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what gas for what compression ratio

Discussion in 'The Garage' started by screamer*145, Nov 12, 2003.

  1. screamer*145

    screamer*145 1/2 ton status

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    ok iam posting this for a friend he just bought a 400ci long block it has 10:1 comp. ratio and he dosn't know what octan gas to use and ehter do I so please help? /forums/images/graemlins/thinking.gif
     
  2. rjfguitar

    rjfguitar 3/4 ton status GMOTM Winner

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    run 91 octane, it's the highest you can get at the pump here in Ca.
     
  3. screamer*145

    screamer*145 1/2 ton status

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    is it just a performance diffrence between the octan or is it better for the motor?
     
  4. FatBoyBlazer

    FatBoyBlazer 1/2 ton status

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    my car(integra) has 10.5:1..i run 93...if im fealing generous she gets the 94 /forums/images/graemlins/thumb.gif
     
  5. FatBoyBlazer

    FatBoyBlazer 1/2 ton status

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    it mighth ping with the low octane in there but the dif in my car between 87 and 93 is a night and day diferance
     
  6. screamer*145

    screamer*145 1/2 ton status

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    so it is only for perfomance?, also what is the ping and why would it do that? /forums/images/graemlins/thinking.gif
     
  7. sled_dog

    sled_dog 1 ton status

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    if it pings thats cause your runnign too low an octane grade and it can damage the motor, its not just a performance thing its a motor safety issue as well. I have about 9.3:1 compression and run 89, I will run 91 exclusively when I change my distributor advance.
     
  8. screamer*145

    screamer*145 1/2 ton status

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    would it be okay if he just put regular in and then added octan booster? also sould he break it in with regular or sinthetic oil? /forums/images/graemlins/thinking.gif
     
  9. A&P

    A&P Registered Member

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    Not really, b/c it usually doesn't mix thoroughly enough. An octane rating in laman's terms is a fuels resistance to ignite, or how quickly it ignites. The higher comp ratio means that through compression, the air fuel mixture is accelerated to a greater heat before ignition. Thus, the fuel will pre-ignite or start burning too soon before the piston reaches the height of it's stroke. If the fuel completely ignites before this, it's like an explosion working against the motor. So, you can either retard the timing with lower octane fuel and possibly use a lower heat range spark plug, or use higher octane fuel. It all depends on the configuration of the engine. I'd recommend using the top octane pump gas, and setting the engine between 8-10 degrees BTC. Then work your way down the octane scale. Don't mix fuel grades. Finish the tank off, and then use the next grade lower and feel it out. You might be able to get away with middle grade (89 octane), and dial the timing up to where it starts pinging, then dial back 2 degrees. You should be good then. BTW, pinging sounds like a clicking, like a valve click, and the motor will run rough. Also, remember that the higher the timing is advanced without pinging, the more power it will be capable of making. I hope this was helpful. PM me if you have any more questions.
    -Ben
     
  10. rjfguitar

    rjfguitar 3/4 ton status GMOTM Winner

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    To add to what A&P said... with a nice built up motor, I would not screw around and try to run 87. This is an important issue for a higher comp motor and should be taken seriously because severe pinging can blow a motor up. As far as running a lower grade and adding octane booster... thats dumb for a couple of reasons: for one if you buy 10 gallons of regular at $1.25 that will be $12.50 and with 91 at say 1.50 that would be 15 bucks. Can you buy octane booster for 2.50? No. Plus the higher grades are a better "more refined" fuel.
     
  11. Z3PR

    Z3PR Banned

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    I use then highest oct. at the pumps in all my vechiles. I may be wrong, but it seams like they get better mpg's with the 91 or higher oct gas. /forums/images/graemlins/thinking.gif
     
  12. dyeager535

    dyeager535 1 ton status Premium Member

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    Not to muddy the issue, but cylinder head design, cylinder head material, and induction method are all relevant when talking compression ratio and octane needed.

    If it's stock 400 heads, then higher octane will be needed. But if it's a different chamber design, and/or aluminum, with fuel injection system (most of which retard the timing automatically) usually a lower octane can be used. Some of the newer motors ARE up around 10:1 and running 87 octane, but with older head designs, it's just not possible.
     
  13. screamer*145

    screamer*145 1/2 ton status

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    [ QUOTE ]
    Not really, b/c it usually doesn't mix thoroughly enough. An octane rating in laman's terms is a fuels resistance to ignite, or how quickly it ignites. The higher comp ratio means that through compression, the air fuel mixture is accelerated to a greater heat before ignition. Thus, the fuel will pre-ignite or start burning too soon before the piston reaches the height of it's stroke. If the fuel completely ignites before this, it's like an explosion working against the motor. So, you can either retard the timing with lower octane fuel and possibly use a lower heat range spark plug, or use higher octane fuel. It all depends on the configuration of the engine. I'd recommend using the top octane pump gas, and setting the engine between 8-10 degrees BTC. Then work your way down the octane scale. Don't mix fuel grades. Finish the tank off, and then use the next grade lower and feel it out. You might be able to get away with middle grade (89 octane), and dial the timing up to where it starts pinging, then dial back 2 degrees. You should be good then. BTW, pinging sounds like a clicking, like a valve click, and the motor will run rough. Also, remember that the higher the timing is advanced without pinging, the more power it will be capable of making. I hope this was helpful. PM me if you have any more questions.
    -Ben

    [/ QUOTE ]


    so what your saying is set the timeing at 8-10, fill it up with the highest octan and drive it till its gone fill it up with 83 and if I start to here a ping back the timeing off two degrees! /forums/images/graemlins/confused.gif
     
  14. screamer*145

    screamer*145 1/2 ton status

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    just so you know this motor has never been started before so any info on the first start up will /forums/images/graemlins/1zhelp.gif out a lot!
     
  15. Triaged

    Triaged 1/2 ton status

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    To clear up a few things:
    1) Higher octane gas is harder to ignite
    2) You will get worse gas mileage if you use a octane higher than the motor needs.
    3) You will get less power if you use a octane higher than your motor needs.
    4) A motor with more quentch (less piston to head clearance) will tolerate less octane.
    5) Octane requirments depend on dynamic compression ratio (DCR) alot more than they depend on static compression ratio (SCR).
    6) Dynamic compression ratio (DCR) is the compression ratio of the motor measured after the intake valve closes. That means it depends ALOT on what cam you are running.
    7) If you want to calculate the DCR this http://home.earthlink.net/~triaged/ea30.zip will do it for you. You have to click "extra calculations" to see it.
    8) I think I have heard 8.5-9.5 DCR for 91 depending if your heads are iron or aluminum
     
  16. A&P

    A&P Registered Member

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    What I meant by that was, pour in the good stuff, advance it til' it starts to barely ping, then back off 2 degrees. If it's not at 8 or higher, then use the next lower octane fuel and keep on keeping on til' you're at the limit and it feels good. There ya go. Sorry for the slop. I'm beat.
    Hope it helps.
    -Ben
     
  17. screamer*145

    screamer*145 1/2 ton status

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    thanks for the help! on that but even no this motor has never been started should i follow that same process /forums/images/graemlins/thinking.gif
     
  18. 84_Chevy_K10

    84_Chevy_K10 Banned

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    [ QUOTE ]
    Plus the higher grades are a better "more refined" fuel.

    [/ QUOTE ]

    Not true. Higher octane fuels just burn slower. There is nothing else different about them. That is a myth.

    [ QUOTE ]
    4) A motor with more quentch (less piston to head clearance) will tolerate less octane.

    [/ QUOTE ]

    That is not true. A motor with the proper quench distance will have less tendancy to detonate despite having a higher compression ratio. Ford discovered in the '30s that an 8:1 motor would knock less than a 4:1 motor with the proper quench distance.

    As said above also, dynamic compression ratio needs to be taken into consideration. If you use a bigger cam that bleeds off compression at lower RPMs where engines tend to detonate, then you might be able to run lower octane.

    As also said, if you run a better head, (Vortecs come to mind but there are others) fuel injection, aluminum heads, or even simply pay close attention to quench distance, you can build and engine that will still run on 87 or 89 and still have 10:1 CR.

    I wouldn't waste your money on octane booster either. You can make it much cheaper than they sell it for...but I wouldn't do that either unless the highest grade premium pump gas that they sell in your area is insufficient.

    Always break an engine in on conventional motor oil. Do not use synthetic.
     
  19. screamer*145

    screamer*145 1/2 ton status

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    He has no problem runnig high octane gas we just wanted to know I cant tell you the specs. of the cam as of now cuse I dont know them off the top of my head, but will post them soon, as for the heads they are stock heads but they have been ported and polished and has a real nice valve job done to them! /forums/images/graemlins/grin.gif
     
  20. rjfguitar

    rjfguitar 3/4 ton status GMOTM Winner

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    [ QUOTE ]
    [ QUOTE ]
    Plus the higher grades are a better "more refined" fuel.

    [/ QUOTE ]

    Not true. Higher octane fuels just burn slower. There is nothing else different about them. That is a myth.



    [/ QUOTE ] Ah, not true again. Higher grade is cleaner than low grade. hold a glass of both up and you will see that the regular is darker.
     

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