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Dyno Test Question

Discussion in 'The Hokey Ass Message Board' started by geez63, Jun 20, 2009.

  1. geez63
    Joined: Mar 7, 2009
    Posts: 55

    from Maryland

    I wanted to add some horse power to my 350, 290hp, Chevy crate motor. I read an article in Chevy Performance Magazine about them experimented with different add-ons to see which ones gave them the most performance gain for the least amount of money. After reading that article and getting some other professional opinions , I decided to replace my stock 350 heads with a set of Vortec heads, and the intake manifold with an Edelbrock Air Gap manifold, I also replaced the stamped rocker arms with a set of roller rockers. I kept my Holly 670 carb. After the upgrade, the engine ran great and you could tell it was producing a lot more power. I took my car to have it dyno tested at a local race engine and chassis builders shop. The owner is well respected and very competent at what he does. After having it dynoed I was very surprised when the dyno showed that it was only producing 195hp to the rear wheels. I started out with a 290hp motor, after the upgrades it was obvious that it was producing considerably more power, but it was only putting 195hp to the rear wheels. The motor wasn't out of tune, the timing was right, the air fuel mixture was right, 195hp was just the best that it could do. Any comments or opinions from anyone who's had a similar experience would be appreciated.
  2. roughidle
    Joined: Feb 1, 2009
    Posts: 549

    from iowa

    I forget the percentage, but there is a parasitic loss of horspower when measured from the wheels. Basically that means the trans, driveshaft, and wheel/tires eat up alot of power before it ever hits the pavement.
  3. Was the 290 figure certified? Did you have it tested before the changes?

    As far as loss to the drivetrain, I would guess 20% at most. If you did have 290 HP and now only 195, something is wrong. Perhaps the feel of more power is your excitement of adding the new parts.
  4. A Chopped Coupe
    Joined: Mar 2, 2004
    Posts: 1,133

    A Chopped Coupe

    First, the 290HP figure is rated by GM is at the crank at 5100rpm with 326lb-ft of torque. The motor has a very mild cam and low compression.
    What did the power/torque curve look like............anything like the one that came with your crate motor.
    So, what kind of exhaust do you have, what kind of transmission and what is the gear ratio in the rearend.
    You should have done a dyno run before you did all this work...............the only way to get a good base line is to put it on the dyno before you start adding stuff.
    What was the air fuel ratio when you ran it on the dyno, did you fool around with timing to see if you get more.
    Not sure what kind of figures this magazine got, but all they did was to change the heads and intake manifold without changing the cam, timing, carb, ignition???????????????
    What size are the combustion chambers and are the valves still 1.94/1.50.
    Whenever you put a car on a chassis dyno you can subract anywhere between 15 to 35% for parisitic loss through torque converter, transmission, driveline and rearend.
    I had a stock 97 Mustang Cobra with 5spd rated at 305HP. It put to the rear wheels 256HP stock...................which is a 18% loss. I am sure if it would have been a C4 transmission it would have been over 20%. I put some long tube headers, catback exhaust, messed around with the fuel injection, cool air intake, gears and gained over 30HP to the rear wheels with some fine tuning, which is more like 345HP at the crank.
    If you are running a 350/400 automatic trans you could be as high as 35%...........they are among the worst for paristic loss.
    My guess is that the motor before you did the mods put less HP to the rear wheels, but how much less you will never know now.
    After drag racing SBC's for years, and building some SS motors that put out over 400HP at the crank, I think you let some magazine article steer you in the wrong direction.


    Joined: Jul 4, 2006
    Posts: 254


    Assuming you actually had the "290 HP", when it's measured on a chassis dyno you will lose between 16% and 18% at the wheels! This should have placed you in the neighborhood of 237 and 243 HP to the ground!

    These are "nominal" numbers we see from the dyno-room to the local chassis dyno's! Occasionally we get to "validate" these numbers with this procedure. Most times they are done at the strip.

    We recently delivered a 540" BB with 686 HP on the engine dyno, tested the ride a few weeks later and showed 576 HP "to the ground"!

    We also recently delivered a "bone-stock" 350" for the local roundy-round track with a "flat-top" L2256F pistons, #882/76 cc heads (8.7:1 C.R.), 1.940"/1.500" valves, Comp 12-521-5 cam (246/250 @ .050" x .420"/.420" x 106 L/S), stock rockers, and a stock cast iron Q-Jet intake. Unit made 325 HP with no add'l work. None was allowed, it had to make the class rules! No decking was allowed and only .010" max milled from the heads!

    Thanks, Gary in N.Y.

    P.S. We haven't had the opportunity to test any of those "290 HP" units to confirm G.M.'s numbers!
  6. RodStRace
    Joined: Dec 7, 2007
    Posts: 2,647


    Dynos are NOT like a yard stick. They don't all measure the same, and they don't all start at zero.
    I hear a bit of desperation in your voice, the same one that is heard from most people when they run their stuff on a chassis dyno.
    Take as just a dumb number, unless you are going to make changes and return to retest under the same conditions.
    Sure, it stinks that you won't be able to toss out that number in bench racing sessions, but it's how it feels to you that's important.
  7. 29NashRod
    Joined: Jan 18, 2009
    Posts: 66

    from Portland

    The amount of horsepower your engine makes greatly depends on the air density on the day that it was dynoed. Usually, factory horsepower numbers and magazine engine builds convert their numbers to show what the engine "would make" on a standard day at sea level. Standard conditions are usually considered to be a barometric pressure of 29.92 inches of mercury, and 54 degrees F at sea level. So if your engine was tested at a higher elevation, or on a day that the barometric pressure was lower, or the temperature was higher than standard, the air going into your engine would have been less dense, and your engine is not going to produce the horsepower numbers that were advertised.

    Your dyno shop might not have converted your numbers, which in addition to the parasitic drivetrain losses in your car, could explain your lower numbers.
  8. geez63
    Joined: Mar 7, 2009
    Posts: 55

    from Maryland

    The old heads had a 76cc combustion chamber and the new ones have a 64cc combustion chamber. I have a set of headers with a 2 1/2" exaust system and Flow master 40 series mufflers, a turbo 350 trans. w/ a 2800 stall converter, and 3.90 rear gears. During the 2 hrs. it was on the dyno we adjusted the timing, re-jetted the carb because it was running too rich, put a 1/2" spacer under the carb, ran it with and without an air cleaner. when we finished, we thought we were getting the most out of what we had. After all the adjustments, the torque readings went from 220 ft-lbs torque @ 3800 rpm to 248 ft-lb torque @ 3900 rpm, and the hp readings went from 171 hp @ 4400 rpm to 195 hp @ 5000 rpm. As I said before, this is power to the rear wheels. I've heard that if you multiply the hp at the rear wheels by 1.35 you'll be pretty close to the hp at the motor. I do wish that I had dynoed the motor before the changes were made but that's just wishful thinking now. I don't plan on doing anything more to the car at this time, so it is what it is, I was just surpised at the low hp and torque readings.
  9. lippy
    Joined: Sep 27, 2006
    Posts: 5,589

    from Ks

    Question is, are you happy with the way it performs? Lippy
  10. Darby
    Joined: Sep 12, 2004
    Posts: 426


    And the factory guys have days to dyno-tune and have wide-range O2 sensors and can not only dial in a blueprinted engine, but also bias their operating conditions to take advantage of those SAE correction factors. It's real hard for the average guy, even with dyno tuning, to hit rated numbers. But 100 hp under rating, with higher compression, is a lot. Maybe you need more cam to take advantage of that new CR...

    You say it <b>feels</b> more powerful--is it possible you built more bottom-end torque and kept top-end HP about the same?
  11. rodknocker
    Joined: Jan 31, 2006
    Posts: 2,267


    How long have you been driving the engine,before you started the upgrades?
  12. geez63
    Joined: Mar 7, 2009
    Posts: 55

    from Maryland

    I bought the crate engine new, put it in my 55 Chevy, and drove it for two years ( approx. 1500 miles ) before I did the upgrades. Theirs no question that it made more power after the upgrades.
  13. geez63
    Joined: Mar 7, 2009
    Posts: 55

    from Maryland

    I was very happy with the way my car performed before the dyno test, and even though I was surprised at the lower than expected test results, I'm still happy with what I have, so your right, that is the bottom line. If the time comes when I want more power I can do more upgrades, at least now I have a known baseline that I'm starting from. I appreciate all the comments and opinions.
  14. dumprat
    Joined: Dec 27, 2006
    Posts: 3,130

    from b.c.

    Wouldn't be that the factory "sold" ya a number eh?

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