Final Engine Compression Ratio?

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YR and nm9stheham, if understood correctly the flat protrudes approximately .032 above the deck, this was with my caliper, I overlooked the part of the feeler gauge, going to recheck and confirm
 
OK, re-confirm with the feeler gauges, but that is certainly possible.

yellow rose said:
I think you need the Sped Pro catalog to find the CC's of the dome. I may have that here. I'll look in a bit.

I do know that if you aren't at least .020 out of the deck the dome is either zero or may even be a minus number, IIRC.

One thing on those Pistons is the damn valve notches are HUGE. Especially for the exhaust valve. Just freaking gigantic. Total waste of CR.
Click to expand...
And here I was thinking they weren't all that large compared to the 5 CC notches of something like a KB or Icon. I'd be guessing +5-6 cc's net, with the domes milled as they are, plus notches combined, referenced to the CH.

It'll be interesting to see what you guys find for the unmilled piston.
 
ok, some readings I feel better about using the feeler gauge, if readings are off it would only be in the hundred thousands.

flat @ top of piston - .029 above deck
dome .184 above deck
head - shallow side - .083 deep
 
billsdartgt said:
ok, some readings I feel better about using the feeler gauge, if readings are off it would only be in the hundred thousands.

flat @ top of piston - .029 above deck
dome .184 above deck
head - shallow side - .083 deep
Click to expand...


Nice. Someone actually understood how Chrysler wanted that piston and head to work together.

In that case, you need to do the 1/2 inch down fill method to verify your CR.

If the engine is in the car that's kinda hard to do.

You can calculate your CR with the heads on if the engine is out of the car. You need to be at absolute TDC and you fill the chamber to the bottom of the spark plug hole doing your level best to keep air out chamber.

Then it's simple math from there. Or, you can do like I do and use a slide rule.
 
after the measurements at least it looks like the piston shouldn't be smacking the head, from the online calculators it seems if I had the cc value for the piston dome I could get at least a close number on the static compression, guess I will have to ponder about trying to check the volume, not interested in testing with the fluid right now, I know the car will at least run as is (once back together) and I can keep the timing manageable if needed.

Will see how the ongoing work goes, I'm way deeper in repairs and cash burning than I thought and going to take much longer than expected to get finished up with what I have now.

Thanks for all the replies, some of this thread made my head hurt LOL
 
Good data... I think those pistons are 10cc net volume above the CH when unmilled; seems like I found that reference somewhere; it would be good if someone had that listed in an old catalog to confirm or deny.

Look at the pix in another thread that I linked in post #21 above where it shows the unmilled piston. Then compare to how much your piston has been milled in your pix in post #22 above. I'm taking a stab that about 4 cc have been milled off...just my best guess based on relative sizes of other domes/eyebrows. So that is why I am guessing a net dome of 5-6 cc.

With that and the rest we can get some ballpark numbers to see where you are at for SCR and DCR. With the following:
  • CH +.029" above deck.... please confirm that this is the case, above the deck
  • Unmilled J/X heads 72 cc .... but they are probably 3-5 cc smaller as your measured depth of .083 in the shallow flat area of the head is a lot smaller than normal
  • Net dome 6 cc (my best guess)
  • .050" thick compressed head gasket
  • 4.030" bore
  • 3.58" stroke
Result is SCR=11.5:1

If the info above is even close right, then the 702 cam is a non-starter, even if my guess on the dome volume is off by + 5 cc, you are still at 11:1. DCR is going above 9 without even running the numbers on that cam; 8-ish is a safe rule-of-thimb limit for us normal-mortal gearheads, and a decently tuned engine, for pump premium.

It would take a .100" thick head gasket to get down close to the 10.0 range. Cometic had .098" thick... but we don't know if your heads and block are good 'n flat and if the surface finishes will work adequately with MLS gaskets. But it is possible it you want to spend $130 per gasket.

So that is the best numbers I can get without knowing things for certain. The next best step is the fill method described, plus cc'ing the heads, if you want to be certain.

"head hurting"? LOL welcome to the club... I am debating whether to stroke a 1.9L Opel out to a mere 2.2L or go for the gold and shoot for 2.5L ! All on the stock crank....
 
the .029 is above deck

the dome volume is what I was missing for the online calculators found on the net, using this one https://butlerperformance.com/n-12872-compression-calculator.html with a 10cc volume for the dome the noted SCR moves from 10.31 to 11.07, changing from a 68cc to a 72 cc for the head and using felpro's range of .049 to .059 for compressed gasket thickness.

Values I used:

bore dia. 4.030 (based on piston size)
stroke - 3.58 (stock)
cylinder head volume - 68 up to 72 (I read somewhere these were what had been seen in the past)
Dome volume -10cc
Deck clearance - .029 (I assume the measurement I took at the flat is this number)
Compressed gasket - .049 to .059
number of cylinders - 8

I concur that the current Lunati cam isn't the best for the rest of the engine, sucky part is what happens for the cam change another 400ish up close to 1000 depending on how it goes.

hydraulic or solid?
verify pushrod length - I now have a set for a solid and a hydraulic, no certainty either set will work.
replace springs - I don't know what the ones are in the heads and have no way to verify
upgrade/change torque convertor

total cost to change - cam, lifters, springs (plus gaskets etc.) probably around 400ish (based roughly on the cam Rusty linked to @ Summit)
pushrods if needed - 40.00 to 150?
torque convertor - 400ish maybe?

so a couple of things I know for sure (because this is what I have LOL)

engine will get a set of Dougs 453s Headers
gears will be 3.55s in a 8 1/4
intake will be Edelbrock RPM Air Gap
Carb - Holley 670 elect choke vac secondary's. (I have a Quickfuel Slayer 600 cfm) I think the holley will be a good fit, it ran well on my 351 in my Mustang before I swapped to fuel injection

Obviously with the engine torn down to where it is it would be better to swap cams now rather then later, I'm gonna continue some of the other work while I ponder this some more, perhaps if I have some luck here soon with a sale of a few spare parts I have it will help with the pondering LOL

Thanks for all the reply's and attempt to further my knowledge,
 
billsdartgt said:
the .029 is above deck

the dome volume is what I was missing for the online calculators found on the net, using this one https://butlerperformance.com/n-12872-compression-calculator.html with a 10cc volume for the dome the noted SCR moves from 10.31 to 11.07, changing from a 68cc to a 72 cc for the head and using felpro's range of .049 to .059 for compressed gasket thickness.

Values I used:

bore dia. 4.030 (based on piston size)
stroke - 3.58 (stock)
cylinder head volume - 68 up to 72 (I read somewhere these were what had been seen in the past)
Dome volume -10cc
Deck clearance - .029 (I assume the measurement I took at the flat is this number)
Compressed gasket - .049 to .059
number of cylinders - 8

I concur that the current Lunati cam isn't the best for the rest of the engine, sucky part is what happens for the cam change another 400ish up close to 1000 depending on how it goes.

hydraulic or solid?
verify pushrod length - I now have a set for a solid and a hydraulic, no certainty either set will work.
replace springs - I don't know what the ones are in the heads and have no way to verify
upgrade/change torque convertor

total cost to change - cam, lifters, springs (plus gaskets etc.) probably around 400ish (based roughly on the cam Rusty linked to @ Summit)
pushrods if needed - 40.00 to 150?
torque convertor - 400ish maybe?

so a couple of things I know for sure (because this is what I have LOL)

engine will get a set of Dougs 453s Headers
gears will be 3.55s in a 8 1/4
intake will be Edelbrock RPM Air Gap
Carb - Holley 670 elect choke vac secondary's. (I have a Quickfuel Slayer 600 cfm) I think the holley will be a good fit, it ran well on my 351 in my Mustang before I swapped to fuel injection

Obviously with the engine torn down to where it is it would be better to swap cams now rather then later, I'm gonna continue some of the other work while I ponder this some more, perhaps if I have some luck here soon with a sale of a few spare parts I have it will help with the pondering LOL

Thanks for all the reply's and attempt to further my knowledge,
Click to expand...


I absolutely detest vacuum secondary carbs. Just hate them. They do nothing good and give up power.

And a 600 is pretty small. A well tuned 750 would be far better.
 
YR, I had mech and vac. secondary's, Edelbrock and Holley in the past, also some Rochester quadrajets many years ago, this Holley I have now has been the best carb I have ever owned, I have the available jets, springs, etc. to tune, it is adjustable on all 4 corners and has the billet base and metering blocks,

I think a 750 would be better suited for this car, especially if I up the cam, another 600.00 plus though and I have also thought about it (770 version of what I havehttps://www.summitracing.com/parts/hly-0-86770bl )

Anybody looking to sponsor a new Mopar owner in the pacific Northwest? I will put your name in bold 1/4" letters on the bottom quarter of the car as appreciation! LOL
 
billsdartgt said:
YR, I had mech and vac. secondary's, Edelbrock and Holley in the past, also some Rochester quadrajets many years ago, this Holley I have now has been the best carb I have ever owned, I have the available jets, springs, etc. to tune, it is adjustable on all 4 corners and has the billet base and metering blocks,

I think a 750 would be better suited for this car, especially if I up the cam, another 600.00 plus though and I have also thought about it (770 version of what I havehttps://www.summitracing.com/parts/hly-0-86770bl )

Anybody looking to sponsor a new Mopar owner in the pacific Northwest? I will put your name in bold 1/4" letters on the bottom quarter of the car as appreciation! LOL
Click to expand...


I personally won't buy anything with the name Holley on it. In several places on here I've related why I won't do it.

FWIW the Edelbrock and Qjet are not vacuum secondary carbs. They have mechanical secondaries with an air valve over them.

That's not the same as the Holley system.
 
billsdartgt said:
the .029 is above deck

the dome volume is what I was missing for the online calculators found on the net, using this one https://butlerperformance.com/n-12872-compression-calculator.html with a 10cc volume for the dome the noted SCR moves from 10.31 to 11.07, changing from a 68cc to a 72 cc for the head and using felpro's range of .049 to .059 for compressed gasket thickness.

Values I used:

bore dia. 4.030 (based on piston size)
stroke - 3.58 (stock)
cylinder head volume - 68 up to 72 (I read somewhere these were what had been seen in the past)
Dome volume -10cc
Deck clearance - .029 (I assume the measurement I took at the flat is this number)
Compressed gasket - .049 to .059
number of cylinders - 8
Click to expand...
Yes, use the measurement at the flat for the deck clearance. BTW, using your calculator, I get 11.6 for a 6 cc net dome volume, .029" above deck, .050" head gasket, and 72 cc head chamber. Both the deck clearance and dome volume should be entered as negative numbers in your calculator. And realize your domes have been milled a bit so the 10 cc net dome volume is too much IMHO. And head is probably more like the 68 cc number so now you're pushing over 12:1.

If it really was 10.3, then maaaaybe the 702 cam. But I am pretty sure you are up well past 11:1. Ready for a bigger cam while maintaining good low RPM torque!

Hope it works out well.
 
Im gonna go with the thought that I am at least at a 11.1.1 on the CR, at this point it will be the best path, I don't think getting the setup to do the fluid check will change the inevitable, motor needs a different cam,

on a good note I learned some more things about cams and CR and know more about the engine in my car, still sucks more money is going to have to leave my friendly bank account LOL
 
billsdartgt said:
Im gonna go with the thought that I am at least at a 11.1.1 on the CR, at this point it will be the best path, I don't think getting the setup to do the fluid check will change the inevitable, motor needs a different cam,

on a good note I learned some more things about cams and CR and know more about the engine in my car, still sucks more money is going to have to leave my friendly bank account LOL
Click to expand...


Is Brier on the west side or east side?? Yes, I'm being lazy...don't feel like digging out my atlas!! Yes, I'm THAT lazy...just ask my wife.
 
OR.....another spin......you COULD just run race gas and run "whatever" cam you want. LOL I think that Comp Purple Plus I linked to will work good. It's an updated grind for the .508 cam with Mopar lobes. Plug all that into a dynamic compression ratio calculator and see what you come up with. I bet it will be pump gas friendly.
 
billsdartgt said:
brier is just south of Everett between the 5 and 405 split
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Really?? I used to drive that route all the time and I don't remember that.

Damn, I'm getting old. That's terrible I've either never noticed that, or I just don't remember that place. Dang.
 
yellow rose said:
Really?? I used to drive that route all the time and I don't remember that.

Damn, I'm getting old. That's terrible I've either never noticed that, or I just don't remember that place. Dang.
Click to expand...

its small, Lynnwood is the exit and even has the dame zip
 
Really
hope he's near one cuz that's the easiest answer if compression looks like it does
or
cut the domes
or PROPANE
 
I like the looks of the cam Rusty listed, anybody else have any suggestions? I'm not looking to race just a decent runner I won't explode while putting around
 
at this point not sure Im going to know anything more than I do now

this is the cam I took out of the car when I got it, solid lifter cam

IMG_3323.JPG
 
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