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  1. Detonation 101 
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    Detonation 101

    Detonation or "knocking" arises from the production of an explosion wave in the combustion chamber. This is due to rapid ignition and combustion. A portion of the unburnt air/fuel mixture becomes compressed prematurely, which in turn rapidly increases in temperature until it ignites uncontrollably and almost instantaneously. The shock wave produced has a characteristic of a metallic sound and can vary in magnitude. It can range from a mild form, occurring only at relatively low speed and wide throttle positions (typically known as "pining") to such violent effects that the engine is no longer running in a controlled manner and power output dies.

    Detailed view of detonation

    Now that we know the definition of detonation we need to comprehend that if the temperature of an air/fuel mixture is raised to high, the mixture will eventually explode spontaneously. This is known as spontaneous ignition temperature. But, before this explosion there is an interval called ignition time-lag. If the piston approaching tdc of the combustion stroke takes longer than this period before the spark plug ignites there will be a premature explosion. The longer the time-lag the less chance of spontaneous there is of spontaneous ignition. If the air/fuel mixture is fired at the correct "timed" point and a good flame is achieved power loss through detonation is still possible. The burnt mixture behind the flame front will be at a high temperature and pressure, which will cause compression of the unburnt gas ahead of the spontaneous ignition point. The flame front must move right through the unburnt charge before the end of the ignition time-lag period if not this can lead to overheating and excessive mechanical stressing.

    Fuel choice and detonation


    With any fuel the brake thermal efficiency 1 will decrease if the compression ratio is increased. A properly chosen fuel can help diminish detonation. The advances in modern day fuels have extended their ignition time-lag periods and have decreased the spontaneous ignition temperatures, which allows for the use of a higher compression ratio (safely up to 11:1) to be used in the combustion process under complete control. There are also special fuels made especially for racing applications that allow compression ratios so high that the limit is actually dependent upon mechanical aspects of the cylinder head design.

    Summary
    The benefit of increasing compression ratio is to increase the torque output of an engine. The harm in raising the compression ratio is that cylinder pressures increases in a linear fashion as compression ratio is increased. An 8:1 compression ratio yields compression pressures of about 275 psi, where 15:1 yields compression pressure of about 575 psi. At the time of ignition the pressures will rise 3 or 4 times the compression pressure. Also, when an engine has been modified where even a slight increase in torque is achieved it is important that detonation is not allowed. The noise of detonation of a high-compression ratio engine using the correct grade of fuel is much harder to detect than that of an engine with a much lower compression ratio burning lower octane fuels. 1 Thermal Efficiency -Thermal efficiency is the measure of the efficiency and completeness of combustion of the fuel, or, more specifically, the ratio of the output or work done by the working substance in the cylinder in a given time to the input or heat energy of the fuel supplied during the same time.

    Author: Dennis Adams
     
     

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    Hers a nice addition to this theory by Jim Conforti...

    one of the 'legends' of BMW DME tuning (conservative tunes mind you!)



    Info by Jim Conforti

    What you do depends on your goals.

    Do you intend to make it a daily driver?

    Do you have emissions "issues" to contend with?

    If the answer is yes, go with a Twin Screw.

    Do a few other mods to your brakes, clutch, and suspension.

    (and take the other 5k, and invest it until you want to go to
    more boost later on)

    The most important thing to remember is "EFFECTIVE compression
    ratio". Above ~18.0:1, you'd better be on race fuels.

    (it's a rule-of-thumb, but a good one to remember)

    My personal like is to keep it under 17:1 or so.

    Let's take a std 8-8.5# TS system on a S52B32.

    10.5:1 CR = 16.57 ECR, 420bhp, 345 rwhp

    Raise the boost to 10.5# and you are pushing that 18.0:1 ECR

    If you wanted to run, say 15psi, the approx max of the 1.7 TS
    on a 3.2 you should drop the CR to 8.5:1 using forged
    pistons and this nets an ECR of 17.17:1

    Putting my trusty HP48SX to work says 535bhp/440 rwhp
    on this otherwise stock motor (which we wouldn't do anyway!)

    Above, BillP mentions 25psi. Well, notwithstanding that boost
    level is unrealistic, let's look at the numbers.

    Assuming you can get the same efficiencies at 25psi
    as you can at 10.5psi, and that's one of those big, benny-hill-type
    assumptions, you have at 25psi:

    CR = 6.3:1, ECR = 17.01:1

    CR = 8.0:1, ECR = 21.61:1

    BTW, at 6.3:1 CR, assuming you can get such a motor to start
    and run with the reduced volatility street fuels we have today,
    you'll produce: 705bhp/580rwhp.

    And thus, Bill demonstrates "reductio ad absurdum".



    Which is why when you reach a boost level, the next thing you
    do is to increase the efficiency of the motor to increase the
    power BEFORE you increase boost.

    This also gives you time (since you're in there anyway) to decide
    to change pistons/connrods and then go for decreased CR and
    more boost.

    How would I stage it, if it were my $$??

    1) TS, 8#, stock 10.5:1 CR, 420/345hp (ECR = 16.57:1)

    2) TS, 10#, stock 10.5:1 CR, some breathing/exh mods, 500/415hp (ECR = 17.64:1)

    then if you really want to go faster still.

    3) Beef up motor , CR = 8.5:1, TS, 15#, same breathing/exh mods
    and produce 605/500 (ECR = 17.17)

    at which point you will have so far exceeded the power handling
    of the chassis/brakes/fittable tires/etc that it isn't funny.

    But there are the numbers.

    FWIW, the larger TS blowers will produce MUCH more boost,
    with Jim Bell of Kenne-Bell producing over 600rwhp on a Cobra
    and that Opcon blower can produce from 8# thru 22# on
    that application. Note that the higher boosts are all on racefuel.

    The only problem? They won't fit in a BMW L6 powered 3 series.

    You have the hood, starter, engine block, shock tower, firewall
    and oil filter to contend with for starters

    Considering you have a range of 345-500 rwhp to choose from
    with essentially the same hardware, along with differing "other"
    mods, I don't see it as a problem.

    Hope this helps you.

    FWIW2, this holds for any FI system with good efficiencies and
    a very efficient intercooler."
    JMC
     
     

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    and a very nifty site that will calculate your effective compression ratio if you enter in a few simple figures to its calculator

    Psi of boost, current Compression Ratio, Altitude (*if known or relevant)

    Calculator
     
     

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