An effective compression ratio calculator (using actual measurements) is your friend.

https://uempistons.com/p-27-compression ... lator.html

Set the effective compression to 8 to 1 and you will have a 'spunky' engine the runs on pump gas.
Guessing or "wishful thinking" will get you... ??? ... less then optimal performance.
DD

Thanks for posting the link Doc. This is very helpful. Other calculators I have found ask for an SCR to be input. I'm understanding now that SCR is the end point, not the beginning. This calculator makes that apparent.
The calculator asks for: Intake Closing Point (degrees) ABDC @ 0.050 lift plus 15 degrees.
Is this 15 degrees true for all camshafts, or is this a rule of thumb to cover most cams?
I assume this addition is done to get close to 0.000 lift where the intake valve is completely closed, correct?

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1975 Dodge D100 225 c.i., HEI, Offenhauser Four Barrel Intake, Parallel-mounted 500 CFM Edelbrock, Split Cast Iron Exhaust, 904 auto, 9.25" 3.55:1

Good news! Rick called it--broken head gasket between two and three. I ordered some ARP head bolts (they wouldn't give me an AARP discount, though) in hopes that new bolts will keep that from happening again. The old bolts were indeed pretty old.

Thanks for your help, everybody. I will let you know when it's running again.


Joe

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Joe Hoppe

71 D100 225 Super Six 727 AT on 2008 Crown Vic CopCar frame

64 Valiant Wagon 225 904 AT 3:23 8.75"

If you blew a head gasket with in 2000 miles of a rebuild,
I would place the fault with either:
1) improper torque applied to the head bolts
Or
2) bad surface finish or profile on the block or head.

You will be able to inspect for issues with the surfaces,
Remember too smooth can be as bad as too rough for
block and head decks.

If nothing suspect is found on the surfaces, torque or torque
sequence is the likly cause.
Bad torque can be caused by:
Wrong specification used
Operator error
Torque wrench out of calibration
Bolts not lubed or dirty or bad threads
Threaded holes in block dirty or defective

Regardless, I would be in serious discussions with the
shop that built the motor. Slants with near stock compression
should not have head gasket issues. The shop that built the motor
screwed something up. The blown head gasket is your proof.
They owe you.

_________________
Doo Ron Ron and the Duke of Earl are friends of mine.....
https://www.youtube.com/watch?v=uX8Nj8ABEI8

Was the block's deck resurfaced by the shop?
If so, how much material was removed?

If the block deck was not resurfaced then pull out the dowel pins and lightly sand the deck with a sanding block to help highlight high and low spots. The photo below shows how bad the flatness on the deck can be with the shinny "stripes" being high spots on this block's top deck.
DD

Thanks for the tips, especially on sanding the head.

I neglected to mention that the head had been rebuilt years ago and enough ground off to give me a 9.5:1 compression ratio. Should I use a different than stock head gasket?

Joe

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Joe Hoppe

71 D100 225 Super Six 727 AT on 2008 Crown Vic CopCar frame

64 Valiant Wagon 225 904 AT 3:23 8.75"

Okay, the compression ratio being raised is important info. Do you know what your initial ignition timing and total mechanical advance is? Are you running regular pump gas? Also, what kind of cam are you running? You need that info to determine your dynamic compression ratio (DCR), which is more relevant for this question than your static compression ratio (SCR).

If your total timing (initial + mechanical) is much over 30°, you may have issues running pump gas at that compression level. Trust me, I have a stack of burned head gaskets hanging on the wall of my garage from learning this lesson the hard way. ARP head studs are nice, but they won't help if you're killing head gaskets from pre-ignition.

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Somehow I ended up owning three 1964 slant six A-bodies. I race one of them.
Escape Velocity Racing

Frank--

I am running propane, which has over 110 octane. I don't remember what the timing is currently set at--10 degrees before TDC sounds right, and I haven't done anything fancy with my Mopar electronic ignition's springs or weights. I am running the Erson RV cam that was offered to Slant Six.Org members at a discount many years ago.

I really don't know about DCR. I should look that up. With the above information, do you think I need some type of special head gasket?


Thanks for the reminders.

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Joe Hoppe

71 D100 225 Super Six 727 AT on 2008 Crown Vic CopCar frame

64 Valiant Wagon 225 904 AT 3:23 8.75"

Oh, ok. I don't know anything about propane; maybe some other folks here can chime in on that. They might also know something about the cam you mentioned and the mechanical advance you probably have if your distributor is still stock (I assume a 1971 D-100 from your signature).

At 9.5 SCR, you shouldn't need a special head gasket or ARP studs. I'm just worried about your ignition timing, but with propane I'm not sure if that's such a big deal. Like others have mentioned, it could also be a machining or assembly issue. There's more than one way to blow a head gasket.

_________________
Somehow I ended up owning three 1964 slant six A-bodies. I race one of them.
Escape Velocity Racing

Correct. Finding the actual intake valve closing point (IC) is the key here and that +15 degrees at .050 lift is a "band-aid" way to get close, especially with hydraulic cams.
Me, I mock-up the engine, set the actual valve lash I will be running and find the IC with that setting, then use that number in the calculator.

Basically, I go and find what the engine will actually "see" and do my calculations with that (those) numbers. I also try some different lash settings during the process, especially focused on where the end of the lash ramps are so the effective compression (EC) supports different lash settings. This helps to "fine-tune" the running engine. (loosen the lash and EC goes up, tighten the lash and it goes down slightly)
DD