Compression Ratio Calculation - 1969 L36
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Instructor
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From: Irvine CA
Compression Ratio Calculation - 1969 L36
Hi Everyone,
Unfortunately, my 1969 L36 melted a head gasket and I had to remove the heads off the motor. Has been a great learning lesson so far (I've never done this before) and had a co-worker help me do it. I'm taking the heads to a machinist to deck them who's done work on many Vettes in the past. My question is regarding the compression in my motor. I always knew the engine had high compression and am trying to figure out a ballpark of what the compression is and ultimately what head gasket I need to get in order to knock the compression ratio down to eat pump gas. Here's some info below:
Motor - Bored .030
Pistons - Speed Pro L2268F (Domed single valve relief)
Bore (Inches) 4.250"
Cylinder Heads 106.8cc (1969 iron rectangular port heads)
Piston Head Volume -36.30cc
Dome Height .300"
The one area I'm totally confused/worried about is deck clearance. Since the pistons are domed, they stick out of the block about 0.25" at 0 degrees TDC. The flat portion of the piston is a bit lower than even with the block at 0 degrees TDC. Am I supposed to ignore the dome and measure the lowest point of the piston to determine deck clearance? I'm really worried I have way high compression and am concerned a thick head gasket cannot solve my problems.
Appreciate any help - Thanks!
Unfortunately, my 1969 L36 melted a head gasket and I had to remove the heads off the motor. Has been a great learning lesson so far (I've never done this before) and had a co-worker help me do it. I'm taking the heads to a machinist to deck them who's done work on many Vettes in the past. My question is regarding the compression in my motor. I always knew the engine had high compression and am trying to figure out a ballpark of what the compression is and ultimately what head gasket I need to get in order to knock the compression ratio down to eat pump gas. Here's some info below:
Motor - Bored .030
Pistons - Speed Pro L2268F (Domed single valve relief)
Bore (Inches) 4.250"
Cylinder Heads 106.8cc (1969 iron rectangular port heads)
Piston Head Volume -36.30cc
Dome Height .300"
The one area I'm totally confused/worried about is deck clearance. Since the pistons are domed, they stick out of the block about 0.25" at 0 degrees TDC. The flat portion of the piston is a bit lower than even with the block at 0 degrees TDC. Am I supposed to ignore the dome and measure the lowest point of the piston to determine deck clearance? I'm really worried I have way high compression and am concerned a thick head gasket cannot solve my problems.
Appreciate any help - Thanks!
Last edited by Dammakins; May 16, 2018 at 12:09 AM.
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From: SW Florida.
Deck clearance is distance below (usually) the deck the flat part of piston is. Post a pic of your pistons. And the casting numbers of your heads. Very few engines have too high compression ratios unless they were built for the drag strip with huge domed pistons.
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You measure the distance the pistons are below the deck. Probably .015"-.020" or so. Basically has same impact as a thicker gasket. You're typically looking for something around .040" TOTAL clearance from piston to head. So a piston is .020" below the deck....it would need a .020" gasket to get .040".
For calculations just input the measurement of piston below the deck. You'll also want to validate head CC's. They often end up larger than you think if a few valvejobs have been done.
Using your numbers I'm getting around 11.44 to 1. I used a .041" gasket and .020" deck height...for a total .061" (bad).
JIM
For calculations just input the measurement of piston below the deck. You'll also want to validate head CC's. They often end up larger than you think if a few valvejobs have been done.
Using your numbers I'm getting around 11.44 to 1. I used a .041" gasket and .020" deck height...for a total .061" (bad).
JIM
Safety Car
Joined: Nov 2008
Posts: 3,717
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From: Seattle Washington
Its been so long since I did this I had to google it to remember. Since Your other measurements seem to be very accurate I assume You are using a Burette and are CCing the Head volume. To do a Domed Piston You lower it 1" in the Cylinder and then do a Math problem to figure it out. Google will quickly give You a couple of different ways to do it. So to answer Your question Yes I have done it before, and no I can't remember how I did it. This was before Google, and Google has its advantages.
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From: SW Florida.
Ok, the L36 is 10.25 to 1 with 101 or 105cc chamber heads. Look like about a 20cc dome. Why do you feel that compression is too high? A set of 118cc 049 heads would drop you to around 9.5 or so. Common, easy to find. I just tossed a set, but they spent 30 years using the Hudson River as cooling system instead of antifreeze.
Thread Starter
Instructor
Joined: Feb 2014
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From: Irvine CA
You measure the distance the pistons are below the deck. Probably .015"-.020" or so. Basically has same impact as a thicker gasket. You're typically looking for something around .040" TOTAL clearance from piston to head. So a piston is .020" below the deck....it would need a .020" gasket to get .040".
For calculations just input the measurement of piston below the deck. You'll also want to validate head CC's. They often end up larger than you think if a few valvejobs have been done.
Using your numbers I'm getting around 11.44 to 1. I used a .041" gasket and .020" deck height...for a total .061" (bad).
JIM
For calculations just input the measurement of piston below the deck. You'll also want to validate head CC's. They often end up larger than you think if a few valvejobs have been done.
Using your numbers I'm getting around 11.44 to 1. I used a .041" gasket and .020" deck height...for a total .061" (bad).
JIM
Thread Starter
Instructor
Joined: Feb 2014
Posts: 154
Likes: 1
From: Irvine CA
Ok, the L36 is 10.25 to 1 with 101 or 105cc chamber heads. Look like about a 20cc dome. Why do you feel that compression is too high? A set of 118cc 049 heads would drop you to around 9.5 or so. Common, easy to find. I just tossed a set, but they spent 30 years using the Hudson River as cooling system instead of antifreeze.
Regarding the heads - You noted a lower CC volume than what I have. The heads on my motor are rectangular ported. Does this mean I do not have the originals?
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From: SW Florida.
The 3931063 is the orig head on an L36. It is an oval port head. The LS6 was onof the few rectangle ported heads. And they are smallish semi-closed chambers.
Clean up that number on top of the piston and google it. Those are speed pro pistons. That number will get you your dome volume. I have a set of 215 heads essentially the twin to the 063 head.except somebody hugged out the ports to try to make them rectangles. Here is a true rectangle port.
Clean up that number on top of the piston and google it. Those are speed pro pistons. That number will get you your dome volume. I have a set of 215 heads essentially the twin to the 063 head.except somebody hugged out the ports to try to make them rectangles. Here is a true rectangle port.
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From: SW Florida.
Ok, just saw piston numbers in first post. 36.3 some. We will assume block wasn't decked and you are still at .020 stock deck height. My second pic shows diff berween 118 open chamber and 101 or 105 semi-closed chamber. Google bathtub ports to see a really tight closed chamber that flows like crap cuz the walls shroud flow to and from the valves. Also shows the hogged-out oval trying to be rectangle ports.
Burning Brakes
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To the O.P., with those 063 (oval-ports) on top of those L2268F pistons you are most likely closer to 12.5:1 for a final static C.R.
Those pistons were also designed for the square-port heads, if they clear the chambers (which they appear to do) you're OK to use them with the ovals.
Most of those Vette OEM 063 heads we've done here have measured under 100 cc's when finish machined, between that 36 cc/.300" (nominal) dome and those 063 heads (assuming them to be under 100 cc's) you are well north of 12:1, probably about 12.5??
Good luck finding a gasket to get you on pump gas!
Thanks, Gary in N.Y.
P.S. Using "nominal numbers" it would probably take a .100" thick gasket to get you under 11:1, I would recommending changing the pistons to a lower dome, around a 24 cc will make you more "pump-gas" friendly.
Those pistons were also designed for the square-port heads, if they clear the chambers (which they appear to do) you're OK to use them with the ovals.
Most of those Vette OEM 063 heads we've done here have measured under 100 cc's when finish machined, between that 36 cc/.300" (nominal) dome and those 063 heads (assuming them to be under 100 cc's) you are well north of 12:1, probably about 12.5??
Good luck finding a gasket to get you on pump gas!
Thanks, Gary in N.Y.
P.S. Using "nominal numbers" it would probably take a .100" thick gasket to get you under 11:1, I would recommending changing the pistons to a lower dome, around a 24 cc will make you more "pump-gas" friendly.
Race Director
Joined: Jul 2015
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From: SW Florida.
Your cr is 11.5 with those heads and pistons and .040 head gasket. What is this car? Driver, or show piece? Does it have to be orig heads to make you happy? Cuz once you get them decked to repair the flame-cut damage, cr is gonna go up. Is short block good, or are pistons coming out? If you dont have to rebuild block, a set of 119cc heads will put you right around 10.1 cr. Which is about where you want to be. Let's see pic of intake ports. Head and intake manifold. Also, head work gets EXPENSIVE! These days new heads cost no more than a serious valve job. You want rectangles? New iron can be bought from Chebby for under 1k a set. And there are zillions of aftermarket options out there. Also measure your current intake valves. 2.06 are stock oval port valves. 2.19 in rectangle heads.
Le Mans Master
Joined: Aug 2017
Posts: 8,336
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From: Cool Northern Michigan
Before you ship those heads off to the butcher, I would put a chunk of PlayDough on the #1 piston, install old headgasket, install head with just a couple bolts. Set valvelash, crank it over by hand, remove head, measure piston to valve clearance. You need to know your baseline before going to the next step. You should have at least 0.080 between the two.
Last edited by HeadsU.P.; May 16, 2018 at 06:51 AM.
Thread Starter
Instructor
Joined: Feb 2014
Posts: 154
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From: Irvine CA
The 3931063 is the orig head on an L36. It is an oval port head. The LS6 was onof the few rectangle ported heads. And they are smallish semi-closed chambers.
Clean up that number on top of the piston and google it. Those are speed pro pistons. That number will get you your dome volume. I have a set of 215 heads essentially the twin to the 063 head.except somebody hugged out the ports to try to make them rectangles. Here is a true rectangle port.
Clean up that number on top of the piston and google it. Those are speed pro pistons. That number will get you your dome volume. I have a set of 215 heads essentially the twin to the 063 head.except somebody hugged out the ports to try to make them rectangles. Here is a true rectangle port.
Thread Starter
Instructor
Joined: Feb 2014
Posts: 154
Likes: 1
From: Irvine CA
To the O.P., with those 063 (oval-ports) on top of those L2268F pistons you are most likely closer to 12.5:1 for a final static C.R.
Those pistons were also designed for the square-port heads, if they clear the chambers (which they appear to do) you're OK to use them with the ovals.
Most of those Vette OEM 063 heads we've done here have measured under 100 cc's when finish machined, between that 36 cc/.300" (nominal) dome and those 063 heads (assuming them to be under 100 cc's) you are well north of 12:1, probably about 12.5??
Good luck finding a gasket to get you on pump gas!
Thanks, Gary in N.Y.
P.S. Using "nominal numbers" it would probably take a .100" thick gasket to get you under 11:1, I would recommending changing the pistons to a lower dome, around a 24 cc will make you more "pump-gas" friendly.
Those pistons were also designed for the square-port heads, if they clear the chambers (which they appear to do) you're OK to use them with the ovals.
Most of those Vette OEM 063 heads we've done here have measured under 100 cc's when finish machined, between that 36 cc/.300" (nominal) dome and those 063 heads (assuming them to be under 100 cc's) you are well north of 12:1, probably about 12.5??
Good luck finding a gasket to get you on pump gas!
Thanks, Gary in N.Y.
P.S. Using "nominal numbers" it would probably take a .100" thick gasket to get you under 11:1, I would recommending changing the pistons to a lower dome, around a 24 cc will make you more "pump-gas" friendly.
Le Mans Master
Joined: May 2006
Posts: 9,993
Likes: 1,136
From: League City Tx
I would not put a .093" head gasket on there....if they even make one that thick. Its likely going to cause issues with the intake manifold bolting up and sealing correctly. You would need some equally (or close to it) thick intake manifold gaskets etc etc..
Not to mention your pushrod lengths will be way off, your rocker arm geometry will get thrown off etc etc.... Just asking for problems trying to space the heads up that much.
If you don't have the money to put pistons in it, a cheaper/easier option would be to find a set of 049 or 781 open chamber, oval port heads..... They'll drop the compression down a good it, and they flow better... You can usually find them used pretty cheap...Then have a valve job and put new springs on them. I bet you could do this for $1k or so..
Not to mention your pushrod lengths will be way off, your rocker arm geometry will get thrown off etc etc.... Just asking for problems trying to space the heads up that much.
If you don't have the money to put pistons in it, a cheaper/easier option would be to find a set of 049 or 781 open chamber, oval port heads..... They'll drop the compression down a good it, and they flow better... You can usually find them used pretty cheap...Then have a valve job and put new springs on them. I bet you could do this for $1k or so..
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From: SW Florida.
Does anybody have an opinion on these heads? ProComp castings. Looks like they use good valves, etc.http://vi.raptor.ebaydesc.com/ws/eBa...=0&cspheader=1
