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Technical Gross versus net BHP

Discussion in 'The Hokey Ass Message Board' started by john W., Nov 24, 2021.

  1. john W.
    Joined: May 16, 2017
    Posts: 126

    john W.
    Member

    OK, Ive searched far and wide over the webby for a decent answer and found none.
    As far as I can tell, converting a gross HP figure to SAE net is somewhere around a 15-25% drop. So which is it?
    Does anyone know the correct % figure for loss?
    This is for flywheel BHP.
    Thanks.
     
  2. oldiron 440
    Joined: Dec 12, 2018
    Posts: 2,871

    oldiron 440
    Member

    It totally depends on what the engine drives, alternator, cooling fan, power steering, AC, type and weight of oil used, type of exhaust system etc.. that's why there a range of power loss in net hp....

    The percentage is going to depend on the amount of gross hp also, if you have a 450 gross hp engine or a 175 gross hp engine and 80 hp of parasitic power loss for both engines the percentages are different...
     
    Last edited: Nov 24, 2021
  3. squirrel
    Joined: Sep 23, 2004
    Posts: 51,157

    squirrel
    Member

    1972 was the year the ratings changed. Here's an example....look closely at compression ratios, some changed, some did not. The ones that are the same, should be the same engine.

    71p.jpg
     
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  4. Tow Truck Tom
    Joined: Jul 3, 2018
    Posts: 448

    Tow Truck Tom
    Member
    from Clayton DE

    I can't tell ya why,,, But somewhere once, I read that a 12 bolt robs less power than a nine inch.
     

  5. guthriesmith
    Joined: Aug 17, 2006
    Posts: 6,649

    guthriesmith
    ALLIANCE MEMBER
    1. H.A.M.B. Chapel

    Yep, what @oldiron 440 said. Gross is based on no accessories and net is as-installed including losses from fans, alternators, etc. that rob hp. It can vary widely on percent based on what accessories are on an engine.
     
    Last edited: Nov 24, 2021
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  6. squirrel
    Joined: Sep 23, 2004
    Posts: 51,157

    squirrel
    Member

    I did some comparisons and calculating on those engines in my previous post, as well as several from Chevy and Ford. The drop in rated HP from Gross to Net varies from 11% to 35%. Generally, the smaller or more "tuned" the engine is, the less drop. The big lazy engines dropped the most.

    If you're trying to figure out how a couple of engines compare, based on their factory numbers, you're probably not going to get too accurate of a result.
     
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  7. mohr hp
    Joined: Nov 18, 2009
    Posts: 426

    mohr hp
    Member
    from Georgia

    In 1971 GM published both figures for their engines. The LS-5 was 365 gross, 285 net. All available from the GM heritage archives.
     
  8. john W.
    Joined: May 16, 2017
    Posts: 126

    john W.
    Member

    Yeah
    about a 15-25% drop is around good.
    Thanks for the help!
     
  9. Relic Stew
    Joined: Apr 17, 2005
    Posts: 1,161

    Relic Stew
    Member
    from Wisconsin

    Tooth contact area. It's one of the things that make a 9" strong. The high hypoid gears spread the load over more teeth, but it also means more friction.
     
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  10. squirrel
    Joined: Sep 23, 2004
    Posts: 51,157

    squirrel
    Member

    I think it has more to do with the large offset between the gear centerlines...more sliding action when they mesh
     
  11. indyjps
    Joined: Feb 21, 2007
    Posts: 4,893

    indyjps
    Member

    The move from gross to net HP ratings - insurance rates also had a big factor. Consider the gross HP ratings best case and the net HP as low as possible to help insurance.
     
  12. gimpyshotrods
    Joined: May 20, 2009
    Posts: 20,185

    gimpyshotrods
    ALLIANCE MEMBER

    RWHP.

    The rollers do not lie. Only what makes it to the ground matters.
     
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  13. squirrel
    Joined: Sep 23, 2004
    Posts: 51,157

    squirrel
    Member

    Go to the drag strip...the clocks don't lie (usually). Making the car accelerate is what it's all about, right? Racing dynos is kind of boring....
     
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  14. Truckedup
    Joined: Jul 25, 2006
    Posts: 4,461

    Truckedup
    Member

    Or pushing through the air doing land speed racing.....Dynos are a good tuning tool...Or a good tool for egos...;)
     
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  15. MeanGene427
    Joined: Dec 15, 2010
    Posts: 2,088

    MeanGene427
    Member
    from Napa

    Yep, there is a slight disadvantage in the loss to friction, but the strength advantage more than makes up for it- with good parts, they are pretty bulletproof
     
  16. gimpyshotrods
    Joined: May 20, 2009
    Posts: 20,185

    gimpyshotrods
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    The drag strip is not available 6-days-a-week, and when it is, they won't let me make 20 passes.

    And no, accelerating is not what it is all about.

    Less than 2% of the cars that I have built or tuned have ever set tire on a drag strip. They are all street cars. They have to do steady-state 75 as well as they do 15.
     
    Last edited: Nov 25, 2021
  17. gimpyshotrods
    Joined: May 20, 2009
    Posts: 20,185

    gimpyshotrods
    ALLIANCE MEMBER

    Tuning.

    It is not about ego, it is about maximum efficiency.

    I have taken 120hp engines and made them 200hp engines, and 600hp engines and made them 750hp engines.

    In both of those two examples, they both made more power AND used less fuel doing it.

    If y'all want to be down on power, and up on fuel usage, that's your own business.
     
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  18. theHIGHLANDER
    Joined: Jun 3, 2005
    Posts: 8,630

    theHIGHLANDER
    Member

    It takes approx 550 RWHP to move a 3500# car to 125 MPH in 10.41 seconds. Back into that with math and it's about 710 BHP. I came to these numbers using some old engine software and math formulas for my bracket racer. The engine builders program said I built a 746 HP BBC. I "backed into it" with an old math formula then added parasitic drag (known) for alternator, trans and rear axle. Once done I multiplied each by .95 to remove the hard on factor. Work over time. HP is always a calculated number, TQ is always a measure. I've always found it interesting that every dyno crosses HP and TQ at 5252 in the graph/results.

    Back in the day TQ numbers were used to maintain "optics" for insurance and truth in advertising a little bit too. The L-88 HP rating was quoted from peak TQ (which was 430 HP). Actual measures of the era found numbers approaching 600 BHP. Very easily accurate if you know what those are made of. I always said HP was discussed at the bar, TQ was discussed with a time slip. Yeah, I like this topic.
     
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  19. RmK57
    Joined: Dec 31, 2008
    Posts: 2,157

    RmK57
    Member

    I've found what happens at the drag strip is their 600 hp engine runs in the 12's. Wrong torque converter, wrong gear, wrong shocks, the list is endless. They look surprised or more disappointed when they pick up their time slip.
     
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  20. jimmy six
    Joined: Mar 21, 2006
    Posts: 11,210

    jimmy six
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    Might be pushing a 55 Chevrolet instead of an 84 Firebird. Aero does make a difference too.
     
  21. 2OLD2FAST
    Joined: Feb 3, 2010
    Posts: 4,060

    2OLD2FAST
    Member
    from illinois

    Did anyone mention parasitic transmission loss ?
     
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  22. Truckedup
    Joined: Jul 25, 2006
    Posts: 4,461

    Truckedup
    Member

    I believe it was Bill Jenkins quote " spend all Saturday on the dyno and lose on Sunday" Other words, as the guy said above there's more than just power needed to win
    My LSR bikes have spent hours on the dyno to get em close,and the final tweaks are made at the track..And the rider/driver factor..
     
  23. guthriesmith
    Joined: Aug 17, 2006
    Posts: 6,649

    guthriesmith
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    1. H.A.M.B. Chapel

    I did, but then reread the question and saw that it asked for net hp at the flywheel.
     
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  24. gimpyshotrods
    Joined: May 20, 2009
    Posts: 20,185

    gimpyshotrods
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    I did. A rolling road-bed dyno takes into account everything, and even the AC, if you turn it on.
     
  25. gimpyshotrods
    Joined: May 20, 2009
    Posts: 20,185

    gimpyshotrods
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    Cool.

    I don't build race cars. I am pretty sure that the OP is not building one, either.

    Come to think of it, nobody has asked for a carburetor-equipped car in quite some time. Unless someone does, it would appear that I have entered the EFI-only era.
     
    Last edited: Nov 25, 2021
  26. theHIGHLANDER
    Joined: Jun 3, 2005
    Posts: 8,630

    theHIGHLANDER
    Member

    It's a'ight @gimpyshotrods I'm still carbs, don't have to go it alone...;)


    In other news, I don't think I'm too far out on a limb here but just about any 60s thru 70s 4bbl engine can be figured for 275-300 BHP. It'll come in low and with a higher rear gear have enough TQ to move anything swiftly. Might not sing alto at the stoplight opera but still as much fun as you can have while still clothed. Just sayin, next...
     

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