Dismiss Notice

Welcome To CK5!

Registering is free and easy! Hope to see you on the forums soon.

Score a FREE t-shirt and membership sticker when you sign up for a Premium Membership and choose the recurring plan.

Cranking Compression vs. Octane

Discussion in 'The Garage' started by bigburban383, May 10, 2004.

  1. bigburban383

    bigburban383 1/2 ton status

    Joined:
    Dec 30, 2003
    Posts:
    1,242
    Likes Received:
    0
    Location:
    Michigan, Royal Oak
    Apparently my engine knocks a lot, even though I don't hear it, I know this because the sensor tells me. I checked the cranking compression on a few cylinders and I am getting 150 psi. Would 93 octane be good enough for pressures like that, I have no clue of the compression ratio. I have dished pistons and unshrouded 305 heads.
     
  2. sled_dog

    sled_dog 1 ton status

    Joined:
    Sep 3, 2002
    Posts:
    16,870
    Likes Received:
    0
    Location:
    Austin, TX
    how much of a dish? Unshrouded 305 heads are probably like 59/60ccs. With flattops that would be about guessing at 10.5:1 compression. You are probably in the mid or upper 9s, I'd say try a little bit of 89, then try a little bit of 93, whichever gives better results(power,knocking,etc.) go with that. If it runs good on 89 there is no reason to run 93.
     
  3. pauly383

    pauly383 Daddy383 Staff Member Moderator

    Joined:
    Jul 22, 2001
    Posts:
    16,217
    Likes Received:
    1
    Location:
    Mesa , Arizona USA
    How dished are the pistons? Stock 83 305 heads on a 305 produced 9.2:1 compression, hence the need for ESC. Bigger piston diameter on your engine will lower the compression a bit. /forums/images/graemlins/k5.gif
     
  4. johnc

    johnc Registered Member

    Joined:
    May 6, 2004
    Posts:
    68
    Likes Received:
    0
    Location:
    O.C., CA.
    http://www.carcraft.com/techarticles/982/
    I hope this can help... /forums/images/graemlins/dunno.gif Generaly 10.0 to 1 will work on premium with cast iron heads without pinging /forums/images/graemlins/waytogo.gif
     
  5. johnc

    johnc Registered Member

    Joined:
    May 6, 2004
    Posts:
    68
    Likes Received:
    0
    Location:
    O.C., CA.
    [ QUOTE ]
    How dished are the pistons? Stock 83 305 heads on a 305 produced 9.2:1 compression, hence the need for ESC. Bigger piston diameter on your engine will lower the compression a bit. /forums/images/graemlins/k5.gif

    [/ QUOTE ] actualy a bigger piston diameter will raise your compression due to the greater volume being compressed /forums/images/graemlins/deal.gif /forums/images/graemlins/waytogo.gif
     
  6. pauly383

    pauly383 Daddy383 Staff Member Moderator

    Joined:
    Jul 22, 2001
    Posts:
    16,217
    Likes Received:
    1
    Location:
    Mesa , Arizona USA
    [ QUOTE ]
    actualy a bigger piston diameter will raise your compression due to the greater volume being compressed

    [/ QUOTE ]

    Then the compression ratios of all those smog 350's in the 70's with 76cc heads would of been really high, right?
     
  7. 84_Chevy_K10

    84_Chevy_K10 Banned

    Joined:
    May 30, 2001
    Posts:
    17,669
    Likes Received:
    0
    Location:
    IL, USA
    [ QUOTE ]
    [ QUOTE ]
    actualy a bigger piston diameter will raise your compression due to the greater volume being compressed

    [/ QUOTE ]

    Then the compression ratios of all those smog 350's in the 70's with 76cc heads would of been really high, right?

    [/ QUOTE ]

    No. Larger chamber heads will reduce compression ratio. 76 cc heads are huge.
     
  8. johnc

    johnc Registered Member

    Joined:
    May 6, 2004
    Posts:
    68
    Likes Received:
    0
    Location:
    O.C., CA.
    [ QUOTE ]
    [ QUOTE ]
    [ QUOTE ]
    actualy a bigger piston diameter will raise your compression due to the greater volume being compressed

    [/ QUOTE ]

    Then the compression ratios of all those smog 350's in the 70's with 76cc heads would of been really high, right?

    [/ QUOTE ]

    No. Larger chamber heads will reduce compression ratio. 76 cc heads are huge.

    [/ QUOTE ] /forums/images/graemlins/waytogo.gif /forums/images/graemlins/waytogo.gif /forums/images/graemlins/waytogo.gif /forums/images/graemlins/waytogo.gif /forums/images/graemlins/waytogo.gifExactly /forums/images/graemlins/peace.gif
     
  9. pauly383

    pauly383 Daddy383 Staff Member Moderator

    Joined:
    Jul 22, 2001
    Posts:
    16,217
    Likes Received:
    1
    Location:
    Mesa , Arizona USA
    After a re-read of that article it makes sense to me but its funny. Overboring an engine RAISES compression by ADDING volume, however using larger chamber in the heads ADDS volume and LOWERS compression. That is the part I forgot about. /forums/images/graemlins/k5.gif
     
  10. sled_dog

    sled_dog 1 ton status

    Joined:
    Sep 3, 2002
    Posts:
    16,870
    Likes Received:
    0
    Location:
    Austin, TX
    I try not to question, a 400 can have the same compression with 76cc heads that a 350 has with 64s, I just think of that and well thats that.
     
  11. bigburban383

    bigburban383 1/2 ton status

    Joined:
    Dec 30, 2003
    Posts:
    1,242
    Likes Received:
    0
    Location:
    Michigan, Royal Oak
    These are specs from the 383 "Street Friendly" kit in my engine:

    9.0:1 with 64cc heads
    8.5:1 with 76cc heads
     
  12. johnc

    johnc Registered Member

    Joined:
    May 6, 2004
    Posts:
    68
    Likes Received:
    0
    Location:
    O.C., CA.
    /forums/images/graemlins/waytogo.gif /forums/images/graemlins/waytogo.gif /forums/images/graemlins/waytogo.gif you got it ! Sounds wierd though at first, I'll give you that one! /forums/images/graemlins/wink.gif /forums/images/graemlins/thumb.gif
     
  13. sled_dog

    sled_dog 1 ton status

    Joined:
    Sep 3, 2002
    Posts:
    16,870
    Likes Received:
    0
    Location:
    Austin, TX
    you are pushing probably 9.5:1+, like I said 89 might work but I'd say much more towards 93. any pictures of your chambers? I remember them a while back but forget now.
     
  14. 84_Chevy_K10

    84_Chevy_K10 Banned

    Joined:
    May 30, 2001
    Posts:
    17,669
    Likes Received:
    0
    Location:
    IL, USA
    Actually he's closer to 10:1 or right at it. I would be willing to say that with 305 heads, 10:1 CR, and any kind of torquey cam, that engine is going to knock like a Diesel under load no matter what pump gas you choose.
     
  15. sled_dog

    sled_dog 1 ton status

    Joined:
    Sep 3, 2002
    Posts:
    16,870
    Likes Received:
    0
    Location:
    Austin, TX
    I'm asking about his heads cause, if he has a nice polished chamber it would be a help. I said a little less than 10:1 cause he elarged the chamber by unshrouding the valves. You are right though, it will be tough to get around.
     
  16. Beast388

    Beast388 1/2 ton status

    Joined:
    Nov 6, 2001
    Posts:
    3,039
    Likes Received:
    0
    Location:
    Clinton, IA USA
    Since your engine and my engine are the same CID, I opened up the compression ratio calculator that I wrote in Excel and checked a few things.

    With a 4.06 bore, 3.75 stroke, 12cc dish piston, 72cc head etc etc my comp ratio is 9.463:1. I changed nothing but the head cc...which I changed to 58cc to reflect the 305 head......and the comp ratio jumped to 10.9:1! /forums/images/graemlins/eek.gif /forums/images/graemlins/doah.gif /forums/images/graemlins/yikes.gif

    You probably did not square deck the block for good quench which will lower the comp ratio, but make the engine more prone to detonation.

    I would guess you are between 10-10.5:1 which is WAY too much for a street motor in a heavy Burb. Loose the 305 heads and find a good set of 350 heads with either 64 or 76cc's. You'll be much happier with the engine. /forums/images/graemlins/waytogo.gif
     
  17. bigburban383

    bigburban383 1/2 ton status

    Joined:
    Dec 30, 2003
    Posts:
    1,242
    Likes Received:
    0
    Location:
    Michigan, Royal Oak
    [ QUOTE ]
    you are pushing probably 9.5:1+, like I said 89 might work but I'd say much more towards 93. any pictures of your chambers? I remember them a while back but forget now.

    [/ QUOTE ]

    [​IMG]
     
  18. Triaged

    Triaged 1/2 ton status

    Joined:
    Feb 21, 2001
    Posts:
    3,808
    Likes Received:
    0
    Location:
    CA (LA/OC area)
    Those pistons must have a big dish in them (like 20+cc's) to get the compression #'s you posted in a 383!

    Tearing into the motor might not be something you want to do...but putting in a larger cam will help bleed off some pressure and you might be able to get away with the cheeper gas. Cam's don't cost a hole lot so you shouldn't have that long to break even...on the plus side you will have some more HP which is always fun /forums/images/graemlins/waytogo.gif
     
  19. bigburban383

    bigburban383 1/2 ton status

    Joined:
    Dec 30, 2003
    Posts:
    1,242
    Likes Received:
    0
    Location:
    Michigan, Royal Oak
    [ QUOTE ]
    Since your engine and my engine are the same CID, I opened up the compression ratio calculator that I wrote in Excel and checked a few things.

    With a 4.06 bore, 3.75 stroke, 12cc dish piston, 72cc head etc etc my comp ratio is 9.463:1. I changed nothing but the head cc...which I changed to 58cc to reflect the 305 head......and the comp ratio jumped to 10.9:1! /forums/images/graemlins/eek.gif /forums/images/graemlins/doah.gif /forums/images/graemlins/yikes.gif

    You probably did not square deck the block for good quench which will lower the comp ratio, but make the engine more prone to detonation.

    I would guess you are between 10-10.5:1 which is WAY too much for a street motor in a heavy Burb. Loose the 305 heads and find a good set of 350 heads with either 64 or 76cc's. You'll be much happier with the engine. /forums/images/graemlins/waytogo.gif

    [/ QUOTE ]

    Oh My Got, 10.9:1, I suppose that is the problem. I'm an idiot! Thank you Chevy for ESC and Knock Sensors /forums/images/graemlins/bow.gif
     
  20. 84_Chevy_K10

    84_Chevy_K10 Banned

    Joined:
    May 30, 2001
    Posts:
    17,669
    Likes Received:
    0
    Location:
    IL, USA
    [ QUOTE ]
    You probably did not square deck the block for good quench which will lower the comp ratio, but make the engine more prone to detonation.

    [/ QUOTE ]

    Decking the block into a good quench zone does the exact opposite. It will INCREASE compression ratio, and make the engine LESS prone to detonation.

    Even with perfect quench distance in a 383 with a torquey cam and 305 heads and 10:1 static CR, it's going to be VERY knock prone.
     

Share This Page