[Z06] Compression Ratio
Thread Starter
Heel & Toe
Joined: Apr 2002
Posts: 16
Likes: 0
From: Madison WI
Compression Ratio
I'm not sure if this belongs here, but I didn't see any forum that seemed better.
Anyway, I want to know more about compression ratio. I somewhat understand what it is, but I want to have a better understanding than I already do. For one, how exactly do you determine compression ratio of an engine? Say you modify it, how would you know if or what the compression ratio changed to?
I kind of have a vague understanding, such as I know if you stroke your engine it will have higher compression due to the pistons compressing the air in the head more because they come up higher, or that a thicker head gasket will lower compression ratio because it increases the volume of air between the piston and the top of the head, right? Nitrous would increase compression too wouldn't it? I know that's a lot of questions, but I've never really heard a good explanation of compression ratio and how it is measured, or even what exactly the numbers mean. Thanks!
[Modified by Samjham, 6:38 PM 8/7/2002]
[Modified by Samjham, 6:38 PM 8/7/2002]
Anyway, I want to know more about compression ratio. I somewhat understand what it is, but I want to have a better understanding than I already do. For one, how exactly do you determine compression ratio of an engine? Say you modify it, how would you know if or what the compression ratio changed to?
I kind of have a vague understanding, such as I know if you stroke your engine it will have higher compression due to the pistons compressing the air in the head more because they come up higher, or that a thicker head gasket will lower compression ratio because it increases the volume of air between the piston and the top of the head, right? Nitrous would increase compression too wouldn't it? I know that's a lot of questions, but I've never really heard a good explanation of compression ratio and how it is measured, or even what exactly the numbers mean. Thanks!
[Modified by Samjham, 6:38 PM 8/7/2002]
[Modified by Samjham, 6:38 PM 8/7/2002]
Le Mans Master
Joined: Jun 2000
Posts: 5,555
Likes: 9
From: Indiana
Re: Compression Ratio (Samjham)
Imagine the inside of the cylinder with the piston at the bottom of its stroke. Above the piston is a certain volume, defined by the volume of the cylinder (using bore as the diameter and stroke as the height) + the volume of the combustion chamber.
Now imagine the piston at the top of its stroke. The gas in the cylinder has been compressed to squeeze into the volume of the combustion chamber alone. The ratio of these two volumes is the compression ratio... eg. 10.5:1.
You can lower the compression ratio by increasing the volume of the combustion chamber (different heads, dished pistons) .. conversely you can increase the compression ratio by reducing the volume of the combustion chamber (domed pistons, different heads). And, as you noted, you can change the stroke .. lengthen for higher comp ratio, shorten for lower ..
Anyway .. not rocket science. Just the ratio between the volume with the piston at the bottom of its stroke to the volume with the piston at the top.
Hope this helps.
[Modified by Jerry Velders, 8:53 AM 8/8/2002]
Now imagine the piston at the top of its stroke. The gas in the cylinder has been compressed to squeeze into the volume of the combustion chamber alone. The ratio of these two volumes is the compression ratio... eg. 10.5:1.
You can lower the compression ratio by increasing the volume of the combustion chamber (different heads, dished pistons) .. conversely you can increase the compression ratio by reducing the volume of the combustion chamber (domed pistons, different heads). And, as you noted, you can change the stroke .. lengthen for higher comp ratio, shorten for lower ..
Anyway .. not rocket science. Just the ratio between the volume with the piston at the bottom of its stroke to the volume with the piston at the top.
Hope this helps.
[Modified by Jerry Velders, 8:53 AM 8/8/2002]
