Quick question on calculating compression ratios
03-15-2018, 07:24 PM
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Quick question on calculating compression ratios Hello all,
I've been researching doing complete custom build from a bare block and I'm wondering about calculating piston deck clearance.
The source says to take divide the stroke in half and add it to the rod length and subtract that from the deck height. So, let's say I'm using a LQ9 block with a deck height of 9.240 inches, rod length of 6.125 inches with a stroke of 4.000 inches, that leaves me with a compression height of 1.115 inches. Based on a deck clearance calculator, that gives me 0 clearance. Is that correct? And is a 0 clearance bad?
If using LS3 heads with a cylinder head volume of 68 cc, bore of 4.000 inches, flat top pistons with a dome volume of -3 cc and compressed head gasket thickness of 0.054 inches, that comes out to an 11:1 CR.
My main question is, should I have some deck clearance? I usually see people say the have some x/1000 of an inch clearance which is why I'm asking.
Thanks.
I've been researching doing complete custom build from a bare block and I'm wondering about calculating piston deck clearance.
The source says to take divide the stroke in half and add it to the rod length and subtract that from the deck height. So, let's say I'm using a LQ9 block with a deck height of 9.240 inches, rod length of 6.125 inches with a stroke of 4.000 inches, that leaves me with a compression height of 1.115 inches. Based on a deck clearance calculator, that gives me 0 clearance. Is that correct? And is a 0 clearance bad?
If using LS3 heads with a cylinder head volume of 68 cc, bore of 4.000 inches, flat top pistons with a dome volume of -3 cc and compressed head gasket thickness of 0.054 inches, that comes out to an 11:1 CR.
My main question is, should I have some deck clearance? I usually see people say the have some x/1000 of an inch clearance which is why I'm asking.
Thanks.
03-19-2018, 09:36 AM
#2
Hello all,
I've been researching doing complete custom build from a bare block and I'm wondering about calculating piston deck clearance.
The source says to take divide the stroke in half and add it to the rod length and subtract that from the deck height. So, let's say I'm using a LQ9 block with a deck height of 9.240 inches, rod length of 6.125 inches with a stroke of 4.000 inches, that leaves me with a compression height of 1.115 inches. Based on a deck clearance calculator, that gives me 0 clearance. Is that correct? And is a 0 clearance bad?
If using LS3 heads with a cylinder head volume of 68 cc, bore of 4.000 inches, flat top pistons with a dome volume of -3 cc and compressed head gasket thickness of 0.054 inches, that comes out to an 11:1 CR.
My main question is, should I have some deck clearance? I usually see people say the have some x/1000 of an inch clearance which is why I'm asking.
Thanks.
I've been researching doing complete custom build from a bare block and I'm wondering about calculating piston deck clearance.
The source says to take divide the stroke in half and add it to the rod length and subtract that from the deck height. So, let's say I'm using a LQ9 block with a deck height of 9.240 inches, rod length of 6.125 inches with a stroke of 4.000 inches, that leaves me with a compression height of 1.115 inches. Based on a deck clearance calculator, that gives me 0 clearance. Is that correct? And is a 0 clearance bad?
If using LS3 heads with a cylinder head volume of 68 cc, bore of 4.000 inches, flat top pistons with a dome volume of -3 cc and compressed head gasket thickness of 0.054 inches, that comes out to an 11:1 CR.
My main question is, should I have some deck clearance? I usually see people say the have some x/1000 of an inch clearance which is why I'm asking.
Thanks.
The general rule of thumb is that .040" quench is safe and gets you 90% of the benefits, however, you can go a little tighter or a little looser depending on the application.
When you shop for your rotating assembly, you should be able to find the compression height of the piston and calculate the deck clearance from there. For example, a lot of Wiseco pistons have a 1.110" compression height for their 4" stroke combos, which put the piston .005" in the hole with a 9.240" deck height. However, the LQ blocks usually have a 9.230" deck height, so you would really see the piston .005" above the deck. If your machinist surfaces the deck of the block to get it straight and square, you might end up with the piston .010" out of the hole. With the .054" gasket, you would be sitting pretty with .044" quench.
03-19-2018, 09:50 AM
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The stock LS engines all have the piston protrude above the deck by about .006", so you can say it has "negative deck clearance." What you want is to make sure you have adequate clearance between the piston and cylinder head, aka "quench". This is calculated by adding the head gasket thickness to the deck clearance. So at zero deck and a .054' gasket, you have .054" quench. If the piston is .010" below the deck (positive deck clearance), you have .064" quench. If the piston is .010" out of the hole (negative deck clearance), then you have .044" quench.
The general rule of thumb is that .040" quench is safe and gets you 90% of the benefits, however, you can go a little tighter or a little looser depending on the application.
When you shop for your rotating assembly, you should be able to find the compression height of the piston and calculate the deck clearance from there. For example, a lot of Wiseco pistons have a 1.110" compression height for their 4" stroke combos, which put the piston .005" in the hole with a 9.240" deck height. However, the LQ blocks usually have a 9.230" deck height, so you would really see the piston .005" above the deck. If your machinist surfaces the deck of the block to get it straight and square, you might end up with the piston .010" out of the hole. With the .054" gasket, you would be sitting pretty with .044" quench.
The general rule of thumb is that .040" quench is safe and gets you 90% of the benefits, however, you can go a little tighter or a little looser depending on the application.
When you shop for your rotating assembly, you should be able to find the compression height of the piston and calculate the deck clearance from there. For example, a lot of Wiseco pistons have a 1.110" compression height for their 4" stroke combos, which put the piston .005" in the hole with a 9.240" deck height. However, the LQ blocks usually have a 9.230" deck height, so you would really see the piston .005" above the deck. If your machinist surfaces the deck of the block to get it straight and square, you might end up with the piston .010" out of the hole. With the .054" gasket, you would be sitting pretty with .044" quench.