Lost HP somewhere

Discussion in 'Engine Combos and Dynometer Database' started by Tom, Nov 5, 2017.

  1. Tom

    Tom 69 vette big block

    I built my first BBC stroked to 496 for my 69 corvette, Had it on a mustang Chassis dyno but was not impressed.
    I thought I should have a lot more HP and torque. The Torque and HP curves are wavy at best. Maybe you can help me find where I went wrong.
    Dyno results Torque=468. HP= 498. at 7.7 lbs of boost.
    My setup
    Dart Big M block to 496ci
    9 to 1 comp ratio
    Edelbrock RPM heads #60555 not ported. 315 long intake runners, 300 short intake runners, 118 chamber.
    1.7 roller rockers.
    Comp Cams roller ADV DUR: 293-383 DUR @ .050: 238 248. LOBE SEP: 115. Comp tech spec'd out for my car.
    Blower shop 250 blower for BBC. (to keep it legal for Pa).
    BDS 8 nozzle 800 HP bug catcher. with FAST ECU.

    Attached Files:

  2. Grumpy

    Grumpy The Grumpy Grease Monkey Staff Member

    Id love too help but I can't clearly read , some parts of the dyno chart,
    if you can post a clearer picture and more info PLEASE do that!
    with more details on what your currently dealing with, now, we can reasonably easily get a good grasp on the likely areas that need looking into to boost performance.
    having boost or a pressurized intake manifold with the correct fuel/air ratio goes a long way toward increasing performance, but remember what goes in is comparatively cooler and denser that the larger exhaust volume so you need a low restriction exhaust to allow the engine to operate efficiently, I.E. what goes in has to go out and its volume has increased significantly in the process.
    IDEALLY,Id want to see the fuel/air ratio, plenum boost pressure, taken at every 1000 rpm,

    Id like to know how the cam was degreed in,
    what was your ignition advance curve, fuel pressure , exhaust back-pressure , picture's of the spark plugs and as much other data as I could get.
    personally I think that the baby supercharger is going to restrict power at anything over maybe 4500 rpm, your obviously going to need an over drive blower pulley ratio to allow a 250 roots style supercharger to provide boost pressure on a 496 displacement engine,and by the time a 250 supercharger spins over about 6000 rpm its losing efficiency.
    Yes your correct, Using the heads and cam and engine displacement , you put together, should as you expected produce better power.

    from what I can see the engines too lean in the lower rpms and too rich in the higher rpms.
    theres no reason some tuning and tweaking should not improve the results,
    but you'll obviously need to check exhaust back pressure, change the fuel ratio curve and check other factors, a easier to read dyno chart will help

    if you were under the impression the side pipe muffler inserts were not restricting power try a dyno and careful engine re-tune with them removed, you may be amazed at the difference in power above 4000 rpm.








    Last edited: Nov 10, 2017
  3. Grumpy

    Grumpy The Grumpy Grease Monkey Staff Member

    BTW why did you say,
    "Blower shop 250 blower for BBC. (to keep it legal for Pa)."

    if you look into this a bit youll see many people select a centrifugal supercharger
    they can be mounted low to clear the hood and still provide good power,and they allow easier use of an inter-cooler that can allow higher boost before you get into potential detonation issues
    for bbc engines as the efficiency climbs as the rpms build, most people don,t really need a great deal of off idle torque with a big block application, but they do want increased power from about 3500rpm-5500rpm plus










    Last edited: Nov 10, 2017
  4. Tom

    Tom 69 vette big block

    Thanks for getting back to me Grumpy.
    Here is a better dyno picture.
    I said "to keep it legal in Pa" because in Pennsylvannia we have stupid DOT laws.
    I had to fight to get a special inspection for the little blower sticking out the hood and for the 671 I bought
    there is NO way Pa would pass that big thing sticking out my hood, I was rejected by 5 garages so I bought this smaller 250.
    So the 671 is now man cave stuff.
    Being the BBC engine is in a 69 corvette I did not have the room to go centrifugal.
    I will try to get all the info you want through my F.A.S.T. XFI 2.0 program and post it like fuel and spark tables, ignition advance curve.
    This will be what the dyno shop tuned it at. I thought they did a good job but I think there should be more.
    I also will read the links you posted .
    I did check the cam with a degree wheel and used the Intake centerline method, it has 2 deg built in, so I left it straight up for now.
    I have large tube Hooker side pipes with straight through mufflers so I dont think I have a restriction.

    Attached Files:

  5. Grumpy

    Grumpy The Grumpy Grease Monkey Staff Member

    if you get the chance pull the dyno with the muffler inserts on the side pipes removed,
    that set of muffler inserts is easily costing you power worth an extra 40-70 hp,
    obviously depending, on the quality of the engine tune,
    if they get removed,matching the new config.
    on the engine with them removed,vs inserts being used.
    yes the required engine tune will change with the much lower exhaust restriction,
    once the muffler inserts are removed,
    (youll generally, but not always, find it needs to be jetted a bit richer,
    in the 3500 rpm and up power band with the inserts removed,
    due to increased scavenging efficiency)
    or put differently just removing the insert won,t result in you seeing the full benefits that carefully matching the new config would produce.


    Last edited: Nov 8, 2017
  6. Tom

    Tom 69 vette big block

    my side pipe mufflers are Ecklers Part #: 25-281309-1
    I want to get another dyno pull soon and I will take the mufflers out.
  7. Mark Bradley

    Mark Bradley Da guy in Newbury

    I have looked at both images and the only scale seem to be AFR & RPM.
    Can you show Rpm and torque?
    I’m running a NA 496 and am interested in your data.
  8. Tom

    Tom 69 vette big block

    Mark I only have that pic of the dyno screen as my phone died and his printer did not work,,,hmmm
    I am working on posting some of my FAST 2.0 maps that the dyno shop changed for me.
    What I dont understand is If I have 7.7 LBS of boost behind the intake valves than I must have the wrong cam?
    Last edited: Nov 8, 2017
  9. Mark Bradley

    Mark Bradley Da guy in Newbury

    Boost is not everything.
    AFR, timing and flow (including exhaust) all will drasticly impact the situation.
    If your running injection do you have the ability to log data?
    Are those numbers rear wheel numbers and are they atmospherically corrected?
    The only fair comparison would be if they are reporting corrected values.
    I was a little less than impressed with mine also initially although when I looked at my broad power curve and said to “hell with a huge power spike” if I can get a broad driveable torque curve.
    My timing starts at 18° and is all in by 25/2600rpm with 38°
    I’m running 10.2cr and pump gas.
    Below is my initial dyno run..

  10. Mark Bradley

    Mark Bradley Da guy in Newbury

    As for his printer malfunction you should ask him to give you his log files. My guy burned me a dvd and I walked out the door wit the data.
  11. Mark Bradley

    Mark Bradley Da guy in Newbury

    Hydrolic or mechanical roller cam?
    What is the cam grind?
    I’m using the Comp Cam HR294.
    The numbers you put up at the top seem small but I may be missing something (lost in translation).
  12. Tom

    Tom 69 vette big block

    This is rear wheel HP. Do not know if it was corrected, he did say quote "you know this is a mustang dyno". Do mustang dyno's measure HP different than others.
    But the F.A.S.T. XFI will only put the fills in GCT format and cant post here or I just dont know how. so I use my cell to post it.
    I am running a Hyd. roller comp cams. custom made by one of there cam experts.
    duration at 050 is 238-248
    lobe lift is .3610-.3810
    lobe Separation is 115
    intake center line is 113
    This weekend I will take some pics of my maps the shop corrected and changed as the car was on the dyno.
  13. Mark Bradley

    Mark Bradley Da guy in Newbury

    Got it... custom grind.
    So the lobe lift is .361 & .381 x rocker ratio.
    1.7:1 (typical) ratio
    .361 x 1.7 = .613 and 238°
    .381 x 1.7 = .647 and 248°
    242°/248° @.50
    110° lobe separation
    106° center line
    .540”/.560” lift
    Your 115° LSA over my 110° tends to do a lot to the behavior.
    According to CompCam your 115° will have a broader power curve although overall lower than the same grind with narrower LSA.
    It also moves peak higher up the RPM range and does make for a smoother idle.
    An indicator of the cam will be the RPM count between peak HP and peak torque. As I have read the peaks tend to be about 1300-1500rpm apart.
    Mine is ~1800rpm apart. Will be interesting to see where yours comes up.

    Attached Files:

    Last edited: Nov 10, 2017
  14. Mark Bradley

    Mark Bradley Da guy in Newbury

    BTW... thanks for the conversation. It is making me think and it HURTS!

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