Clone spark plug??

Bumpy

Member
Can anyone shed some light on a hotter plug that works with the clones? Especially a restricted engine with a very rich carb?
A hotter extended nose plug will burn a too rich mixture off better and longer
before fouling out.
196cc/27cc (I think that's the head cc)= 7.25 compression ratio. At 26cc it's 7.5.
There probably isn't a plug that would ever be considered/found too hot for that
low of a CR.
 
Actually your ratios are 8.25 and 8.54. The formula is; cylinder cc + head cc / head cc. And when choosing a plug, you don't just consider the compression ratio, you have to consider the load on the engine as well. Fact is, the Briggs & Stratton engine, in every day usage, seldom experiences anywhere near the load it experiences in a kart race.
 

Bumpy

Member
By definition, the compression ratio is the total swept volume of the cylinder with the piston at bottom dead center (BDC), divided by the total compressed volume with the piston at top dead center (TDC).
aka 196 divided by 26
My bad.
AL is correct. I overlooked that little word 'TOTAL' which would include the combustion chamber.
 
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flattop1

Dawg 89
So 196 +26 ÷26= 8.358
196÷26 = 7.538
Swept volume would have to include cylinder head volume , would it not ?
The cyinder may hold 196 but the cylinder and head are 222 .
 

Bumpy

Member
Swept volume is the piston from BDC to TDC or vice versa.
From the book -
Swept volume is the displacement of one cylinder. It is the volume between top dead center (TDC) and bottom dead center (BDC). As the piston travels from top to bottom, it "sweeps" its total volume.

If you have a pop-up piston, then you would need to know the volume of the pop-up to subtract from the CH cc's and work that in with the swept volume.
Other things affect CR also such as gasket volume, dish volume ( in the case of the Clone piston dish), distance down to the first ring and how deep the piston is in the hole.
 
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Bumpy

Member
Gasket volume and distance down to first ring is just for nerds.
I R 1
The engine has the same swept volume with or without the head.
Pop-up pistons, shaved heads and really thick head gaskets change head volume and thus the CR.
A .009 head gasket with the piston .009 in the hole has a volume of .1 cubic inch or a little less than 2cc. (I think, I don't do maff.)
 
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flattop1

Dawg 89
By definition, the compression ratio is the total swept volume of the cylinder with the piston at bottom dead center (BDC), divided by the total compressed volume with the piston at top dead center (TDC).
aka 196 divided by 26
Swept volume is the piston from BDC to TDC or vice versa.
From the book -
Swept volume is the displacement of one cylinder. It is the volume between top dead center (TDC) and bottom dead center (BDC). As the piston travels from top to bottom, it "sweeps" its total volume.

There is no need to work Cylinder Head volume into the calculation for swept volume because CH cc is constant. The only reason to involve CH cc's is if you have a pop-up piston. Then you would need to know the volume of the pop-up to subtract from the CH cc's and divide that into the swept volume.
Other things affect CR also such as gasket volume, dish volume ( in the case of the Clone piston dish), distance down to the first ring and how deep the piston is in the hole.
Here we have the same misunderstanding .
Definion #1 total swept volume .
Definition #2 swept volume is the displacement of one cylinder .
Screenshot_20191009-154621_Google.jpg
 

95 shaw

Premium User
The simplest way to view this.

Fill the cylinder with the head on and piston at BDC with fluid, taking a measurement of the volume required to fill it.
Move the piston to TDC, catching the removed fluid. measure this volume. Subtract to find volume remaining in head.
Divide the first volume by the remainder in the head to find compression ratio.

You are compressing total first volume into area left in the head.

Total area/ remaining area = compression ratio:1
 

Bumpy

Member
Yeah.
I probably should delete my posts. :unsure:

AL is correct. I overlooked that little word 'TOTAL' which would include the combustion chamber.

This might explain a car engine I know about that a top builder in the general area built with an 11:1 compression
ratio that turned out to be in the 13's. No. The engine didn't last too long.
 
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Here's something (in Nine Sheets) to calculate your combustion chamber volume. You have to cc the head 1st. This doesn't allow for the space around the piston, to the top of ring. You could easily add that to your calculation. "ITH" = in the hole. You can use all metric numbers if you would like, but you can't change between metric and decimal. Head gasket hole, thickness and "in the hole" all have to be one or the other.
calculatn head vol.jpg
 
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