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Thread: Base Gasket's effects on compression

  1. #1
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    Base Gasket's effects on compression

    200E engine

    My search skills suck donkey. I can't find squat on the subject.
    Obviously deleting the base gasket can bump compression. It's done in chainsaws all the time.

    Does anyone know by how much in reference to the ATC200 engines?
    The base gasket that came with the kit is .019 thick. It's getting a new 10.25:1 Wiseco and I'd like to get it closer to 11:1.
    12:1 would require special fuel and play hell on the pull rope. I should probably leave the 10.25:1 alone...but you know me. I always gotta be F'ing with stuff.
    I think I have some .010 thickness gasket material somewhere. Thought about splitting the difference and making a gasket.

    I don't want it any more than 11:1. 92 octane non-ethanol is the best fuel I can feasibly get around this podunk hole.
    I was born and raised on Venus & I may be here a while.....

  2. #2
    barnett468 is offline FACT ! I have no edit button Arm chair racerThe day begins with 3WW
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    Quote Originally Posted by Gabriel View Post
    200E engine

    My search skills suck donkey. I can't find squat on the subject.
    Obviously deleting the base gasket can bump compression. It's done in chainsaws all the time.

    Does anyone know by how much in reference to the ATC200 engines?
    The base gasket that came with the kit is .019 thick. It's getting a new 10.25:1 Wiseco and I'd like to get it closer to 11:1.
    12:1 would require special fuel and play hell on the pull rope. I should probably leave the 10.25:1 alone...but you know me. I always gotta be F'ing with stuff.
    I think I have some .010 thickness gasket material somewhere. Thought about splitting the difference and making a gasket.

    I don't want it any more than 11:1. 92 octane non-ethanol is the best fuel I can feasibly get around this podunk hole.

    Using a thinner base gasket will not increase the compression by much, and if you want to increase it by .75 you will likely have to loose around 1 mm of thickness somewhere which isn't practical, but if you want to know almost exactly how much it will change it by using a thinner gasket, you can do the following. Also, lowering the head will change the cam timing slightly.

    Measure the volume of the combustion chamber in cc's.

    Determine the volume of the high compression piston, but that info should be in the spec sheet.

    Measure the thickness of the head gasket.

    Get the bore and the stroke numbers.

    You can then do a google search for compression calculators like the one below that have an option for 1 cylinder and for metric measurements.

    Enter all the info it requests to get the static/uncorrected compression. This will be very close to the actual static/uncorrected compression, however, the piston ring height is another factor that can be used to get a slightly more accurate measurement which is unnecessary for your app.

    https://speedmaster79.com/tools/engi...ression-ratio/

    After you have the compression ratio, simply change the head gasket thickness value by the amount you are changing the base gasket thickness.

    EXAMPLE - Subtract the thin base gasket thickness from the thick base gasket thickness, then subtract this number from the head gasket thickness you entered in the calculator.

    If you use a calculator that asks for "deck height", you can use 0 unless you have measured what it is.




    .

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    barnett468 is offline FACT ! I have no edit button Arm chair racerThe day begins with 3WW
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    Quote Originally Posted by Gabriel View Post
    I don't want it any more than 11:1. 92 octane non-ethanol is the best fuel I can feasibly get around this podunk hole.
    You can use octane boosters to help some if you want. The best one is probably TEL. It is real lead additive so you might kill off a few fish or other animals if you use enough of it.

  4. #4
    barnett468 is offline FACT ! I have no edit button Arm chair racerThe day begins with 3WW
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    POST CORRECTION POST CUZ I AIN'T GOT NO EDIT BUTTON

    "You can then do a google search for compression calculators like the one below that have an option for 1 cylinder and for metric measurements."

    Should read:

    "You can then do a google search for compression calculators like the one below that have an option for metric measurements."


    The number of cylinders is irrelevant for determining compression ratios, and is only for determining engine displacement, therefore, you can choose any option they provide in this space for your app.

    .

  5. #5
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    Get a standard CRF230 3 piece head gasket. Seperate the layers and use one of the thinner layers as the head gasket. Throw the thick centre piece away. You end up with more compression and a spare gasket.

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    Seems this subject got covered well. Also yea, since it's an over head style cam setup, the timing would be modified slightly. I've seen people use the cam gears on cars that are adjustable to offset the change, but I doubt it's enough to bother with that much.

    Also it's a good note, in the chain saw world, the timing is also effected by dropping the cylinder down, the port timing also lowers. Along with a base gasket delete, a port job is in order to at least bring it back to factory specs. The one saw my dad built (base gasket delete + porting) is something like 210psi compression and is a bit of a bear to start, but it really rips the wood too. If I recall correctly, that machine he had to sand down the edge of the piston slightly because it touched the dome of the top of the cylinder.

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    Scootertrash is offline Just Too Addicted: Protecting Our Community The day begins with 3WW
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    Wouldn't shaving the head accomplish the same thing? Or would it decrease the cc volume of the head too much?
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    Quote Originally Posted by ps2fixer View Post
    since it's an over head style cam setup, the timing would be modified slightly. I've seen people use the cam gears on cars that are adjustable to offset the change, but I doubt it's enough to bother with that much.
    You might be surprised how much it can effect the way an engine runs. Besides the effect of any raising or lowering of the head, due to the thickness of the head or base gasket, the factory tolerances for the camshaft, the height of the cylinder, the head mating surface to the centerline of the cam journals, the valve lash, the timing gears, the cam chain and the adjustment of the cam chain all have an effect on the timing.

    The only engines I bother to time are 4 cylinder bikes, but that’s only because slotted cam gears are easily acquired for them. It’s nothing to find the stock timing off by 6 degrees on a mid 80’s In-line four.

    Gabriel, I’ll go out in a limb and say that there is probably more overall power to be gained by slotting a cam gear and having your timing set to the factory recommendation than by increasing your compression and you won’t need to worry about running pump gas.

    If you do decide to remove a gasket and run your engine like some Rotax and Ducati’s do, you’d better make sure the surfaces are lapped and/or sealed and that if you use sealant not to plug any oil galleries.

    Providing your engine turns counterclockwise when you look at it from the left, any reduction to the distance between you crankshaft and your camshaft will advance your cam timing which typically results in more low end power and less top end power. So if your goal is to get more top end rpm from your engine by raising your compression you would need to retard you cam timing after reducing the height of the head or cylinder.
    Last edited by El Camexican; 03-16-2019 at 09:45 AM.
    It sucks to get old

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    Yeah, I've thought about this at length and have decided to put it back the way it's supposed to be.

    Funny you mention slotting the cam gear. I just finished making my needle pointer attachment for a degree wheel yesterday. I may or may not play with cam timing BUT I will see where it is before I finish assembly. Somewhere around 5 degrees cam advance gives more bottom end but suffers on top end. This engine will likely never see much use beyond 4500 rpm so If it doesn't prove too much trouble, that MAY be what I do. Compression can also be affected by cam timing so there's that too. I have much to read before I do any of this though/
    I was born and raised on Venus & I may be here a while.....

  10. #10
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    Quote Originally Posted by Gabriel View Post
    Somewhere around 5 degrees cam advance gives more bottom end but suffers on top end./
    I'd be inclined to say 2 degrees is enough to start noticing a difference.

    Quote Originally Posted by Gabriel View Post
    Compression can also be affected by cam timing so there's that too./
    Moving around the timing of a given cam shouldn't effect the measured compression, but changing the duration of a cam will. The only way to change duration is to change out the existing cam for one with a different duration, or adjust your valve lash. A looser valve lash will retard the timing as well decrease lift and duration and a tighter valve lash will advance the timing as well increase lift and duration, but don't expect to notice a change in your seat of the pants dyno from such an a adjustment. Those kinds of small tweaks need controlled dynos pulls, or well documented 1/4 miles passes to be seen.

    Quote Originally Posted by Gabriel View Post
    I have much to read before I do any of this though/
    Have fun, it's almost as confusing as 2 stroke timing, but at least when you're messing with cams instead of ports you can undo your mistakes without having to buy a new cylinder.
    It sucks to get old

  11. #11
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    Quote Originally Posted by El Camexican View Post

    Have fun, it's almost as confusing as 2 stroke timing, but at least when you're messing with cams instead of ports you can undo your mistakes without having to buy a new cylinder.
    I thought I wanted to learn port timing a while back. Turns out...I didn't. Not wanting to ruin anything of value I tried it with a couple of Ryobi weedeaters. Deleted bas gaskets and monkey'd with the ports. They're both in the landfill. SURPRISE.
    I love it when people say 2 cycle is simple because they don't have valves. Morons.
    I was born and raised on Venus & I may be here a while.....

  12. #12
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    2 cycles are simple, but the valves are replaced with ports xD. The biggest problem I suspect most new people make on porting is going too far, or doing too much. Like when my dad ports a saw, first thing he does is looks for well known saw builders that publish their numbers. If he can find numbers, he follows them and so far has had great results. If not, he takes measurements before mods, then deletes the base gasket etc and adjusts the ports to match factory + widen them slightly and smooth the path out a little. Dolmar's are pretty good for their transfer ports, so generally he doesn't touch them. I know at least one of his saws he had to bore the hole the gas comes into the carb a little bigger because he had a fuel starvation problem after porting and I guess it was a known issue with that carb.

    In the 4 stroke world, it's probably best to just cleaning up casting marks, port matching bolt on parts (intake etc) and not try to really change sizes etc unless you know what you're doing.

    When learning about porting, it's best to start with throw away engines, since it's likely you'll mess up. I think the first machine my dad ported was a leaf blower. If I recall correctly he went a little bit more than he should have, but it still rev'ed out better under load than stock. Also worth pointing out, you need to start with a healthy engine or the porting is kind of pointless. While it's apart, check if the ring is worn, cylinder health, wear on piston etc. It's fairly common to remove the 2nd ring on chain saws due to their nature of normally being in the high rpm range when in use, so less drag and a tiny bit of compression loss nets to more power in their case. Low rpm like a 4 stroke probably wouldn't benefit from it though. Also don't use low quality cheap crap rings on ebay, get ones of a name brand that's known to at least be fairly good quality.

    Anyway, the question about about base gasket deleting vs shaving the head is kind of spot on, the end result is more or less the same, however removing a gasket and using a sealer vs paying a machine shop to mill the head down, the cheaper option is remove the gasket. Less control over how much the cylinder is dropped with that option though.


    About the timing of the cam vs dropping the cylinder, I'd be interested in the numbers, like say the 200 series engine has the cylinder drop 10 thousands of an inch, how much does that effect the timing of the cam? I'd figure it would be something like 0.1 degrees or something like that, but maybe it's a larger effect than I'm thinking? Don't really have the time to try to figure out how to calculate it, but would be kind of a neat thing to know the formula of. Maybe a calc exists online, I just didn't see one in a quick search though. I suspect cam gear diameter, crank gear diameter, distance apart, and amount it drops by would be the main details needed.

  13. #13
    barnett468 is offline FACT ! I have no edit button Arm chair racerThe day begins with 3WW
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    Quote Originally Posted by glamy View Post
    What kind of farming are you gonna use it for Gabe
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    barnett468 is offline FACT ! I have no edit button Arm chair racerThe day begins with 3WW
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    Quote Originally Posted by Scootertrash View Post
    Wouldn't shaving the head accomplish the same thing?

    Or would it decrease the cc volume of the head too much?
    Yes, "shaving" the head will have the same effect.

    You can "shave" as little as around .0015" from a head, and this will change the compression less that using a head or base gasket that is .010" thinner.

  15. #15
    barnett468 is offline FACT ! I have no edit button Arm chair racerThe day begins with 3WW
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    "less that using"

    should be:

    "less then using"

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