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Thread: How much effect does static compression ratio have on dynamic compression?

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    Junior Member Backyard Mechanic DrNick's Avatar
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    Default How much effect does static compression ratio have on dynamic compression?

    Something that has just been on my mind today.

    Assuming that other factors stay the same, such as cam timing/overlap etc and the static compression ratio of an engine is changed, how much of an impact does this have on the dynamic compression figures?

    Also, does this directly or indirectly have an impact on exhaust gas velocity.

    Not talking about forced induction here, just in common conditions such as idling/decellerating with high manifold vacuum or WOT with close to atmospheric pressure.

    The reason why it came to mind is that my car has recently started popping/afterfiring since I reconditioned the cylinder head. All that I have changed is that I have decompressed the engine a bit down from 10:1 to 9.2:1 in preparation of my turbo conversion. Everything else is still intact. It occured to me that maybe the exhaust is exiting a bit slower and changing the dynamics of the system.

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    Balloon Slayer Backyard Mechanic jimmmayyy's Avatar
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    Default Re: How much effect does static compression ratio have on dynamic compression?

    I'd say so. The valve opening/closing events would now be slightly out with a drop in 8 pts.

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    Not known unless ur blown Backyard Mechanic
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    Default Re: How much effect does static compression ratio have on dynamic compression?

    The dynamic compression is influenced by your intake valve timing and your compression ratio.
    You original valve timing is to suit a 10:1NA motor, now that its 9.2:1 your DCR could be too low? That might cause popping?
    Do some maths with your intake closing time and new compression and find out what your new DCR is.
    Wallace racing has a decent calc.
    Remember that dynamic compression is just what your motor is actually seeing during compression stroke..
    You may have 9.2:1 currently but with a very late intake closing timing the motor could only be seeing about 7.0:1 ~ 8:1
    The earlier the intake valve closing the higher your dynamic compression ratio will be.
    A safe area to shoot for is 7.5:1 ~ 8.5:1 any more than that and you'll be seeing a motor that isn't going to last long any less and you'll be driving a total dog.
    A low DCR could be your problem, try retarding the intake cam and making the intake event a little sooner?
    Last edited by Radar; 10-01-2010 at 08:33 AM.

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    Bull now in china shop! Domestic Engineer NME308's Avatar
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    Default Re: How much effect does static compression ratio have on dynamic compression?

    Quote Originally Posted by jimmmayyy View Post
    I'd say so. The valve opening/closing events would now be slightly out with a drop in 0.8 compression pts.
    That will depend entirely on the method used to drop the compression also assuming this engine being referenced is an overhead cam design... On an overhead cam design lowering or raising the deck height (shaving the head or inserting a shim/ultra thick head gasket) will change the valve timing slightly. A pushrod design merely needs the rockers readjusted or at worst chuck in a different length set of pushrods!



    Quote Originally Posted by Radar View Post
    The dynamic compression is influenced by your intake valve timing and your compression ratio.
    You original valve timing is to suit a 10:1NA motor, now that its 9.2:1 your DCR could be too low? That might cause popping?
    Do some maths with your intake closing time and new compression and find out what your new DCR is.
    Wallace racing has a decent calc.
    Remember that dynamic compression is just what your motor is actually seeing during compression stroke..
    You may have 9.2:1 currently but with a very late intake closing timing the motor could only be seeing about 7.0:1 ~ 8:1
    The earlier the intake valve closing the higher your dynamic compression ratio will be.
    A safe area to shoot for is 7.5:1 ~ 8.5:1 any more than that and you'll be seeing a motor that isn't going to last long any less and you'll be driving a total dog.
    A low DCR could be your problem, try retarding the intake cam and making the intake event a little sooner?

    Too much reading Radar!!! Get out there into the garage and get your hands dirty!

    Quote Originally Posted by DrNick View Post
    The reason why it came to mind is that my car has recently started popping/afterfiring since I reconditioned the cylinder head. All that I have changed is that I have decompressed the engine a bit down from 10:1 to 9.2:1 in preparation of my turbo conversion. Everything else is still intact. It occured to me that maybe the exhaust is exiting a bit slower and changing the dynamics of the system.
    Not really enough info but I'll put forward a couple of suggestions anyhow!

    If the head was raised by decompression shim or extra thick head gasket it 'MAY' have altered the timing just enough to allow unburnt gasses to exit the chamber and explode in the exhaust.

    If the compression was lowered by either the above method or using pistons sitting further slightly lower then the squish area of the chamber will have been compromised - not that this bothers agricultural tuners like myself! What this will mean is the mixture may not be fully combusting in during the compression/firing event before being exited into the exhaust once again exploding there instead. More ignition timing should help in this case to ensure the mixture has sufficient time to burn fully in the chamber, meaning also better power and economy.

    Even if the compression were lowered with the recommended proceedure of properly designed pistons with larger CC dish the forces acting in the chamber will be less violent and may be resulting in a slower burn rate. As per last paragraph I recommend more ignition timing!

    Steps down from soapbox.

    Cheers,
    Jason
    3TC Compound Turbo 1976 TA23 - Members Ride Thread HERE
    479RWHP on 50psi and 70psi hasn't broken her at the track!

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    Junior Member Backyard Mechanic DrNick's Avatar
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    Default Re: How much effect does static compression ratio have on dynamic compression?

    Some real food for thought there. Thanks for the enlightening responses.

    I had totally not thought about the fact that my valve timing would be affected by the decompression. I knew there is a relation, but I didn't think it was significant.

    The engine is a 2JZGE from JZA80 Supra. I have replaced the stock 0.2mm head gasket with a 1.3mm MLS gasket to decompress. I had to deck the head by 0.15mm so the head has been raised by 0.95 to decompress. This would result in a static CR a bit higher than 9.2 actually but not much.

    I did some googling on the effect of changing deck height on valve timing but found posts which completely contradict each other. Does raising the height of the head with a thicker gasket retard or advance the valve timing?

    When I was deciding on the target compression ratio for my turbo conversion I also thought 9.2:1 (or higher) was really high, however I have seen dozens of builds in the US forums with the same setup so I decided to run with it. None of them (from memory) discuss changing the valve timing, so I wonder now if the resulting valve timing is helping to stave off detonation under boost?

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    doctor ed Conversion King ed's Avatar
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    Default Re: How much effect does static compression ratio have on dynamic compression?

    have a play with this:

    COMPRESSION RATIO CALCULATOR
    (Static and Dynamic compression ratio)
    (considers cam timing and rod ratio)

    http://www.kb-silvolite.com/calc.php?action=comp
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    Balloon Slayer Backyard Mechanic jimmmayyy's Avatar
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    Default Re: How much effect does static compression ratio have on dynamic compression?

    NME308... it is 8pts. a point by definition is 0.1, or one point.

    The problem could be adjusted out by dialling in more ignition advance, and that will work fine.

    If you were a perfectionist then adjustable cam pulleys would get the valve events closer to where they should be.

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    Bull now in china shop! Domestic Engineer NME308's Avatar
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    Default Re: How much effect does static compression ratio have on dynamic compression?

    Quote Originally Posted by jimmmayyy View Post
    NME308... it is 8pts. a point by definition is 0.1, or one point.

    In the mathmatical world maybe...

    But in the performance engine world a point of compression is a full unit i.e. 1
    To take an engine of 9.2:1 up a compression point is to end up with 10.2:1 compression.

    Cheers,
    Jason
    3TC Compound Turbo 1976 TA23 - Members Ride Thread HERE
    479RWHP on 50psi and 70psi hasn't broken her at the track!

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    Junior Member Too Much Toyota oldcorollas's Avatar
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    Default Re: How much effect does static compression ratio have on dynamic compression?

    Quote Originally Posted by DrNick View Post
    The engine is a 2JZGE from JZA80 Supra. I have replaced the stock 0.2mm head gasket with a 1.3mm MLS gasket to decompress. I had to deck the head by 0.15mm so the head has been raised by 0.95 to decompress. This would result in a static CR a bit higher than 9.2 actually but not much.
    so you're removed any squish that was assisting combustion before? or 2J head doesn't have any?
    "I'm a Teaspoon, not a mechanic"
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    doctor ed Conversion King ed's Avatar
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    Default Re: How much effect does static compression ratio have on dynamic compression?

    lowered compression to reduce detonation, and removed squish to increase detonation

    sounds sensible to me

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    Balloon Slayer Backyard Mechanic jimmmayyy's Avatar
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    Default Re: How much effect does static compression ratio have on dynamic compression?

    Quote Originally Posted by NME308 View Post
    In the mathmatical world maybe...

    But in the performance engine world a point of compression is a full unit i.e. 1
    To take an engine of 9.2:1 up a compression point is to end up with 10.2:1 compression.

    Cheers,
    Jason
    i have honestly never, ever heard it being interpreted that way..guess they do it different up your way :/

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    Junior Member Automotive Encyclopaedia
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    Default Re: How much effect does static compression ratio have on dynamic compression?

    Quote Originally Posted by DrNick View Post
    recently started popping/afterfiring since I reconditioned the cylinder head. It occured to me that maybe the exhaust is exiting a bit slower and changing the dynamics of the system.
    Did it do this Immediately after it was put back together or has it taken a while??
    Yes, the cam timing was advanced from the thicker gasket.
    What's the compression?
    If the symptom wasn't immediate, you've got some bad valves now, or the cam timing is off more then then gasket would affect.
    'I've scrapped better.' John stated when asked about the car by the guy with the silver tipped cowboy boots!

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    Junior Member Backyard Mechanic DrNick's Avatar
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    Default Re: How much effect does static compression ratio have on dynamic compression?

    This is very interesting - It only happens once in a while and has done it the whole time so I haven't got any bad valves.
    I still haven't quite got my head around this. If the cam timing is advanced now due to thicker gasket, then wouldnt this make the DCR go up rather than down? (If the intake valve is closing early) Or are we just concerned with when the valve opens rather than closing.

    Also the changes in timing would apply to both IN and EX cams equally, so why doesnt this even out and just shift the power band around instead of doing anything more than that?

    Is there a way I can calculate how many degrees of timing it has moved, is it just as simple as saying ok I have 1mm thicker head gasket so 1mm of length is equal to X degrees of cam pulley rotation and then work it out from there?

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    Not known unless ur blown Backyard Mechanic
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    Default Re: How much effect does static compression ratio have on dynamic compression?

    I just read this and figured it applied directly to you so here goes:

    "Since it is the close collision between the piston and the cylinder head that reduces the prospect of detonation, never add a shim or head gasket to lower compression on a quench head engine. If you have 10:1 with a proper quench and then add an extra .040" gasket to give 9.5:1 and .080" quench, you will create more ping at 9.5:1 than you had at 10:1. The suitable way to lower the compression is to use a dish piston."

    http://racingsecrets.com/article_racing-10.html

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    Junior Member Too Much Toyota oldcorollas's Avatar
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    Default Re: How much effect does static compression ratio have on dynamic compression?

    the 2J is not a quench head (and already have a dished piston) keep reading...
    "I'm a Teaspoon, not a mechanic"
    "There is hardly anything in the world that a man can not make a little worse and sell a little cheaper" - John Ruskin (1819 - 1900)

    AU$TRALIA... come and stay and PAY and PAY!!! The moral high horse of the world!

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