Thread: compression issues

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hey guys I have 2 sets of castiron heads with the same casting number 14102193.
My book lists them as for 350 1987-up. There is no more info.

One set of heads has GM17 cast into it under the valve cover and the chamber is 76cc's.

The second set has GM23 cast in the same place and is 64cc. Can anyone fill in the gaps here?
I used one to mock up while the other was at the shop and now I have 11:1 using the fresh heads and .125 dome pistons.
I really wasn't looking for that much in my truck and the 76cc's are in questionable condition following a recent implosion. HELP? I was shooting for 10:1 but I don't think a thicker head gasket is going to fix this one.........
I have retarded the cam to bleed off cylinder pressure but that is really a bandaid fix.....unless I can better understand why it will work for sure.

I am running a 4.030" bore on speed pro pistons 0.125 dome (3.5cc) with hastings moly rings .020" gap. piston to deck is 0.020". cast steel crank 3.48". Cam is 214/224 @ 0.050 installed at 105 centerline. Intake closes 34*ABDC

Time and money are both issues. The pistons have been installed but not run, maybe the shop will exchange? I really wasn't planning on building at this time and funds are nonexistent due to the kids just going back to school and the holidays coming. ANY USEFUL IDEAS TO PUT ME BACK INTO THE PUMP GAS ARENA?

I,m no expert but i would either get flattop or dish pistons or maybe go with 400sbc open chamber heads. Am interested in hearing what others that have more knowledge than i have have to say maybe i will learn something along with you getting the help you need. Hope others respond with the answers that you need.

i would never try to move the cam .it will be a pig? why not just buy a hyper cast pistons or look in to cutting the piston domes off

i agree that the piston is where the answer lies. The head gasket approach would really mess up my quench area. Today I am going to assemble and cc for a true net chamber volume. Then I am going to see what I can do about the piston to help out. possibly I can work something out with the machine shop and just bite the bullet for a little more money and time.
I would really be interested for more info on why the heads with the same casting number have different chamber volumes.

If the pistons can be cut down as flat tops the SCR would then be 10.48. According to the calculator I found last night the DCR would then be 8.35. the calculator used the advertised measurements and calculated seat times. What do you guys think?
Sure would be a good question for tech. I have been using the search feature and have gotten a lot of good info. I have not found an "established" limit on the DCR. I know it depends on a lot of the characteristics of my motor but it is always nice to have a basic target in mind.

if the pistons are not hollow domes they can be trim and i have run more dcr and got way with it

Originally Posted by pat mccarthy

if the pistons are not hollow domes they can be trim and i have run more dcr and got way with it

Thanks Pat. I shall have to take a look at the domes to see......could be buying pistons in the near future.

Hey did you see any of that storm this week? Twister went through just north of my jobsite and wiped out a good stretch.

Originally Posted by threearmsinjune

Thanks Pat. I shall have to take a look at the domes to see......could be buying pistons in the near future.

Hey did you see any of that storm this week? Twister went through just north of my jobsite and wiped out a good stretch.

no i did not it was about 40 miles from us were the twister hit

I think I have a solution. I am going to take the 76cc heads to the shop and have the checked out. This combination was intended for these heads and I want to keep it that way if I ever need to replace them. The 64's can either be sold or shelved. Maybe they would be good for a claimer racer?

Originally Posted by pat mccarthy

no i did not it was about 40 miles from us were the twister hit

I am a pipewelder and my trench was filling with water faster than we could pump it out. We had a 20" gas main open and had to seal it up. It really made it stressfull conditions for xray quality welds.

Glad to hear you were ok up there in the north.

Originally Posted by threearmsinjune

I am a pipewelder and my trench was filling with water faster than we could pump it out. We had a 20" gas main open and had to seal it up. It really made it stressfull conditions for xray quality welds.

Glad to hear you were ok up there in the north.

that why you guys are the bad boys when it comes to welding .i have talk to some pipe welders i would have a hard time with the job

With a cam of 38 abdc closing point on the intake valve,you do not have that low of a dcr ,.....mine at 9.5to1 and a abdc of 60 degrees ,my dcr was 8.5 to1....... my calculations show a dynamic of 10.22 to 1 dynamic. You will need atleast 68 abdc closing point on the cam,70 abdc would be better.
I also show 10.55 to 1 compression ratio for your combo which makes the dynamic actually be 9.81 to 1 with the current abdc of 38 degrees.

A cam with about 64 abdc closing point ,puts you at 8.46 to 1 dynamic compression on your current combo.. domes and 64 cc heads.

I think Denny posted a good link a while back on SCR, DCR, and practical limits for pump gas... I think 8.5:1 DCR is nominal tops for pump premium but some factors can push that up like high altitude, aluminum heads, etc... mine is at 8.9:1 (10.8:1 SCR) and will run on 91 octane but I have to take out a bunch of timing (32-33* total) to do it... On 100 octane unleaded I run 38-39* and it screams.

-Chris

I took physical measurements this morning and triple checked them:
BORE: 4.030
STROKE: 3.48
GASKET: 4.135 X 0.040
ACTUAL CHAMBER: 63.0cc
PISTON IN BORE: -0.3cc

SWEPT VOLUME (4.030)(4.030)(3.48)(PI/4)(16.387)=727.4cc
GASKET VOLUME (4.135)(4.135)(0.040)(0.7854)(16.387)=8.80cc
NET CHAMBER 63.0+8.80-0.3=71.5cc

SCR (727.4+71.5)/71.5=11.17

INTAKE CLOSED ON SEAT @65*ABDC MEASURED STROKE LEFT=2.697"

THIS WOULD CHANGE THE ABOVE SCR TO 8.88 FOR A MEASURED DCR.
IS THIS PRACTICAL FOR 89 OCTANE OR BETTER?

I still do not know why the two sets of heads under the same casting #'s and with no previous machining came up different, but I have them in my hands and know it is so. Now what can I do with what I have? I was thinking of putting the 76ers together but I dont like the numbers much. I could have the 64's machined for 2.02/1.6 valves with an unshroud sweep. this would open up the net chamber some but would it be enough? An additional 3.5cc would bring the DCR down to 8.516. Any thoughts?

Originally Posted by threearmsinjune

I took physical measurements this morning and triple checked them:
BORE: 4.030
STROKE: 3.48
GASKET: 4.135 X 0.040
ACTUAL CHAMBER: 63.0cc
PISTON IN BORE: -0.3cc

SWEPT VOLUME (4.030)(4.030)(3.48)(PI/4)(16.387)=727.4cc
GASKET VOLUME (4.135)(4.135)(0.040)(0.7854)(16.387)=8.80cc
NET CHAMBER 63.0+8.80-0.3=71.5cc

SCR (727.4+71.5)/71.5=11.17

INTAKE CLOSED ON SEAT @65*ABDC MEASURED STROKE LEFT=2.697"

THIS WOULD CHANGE THE ABOVE SCR TO 8.88 FOR A MEASURED DCR.
IS THIS PRACTICAL FOR 89 OCTANE OR BETTER?

I still do not know why the two sets of heads under the same casting #'s and with no previous machining came up different, but I have them in my hands and know it is so. Now what can I do with what I have? I was thinking of putting the 76ers together but I dont like the numbers much. I could have the 64's machined for 2.02/1.6 valves with an unshroud sweep. this would open up the net chamber some but would it be enough? An additional 3.5cc would bring the DCR down to 8.516. Any thoughts?

Have you cc'd the heads to make sure of the combustion chamber size??

the actual head cc is 63.0 measured three times this afternoon.
the piston was measured in the bore to account for dome, valve reliefs and deck height.