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Thread: How do i lower the compression on an M30 ?

  1. #1
    Join Date
    Jan 2004
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    128

    Default How do i lower the compression on an M30 ?

    Hi all,

    As a sort of a follow on from an earlier thread i posted about putting a supercharger on my 535....

    I was wondering if any one know's of any piston's or con rod's from anouther M30 that will reduce my comp' ratio ?

    There's lot's of capacity variation's on the M30 over the year's(2.5,2.8,3.0,3.2,3.3,3.5) this was achieved through various stroke and bore permutations.

    But which combination whould make the piston sit lower in the bore at TDC ?

    If the piston sat say 3 mm lower in the bore at TDC than the standard set up then how much whould this lower the comp' by ?

    I suppose the ideal set up whould be the piston's and con rod's from the Alpina twin turbo,as this whould lower the comp'(7.2:1 ? V's 9.0:1 as standard) and be tough enough to handle some serious power with out failing.But can you imagine how epensive they whould be ? $$$$$$$$££££££££

    The chap in the following link( http://www.cardomain.com/id/pphilpot08 )has what sound's like the Alpina set up with his turbo'd M30 that he's running 20 psi of boost that he's estimated to be producing 700 bhp !!!!

    Thank's for your time and valued input guys

    Andrew

  2. #2
    Join Date
    Feb 2004
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    4,894

    Default

    Mmm... Normally you would not need to lower compression when using low pressure supercharger. I remember about this long time ago about those supercharged and turbocharged 535i... can't remember who did it... famous company.

    To lower compression ratio, you would need shorter connecting rod or a different crankshaft. However, to do high pressure supercharging or turbocharging, you need stronger crankshaft, connecting rod, and piston. This gets into $$$$. JustinRed5 would have alot more experience in this matter.

  3. #3
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    Jan 2004
    Location
    NY
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    478

    Default

    Have an engine shop redish your pistons, or have new ones custom made. To be honest, there's really no need to lower the comp ratio even if you plan on turbocharging your engine, unless you plan on really ridiculous amounts of pressure.

    Whatever you do, don't use a thicker headgasket- this is the worst way to go about it.

    Custom Turbocharged 1991 535iM - 135k mi.

  4. #4
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    Default

    well my 91 535i has a 9.0:1 which is extremely low for a stock motor.
    u can install a thicker head gasket...but for 10 psi of boost 9:1 should
    be fine. so long as u don't live on teh sun =]

  5. #5
    Join Date
    Jan 2004
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    1,171

    Default don't know how mathmatical you are but here is...

    a derivation I posted some time ago on how much compression is changed by milling the head of an M-30 engine and maintaining the same head gasket thickness. The following is based upon certain boundary conditions and therefore is theoretical but can be adjusted to back out compression difference based upon different piston height/stroke change etc as compression is simply a value X [how many atm or bar/14.7 psi.]
    Since bore size wouldn't change for your calculation, and compression/pressure is proportional to stroke or piston height, it is pretty easy to back out the relationship if you care to. As a sidebar, you can crib a bit off of the 3800 V6 motor that GM offers in both NA and supercharged form. If you have driven a Pontiac GTP for example with roots style Eaton supercharger...they are quite fast at least to 120 mph or so. GM does opt to change the pistons to sag the compression for the supercharged v6 which max runs about 8 psi of boost....I believe a good practice for any blow motor if you want longevity. With a supercharger the tradeoff isn't as pronounced as with lowering the compression on a turbo motor because prior to spool with a turbo you are running on low compression. With a supercharger, boost occurs at a lower RPM hence more low end torque with a lower compression motor.

    In any event the following provides a guideline if you want to take the dive and calculate the relationship between compression and stroke and/or piston height reducing combustion chamber cc's:

    Solving for M-30 compression ratio using Boyle's Law:

    You can back out the equation combustion chamber (cc) volume knowing the bore X stroke of the big six based upon known compression ratio as follows:
    P1 x V1 = P2 x V2
    P1 = std atmospheric pressure = 1 atm = 14.7 lbf/in2
    V1 = Pi x B(Sq)/4 x S where Pi~3.1416 B=bore = 3.62 in
    S=Stroke = 3.38 (the big six is wonderfully oversquare!)
    P2 = 9:1 compression = 9 x 14.7 lbf/in2 = 132.3 lbf/in2
    V2 = combustion chamber size
    Solving for combustion chamber size V2 which is 9 x smaller than bore X stroke per Boyles law:
    V2 = P1 x V1 /P2
    plugging in quantities above:
    V2 = V1 / 9 = 34.78 / 9 = 3.87 in3 (combustion chamber size)
    Pi x B(Sq)/4 X H, (where H = combustion chamber Height) = V2
    solving for combustion chamber height...assuming a cylindrical
    combustion chamber shape..could also assume a wedge or hemispherical combustion chamber:
    H= V2 / piston surface area:Pi x B(Sq)/4
    we now know V2= combustion chamber volume and piston surface area so we can now solve for combustion chamber Height H:
    H= .38 inches
    we now know the height of the combustion chamber and what you want to calculate is the change or delta in combustion chamber height..will call it H'...due to milling the head .012" so:
    H'=.38 - .012 = .368" (new combustion chamber height with milled
    head)
    OK...now you know everything to solve for "change in compression" due to milling the head .012". Employing Boyle's Law once again:
    but first you have to solve for new combustion chamber volume due to milling the head..will call it V2'
    V2'= .368/.38 X 3.87 in3 = 3.75 in3
    therefore using Boyle's Law once again to solve for new compression:
    P1 x V1 = P2' x V2'
    where:
    P1 = 14.7 lbf/in2
    V1 = 34.78 in3
    P2' = new compression
    V2' = combustion chamber size with milled head = 3.75 in3
    solving for P2':
    P2' = 14.7 x 34.78 / 3.75
    P2'= 136.3 lbf/in3
    to convert to unitless compression ratio, divide by 1 atm = 14.7 psi:
    P2' therefore = 9.27 : 1 compression ratio
    In summary, if you mill the head on a big six by .012" you will increase the compression ratio approx. 0.27 of a point.

  6. #6
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    Dec 2003
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    I think the best think you can to do is find the block from a 745 turbo. M30 based but came with 8:1 forged pistons, oil sprayer in the crank, etc.

    BTW, Pat P. sometimes drops by here - he's the guy with the serious turbo setups.

    Jeff
    Bellevue WA
    90 535iM - not much stock remains. 3.7 liters, ported head, cammed, 3.73 diffy, M5 brakes, MAFed, yadda yadda yadda
    86 Porsche 951 - Track Toy

  7. #7
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    Default he has a SEROUS e34 m30 turbo too

    BTW, Pat P. sometimes drops by here - he's the guy with the serious turbo setups
    same dude as in the car domain link
    Gone but not forgotten

  8. #8
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    take the plugs out for proper position and then drill a small hole through the piston, that should suffice.
    95 E34 530I V2.37
    ===========
    Those who make peaceful revolution impossible will make violent revolution inevitable.

    John F. Kennedy

  9. #9
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    Default

    or you could drill a hole through the head and cap it off with a valve, then you could fine tune the amount of compression, without all of the engineering expence of the varible compression ratio motor saab has been devloping for the last few years

    Quote Originally Posted by 632 Regal
    take the plugs out for proper position and then drill a small hole through the piston, that should suffice.
    all america wants is cold beer warm cat and a place to take a poop with a door on it

  10. #10
    Join Date
    Jan 2004
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    Default

    Thank you every one for your reply's !

    From your ideas i've got a few option's that need further investigation.

    If i use a fairly light boost pressure of say 6-8 psi then i should be able to stay with the standard compression ratio(CR).But judgeing by the feed back i got from Rob (in the previous thread i started about super charger's(SC)).He had a SC M30 with a similar level of boost but he still had a head gasket go in a big way on his motor. So may be for piece of mind/longevity sake i should lower the CR from the start.Then later if i want to run a higher boost level i've less chance of it going bang !

    I like the idea of having my piston's milled out as this will lower the CR with out breaking the bank.I think i'm going to get a 2nd hand short engine so i can have a machine shop look at the piston's to see if they can be milled out safely or not.I will also have the bottom end/piston's/bore's/bearing's etc reconditioned and balanced.

    Thank's George M for your technical post on CR,it must have took a fair bit of time and effort to write.Though i must admit that certain part's of it are a bit beyond my math's ability to understand but i've printed it out and hope it will be more clear if i re-read it a couple of time's.

    I'll have to find out some more info on 745's as i don't know any thing about them.

    As for drilling either the piston's or the head......

    I understand the principle's as to how it whould lower the CR but i don't think this is an easily realised solution to lower the CR and to get it to work properly.

    So a few thing's for me to think about and some other stuf i need to reserch(745's) before i make any commitmant's.

    Please keep the ideas and advice comming

    Thank's

    Andrew

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