Compression ratio?

Discussion in 'Engine, Intake, Exhaust and Drive Line' started by Night Runner, Sep 1, 2019.

  1. Night Runner

    Night Runner SHO Member

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    So I’m just gathering info for the future. I’m wondering what compression ratio in a 3.0 SHO would be 92 octane. I would like to know so I could run premium pump gas so I don’t have to buy any race gas. Save some money
     
  2. luigisho

    luigisho SHO Member

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    The compression ratio is 9.8:1. It does not matter what the fuel is the compression ratio is the same. Predetonation and pulling timing is another matter. You can go as cheap as you want as long as it's not pinging. I use premium and midgrade sometimes. Others have used lower octane and say their cars run fine
     
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  3. Night Runner

    Night Runner SHO Member

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    Well I wanna up compression in the future what’s the highest compression I can use that utilizes pump gas. Sorry it seems I wasn’t too clear
     
  4. luigisho

    luigisho SHO Member

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    How would you like to up compression? It's not an electronic tuning option, so you can reduce the clearance from the head and valves to the pistons by milling or you can install new pistons... I'm not clear on what you are saying.
    http://blog.jepistons.com/how-to-calculate-engine-compression-ratio-and-displacement

    There are modern fuel adaption tables run in the computer to increase and decrease timing and to even use 'flex fuel' lines, injectors and sensor input for timing in some newer models of Ford vehicles.

    This model is old and does not have variable valve timing or a bunch of fuel options. It uses sensors with cruder tables and advances and retards timing according to several sensors and for fuel grade you are asking about it's basically knock sensor detecting preignition knock and pulling timing or not.
    Will your car run better on premium? Probably. It's recommended fuel is premium. At the time of production that was a higher than normal compression ratio and higher octane fuels were recommended for greater than 9:1 ish c/r.
     
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  5. Night Runner

    Night Runner SHO Member

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    So the higher the compression ratio the less space in the cylinder the higher octane needed I’m trying to get to the highest compression needed for 92 octane which I run already I just want a higher compression
     
  6. luigisho

    luigisho SHO Member

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    Basically. Here is an easy definition from the link above
    By definition, the compression ratio is the total swept volume of the cylinder with the piston at bottom dead center (BDC), divided by the total compressed volume with the piston at top dead center (TDC).

    So very basically without math, the idea is that to increase compression ratio you would have to decrease the amount of space at the top of the piston stroke. You can accomplish this in a few ways but the easiest available is to lower the top deck (the head mating surface at either the head or the piston deck) or increase the piston height. What you have to determine is how much change will turn your non interference motor to interference where a piston has the chance to hit the valves if something goes awry. That will be very likely considering the small amount of room. The pistons shou;d have the correct valve reliefs cut in so it will run fine unless timing goes crazy like a broken timing belt etc. Maybe the JE pistons folks already have that info. People have made higher and lower compression engines over the years around here. But very few
    here is the engine deck area
    https://help.summitracing.com/app/answers/detail/a_id/5002/~/engine-deck-clearance
     
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  7. luigisho

    luigisho SHO Member

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    Not worth the hassle and money to me, but if you're interested in it there should be some archived posts about doing it on the forum
     
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  8. NoSlo

    NoSlo GoldMember

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    The SHO is already "premium fuel recommended". It will run on regular, but only because it has a knock sensor that can scale back the fuel if detonation is detected, giving a bit less horsepower.

    You won't get more horsepower by getting 100 octane gas. Higher octane rating fuel is just more resistant to knocking. Horsepower is limited by the amount of intake oxygen. This is why you squirt nitrous oxide, not squirt gas.

    The predisposition to knock is also affected by RPM, throttle and mixture, valve lift and overlap, charge cooling and dilution from PCV and EGR, along with cylinder deposits and spark plug temperature. A 200K engine is going to have higher compression from reduced head space and more preignition simply from cylinder carbon deposits.

    If you want higher compression, just replace 6 x 3 cylinder rings and clean the ring grooves for measurable results on a cylinder compression test. Totally not worth the effort and parts cost.
     
    Last edited: Sep 2, 2019
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  9. rubydist

    rubydist SHO Master Staff Member Super Moderators

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    and milling the heads is not a recommended option because by the time you mill off enough to effect the compression ratio, you have affected valve timing in a way that is very difficult to correct.
     
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  10. Irish Pride

    Irish Pride Irish Inside Staff Member Super Moderators

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    SHOnut offers Wiseco 10.6:1 pistons. I was using these Pistons in my old white 89 for years. I only used 93 octane fuel and would still get some pinging on really hot days. It really needed a tune to reach its full potential. I was running an 185° thermostat with water wetter in the coolant to help keep the pinging down. I also switched to copper plugs one stage cooler. That car had great power and a really nice torque curve. Pulled really nice. I have since removed the 10.6:1 pistons for new 11.5:1 Wisecos. I'm going to get a proper tune this time around and might need to switch over to E85 when it's all said and done. We'll see.

    FYI, I think the 10.6:1 and up pistons are only available in a 3.3L(93mm) bore. You'll have to look at SHOnut to see. My block is a 3.3 liter.

    -Chad
     
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  11. Night Runner

    Night Runner SHO Member

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    I plan on putting a 3.2 in it with the mtx already have the engine too don’t plan on boring it out but this helps a lot thanks guys
     
  12. SHOdded

    SHOdded SHO Member Supporting Member

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    If you truly want to do a power build, you should talk with yamahasho. He and Zach have put together a monster SHO as you may be aware. Don't go bits and pieces. Plan it out so it works right the first time, every time.
     
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  13. SM105K

    SM105K Land Yacht Pilot Supporting Member

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    This, and embrace the money pit it will become.
     
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  14. luigisho

    luigisho SHO Member

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    That's the truest statement on this forum
     
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