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Thread: Compression

  1. #1
    Team Member Roflhat's Avatar
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    Default Compression

    Hi,

    looking for some advice regarding the compression on my tohatsu 50.
    It was at 140psi top, 135 middle, 140 bottom, then I took it apart to modify it.
    I had 1mm removed off the head (which should raise it 30psi), but also opened up the exhaust port quite a bit.
    Did a compression test yesterday and got 140, 135 and 140 again.

    I'm going to do a squish test today.
    My question is (if the squish test shows its OK) is it safe to remove more from the head to remove compression?
    I know it's safe to run 180psi on a stock engine here with our fuel.

    Cheers

  2. #2
    Team Member smittythewelder's Avatar
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    Think about compression in two ways, static and dynamic. The compression you read on the guage when roping the motor over doesn't necessarily tell you what you want to know (octane requirements, for one thing) about the engine when running hard. Any change you make (such as port timing, exhaust pipe alterations, bigger carb) that improves cylinder filling thereby affects dynamic compression. With such changes you might see a lower static compression reading, yet find that the engine now needs higher octane fuel (and almost surely needs a change in the spark advance curve).

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    And richer mixture

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    Team Member Roflhat's Avatar
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    Thanks very much, this is the kind of information I was after.
    Timing is one part I'm not too sure about. With the mods lister (bigger carbs, exhaust, bigger ports) do I want to be advancing or retarding the timing?
    At the moment the acceleration is quite slow and it's reaching about 6300rpm top, was hoping for more.
    Cheers

  5. #5
    Team Member smittythewelder's Avatar
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    I just wrote out a long explanation of some principles of ignition timing, only to have this bleeping laptop, this steaming pile of bleeping bleep, wipe it all out for no reason. So what you want to do is Google "How does ignition timing work?," or something similar. There are a LOT of factors in establishing an ignition advance curve.

    If you have fixed timing, which is usual for racemotors that spend most of their time at WOT, and IF you are not seeing any signs of excess advance right now (such as grey flecks of "death ash" on the spark plugs)(which is aluminum from overheated pistons) then you could add some ignition advance and maybe help acceleration, if timing was the problem there. But as you do this, watch the plugs. The changes you've made do not call for advancing timing (rather the opposite, actually), but real-world testing might be the best answer for you. As you'll see if you read up on it, an engine's timing requirements vary with rpm, load, lots of things. The same is true of carburetion, which is why carburetors usually have several modes of adjustability. Your alterations affect all of this, each alteration doing something different.

    You are hardly the first guy to try making changes known to improve performance and being disappointed in the results. Countless would-be hot-rodders have spent their money on "tuned" exhaust systems, hot cams, super-duper manifolds and carbs, only to find that they aren't getting the advertised results. But this doesn't, usually, mean that they've been scammed. It's usually that they have not re-curved their ignition timing and done all of the carburetor adjustments needed to meet the new requirements of their altered engines. Unfortunately, it's a lot easier to bolt on speed parts, or even to carve on the ports of a 2-stroke, than to re-tune an engine for the new combination. I wish I were better at it. Factory engineers spend hundreds of hours on dynos and test-tracks getting the right settings for production engines. Any alterations you make to a production engine means all of that factory tuning is out the window, and you're on your own for dialing in your new best settings.

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    Sam Cullis Mark75H's Avatar
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    Exactly correct.


    With direct regard to this project, additional tightening of the squish might increase turbulence and increase combustion speed needing later ignition, or it might cool everything down and call for earlier ignition.

    You will have to test it yourself and figure out which combination you have dialed in.
    Since 1925, about 150 different racing outboards have been made.


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    Team Member smittythewelder's Avatar
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    [QUOTE=Roflhat;141105]
    I'm going to do a squish test today.
    My question is (if the squish test shows its OK) is it safe to remove more from the head to remove compression? (end quote)

    Not sure what you're saying here. If you shave the deck surface of the head to get a tighter squish-height, you'll also increase the compression a little by reducing volume a little. IIRC, there's a good description of squish in the tech section of speedomotive.com. Generally, the tighter you make the squish, short of the piston contacting the head, the more effective it is in quelling detonation, even though you are increasing compression at the same time. Somebody who knows your engine, and how high you want to rev it, can advise you on how tight to get the squish. As a wild guess, which you better confirm with someone who knows the engine, I'd guess you'd be safe enough, meaning you shouldn't have the piston hitting the head, at about a measured one millimeter. I take it you know how to measure this with a length of soft solder and a micrometer . . . ??

    Having a tight, therefore effective, squish-height is always a good thing so far as I know. If you find that your total combination of alterations of the engine, any engine, are giving you detonation on the fuel you want to use regardless of your tuning efforts, one thing you shouldn't do is reduce compression by means of a thick head gasket or doubled head gasket. This will certainly reduce compression, but it will also raise the squish-height, and doing that might even cause more detonation. So get the squish-height right and leave it alone, and if you have to reduce compression, do it by taking material out of the middle part of the combustion chamber. Actually, when you start a hop-up project like this, probably the first thing to do is get the squish-height down to a known good dimension, and then start experimenting with the intake side, the exhaust system, and the porting (or cam and valves in a 4-stroke), re-tuning as you go. Tightening down on the compression is about the last change to make, again because all those other changes (except squish) improve cylinder filling and dynamic compression.

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    Interesting question. Take Merc 850 (fours): The book calls for 27 degrees max advance, which is a ton more than for the similar cylindered triples and in-line sixes. Further investigation reveals that the early 850s (with the 4 exhaust ports) had a higher compression ratio; and one doesn't hear of detonation problems with them (vs. the in-line sixes with only 21 degrees max advance).

    Hope that helps.

    Jeff
    "We live at the bottom of an ocean of air." - General Marvage Slatington

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    Team Member smittythewelder's Avatar
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    Had to come back to this. Because I don't know you or what you already know about all of this, Roflhat, some clarification: Not all piston engines have a squishband/squish-pad/squish-area; probably more don't than do. The squish (Brit. "quench") combustion chamber was the invention of the great WW1-era English engineer Harry Ricardo. It was so effective in preventing detonation that engine designers were able to nearly double the then-very-low compression ratios needed to run on the very low octane fuels of the day. The resulting boost in fuel efficiency and power was so helpful to the military that Ricardo was knighted, and soon variations on Ricardo's squish heads came into worldwide use, especially in cars. This situation pertained up to the advent of emissions controls in the late-1960s. Unfortunately, squish heads produced a small amount of unburned hydrocarbons, and the high compression and high temperature combustion permitted by squish heads also produced excessive nitrogen oxides. So the manufacturers' new smog-motors generally went from "closed chamber" to "open chamber" heads with little or no squish, lowered compression and consequent loss of power and fuel efficiency. By the 1980's the manufacturers were able to get most of this back, but not with squish. So after this long-winded clarification, the question is, does whatever motor you're working on really have a squish area at all?

    So, are you Scots going to split or what?

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    Team Member Roflhat's Avatar
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    thanks for the help, lots of good reading here. I really don't know much about this at all, just trying to understand before I go off and make a mess of something.
    I've made a thread for all the work I've done on the engine - http://www.boatracingfacts.com/forum...-My-Thundercat
    Just to give you an idea of how everything looks at the minute.

    I know how to measure the squish I think, I tried it today with some solder but the thickest stuff we had was 1.15mm. So I tried some plastocene around the solder and it looks like there is 3mm? Although this seems like too large a value to me.

    The engine seems to be running really well at the minute, the timing link and throttle link are bang on (for a stock engine at least). It's idling well and running smoothly, I'd just like another 1000 or so RPM up top...

    Thanks for the help so far, great learning here too!

    I'm for independance, not sure which way it's going to go yet though, have to wait and see.

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