Compression Ratio's Type 4

Discussion in 'Mech Tech' started by Deefer66, Nov 24, 2019.

  1. We've been here before but I thought I'd start again...

    Got a 2056 going together and putting in 73/86 cam. It's got 1800 heads which would give low CR as they have bigger combustion chambers (56cc) but they were skimmed to tidy up and took 0.75mm off so chambers now down to 48cc.
    AA pistons advertise cuts out at 7cc but measure to 4!. Deck height on 2 and 3 (1 and 4 doing today) 1.1mm so compression ratio at 9.5 static 8.8 dynamic. If I open chamber by 4cc I can get down to 9 static and 8.3 dynamic and keep a tight deck height. Anyone know if this still too high.

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  2. 77 Westy

    77 Westy Supporter

    I’m sure you know Aircooled.net say it works best with 8-8.25:1 CR; I think they’re too conservative, possibly due to the fuel quality in the US, but it’s a mild cam and 9.0:1 static is probably too high. I run the C25 with 9.0:1 (96mm flat top pistons, 1.6mm deck, 62cc combustion chambers) but is has more duration than the 73/86, and I use 98 octane fuel, usually E5.

    I’d aim for 8.5:1 static with a 73/86 - it’ll be interesting to hear what others say.
     
  3. Not sure what CR aircooled are referring to.. suppose its Static as it involves "math" to do Dynamic...

    [​IMG]

    All measured up , going to check combustion chambers again now... but a teaspoon of material out of the head... cant be hard

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  4. I'm building the following:
    2056 with 41 x 34 valves heads
    Fully balanced
    8.5:1CR static
    SCAT C35
    IDF 40's with 30mm venturi's

    I was recommend by VW Heritage 7.5:1 with 95 octane with 28 degrees maximum advance @ approx 3500rpm
    As mentioned above Aircooled.net recomended 8, 8.5 - 1.

    To keep the generated heat down as much as possible I'll use 95 octane with a max CR of 8.5

    Comments would be appreciated
     
    Deefer66 likes this.
  5. MorkC68

    MorkC68 Administrator

    Our 1911 has a static of 8.1:1 using 95 octane fuel.

    God knows what cam, it should be close to stock, if not stock - CoolAir supplied one appropriate to the build.
     
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  6. 77 Westy

    77 Westy Supporter

    This isn’t a bus engine is it? Heat won’t be a problem in a car. A bus engine has to push twice the weight of a car and has the aerodynamics of a barn door – even if it isn’t a barn door bus.

    Aircooled.net recommend 8.25-8.5:1 for the C35. I’d aim for at least 9.0:1 but if you want to stay within Aircooled.net recommended CR go for the very top end of their range.

    VWH are just wrong; nowhere near enough compression for a C35 to work, 7.5:1 is even low for a standard cam – did they know what the engine was for?
     
    snotty likes this.
  7. I'm building the following:
    2056 with 41 x 34 valves heads
    Fully balanced
    8.5:1CR static
    SCAT C35
    IDF 40's with 30mm venturi's

    I was recommend by VW Heritage 7.5:1 with 95 octane with 28 degrees maximum advance @ approx 3500rpm
    As mentioned above Aircooled.net recomended 8, 8.5 - 1.

    To keep the generated heat down as much as possible I'll use 95 octane with a max CR of 8.5

    Comments would be appreciated

    Thank you 77Westy. you just cant beat the hard facts of experience. Its going in the back end of a Karmann Ghia if I sort the cracked head problem and barrel to case face issues. lol I had thought about startinjg an engine build post, more for my benefit than most readers.

    Throwing caution to the wind I could increase the CR and use a better octane fuel if needed.
     
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  8. 77 Westy

    77 Westy Supporter

    No need to throw caution to the wind, you just need to know how the cam changes what static compression is required to end up with the dynamic compression the engine needs to work efficiently. A standard cam doesn’t have much duration and the dynamic compression is similar to the static. But when you fit a cam with more duration you need to start with a higher static CR to end up with the same dynamic CR as standard.

    John Maher (JMR) can explain it far better than me, have a look at this: http://johnmaherracing.com/tech-talk/what-effect-does-compression-ratio-have-on-bhp/
     
  9. Well after taking a grinding bit to the head I've got another 2cc! That 4cc piston "dish" is a killer so I'm going to have to loose the tight deck and stick a .254 shim in ..


    [​IMG]

    If I can find another 2cc... they're hard to come by...

    [​IMG]


    That's about as close to dynamic of 8 :1 I can get.. maybe my "effective" will be a bit lower too with stock exchangers....


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    Last edited: Nov 25, 2019
    77 Westy likes this.
  10. Mines only going to be pushing g half a bus

    [​IMG]

    Still a brick though...

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  11. 77 Westy

    77 Westy Supporter

    1.4mm deck is fine. 1.6mm is generally recognised as being the maximum for an effective squish.
     
  12. I was just on a quest to get as low as I could as I'd read somewhere it helps keep temperature down if you get closer to the minimum, which would go someway to offset the highest tempnfrom increasing compression. Maybe I should consider an external oil cooler too, seems the done thing once away from stock, though not sure that helps the heads so much.

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  13. On a C35 you get intake closing at 51° after bottom dead centre so with stock crank length I make your Dynamic CR a 0.872 of Static.. if it helps... check yourself too tough, so about your Dynamic would be 7.4.. like 77Westy says prob up it a bit for more power or keep it low for cooler

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  14. 77 Westy

    77 Westy Supporter

    An oil cooler won’t affect cylinder head temperature much, if at all, IMO. I have a 28-row external cooler to cool the oil, not the heads.
     
    Deefer66 likes this.
  15. It is an aircooled engine after all ..not oil cooled. I'll just try and keep as much of the air inside the tin until its past the heads.

    Guess just a little bit cooler oil splashing the back of the pistons helps take some of the heat away.

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  16. to encourage better piston cooling I followed the attached
    8c1zhQ2.jpg
     
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  17. Is that both sides?

    Unfortunately my milling machine is an angle grinder followed by a chainsaw file so might not look as tidy as yours..

    They don't look that tidy to start with as I dropped off the wrong set to be re-bushed so I've got one set that are great small end but a bit tatty sides to big end or a set with tidy big end sides and bushes slightly worn .

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  18. Yes both sides, a used a small square file, making sure good access and egress for the oil from bearing up towards the piston. i finished off with a rotary brush smoothing the edges and taking any "loose" burrs away.
     
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  19. So so cool
     
  20. 77 Westy

    77 Westy Supporter

    Per the 1990 Technical Bulletin 10-90 T01. Mine look like this, but with a half round groove rather than a V.
    IMG_4840 con rods (Medium).JPG
     
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