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Thread: AUTO-NERDS: T series compression ratios

  1. #1
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    AUTO-NERDS: T series compression ratios

    So after much fun discussion and bouncing ideas back and forth I decided it was time to revisit the 2tc piston dome heights on both the golfball and flat-dome style pistons.

    The golfball pistons started showing up from 1975 to 1979 with no obvious rhyme or reason as to why each car got them or didn't. the only thing I ever noticed was for some reason everyone built in 1977 got them.

    For reference Golfball on the left, flat-dome on the right. All 1971-74 corollas got the flat-dome.



    I made up a test rig by using plastic to create a water tight seal around each style of piston. This took several attempts to get right. The execution consists of filling each imaginary cylinder until the water reaches the top of each dome. I repeated the process three times with each piston to ensure accuracy. Then calculate the total volume of each created cylinder and subtracted the amount water needed to fill them. This results in a close measurement of the dome cc volume.

    The test rig:



    And the results???

    The golfball piston came in everytime with a dome volume of 20cc. The surprise here was the flat-dome with a volume of 22.9cc.

    __________________________________________________ __________________________________________________ _______________________

    Now the math. We now know the stock 2tc cars have a cylinder volume of 397cc. For this we will assume a head gasket of 1.5mm thick and a 68cc combustion chamber with an 85mm bore on the gasket. Lets correct the values.

    Golfball engine like Warrens:

    397cc displacement - 20cc dome = 377cc displacement

    3.14 (4.25x4.25) x 0.15h = 8.51cc gasket volume

    68cc - 20cc dome + 8.51cc = 56.51cc compressed area

    377cc + 56.51 divided by 56.51 = 7.67:1 compression ratio.

    Flat dome:

    397cc displacement - 22.9cc dome = 374.1cc displacement

    gasket volume same as above = 8.51cc

    68cc - 22.9cc dome + 8.51cc =53.61cc compressed area

    374.1cc + 53.61cc divided by 53.61 = 7.97:1

    This is the conventional method of calculating the cr and as you can see, toyota lied.

    Now I know for certain there are many casting variations from as low as 63cc to as high as 69mm combustion chambers so these numbers are very very variable.

    So lets discuss it!!!

    Oh and check the solution from guys like weisco: http://www.brdracing.com/images/pist...co_pistons.jpg

    Edit: with these values it makes an 85mm bore 3tc have an 8.8:1 ratio.
    Last edited by sunray; 02-19-2012 at 08:11 PM.


    Or you could have said "sorry, my kidnap victims keep kicking my elbow, it made me drop my crack pipe and swerve"

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  2. #2
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    So I assume this is part of the reason why people say they are so tuff when it comes to boost. Neat.
    BCWORKS

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    Quote Originally Posted by Warren View Post
    So I assume this is part of the reason why people say they are so tuff when it comes to boost. Neat.
    Could be but it also helps that they have a heavily reinforced bottom end when compared to just about every other japanese engine. And the hemi head helps too as detonation is far far less likely.


    Or you could have said "sorry, my kidnap victims keep kicking my elbow, it made me drop my crack pipe and swerve"

    -bangnscrape

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    Here's a question:

    Did you verify the pins are in the same place on both pistons. I imagine a couple of thou of difference might make the end numbers change somewhat.

    Are there significant squish/swirl differences between each that would make one better than the other? Piston strength?

    yeah

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    Quote Originally Posted by toastboy View Post
    Here's a question:

    Did you verify the pins are in the same place on both pistons. I imagine a couple of thou of difference might make the end numbers change somewhat.

    Are there significant squish/swirl differences between each that would make one better than the other? Piston strength?

    yeah
    The pins are identical. The flat ones have more material and appear stronger. The golfballs look like they direct the flame front better. Same size skirts.


    Or you could have said "sorry, my kidnap victims keep kicking my elbow, it made me drop my crack pipe and swerve"

    -bangnscrape

  6. #6
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    So
    The dome on my wiseco 10:1 pistons was 20cc
    89mm bore
    89mm Cometic gasket .040" thick
    And my combustion chamber was 60.3 on my last head.

    What was my comp ratio?
    My iPhone app says 11.54:1

    I guess this variance and inaccuracy is why you must measure your own parts when building any performance engine and not just use what someone told you would work.

    I toyed with making a plaster cast of the tops of my old venolia? Pistons I had in the first 3t I built.
    But thought it might be too pourous. Could make a pretty good negative cast using some fiberglass body filler, but it shrinks too when it dries.

    The hemi chamber is huge!
    I had a huge amount off my head, the 1mm over sized valves stuck out below the head surface on my last version.
    The 10:1 pistons were more like 9:1 with a stock head.

  7. #7
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    Sunray = Toyota Detective

    Even with the slight bump, Toyota still lied. I'm shocked. Nice work.
    1987 Corolla GTS Hatch | 1985 Corolla GTS Coupe | 1987 Corolla GTS Coupe | 1992 Tercel CE | 1992 Paseo | 2003 Corolla CE | 1974 Corolla 4 Door | 1978 Corolla Coupe | 1979 Corolla Hatch SR5 | <3||||||1990 Miata |||||| <3
    I like Toyota's wanna fight about it?

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    485.24cc cylinder
    6.22cc gasket
    60.3cc head
    20cc piston dome

    total combustion volume = 46.52

    10.2:1, Paul
    1987 Corolla GTS Hatch | 1985 Corolla GTS Coupe | 1987 Corolla GTS Coupe | 1992 Tercel CE | 1992 Paseo | 2003 Corolla CE | 1974 Corolla 4 Door | 1978 Corolla Coupe | 1979 Corolla Hatch SR5 | <3||||||1990 Miata |||||| <3
    I like Toyota's wanna fight about it?

  9. #9
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    Quote Originally Posted by pitobread View Post
    485.24cc cylinder
    6.22cc gasket
    60.3cc head
    20cc piston dome

    total combustion volume = 46.52

    10.2:1, Paul
    Close but you have to correct the values.

    With 89mm bore and a 20cc dome you get a volume of 465.00cc

    Gasket area is 6.34cc plus 60.3cc chamber subtract 20cc dome = 46.64 compressed area

    Volume 465 + 46.64 compressed area divided by compressed area = 10.97:1

    Paul, the reason those apps and calculators get it wrong is because they do not correct the cylinder volume by subtracting the dome cc's.


    Or you could have said "sorry, my kidnap victims keep kicking my elbow, it made me drop my crack pipe and swerve"

    -bangnscrape

  10. #10
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    More confusion, since you are subtracting the dome from cylinder volume AND from chamber volume aren't you removing an extra 20cc from the calculation? Because all the calculations are based of static numbers this makes sense to me?

    Pump analogy, if we had a flat cylinder head and domed piston, put the engine at BTDC, fill the cylinder with water and crank it over you would never get full stroke and never clear the entire volume of water. Would it not be safer to assume the piston top is flat for calculating the cylinder volume them remove/add the dome/dish from the overall combustion cc's? That seems to make sense in my head anyway. But math is not entirely my forte.
    1987 Corolla GTS Hatch | 1985 Corolla GTS Coupe | 1987 Corolla GTS Coupe | 1992 Tercel CE | 1992 Paseo | 2003 Corolla CE | 1974 Corolla 4 Door | 1978 Corolla Coupe | 1979 Corolla Hatch SR5 | <3||||||1990 Miata |||||| <3
    I like Toyota's wanna fight about it?

  11. #11
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    When the piston is at bottom dead center (bdc) the total volume of the cylinder is it total theoretical volume minus the dome as the dome reduces the effective volume.

    The combustion chamber volume is the total measured chamber volume found by cc'ing the chamber and subtracting to the dome cc volume. You then need to add the gasket volume.

    Makes sense??? You see that with the dome there the cylinder cannot displace the full cylinder volume so thats not the volume.



    I guess you could argue the point like you say using a piston dish as they test case. I guess really the dome/dish should not affect overall displacement but it must play a role. If we used your thought then the iphone app is right at 11.4:1. That figures assumes that with a dome the cumbustion chamber becomes part of the cylinder which is a reasonable assumption.

    I think what we are missing here is what paul says: Hemi chambers are huge. By contrast the 4age pentroof chambers are tiny. Chrysler knoew this in the 1960's so they made ridiculous pyramid style piston dome that reduce the final volume substantially.
    Last edited by sunray; 02-20-2012 at 06:55 PM.


    Or you could have said "sorry, my kidnap victims keep kicking my elbow, it made me drop my crack pipe and swerve"

    -bangnscrape

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    Or you could have said "sorry, my kidnap victims keep kicking my elbow, it made me drop my crack pipe and swerve"

    -bangnscrape

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    I suck at math, I don't entirely get it. Maybe I am just thinking to far into it. I have resorted to wikipedia- This space reserved for future not understanding.

    http://en.wikipedia.org/wiki/Compression_ratio

    Even my values up top don't get me my number I calculated earlier. Suck.
    1987 Corolla GTS Hatch | 1985 Corolla GTS Coupe | 1987 Corolla GTS Coupe | 1992 Tercel CE | 1992 Paseo | 2003 Corolla CE | 1974 Corolla 4 Door | 1978 Corolla Coupe | 1979 Corolla Hatch SR5 | <3||||||1990 Miata |||||| <3
    I like Toyota's wanna fight about it?

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    Nope, your way completely doesn't add up to me. I thought about it all day, it haunts me. It still seems like you are removing 20cc extra displacement from everything (and also your overall displacement, more about that in a second.

    So I am going to break it down in the dominant formulas I have found on the internetz and books. 1st one is the one in my red seal book. I am using a stock 3tc 85mm bore, 78mm stroke, 1mm 85mm headgasket, 68cc combustion chamber and 20cc dome unless otherwise specified. Stock displacement is 1770cc

    CV (combustion volume) + SV (swept volume) / CV (combustion volume)

    SV is equal to displacement (as it has to be, it is the volume the piston displaces in it's travel) To get displacement, there are 2 ways to do this.

    bore x bore x stroke x 3.1416 / cylinders OR .7854 (bore x bore) x stroke x cylinders(we only want to know 1 cylinder)
    8.5cm x 8.5cm x 7.8cm x 3.1416 / 4 = 442.6cc OR .7854 (8.5cm x 8.5cm) x 7.8cm = 442.6cc

    Both will give you the same number as shown above. So then we need to calculate our combustion volume, for that we need to figure out our gasket area using the same cylinder volume shown above. 8.5cmx8.5cmx.1x3.1416/4 =5.67cc

    Add my 68cc combustion and 5.67cc gasket and subtract the 20cc dome and I have my CV, 53.67cc.

    Complete the formula 53.67cc+442.6cc = 496.27cc/53.67cc and we get 9.24:1 CR.

    The other formula is this(essentially the same thing though) Taken from a book The step by step guide to engine building by Rick Vaughn

    CR=(c-p+g+d+v) / (c-p+g+d) c=combustion chamber volume p=piston dome volume g=head gasket volume d=deck height (not applicable) v=cylinder swept volume
    CR=(68-20+5.67+0+442.6) / (68-20+5.67+0)
    CR=(496.27) / (53.67) = 9.24:1

    Now if I do it the way you describe I do all the same above 442.6 SV and subtract my dome volume from it 442.6-20=422.6cc SV then do the equation I get a CR of 8.87:1

    However if I work back using your SV used to formulate that CR I should be able to add up to my original displacement, because SV=cylinder displacement. Add it up though and you get 1690.4cc which is not the 1770cc that it should be. Every book/article/paper I have looked at today have told me that SV is calulated the same as displacement and that they are interchangeable. And it makes sense that it really shouldn't be removed, the dome of the piston doesn't dissapear once it goes over deck height, it still travels the same and displaces the same amount as if it were flat top and bottom. If I had a cylinder with a inward dome at the bottom and a outward dome at the top of the same CC it would be the exact same displacement as one that is flat on both ends.

    If we were to flip it to a dish, and run the piston through the travel, we still would not displace the volume in the dish; it wouldnt be counted as extra cylinder displacement. Combustion volume yes, displacement no.
    Last edited by pitobread; 02-21-2012 at 09:00 PM.
    1987 Corolla GTS Hatch | 1985 Corolla GTS Coupe | 1987 Corolla GTS Coupe | 1992 Tercel CE | 1992 Paseo | 2003 Corolla CE | 1974 Corolla 4 Door | 1978 Corolla Coupe | 1979 Corolla Hatch SR5 | <3||||||1990 Miata |||||| <3
    I like Toyota's wanna fight about it?

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