1300 Compression ratio

Discussion in 'Workshop Forum' started by B0b, Nov 15, 2019.

  1. B0b

    B0b Daily Driver

    In the thread about balancing I got off on a compression tangent; moving it here.

    I have measured the head volume again - 31.0 to 31.5 cc across all four.
    This is a NA head, with the full diameter recess, 1.5mm deep.
    The pockets in the new 87mm pistons are 2cc
    The deck height of the stock pistons is - 0.1mm in the center, at the top of the 0.6mm pimple.
    The new pistons are flat and the compression height is 34.9, 0.2mm taller than stock.

    The CR calculation using stock numbers gives me 8.12:1, using a gasket thickness of 1.2.:
    Using the new pistons, I get the following effects:
    • increase bore to 87 8.36
    • account for pistons height 8.91 includes 0.2mm increase in compression height
    • deck block 0.25 9.20 to clean up ; deck height +0.15
    • deck head 0.25 9.51
    • deck head 0.5 total 9.85
    • deck head 0.75 total 10.22
    These numbers seem a bit higher than I found in various archives, but it would appear that getting the CR up around 9.5 by just cleaning up the block and taking 0.5mm off the head.
    Which is great, but these numbers seem a lot higher than I have found in the archives and it worries me that I have missed something.

    Oh yeah, need some advice: when you are finished using cooling oil to CC the head, should it be put back in the bottle? And if so should you mention it to anyone else in the house?
    I didn't ask for advice before I did this, resulting in an interesting discussion:

    Attached Files:

  2. Dr.Jeff

    Dr.Jeff True Classic

    Sin City
    This really won't be any help to you, but it's interesting the CR number you came up with on a stock 1300 (8.12:1) is right about the same number as a couple of others have found measuring the 1500's CR (despite the factory claiming 8.5:1).
  3. B0b

    B0b Daily Driver

    Any opinions about taking .75mm off the block instead of the head?
  4. ghostdancing

    ghostdancing True Classic

    as far as i know you should have machined both block and head to be sure the surfaces are flat and true..but certainly there is much more expertise here on this..
  5. andreav

    andreav True Classic

    It is safe also to remove from head, I do several times 0.8 0.9mm from head and all was ok, but important that head is as possible to original height
  6. JimD

    JimD Waiting for Godot... Moderator

    Missouri, USA
    I made a PDF of the compression page on the old Lalo's Garage site which is no longer active. I will attach it here since it is on topic, but may not be of particular help to B0b. Just some more data points.

    Attached Files:

  7. Dr.Jeff

    Dr.Jeff True Classic

    Sin City
    Thanks Jim. I seem to recall you have brought back a couple of really interesting things that Lalo did, great stuff.

    I might have missed something in his document, but I see he reports the 1500 compression ratio as 8.5:1 (like it says in the manual). If I understood correctly this was calculated by him, following his chamber volume measurement (which were about the same volume as I got on my 1500, 33cc if I remember)? However that 8.5 number does not correspond with the findings from a few others; the consensus seems to be that the 1500 CR is more like 8.1:1. I wonder why the discrepancy? Did he actually do the calculations in this case, or did he assume the manual to be correct? [Not that I'm asking Jim these questions, just asking in general.] The actual stock CR for the US spec 1500 has been something of an debate issue all along.
  8. Dr.Jeff

    Dr.Jeff True Classic

    Sin City
    Regarding the question on removing material from the head vs the block. I believe it has been discussed that with the deep recess in the US spec 1500 head, there is something to be gained (beyond compression increase) by removing it - as in milling the head to eliminate the recess. I think it was about the squish area, chamber shape, flame progression, etc? Hopefully someone can add better information on it; personally I haven't done it (or intend to for my particular project engine's requirements) so I haven't reviewed the prior discussions about this.
  9. Jefco

    Jefco Daily Driver

    Portland OR
    Here's a pic of the bottom of my (1500,FI) MWB performance head from 2015 (sorry it's not sharper), the spec sheet with it said it was milled 0.075 in (about 1.9mm);
    you can just see the dished shape of the roof of the original deep recess combustion chamber (most visible on #4, left) where it was not milled completely flat.
    Last edited: Nov 17, 2019
  10. Dr.Jeff

    Dr.Jeff True Classic

    Sin City
    Do you recall there was something said (maybe by SteveC?) regarding improved effects due to the change in the combustion chamber shape, and not just the increased compression?
  11. B0b

    B0b Daily Driver

    I thought about getting rid of the recess, like the picture, but then I would have to enlarge the combustion chamber from 31 cc ( measured) to 37cc to have a CR of 10:1.
    this would allow unshrouding the valves, but is more work than I was intending.
    By my calcs, it changes the cam timing by about 2 degrees, so an adjustable pulley is likely involved.
    Alternatively, if I just mill 1mm of some combination of the head and block, I get the CR, without as much timing effect.
    However, it would leave the squish dimension at about 2mm, greater than suggested.
    There have been comments about weakening either the block or the head, and I wonder about the best combination:
  12. Jefco

    Jefco Daily Driver

    Portland OR
    I thought since I got the head from a reputable dealer ( Thanks Matt!) they wouldn't go too far with the shaving; they shortened the cambox too.

    I bought a commercially done head because I didn't know a machine shop and had no time to find one; I wanted to improve the performance and since I had to go into the bottom end anyway (thrust washers) decided to refresh bearings & rings and it just kinda snowballed from there.
  13. B0b

    B0b Daily Driver

    The picture of the head is helpful.
    I have exhausted my thinking so have decided to have the block just cleaned up, likely resulting in a positive deck of about 0.1mm.
    Once that is done and checked, I'll open up the combustion chambers to get the volume needed for a CR of 9.5 or 10 , based on milling the head flat.
    The edge of the recess will be handy as a guide.
    With this approach, the squish dimension is basically the gasket thickness minus the 0.1mm positive deck, which should be close to various recommendations.
    Then just deal with the cam timing.

    Does anyone know if taking 1.75mm off the head on a 1300 will allow the tensioner to work?
  14. fyremanbill

    fyremanbill Daily Driver

    Los Angeles, CA
    Ideally, you want the quench area (flat part) of the head as close to the piston at TDC as possible without risk of contact. This tight tolerance produces higher velocity turbulence in the intake charge as the mixture is "squished" out into the combustion chamber at TDC. Benefits include better fuel/air mixing, fuel vaporization, and faster flame propagation.
    Additionally, the relatively cool surfaces of the head and piston help keep the end gasses from reaching their auto ignition temperature and detonating. The tighter the piston to head clearance, the better this detonation suppressing affect is.
    So in this case, cutting the head to remove the recess would have double benefit. Decking the block to get the clearance down to .75 to 1 mm should get you the best possible combination. Then unshroud the valves to reduce CR to below 10:1
  15. Simon Oaten

    Simon Oaten Daily Driver


    on an 1116cc engine (same block deck height) - with similar amt machined off - no, the tensioner wont take up all the slack.i made a "sleeve" that goves over the OD of the bearing, approx. 4mm wall thickness, press fit, with Loctite bearing mount.

  16. B0b

    B0b Daily Driver

    Dang, that's not what I wanted to hear, assuming a NA 1300 block/head yields the same result.
  17. Dr.Jeff

    Dr.Jeff True Classic

    Sin City
    The T-belt tensioner issue with milled heads is easily corrected on 1500 engines by using a (larger diameter) 1300 tensioner. However as Simon says, it isn't as easy on a 1300. I recall some discussion on this awhile back. Someone modified the tensioner mount to allow more movement. Another did what Simon did by adding a sleeve to it. There may have been other ideas as well. Unfortunately I don't have a 1300 so did not save the thread. But hopefully it can be found by searching (although I don't have a lot of luck with the search function).

    Also keep in mind the T-belt length is additionally affected by milling the cam box. Some sources like to mill quite a bit off the cam box to allow thinner than stock (valve clearance) adjusting shims - for reduced weight. Personally I don't think that's worth the effort, but just wanted to throw it out.
  18. B0b

    B0b Daily Driver

    Once I get the block back, and can check the deck height with the new pistons, I'll be able to verify the compression ratio calcs.
    Then a dry assembly before touching the head will let me determine how much I can deck the head without causing tensioner problems.
    I suspect that I'll wind up leaving some of the recess. That will reduce the amount of work unshrouding the valves, and will compromise the squelch height but all I am really after is a CR increase.

Share This Page

  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.
    Dismiss Notice