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Old 08-31-2012, 09:53 AM
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Compression Ratio Calculation

Hello,

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

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Old 08-31-2012, 10:30 AM
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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.



Quote:
Combustion Chamber Volume In CCs = 58 cc (stock 416 heads specs, unsure if large valves will affect that?)
Large valves will not effect that. Generally factory cast heads are going to be larger than advertised. So unless you actually remove the heads and CC the chambers yourself, you might want to run calculation on the high side, some where in the neighborhood of 60-62cc.

Quote:
Piston Dome Volume In CCs Negative For Dished Pistons (Use '-') = ??
Enter Piston Deck Clearance Negative If ABOVE Deck (Use '-') = ??
*There are flattop pistons=0cc
*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.

Last edited by Silver Surfer; 08-31-2012 at 10:40 AM. Reason: more info added
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Old 08-31-2012, 10:59 AM
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I have built many of these 350 SBC's using the 416 heads with porting and larger valves.
The chambers end up around 61 to 64cc once the chamber wall is relieved for the larger valves.
(1.94x 1.60 best)
(assuming the heads were not shaved)

The only way to determine the exact real CR is to get the real measured head CC chamber volume and the real piston deck clearance @ TDC and the actual gasket thickness. and the piston valve relief volume.

Interactive JavaScript to Calculate Engine's Compression Ratio (CR)

Actual piston deck clearance can range from .055" in the hole @ TDC to 0 decked to a negative
( piston above deck, over decked block)

If it runs on 94 octane gas with 34 to 36deg timing and correct jetting and spark plug heat range don;t worry about it.

The Sunoco/ Petro Can Ultra 94 is pretty good stuff.

You are under 11:1 cr.

Typical head gaskets are 4.166" x .038 to .041"
or 4.10" x .015" stell shim style.
many flat top rebuilder pistons have a modifed pin height 1.54" resulting in a .045" to .055" deck clearance and lower than expected compression ratio.

1.56" is stock.

typical flat top piston valve relief volume is -4cc to -6cc

Running downt he 1/4 mile will tell you how much Horsepower you are making wiht this motor.
MPH and car weight w driver.
This combo should run pretty strong if the 416 heads were ported too.

( I have made well over 400hp with bigger cams and ported 416 heads)
The 224 106LSA .450" lift cam should run pretty good.
Will work best with a 3500 stall and gears for best ET.
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Old 09-01-2012, 09:34 AM
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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.
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Old 09-02-2012, 11:45 AM
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Compresson ratio

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.

Gary
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Old 09-02-2012, 11:46 AM
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Unshrouded

One other question, what does unshrouded mean?
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Old 09-02-2012, 12:55 PM
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You are brewing over and number that will not tell you much.

All that matters is it goes like hell and does not knock.
If you want to try a lower octane fuel to see if its enough, just do it.
Esso 92 or Pioneer 93. It will either rattle or it won't.
I can tell you neither are quite as good as the Ultra 94.
I have tested them all (local pump gas)
and this fuel makes a bit more power too. Key word: A BIT

If you need the detonation protection and like the slight performance gain:
Its worth the slight extra cost at the pump.
Pistons and rebuilds are expensive.

The R45TS plugs are a good city stop and go plug. "stock" heat range

The slightly cooler performance oriented RV8C (R42t) is a bit cooler heat range to give more detonation threshold
with lead foot WOT driving or towing where engine throttle load is higher. .035" gap gets it done very time.

I have been there done that as far as 305HO heads on a 350 goes...so....

If you really want to know the real cr you need to yank the heads off.

Last edited by F-BIRD'88; 09-02-2012 at 01:00 PM.
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Old 09-02-2012, 04:44 PM
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Good Stuff

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?

Last edited by ggevaert; 09-02-2012 at 04:49 PM.
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