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Thread: Can you calculate Compression ratio?

  1. #1
    I definitely ain't a Chief Engine Builder wagonist's Avatar
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    Question Can you calculate Compression ratio?

    There was some talk over the weekend about my hybrid 3S-GTE.

    It has an ST205 block & bottom end with the original ST215 head.
    Neither has been modified & an ST215 head gasket was used.

    Seems that the ST205 had a compression ratio of 8.5:1, but the ST215 has 9.0:1

    So which ratio would mine have now?

    I know there's a few factors that effect it.
    eg rod length & piston height, head design.

    Is there a way to calculate it with the engine in the car & not dismantling it?

  2. #2
    Toymods Club Member Conversion King big_zop's Avatar
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    Default Re: Can you calculate Compression ratio?

    You need max and min volumes to determine compression ratio, and since you have the CR and stroke of each motor, then use that as a starting point.

    Though, I would suspect it would be 8.75 - assuming that all 3S-GTEs have the same stroke so the difference in compression ratio comes from the associated head/HG combo (sicne you used the ST215 HG and head on a ST205 bottom end)

  3. #3
    ......... Carport Converter jeffro ra28's Avatar
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    Default Re: Can you calculate Compression ratio?

    To answer your question, Yes you can if your spark plug hole is at the highest point in the combustion chamber so you can fill with a burret but you may need to pull a cam to get the valves closed at BDC.
    Measure cylinder and combustion chamber volume at BDC and TDC and your done. Use kerosene as your burret fluid. Some additional oil may help temporarily seal the rings while your measure the cylinder volume.

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    Toymods Board Member Chief Engine Builder Hiro's Avatar
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    Default Re: Can you calculate Compression ratio?

    Quote Originally Posted by jeffro ra28 View Post
    To answer your question, Yes you can if your spark plug hole is at the highest point in the combustion chamber so you can fill with a burret but you may need to pull a cam to get the valves closed at BDC.
    Measure cylinder and combustion chamber volume at BDC and TDC and your done. Use kerosene as your burret fluid. Some additional oil may help temporarily seal the rings while your measure the cylinder volume.
    No need to measure at BDC as well, the difference is between volume at TDC and volume at BDC is simply the swept volume of the piston crown, which is PI/4 * bore^2 * stroke.
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    Junior Member Automotive Encyclopaedia
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    Default Re: Can you calculate Compression ratio?

    Forget doing that BDC stuff completely!!!!!
    A disposable plastic syringe is a lot easier to find & cheaper then a burrete/graduated cylinder.
    There's a lot less chance of any seepage at with ATF instead of kerosene/paraffin, just a bit messier expelling it.
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    ......... Carport Converter jeffro ra28's Avatar
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    Default Re: Can you calculate Compression ratio?

    hmm to true,

    What is the specific gravity of ATF compared to kero tho? IS it different enough to fuck your actuall CC's readings?

  7. #7
    Experience shows I'm no Chief Engine Builder Roundy's Avatar
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    Default Re: Can you calculate Compression ratio?

    Specific gravity shouldn't matter as it is measuring volume not mass.

    Compression Ratio = (combustion volume + swept volume)/combustion volume

    Using some algebra and the knowledge of the stock CR you can get the combustion volume.

    Only thing is you need to know the crown/dish volume of the piston to be able to work out the head CC's.
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  8. #8
    I definitely ain't a Chief Engine Builder wagonist's Avatar
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    Default Re: Can you calculate Compression ratio?

    The aim is to hopefully not dismantle the engine at all.
    I'm not overly worried to know, but it'd be nice to know that if I've lost some compression, and with the stronger rods in the ST205 bottom end, then I'd just wind up the boost a little to compensate.
    Especially as the car is about to be retired from work & daily duties, hence fuel economy is less important.

    The bore of both engines is known at 86.0mm
    The stroke of both engines is known at 86.0mm
    From this the swept volume can be calculated.

    The squish (combustion) volume however is the unknown (maybe should've checked that on my head while is was off), as I daresay someone already knows what that volume would be for an ST205.

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    Cressidaspert Carport Converter andrew_mx83's Avatar
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    Default Re: Can you calculate Compression ratio?

    why not google for the head cc of the 215 head, google for the piston dish cc & deck height of the 205 and use something like this:
    http://www.csgnetwork.com/compcalc.html
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  10. #10
    ......... Carport Converter jeffro ra28's Avatar
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    Default Re: Can you calculate Compression ratio?

    Quote Originally Posted by Roundy View Post
    Specific gravity shouldn't matter as it is measuring volume not mass.

    Compression Ratio = (combustion volume + swept volume)/combustion volume

    Using some algebra and the knowledge of the stock CR you can get the combustion volume.

    Only thing is you need to know the crown/dish volume of the piston to be able to work out the head CC's.

    It is a measurement of volume yes but the way i understand it the liquids density is of importance to measure the actual volume. To take it to the extremes compare filling the cylinder with foam (less dense) as compared to water (more dense), it may take 5cc of foam to fill the cylinder or it could take say 50cc (round figures) to fill it with water. Or maybe im just dense myself? LOL

    The only reason i suggest kerosene is because of its low surface tension, it "flows" good and is easy to remove the air pockets from what your measuring. With water, it can be hard to CC things because air pockets get stuck in crevices and can be hard to move. When your CCing a combustion chamber with a closed engine there is really no way to tell if there are air pockets stuck in the sqiush area near your head gasket or what not.
    But then ATF sounds like a good idea aswell, not sure of its surface tension characteristics tho and its ability to "not get air pockets" but it seems it is of similar spec gravity to kero which is around .9, and from what i gather should be as close to 1 (pure water) as possible.

    But then, maybe Steve isnt that picky.

    Sorry for splitting hairs guys but just My 2c

  11. #11
    Experience shows I'm no Chief Engine Builder Roundy's Avatar
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    Default Re: Can you calculate Compression ratio?

    Still shouldn't make a difference (well foam will because it won't stay as foam for ever) but if it stays all the same 50cc of water is the same as 50cc of petrol, it is volume.

    Sure, the 50cc of water will be heavier, but should still be 50cc.
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  12. #12
    ......... Carport Converter jeffro ra28's Avatar
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    Default Re: Can you calculate Compression ratio?

    Yea dude your right sorry. A cubic centimeter is just that, a cubic centimeter. Dont know why i got confused with that bs lol.

  13. #13
    I definitely ain't a Chief Engine Builder wagonist's Avatar
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    Default Re: Can you calculate Compression ratio?

    Except that this side of the planet its a cubic centimetre

    I've sent a message off to someone who may have a Gen 4 apart at the moment, so if we can get the squish volume of that head, then I should be able to find someone who knows how far down the bore the piston sits at TDC for the Gen 3, we should be in business

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