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Thread: turbo's - boost equals how much horsepower??

  1. #1 turbo's - boost equals how much horsepower?? 
    Car Enthusiast CF Newbie
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    I was just wondering if there is any type of formula... like "x" psi of boost equals "x" amout of hp gain, and "x" amout of torque gain. Or even some type of rough estimate for a 4-cyl.

    Also, is there a set range of boost-psi limit that you shouldn't go above on a stock 4-cyl engine? If so, what would that be by any chance?

    Is it okay to run lower than 92 octane fuel on a stock turbo'd motor, if your not set for insanely high psi. Just for a moderate boost? I know there are chaneces of knocking, but some people say that higher octane fuel doesn't do much. What about 87, or midrange, and then just adding octane booster?

    One more thing... I love working on cars, and learning more about them, but I love doing stuff myself, and rather than buy a turbo kit, I would like to get parts and build my own set and ot it on. I don't have a lot of money, so I would have the do it as cheap as possible. Any advice on what parts I should get, and maybe some cheap parts that will still be good in the long run.

    I'm just trying to do my research before putting money into stuff.

    Thanks, Scott
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  2. #2 Found an answer 
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    - If I'm correct, I believe 1psi of turbo boost is aprox 5% of original horsepower (it can vary i guess.)

    - If I'm correct again, I'm going to say that you can run up to 18psi on a stock 4-cly engine if you have a good intercooler. If you don't run an intercooler, you can still get up to about 8psi of boost.

    - And I'm still not sure on if you really have to run 92 octane or not. I would run it if I was doing a large boost, but do you have to run using high octane if you are only doing 5 or 8psi?

    I was going to build my own turbo kit, and get used parts, and make sure they are good. I found a used t3 turbo for $250.00. And an Intake Manifold for the turbo and my civic for $150.00. I also found an Intercooler for $150.00. All the other stuff I would need is piping, and oil lines correct? (Assuming the turbo has a built in wastegate) So I'm thinking that I can get away with a pretty cheap turbo setup as long as I get the turbo looked at by a professional and checked out. Any help would be great.

    Scott
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  3. #3  
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    I would say it really depends on the size of motor, comperssion, other modifications, etc etc. But I'm sure given all that information we could do a few calculations and get an approximate HP per LB of boost #
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  4. #4  
    CF's Pimp CF Senior Member
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    Quote Originally Posted by dono
    - If I'm correct, I believe 1psi of turbo boost is aprox 5% of original horsepower (it can vary i guess.)
    in theory, every 14.7 psi over atmospheric doubles the horsepower output to the flywheel.

    - If I'm correct again, I'm going to say that you can run up to 18psi on a stock 4-cly engine if you have a good intercooler. If you don't run an intercooler, you can still get up to about 8psi of boost.
    12 PSI depending on intercooling, static and actual running compression of the engine, fuel used / fuel pressure, spark duration and temperature.

    6 PSI is pushing it. again it depends on the above variables.

    - And I'm still not sure on if you really have to run 92 octane or not. I would run it if I was doing a large boost, but do you have to run using high octane if you are only doing 5 or 8psi?
    On your stock honda. Yes.

    I was going to build my own turbo kit, and get used parts,and make sure they are good.
    okay. I will post my recommendations.


    I found a used t3 turbo for $250.00.
    Used by who for what? how much endplay is in the impellor shaft? is it rebuilt or modified in any way?

    And an Intake Manifold for the turbo and my civic for $150.00.
    who makes this intake?
    I also found an Intercooler for $150.00.
    Does it leak? have you been able to test it under pressure..?

    All the other stuff I would need is piping, and oil lines correct? (Assuming the turbo has a built in wastegate)
    Yep. you'll need a complete exhaust system depending on your state, and probably need to get the whole kit and kaboodal welded together by an exceptionally talented exhaust assembly technician. or a tony welder. I'd suggest someone if I knew anyone in your area.

    do more research on the turbo before you buy it...

    So I'm thinking that I can get away with a pretty cheap turbo setup as long as I get the turbo looked at by a professional and checked out. Any help would be great.
    Cheap is relative. You may end up paying the same or more than the kit when you have to route all those pipes, oil lines, and have the exhaust shop weld it all together. plus gas and down time on the car. This is my 2 cents..
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  5. #5  
    Grand Ass CF Senior Member
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    There are way to many variables to give an awnser thats even close to accurate. You have to account engine size, type, mods to it, mileage, condition, signle or dual overhead cam, how well the car breathes, the ECU, fuel management system, ect...
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  6. #6  
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    "in theory, every 14.7 psi over atmospheric doubles the horsepower output to the flywheel"

    correct in theory, but properly tuned b series motors aimed at hp will greatly over reach the double hpo for 14.7lbs of pressure.
    many can do as much as 130% or more.

    Tuned you can run up to 18lbs ( as high as i have seen it) on a b18b and 15 on my b18c ( even on 10.6 compression and 94 octane.)

    i dont even recommend a non intercooled motor. just buy an intercool for crying out loud.
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  7. #7  
    CF's Pimp CF Senior Member
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    eh, I'm just stating theory. I left out airflow, CFM, intercooling efficency etc, but I'm sure that applys to more than the B-series honda engines..

    thanks for the update, however
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  8. #8  
    What would THEY do? CF Senior Member
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    The more CFM you flow thru the engine the better.

    Before you try and force more air into make sure that you are flowing enough to handle it, you can crank the PSI but if you dont have the CFM to handle it your gonna break sh!t
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  9. #9  
    CF's Pimp CF Senior Member
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    I'm forced to believe that when the manifold pressure has reached 14.7 over Atmospheric, you're supposedly flowing double the CFM as naturally aspirated. Of course, you could have some extremely well flowing heads and not be set up to actually flow maximum CFM (ala wrong cam / wrong intake / what have you ) and the turbo allows you to flow the maximum CFM X whatever.

    Of course, the turbo / supercharger has to be able to flow at least 200% of the engines maximum CFM for this to be possible...
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  10. #10  
    What would THEY do? CF Senior Member
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    Flowbenchs' are your friend.


    Now im getting into serious engine blueprinting and power,but in the end the one with the most power wins.

    Im just one of those people that do things right the first time.
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