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Hot Rods Hemi compression test question?

Discussion in 'The Hokey Ass Message Board' started by slayer, May 9, 2018.

  1. The last few weekends Dad and trying to start up the fresh rebuilt 392 Hemi stroker in the 1937 Dodge project of ours. We have been trying to cover the basics. It has fresh fuel and sparked when checked with an inductive spark tester. Dad got one of those Joe Hunt mag looking distributors, so i'm questioning the spark part a bit. After trying to fire up the Hemi repeatedly without so much as a sputter i tried a compression test.
    What I did find questionable was the cylinder pressure. The engine is supposed to have 10-1 compression, but the cylinder pressure was high, like 230 lbs. Assuming my compression gauge is good, what would cause the pressure that high?
  2. Deuces
    Joined: Nov 3, 2009
    Posts: 22,134

    from Michigan

    I wonder if cam timing has something to do with it....
    Just thinking out loud...:confused:
    Johnny Gee likes this.
  3. Deuces
    Joined: Nov 3, 2009
    Posts: 22,134

    from Michigan

    Also wish I knew what the Piston tops looked like...
    Johnny Gee likes this.
  4. I should have degreed the cam when we installed it, but didn't. If I screwed up and put the crank sprocket on the wrong key way, would that possibly be a problem? The piston domes didn't look much different then the original pistons we got with the engine.

  5. razoo lew
    Joined: Apr 11, 2017
    Posts: 517

    razoo lew
    from Calgary

    I don’t think that 230 psi is possible for static compression with any rational compression ratio..... Certainly not with 10:1.
  6. Those mag looking distributors have been known to be problematic at best. Cool as hell idea , poor execution.

    230 psi cranking pressure is usually about 13:1 static compression ratio range.

    You've got a few conflicting things going on here. Run down to Autozone and rent a compression gauge. Cross check that for sure.

    Double check your ignition timing, TDC on compression, firing order.
    If that's all even just close it should at least fart with 230 cranking psi.
  7. If that cap is like the old Vertex, the numbers are the firing order of the unit, not the ignition of the engine. You would need to compare the distributor's firing with the engines and install the wires in the correct order. You haven't said what camshaft is in the engine. If this was a 480 ci. stroker with a stock camshaft, maybe it would have that much cranking psi. In case you are unaware: with the ignition disabled, hold the throttle wide open and crank 6 revolutions on each cylinder for an accurate test. If you have an air compressor with a regulator, you can use that to check your tester to see if it's close. Good luck. :)
  8. Johnny Gee
    Joined: Dec 3, 2009
    Posts: 10,123

    Johnny Gee
    from Downey, Ca

    Not for me to really care about but I like learning things I don't know of. Is this what your referring to?

  9. yellow dog
    Joined: Oct 15, 2011
    Posts: 498

    yellow dog
    from san diego

    Plug your intake valve closing ABDC and usual bore, stroke etc into this calculator to check feasibility.

    I once measured 220 across on a crate 327 w/ mild solid cam, so I don't think you are seeing ridiculous
    numbers if cam specs are fairly mild
  10. Interesting ,
    I can see how that might throw someone off,
    You'd think it should still at least cough or fart or something.

  11. Mr48chev
    Joined: Dec 28, 2007
    Posts: 31,304


    With that much compression an hopefully a decent spark that thing should pop, fart or shoot flames somewhere no matter what the timing was.
  12. I was thinking the same thing. I have little faith in the ignition system, but I'm not the one who way over spent on it. That fucking cap did throw me for a loop, but I did correct the firing order. The spark looked week to me when i had an extra plug hooked to #1 plug wire.
    The compression gauge we used is older then me (43), so maybe not exactly accurate.
  13. Was that correction awhile ago or was that corrected just recently after it was posted here? Plugs might've fouled

    If the spark looks weak to you, do you have a different ignition set up you could try, borrow, beg ?
  14. oj
    Joined: Jul 27, 2008
    Posts: 6,320


    If engine is brand new, unstarted etc it'll have lots of assembly lube etc in the cylinders creating an overly tight seal, that will result in high compression numbers, also your lifters may not be pumped up yet so the exhaust valve won't be operating right.
  15. Johnny Gee
    Joined: Dec 3, 2009
    Posts: 10,123

    Johnny Gee
    from Downey, Ca

    That would be like having a lot of overlap like big cams of back in the day. That would bleed off cylinder pressure.
  16. I'm thinking it's going to be best to verify that cranking compression number before speculation on why.

    Lifters that are still squishy will throw off valve timing, shorten duration, decreased lift make the valve opening events late and closing events early, in effect making the cam seem much milder. The swept volume calculations will give you the static compression ratios. The cranking compression relates to dynamic compression ratios, and there is no way that I know of to manipulate the valve timing to get anything above the static compression.

    Cranking compression and static compression do have a relationship, but there's a lot of variables that make things different, intake closing event being a big one.
    73RR and oj like this.
  17. Johnny Gee
    Joined: Dec 3, 2009
    Posts: 10,123

    Johnny Gee
    from Downey, Ca

    This is where I was going in regards to exhaust being hung open if the lifters were pumped up. And yes, Compression ratio and cylinder pressure are separate of one another but work hand in hand to help achieve the correct conditions withing a cylinder to fire off correctly or better yet for maximum affect.
  18. Gotgas
    Joined: Jul 22, 2004
    Posts: 7,055

    from DFW USA

    You might want to degree that cam. A cam designed for a 331/354 has a slightly different lobe offset than a 392. Chrysler kept the cam in the same place relative to the crank while raising the deck height. It would not be a problem to get a low-deck cam into a 392, but the changes in valvetrain geometry would keep the engine from running right. Might be what we're seeing here? Just thinking out loud.
  19. Johnny Gee, that is correct. :)
  20. I have seen too small of a cam, and too much compression ratio, leading to a abnormally high cranking compression (190 lbs. and up). It will simply blow out the spark. However, if your ignition system can light the mixture, it will give you gobs of torque down low.

    I've also seen electronic ignitions not grounded properly and compromise the spark. Make sure the distributor is grounded properly. Sometimes the gasket on the distributor hold down will not let it ground properly and you have to run an external ground strap.

    Then again, you might just need a higher output ignition system with that much cranking compression. ;)
    ottoman likes this.
  21. Thanks for all the input guys. I don't have the cam specs on hand or I would post them. I can tell you its an Iski solid, and no slouch. A ground on the distributor is something I will try when I get back to it Saturday. I pulled all the plugs last weekend and all look new and smell of gas. this engine has never run, built a couple years ago for this project but not run. I also had solid core plug wires not knowing the Hunt distributor doesn't play well with them. A compression test with a new gauge and new wires will go on before anything else is attempted. I will post my findings, running or not. Thanks for all the help.
  22. Johnny Gee
    Joined: Dec 3, 2009
    Posts: 10,123

    Johnny Gee
    from Downey, Ca

    ^^^^^ I hope your not killing the rings and bores from the all the raw fuel that may have gotten thru. Change the oil if you've been cranking it for a long period of time along with any raw fuel you may have used to get it cackle.
  23. 73RR
    Joined: Jan 29, 2007
    Posts: 6,740


    ...I am a bit concerned about all this 'cranking' on a fresh engine. The cam/lifters will not like losing the assembly lube and since you indicate the cam is 'no slouch' can we assume lots of spring? How much/what type of bearing lube?

  24. I share your concern on both the last two posts. I coated the cam and lifter faces with the molly lube included with the cam and have kept cranking time to a minimum. If the starter motor even got warm to the touch we took a brake. I don't want to waste the cam, but don't want to pull the intake and valley cover to re cote everything.
    As for the gas in the oil/cylinder, the plugs smelled of gas, but weren't wet. Nothing came out when we were doing the compression test with all the plugs out. Both of these things have been on my mind since the first fire up attempt failed. The more I think about things, the more I wonder if I'm just not giving it nearly enough gas?
    Tomorrow we will be back at it. I will post results, hopefully a video of it running! Thanks again for all the input.
    Last edited: May 11, 2018
  25. UPDATE! Worked on the Dodge part of the day yesterday. Rechecked the compression with my cousins compression gauge and got 160 pounds this time. I guess my dads 45 plus year old gauge is less then accurate.
    We tried starting it again, this time lost spark on the second try. We spent quite a but of time looking for where a burning electrical smell, only to find the ballast resistor let go. I installed the extra one we had, but its allowing 12.5 volts to the distributor even thou my multi meter shows it having 1.5 ohms. I guess I need to get another resistor.
    With battery voltage going to the we only tried a couple times to start, didn't want to risk frying the ignition module or coil. The first time resulted with cranking like there was to much advance. We retarded the timing a bunch and it started backfiring through the carbs. That was when we called it a night.
    On a side note, I am so done with this distributor! The god damn spark plug wires took forever just to push into the cap! A had to throw 2 wires into the freezer for an hour to get them to shrink enough to slip in position. What a pain in the ass!
  26. Getting the obscure off the table is progress!!!!

    I've found some of those ballast resistors don't reduce voltage until they are good and hot. Couple mins is usually enough. Your mileage will vary there.

    If retard on the distributor finally got you some action, I'd say you're a bit off on the TDC - rotor position - where's #1 game we play or that Dizzy is the cause of a lot of grief.
  27. Update! The Hemi is up and running after correctly installing the camshaft. After putting a degree wheel on the engine, we discovered the cam was not indexed correctly. The Nos Cloyes timing set from the early nineties had two marks for installing the cam straight up. one to designate which key way to use, and one to align with the cam sprocket. Guess what mark I lined up with what?
    Now I just have to deal with the spark advance in the distributor set up for around 40 degrees and things should be all good. Thanks for all the help and suggestions!
    loudbang, Hnstray and Johnny Gee like this.
  28. Johnny Gee
    Joined: Dec 3, 2009
    Posts: 10,123

    Johnny Gee
    from Downey, Ca

    Good to hear it's nothing major. Other than a major head ache though.
  29. All it cost us was a timing cover and water pump gaskets, weeks of our time, sleep, hair loss, sanity, vomiting etc..... I posted the fire up video on the Meltdown drags 2018 thread, but when I try posting here its throwing me fits like the Dodge not wanting to start.
    loudbang and 31Vicky with a hemi like this.

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