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I'm trying to calculate compression (desired is 10:1) and to do that I see that Piston deck clearance is needed.
Block deck height is the distance from the center line of the main journals to the block deck surface. So assume this lsx block that comes in at 9.260 get's machined down to a 9.00-inch deck height. Using 6.125-inch connecting rods, and a stroke of 3.622 inches. I divide the stroke by two and add that to the rod length: 3.622 / 2 = 1.811, and 1.811 + 6.125 = 7.936. Next, subtract that answer from the deck height: 9.00 – 7.936 = 1.064.
So the piston compression height is 1.064 inches.
So does this mean that a piston compression height of 1.064 puts the piston exactly at the top of the engine block?
By this calculator, the deck height clearance is 0?
I'm I correct in that the Head Gasket Bore Diameter for a bore of 4.125 is 4.125 unless otherwise stated on the gasket specs, and that you would enter that value into the calculator here because the calculator already has an entry for compressed head gasket thickness?
That would give a compression of 10:1 if -7cc dish pistons are used. Is this correct? Is there an off shelf -7cc piston for the 4.125 bore and 3.622 stroke using 6.125 rods or do you have to go custom?
Other option is to change the heads. A set of 285cc with 66cc combustion chamber and wisco -12cc pistons with a Cometic 0.045 gasket. That would be 10:1 compression also.
Last edited by 5.7stroker; 08-10-2017 at 02:34 PM.
If you go to most any typical piston manufacturer they'll give you the Comp with a giving cc head and how much the piston sits down in the hole and gasket thickness to keep things simple. Mill the block to get desired comp and or change head gaskets.
I think JE sits .0015 in the hole all piston makers have these.
If you go to most any typical piston manufacturer they'll give you the Comp with a giving cc head and how much the piston sits down in the hole and gasket thickness to keep things simple. Mill the block to get desired comp and or change head gaskets.
I think JE sits .0015 in the hole all piston makers have these.
Oh I understand.....wanting to know the calculations.
If we look at a stock LS1, the block has deck height of 9.24" Using 6.098-inch connecting rods, and a stroke of 3.622 inches. I divide the stroke by two and add that to the rod length: 3.622 / 2 = 1.811, and 1.811 + 6.098 = 7.909. Next, subtract that answer from the deck height: 9.24 – 7.909 = 1.331 So piston compression height is 1.331
Deck clearance is 0 based when you put those values into the calulator:
It's better to run the quench about .040 and get the volume in the chamber or piston.
3.622/2 + 6.125 rod = 7.936 9.240 - 7.936 = 1.304 piston compression distance for zero deck
Some piston manufacturers will let you make one change (volume in this case) (or it could be compression distance just as easy) to a shelf stocking piston part number for a nominal charge. It is actually a full custom piston and delivery takes several weeks, but it's an inexpensive way to hit the number exactly.
Easier and quicker is to have the heads milled and use the piston deal to set deck. The LSX block decks often come in a little higher than 9.240.
The calculator gives 0.000499999999999 deck clearance when you use the info above, so essentially zero deck clearance.
So if someone wants a quelch of .040" in this setup, then they would THEORETICALLY run a head gasket that has a compressed thickness of 0.040", correct? Do they increase the bore diameter of the gasket also for a better seal? Like use 4.135" bore diameter gaskets with compressed thickness of 0.040" since the bore is 4.125" ?
So the correct way is to actually measure how much the piston is actually sticking out, and then base the thickness of the gasket on which piston sticks out the highest. So in the end, you may get close to your theoretically calculated compression, but getting the proper quelch is more important than hitting that theoretical target compression number perfectly. Got it.
Last edited by 5.7stroker; 08-10-2017 at 08:36 PM.
Six inch rod length and nine inch deck height is GEN I SBC, no reason to machine a quarter inch off your LS deck
The 6.098"-6.125" rods are great on the 3.622 stroke and a taller piston (1.34") Comp height is both light and stable in the bore which minimizes ring flutter. Better ring seal is more power
Six inch rod length and nine inch deck height is GEN I SBC, no reason to machine a quarter inch off your LS deck
The 6.098"-6.125" rods are great on the 3.622 stroke and a taller piston (1.34") Comp height is both light and stable in the bore which minimizes ring flutter. Better ring seal is more power
Right, so we keep the deck at 9.24 and if we use Ls7 285cc heads that have a bore of 4.145 and use athena gaskets that are 4.170 bore which I believe have compressed thickness of 0.059, we just use custom pistons the have a -5.114 dish to hit the 10:1 compression. Quench is then 0.059
Is this the correct way to do it? Those athena head gaskets made in Sweden which are supposedly the best the way to go, but are only available in certain bore sizes. Given the cylinder bore size, the bore size of the LS7 285cc heads and the compressed thickness of those athena gaskets, or would you use those or just use a 0.051 compressed thickness cometic gasket that has a bore of 4.160? If spending the money on a 6 bolt setup, I want the correct gasket size given the bore size of the heads being bigger than the bore size of the cylinders. If the quench needs to be brought to 0.040, how would you do it?
Cylinder Bore Size 4.130
Piston Stroke length 3.622
Head Gasket Bore Diameter 4.170
Compressed Head Gasket Thicknes 0.059
Combustion Chamber Volume in CCs 70.000
Piston Dome Volume IN CCs -5.114
Deck height 9.240
rod length 6.125
Amount milled off the head 0.000
Piston compression distance 1.304
Piston Deck Clearance 0.000
Modified Combution Chamber Volume in CCs (still 70.000)
Swept volume cc 795.134
Gasket cc 13.204
Deck Clearance Volume cc 0.000
Total Chamber volume cc 88.418
Compression ratio 10:1
Quench 0.059
Usually the engine builder will use a shelf piston that is easy to replace in case they get nuked, and upgrade the rings and pins for the more extreme applications. Then they will fine tune the compression ratio and quench distance by machining the deck of the block and cylinder heads.
Assuming this is a boosted application, the quench isn't really that important like it would be in a naturally aspirated engine. No need to stress over it as much as you are. So for the goals you've stated, you could use a 4.130" bore piston with a 1.304" compression height and a 5cc dish (JE PN 311970) with a 6.10" rod to put the piston in the hole at TDC. Your machinist could then mill about .015" off the heads to go from 70cc to 67.5cc, and then flat mill the block about .005" so that the pistons will end up about .020" in the hole, which will put you right around 10.0:1 depending on the gasket you use.
Usually the engine builder will use a shelf piston that is easy to replace in case they get nuked, and upgrade the rings and pins for the more extreme applications. Then they will fine tune the compression ratio and quench distance by machining the deck of the block and cylinder heads.
Assuming this is a boosted application, the quench isn't really that important like it would be in a naturally aspirated engine. No need to stress over it as much as you are. So for the goals you've stated, you could use a 4.130" bore piston with a 1.304" compression height and a 5cc dish (JE PN 311970) with a 6.10" rod to put the piston in the hole at TDC. Your machinist could then mill about .015" off the heads to go from 70cc to 67.5cc, and then flat mill the block about .005" so that the pistons will end up about .020" in the hole, which will put you right around 10.0:1 depending on the gasket you use.
Ah, that makes sense. Does your calculation take into account piston diameter at top ring land which usually runs 0.003" to 0.005" less than bore diameter? Does it take into account distance from Top of Piston to Ring Land - which will vary from manufacturer to manufacturer?
So if we keep piston diameter at top ring land as 1.304 and distance from top of piston to ring land as "0", I get 9.76 using this calculator, which matches what I have in the calculator I made in M$ excel.
I'd like to be able to incorporate both piston diameter at top ring land and distance of top of Piston to Ring Land into my calculator which I've attached.
So if the compressed thickness of the gasket is 0.059 and the pistons are 0.020 in the hole, what would the quench be? 0.059 + 0.020 = 0.079 in?
And a quench of 0.079" is not a problem on a boosted car?
Ah, that makes sense. Does your calculation take into account piston diameter at top ring land which usually runs 0.003" to 0.005" less than bore diameter? Does it take into account distance from Top of Piston to Ring Land - which will vary from manufacturer to manufacturer?
No, most won't either.
Originally Posted by 5.7stroker
So if we keep piston diameter at top ring land as 1.304 and distance from top of piston to ring land as "0", I get 9.76 using this calculator, which matches what I have in the calculator I made in M$ excel.
I'd like to be able to incorporate both piston diameter at top ring land and distance of top of Piston to Ring Land into my calculator which I've attached.
1.304" is the compression height from the pin centerline to the piston top. The piston is normally spec'd for a 4.130" bore, meaning the clearance is built in and the major OD is likely to be more like 4.126". At the top ringland, assuming .005" smaller, you're looking at 4.121" OD.
However, without actually measuring the piston, the OD at the top ring land is a guess at best. You're assuming a lot right now. If you're trying to get that precise, which personally IMO is a waste of time, you should also consider whether or not the top ringland has the "anti-detonation" grooves machined into it and how much volume they add.
Keep in mind that most builders don't bother with all that guessing in their compression ratio calculations, so if you've decided you want 10:1 based on what others have built, chances are that you're using a different calculation and it's not comparable to what you've researched.
Originally Posted by 5.7stroker
So if the compressed thickness of the gasket is 0.059 and the pistons are 0.020 in the hole, what would the quench be? 0.059 + 0.020 = 0.079 in?
And a quench of 0.079" is not a problem on a boosted car?
Yeah, it won't be a problem. I'm not a tuner, but many of the guys that do tune the big power adder cars seem to prefer it as it has shown to help widen the margin of error in the tune up.
Got it. So with the ring land specifics being taken out of the equation, we are still at 9.79:1 compression milling the deck, heads, using the 0.059 Athena gasket and -5cc pistons. At that point, if a builder wants to get near 10:1 using those off shelf pistons and wants to use the Athena gaskets what would a builder do? Mill 0.030 off the head instead of 0.015? The gaskets are only available in 4.055, 4.133 and 4.170 bores and they all have 0.059 compressed thickness as the only option. Since the bore of those 285cc heads are 4.145, a builder would normally use a gasket with a 4.160 bore given the bore on the block is 4.130. So the best option with the Athena gasket is 4.170
Now the other option is to use a gasket which has more flexibility in the bore and compression thickness and then you don't have to mil to 0.030, but I hear the Athena gaskets which are supposed to be the best for LS7 heads on boost.
I realize this is a "just give it to the builder and they will figure it out" kind of thing, but i'm just trying to understand how exactly it's figured out.
Last edited by 5.7stroker; 08-12-2017 at 01:34 PM.
Usually the machinist will mill the heads. Since most shelf stock forged pistons have valve reliefs, you don't really have to worry about PTV clearance, especially with a a cam spec'd for a turbo application.
Usually the machinist will mill the heads. Since most shelf stock forged pistons have valve reliefs, you don't really have to worry about PTV clearance, especially with a a cam spec'd for a turbo application.
Great! Can you take a look the calculator I made and run some different values through it and verify that the math used in the cells of the calculator is correct?
Last edited by 5.7stroker; 08-12-2017 at 03:25 PM.
08-10-2017, 01:21 PM
Calculating compression question 
