -5cc to -15cc.
Deck clearance is measured from the top of the block to the top of the piston (edge of the piston, not the center where the dome/dish is). An untouched factory block would have .025" deck clearance. If the block has been decked, then this value would be smaller.
Compression Ratio Calculation
Figuring the static compression is something you do before you assemble your engine. The calculations such as bore, stroke cylinder head volume, effective dome volume, deck clearance, and compressed gasket thickness. If it is a stock 350 you know 4.00 x 3.48, cylinder head volume has to be measured for cc's, effective dome volume-the piston manufacturer list this. Deck clearance is the measurement from the top of the piston to top of block. Examp. dished 12.5cc, flat top w/ two eyebrows 7cc, dome piston -20cc, compressed gasket thickness the manufacturer list this. There is no way to know with the engine assembled.
Thanks for the help everyone, since the engine was assembled when I acquired it and no information available other than the details provided below It is looking like I won't be able to get that info needed readily.
As F-BIRD'88 said, if it is running ok on 94 and, yes, my timing is a confirmed total at 36, and my jetting seems to be ok then I'm alright from the driving pont of view.
However, I had hoped to get the exact compression ratio to 1) figure out what octane rating I do need. (to save some bucks if possible) and 2) to calculate the Gap I really need in my plugs (running R45TS's right now with 0.45 gap).
Is there a way to figure out volume without disassembling the engine? I know the water trick when you have the engine apart but not recommended with a running engine :)
I do not know if the 416's were ported.
I have one other avenue; my aunt thinks she recalls the place that built the motor and I am going to ask them if they keep detailed specs.
One other question, what does unshrouded mean?
Thank You F-Bird'88 for your insight, very useful info.
I'll get a set of the colder plugs and try them out. Thank You.
I do follow what your saying; it works great so just go with it but I like to know the why's as well and understand the technology and/or physics behind it; not necessarily to make it stronger/faster but just to understand the why's (as my wife would say; its one of those annoying traits that she has not been to find and rip out yet :)). This is my first engine build (well, someone else built it, I'm just using an easy to work on truck - i.e. less variables - to gain an understanding) and I want to become more proficient when I get to my future ones.
Retirement is only 6 years away so I'm looking forward to starting a new chapter in my life; which involves cars and restoration.
Either way, the help is appreciated.
So what does unshrouded mean? ;)
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