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  1. #1
    Herman Singh
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    Why Lower Compression??

    I was reading thru some posts and I saw that someone put in lower compression pistons. Why would you do that? Isn't that just robbing power from you? Less compression = smaller stroke, n'est pas?


  2. #2
    Bill Bradley
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    Re: Why Lower Compression??

       Most of the "old school" turbos used a lower CR (2002 Turbo 6.6:1, 1975 Porsche 911 6.5:1, Saab 99 7.2:1) to prevent detontation (no intercooler means a hot charge) and to limit the dynamic compression ratio under boost (7:1 running at 1 bar of boost would effectively be 14:1). These were the turbos famed for their lag and on/off characteristics. Little power at lower rpms, then a rush.

       Modern systems (with better fuel, ignition and boost control) run higher CRs and usually more modest boost and have very flat torque curves (Torque is directly related to cylinder pressure Torque=(cylinder pressure)x(piston area)x(torque arm) ). You get better off boost power and the turbo allows you to pretend like intake restrictions don't exist. On NA cars the restrictions show up as a lower manifold pressure, with a turbo you can have the same manifold pressure at 8,000rpm as 3,000rpm if the turbo is properly sized.

  3. #3
    Bill Bradley
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    and eventually getting to my point...

       No, lower compression pistons do not mean less stroke (it's just a greater volume at maximum compression).

       One might want lower compression pistons to run higher boost, but that's often a "mine's bigger than yours" issue rather than actual performance. With proper engine management Porsche runs 9.4:1 with 0.8bar(about 11.5psi) of boost, Saab runs up to 1.4bar (20psi) on 9.3:1 on pump premium gas. If the engine was originally higher compression than that there might be cause to drop it.

  4. #4
    Herman Singh
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    Re: Why Lower Compression??

    okay, i see what you are saying, but performance wise how would a low CR high boost enging compare to a high CR low boost engine.... hypothetically saying they are evenly matched in compression at TDC.

  5. #5
    Bill Bradley
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    Re: Why Lower Compression??

        All other things being equal they would be the same. Say you have a 9:1 engine with 0.5bar of boost (1.5 atmospheres at the manifold) you'd have an effective compression ratio of 9*1.5=13.5:1. To get the same ratio with a an 7:1 setup you'd need 0.93 bar of boost 7*1.93=13.5:1. If the air was the same temperature (which depends on the compressor efficiency and usually favors the lower boost) you'd make the same power.

       The drawback is that you'll make less power off boost (and take longer to spin up the turbo) with the lower CR and your fuel economy will be worse. While this setup can be fun (the wait..wait..wait..HOLD ON feeling) it actually tends to be slower than high CR route unless you're drag racing or a formula 1 car (higher boost is better at high rpm when cylinder filling becomes an issue)

  6. #6
    John N
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    Bill, you're missing the heating effect of

    compressing air. That ~0.5 bar boost differential is major delta in the intake charge temperature (sorry I can't find my Corkey Bell book).

    It's a little oversimplified, but I think the short form answer is that a higher CR boosted engine will make the same HP as a lower CR engine boosted engine (as per your formula), and it will run better because the intake charge is cooler.


    Cheers!!

    John N

  7. #7
    Jim Derrig 93 750
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    Re: and eventually getting to my point...

    I agree with everything so far (including the correction), but might add that in addition to mimicing a higher compression ratio (but with heat delta as noted), forced induction also mimics greater displacement by placing a denser air/fuel charge is a fixed volume of space.

    Thus, it isn't accurate to think that a 2.0 liter NA engine with a high compression ratio will be as equally POWERFUL as a 2.0 liter forced induction engine running boost equivalent to the compression ratio--the fi engine will be "compressing" a greater charge and should be able to produce more power [all other things being equal, which, of course, they never are]. Put another way, to get your NA engine to have the equivalent of fi, you'd have to increase CR AND increase displacement.

  8. #8
    Bill Bradley
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    Re: and eventually getting to my point...

        Sorry, but Physics says otherwise. Work=delta-PV (the change in pressure times volume) So one atmosphere at a 15:1CR will give _exactly_ the same power as 2 atmospheres at 7.5:1CR. (The NA engine however wins big on Carnot efficiency)
       The "trick" with the turbo (and why it will make more power across a wider range in real actual practice) is that it is easier to get a high Volumetric Efficiency (VE) with a turbo since the maximum pressure you can get across the intake valve of a NA engine is 14.7psi, while a forced induction system can have any amount of pressure differential (just ramp up the boost pressure). Higher VE is why Porsche, BMW, et. al. and other go to things like multivalve heads, variable valve timing & lift, variable intake runner length and variable back-pressure exhaust and can in fact achieve very "turbo-like" performance, but I suspect a Turbo or Super is an easier retrofit :-)

  9. #9
    Bill Bradley
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    Oops, not exactly the same

    In my example the NA engine would actually make about 7% more power since both would have the same P1 (15*14.7psi=7.5*29.4psi) but the NA would have a lower P2 (14.7psi vs. 29.4psi) for a greater delta-P. I am making the assumption that it is an adiabatic cycle but so do most models.

  10. #10
    Jim Derrig 93 750
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    Re: and eventually getting to my point...

    Not an engineer, and after I posted I started thinking along the same lines as your correction, but thought I had accounted for an implicit assumption in your line of reasoning--which is that the we are always dealing with the same amount of potential energy in the air/fuel charge.

    Where I got hung up is with the concept that with an NA engine, the compression occurs AFTER the fuel air charge is introduced, while with fi, a greater charge is introduced prior to compression. Seemed like you'd have more potential energy to work with with the fi. . .

    . . . but oops, maybe you do, but (to borrow your example) with only a 7.5 CR you can't make as much use of that potential energy, can you? so the two do indeed offset, and the NA gets more points for overall efficiency since it is making same power with a smaller charge, yes?

    thanks for the correction.

  11. #11
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    Don't forget, though, that the lower CR engine...

    ...will have more volume in the combustion chamber when the piston is at TDC. Thus, you can fit more compressed air and fuel into it than the higher CR engine.
    - John
    '92 850i Twin Turbo

  12. #12
    Jim Derrig 93 750
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    but i think he means . . .

    that the extra potential energy from the "more volume" cannot be used as effectively due to the lower compression ratio, so what you gain in one hand you lose in the other.

    Gee John, that means that instead of 9psi boost, you could have switched to 17:1 pistons. You'd have the world's only diesel M70 :)

  13. #13
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    I'd certainly save $$ on fuel wouldn't I ? Ha Ha!

    - John
    '92 850i Twin Turbo

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