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compression ratio ????

Discussion in 'The Garage' started by skidpan, Feb 28, 2006.

  1. skidpan

    skidpan 1/2 ton status

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    i am planning on building an engine... I have a couple of small blocks (a 350, and a 305)
    I want to build a stroker, but i dont know high compression i can run on 94 oct.. i have an extra set of 58cc heads, and a set of 64cc heads
    the pistons i plan on running are 60+ Speed-Pro 383 domed pistons. I believe the 58cc heads would put me around 12.5:1
    is this too high for 94? what is the highest compression I can run on 94
    would there be any benifets to using a 305? I believe I would have to modify the block to use the 400 crank... would i be able to use 60+ 350 pistons? would I have to sleve the block? I am just trying to get a high tq sb.
     
  2. 1985_K5_Silverado

    1985_K5_Silverado 1/2 ton status

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  3. tarussell

    tarussell 1/2 ton status

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    First rule of thumb is the old saying "no replacement for displacement" ! So for get about using the 305 block that came with a bore size that was damn near equal to a Pepsi can.
    12.5 CR is way to much for 94 octane . You need to be under 10 to 1 if you want to run that kind of octane level. Newer motors with reverse cooling and electronic controls can get away with higher ratio's but not a motor with standard cooling.
    Fine engine tuners can get away with ratio's in the 10 to 1 range but a lot of that depends on how much cranking pressure the motor has and that is determined by camshaft timing selection.
    Make a strong SBC with as much cubes as possible but do not get caught up on maximum C/R and end up hating driving the truck.
    HTH's Tom
     
  4. bablazer73

    bablazer73 1/2 ton status

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    I would stay around 9.5-1. Use a flat top piston and what ever CC head will get you closest to that 9.5. A flat top piston is Less prone to "hot spots" than dome pistons. this will reduce knocking/preignition.
     
  5. skidpan

    skidpan 1/2 ton status

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    so should i look for a set of 72cc heads? I do plan on boring the 350 to 60 over, and going with a 400 crank. What heads should i go for? The first sb i built ended up with 11:1 and ran great with 92 and even better with 94.
    so should i gat a flatter or perhaps a dished piston? its not going to be goiung in a daily driver, so it can be a little more agressive than most...
    I appreicate the help guys!
     
  6. beater_k20

    beater_k20 Banned

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    agreed. the bore shrouds the valves too much to make any sort of performance. and assuming you want to make good power, use the 58cc heads as doorstops. they're suited well for a 305ci, small bore engine, and that is all.
    incorrect. a 12.5:1 engine can be completely happy with 94 octane, however, the cam will be so big and the powerband so high, that it wont be a street friendly engine.
    "fine engine tuners" huh? please explain why a stock 305 (with 9.5:1) will run fine on 87 octane, without the use of anything out of the ordinary (standard HEI, QJet carb)




    what kind of cam are you looking at running?
     
    Last edited: Feb 28, 2006
  7. pauly383

    pauly383 Daddy383 Staff Member Moderator

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    Mine was rated 9.2 to 1 stock , and I can't give it all the advance it needs , or I want without it pinging on 87 . I took off the ESC , and run a standard HEI for a 76 nova with highway gears .

    I do beleive it was the ESC that made it easier to run higher compression :D
     
  8. skidpan

    skidpan 1/2 ton status

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    I dont really know what cam yet, I have been reading about 4/7 swap cams but i havent figured out which cam ..... and sugestions??
     
  9. beater_k20

    beater_k20 Banned

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    what is a 4/7 cam swap? :confused: is that the cams that are ground to swap 2 cylinders, and make the engine run smoother?
     
  10. beater_k20

    beater_k20 Banned

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    intended application for the truck? gear ratio? tire size? trans? need a little more info...
     
  11. ryan22re

    ryan22re 1/2 ton status

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    the 4/7 swap is designed to help out the intake manifold charge. its for the guys who want that extra .5 hp out of an engine. if the cams cost more than regular cams (which im sure they do) they would be a waste of cash. and being that they are suited to max hp comp engines, i dont think that street/4x4 grinds are avail.
     
  12. 1-ton

    1-ton 1/2 ton status Premium Member

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    Well, if you want to argue, just for the sake of arguing, then yes, an engine with 12.5:1 compression could run on 94 Octane, but how many people have you known that actaully have an engine with 12.5:1 compression, run 94 Octane gas? I can tell you how many I know. NONE! Every person I know that has ever built an engine with that much compression, including my own engines, has always opted to run 100 Octane. An engine built with 12.5:1 compression is not a "run of the mill" every day engine, so you are not going to consciously choose to burn a "run of the mill" every day fuel in it.

    He said 10:1, and not 9.5:1! A .5 increase in compression does make a difference.

    I think everything tarussell posted is, to quote Spock, "quite logical", and the norm for most people that go to the extra effort to build a performance engine for themselves. I think your arguments are based on extraordinary hypotheticals
     
  13. tarussell

    tarussell 1/2 ton status

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    1-ton , You and I are on the same exact page in logic!

    I do agree that a motor "can" run on that low of an octane level but is it going to reach it full potential - NO !

    That 305 running on 87 octane is not doing all that it could either but it can run on such a low number.

    As stated up above this guy is building a motor for torque in a truck application not some fire breathing monster that has no cranking pressure that needs to run on the minimal flame-retardents to make every last bit of power possible.
    I stand by my statements about my opinions on C/R's and what would be a bad idea in this situation.
    Tom
     
  14. Beast388

    Beast388 1/2 ton status

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    Don't go over 9.5:1 if you want to drive on street gas. The loads that a truck engine sees are greater than a street car.

    In my stroked 350....now 388 cid I have a true 9.46:1 compression ratio and I run 93 octane. I cc'd the heads, decked the block and bought the appropriate piston to get that comp ratio. You will never know your true ratio unless you do the same. Head cc, piston cc, gasket, deck height, bore & stroke are all things that affect your final ratio.

    Throw that 305 away and build something solid with that 350 core.:D :D
     
  15. beater_k20

    beater_k20 Banned

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    does it make a difference? hell yes it does, there's no disputing that. however, it does not make the difference between 87 and 94 octane.
     
  16. POWERMAD

    POWERMAD 1/2 ton status

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    What kind of rig that the engine will be built for is the first step in determining what the power requirements will be.

    How heavy is the rig?
    Are ya building a rig to blast the dunes at places like Glamis and such only?
    Badazz DD rig?
    A lil of both?
    What's going to be bolted behind it, gearbox or slushbox?
    Tires and gearing?

    Unless it is a stripped out, trailered only and has to be fed 103 to keep it happy rig I would keep the ratio to no more than 9.5 with a mild cam grind.

    My .060 over 350 is at 9.17:1 and doesn't like 87 at all, gotta watch the gas dude like a hawk. :mad:
     
  17. skidpan

    skidpan 1/2 ton status

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    well the rig is going to be pushed with a sm465 and a 205. i would like to get out to the dunes in it occasionally but it would be more for rocks, and desert trails. I know its taken a while for me to reply to this..
     
  18. Chevy305

    Chevy305 6 Lug 14bsf Status Premium Member GMOTM Winner

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    You can run I'd say a max of 10.5:1 CR on 94 octane. If you spend time to play with timing and the advance curve then you will have no problems at all. Also a cam with more overlap will help too.

    On my 305 I have a 9.5:1 CR and I use 91 or 93 octane. I have only really advanced my timing by about 2 degrees from stock and it runs great. Also I have the stock cam so there is almost no overlap.
     
  19. blazinzuk

    blazinzuk Buzzbox voodoo Premium Member GMOTM Winner

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    A substantial number of things need to be decided before you decide on compression ratio, what heads are you going to run, what ignition, what cam etc. An engine needs to be a whole system. In all honesty when poeple ask me for advice on something like that here. I give them a stack of magazines I have and say pick out the motor you want and duplicate it. Also if you want to run a higher compression have you thought about coatings. They will insulate the combustion chamber and help with the flame propigation. Under a microscope they are way smoother than even a machined metal surface so there are less hot spots. I have never had any expirance with high compression motors in trucks but in cars yes big cams hi po ignition systems and lots of messing around to get them to run decent on 91 (the highest you can get where I am from) I would say build to put the power where you want it. If you want the power down low and a lower compression ratio will do that then build accordingly. If you want a motor that will scream at 8500 rpm than build accordingly. Good luck. I would say go buy some books also. Just because a guy has been building engines his whole life doesn't mean he is up to date on engine technology
     

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