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  1. #1
    SHOBiz's Avatar
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    Choosing between 9.5-1 or 8.6-1 compression

    Pros and Cons for each using 93 octane (US).


  2. #2
    smbstyle's Avatar
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    all depends on what you want to do. what wheel are you going to be using? the less compression you are running, the more boost you can run.

  3. #3
    JD 1's Avatar
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    Quote Originally Posted by SHOBiz View Post
    Pros and Cons for each using 93 octane (US).
    this is simple. if you are wanting to run the 9.5 then you get)
    high octane fuel only
    way stronger bottom end to mid range acell
    only short distance at wot with less top speed

    the 8.6 will slightly improve across the board where the stock pistons left off. You can run 93oct. and the ski for long distance at wot no problem (good top speed)
    If you want to run 87oct with the 8.6 then timing will need to be removed.

    free up the exhaust with along with the 8.6 and you will gain topspeed.

    Id recommend the 8.6 for a rec rider

  4. #4
    Heres everything you want to know....
    Now you can see 93 octane doesnt get you very far, SAFELY...
    http://www.blowerdriveservice.com/techcharts.php

  5. #5
    West Texas RXP skoepp's Avatar
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    Quote Originally Posted by JD 1 View Post
    this is simple. if you are wanting to run the 9.5 then you get)
    high octane fuel only
    way stronger bottom end to mid range acell
    only short distance at wot with less top speed

    the 8.6 will slightly improve across the board where the stock pistons left off. You can run 93oct. and the ski for long distance at wot no problem (good top speed)
    If you want to run 87oct with the 8.6 then timing will need to be removed.

    free up the exhaust with along with the 8.6 and you will gain topspeed.

    Id recommend the 8.6 for a rec rider
    What is stock compresion on the yami???

  6. #6
    Quote Originally Posted by skoepp View Post
    What is stock compresion on the yami???
    Stock Compression Ratio 8.6:1

  7. #7

  8. #8
    SHOBiz's Avatar
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    I don't have so much of an issue using higher octane, but availability is the issue. So, 8.6-1 it is. I can put that on my white board now

    Thanks


    Quote Originally Posted by smbstyle View Post
    all depends on what you want to do. what wheel are you going to be using? the less compression you are running, the more boost you can run.

    I run a MoTec ECU with a E1 wheel, injectors and just about anything else you can think of.


    Quote Originally Posted by JD 1 View Post
    this is simple. if you are wanting to run the 9.5 then you get)
    high octane fuel only
    way stronger bottom end to mid range acell
    only short distance at wot with less top speed

    the 8.6 will slightly improve across the board where the stock pistons left off. You can run 93oct. and the ski for long distance at wot no problem (good top speed)
    If you want to run 87oct with the 8.6 then timing will need to be removed.

    free up the exhaust with along with the 8.6 and you will gain topspeed.

    Id recommend the 8.6 for a rec rider
    Quote Originally Posted by rossnemo View Post
    Heres everything you want to know....
    Now you can see 93 octane doesnt get you very far, SAFELY...
    http://www.blowerdriveservice.com/techcharts.php
    This chart easily explains why my 944 (non turbo) CAR is pretty well worthless for adding boost with my 9.5-1 ratio

  9. #9
    Vern's Avatar
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    Wrong chart for our skis. See the attached from Prochager ... centrifugal SCs with water to air intercooling can run 3 times the boost on pump gas, compared to a roots SC without intercooling ...

    Reliable High Performance
    Clearly, the only type of performance that matters is reliable performance, and detonation is the single biggest threat to engine reliability. The boost range for reliable performance, without detonation, can be determined by looking at the type of supercharger technology being considered, and the compression ratio of the motor. With a lower compression ratio, an engine can safely handle more boost, everything else being equal. Similarly, if the temperature of the compressed air is lower, an engine will have a much higher detonation threshold (the point at which fuel ignites without a spark), and will be able to safely handle more boost. The chart below helps to illustrate how the overall efficiency of the entire supercharger system can be increased by both leading edge supercharger efficiency and the use of intercooling. Simply locate an engine's compression ratio at the bottom of the chart and trace upwards to determine the maximum reliable boost level. The amount of heat produced (adiabatic effiency) by each supercharging technology is what determines the boost limitation. While gear-driven centrifugal is clearly the superior supercharger technology, it is also clear that the biggest benefit comes from intercooling. These calculations assume moderate timing, 92 octane pump gas, and a good supply of fuel to the cylinders. As mentioned previously, detonation is the single biggest threat to engine reliability. It is heat and detonation that cause blown head gaskets and burned pistons, not boost. Achieving maximum performance from a given engine while avoiding detonation requires the right combination of intake air temperature, timing and fuel quality. For example, without intercooling a stock 5.0 with 9.5:1 compression ratio can only hold 5-6 psi of boost before detonation becomes a problem. The only way to safely run more than 6 psi of boost and still make a meaningful increase in power without an intercooler is by using racing fuel to avoid detonation. Many companies also employ "band-aids" such as ignition retard and larger injectors to run 8-9 psi on pump gas, but the resulting increase in performance is only marginal (since both of these band-aids suboptimize) and detonation is frequently still a problem.

  10. #10
    Thanks Vern, huge difference

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