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Chassis dyno - the real story

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  • Chassis dyno - the real story

    I found this on the HSV.ORG.UK forum and thought it was worth posting here
    I have been meaning to do a thread on the Do's and don'ts of putting your car on a rolling road, for some time now, but with the recent spate of new dynos popping up it is now time to post.
    Preparation for the Dyno.................
    1. Make sure your car has enough of the correct fuel in it for your journey and dyno runs.
    2. Check all oil / water before leaving home and make sure to take some spare so you can top up at the dyno.
    3. Look at your tyres and check pressures before you run your car on the dyno (Hub dynos being the exception here)
    4. Find your locking wheel nut socket if you are going to a Hub dyno.
    5. Arrive at least an hour early and open your bonnet to get the car as cool as possible
    6. Check your air intake and filter before the run.
    7. Talk to the dyno operator about what you what to achieve from the runs, as this will have a bearing on time and cost of you runs
    8. Get the car loaded onto the dyno without running the car to much (as the car will heat up very very fast and under perform)
    9. Cooling fans need to be positioned so they point at your cars air intake and radiator not just stuck in front of the car!!!
    10. Don't run the car for more than a minute before the first pull as it will heat up and under perform (leave the bonnet up during the run)
    11. Do the runs in quick succession to avoid heat build up (three runs should produce a good average)
    12. Ask for a print out of the runs showing Torque and BHP in the same scale on the same sheet and taken from the wheels NOT the flywheel.....(fig 1)
    13. Ask for a print out of the runs showing BHP and AFR on the same sheet. (fig.2)
    Below is some of the reasoning behind the suggestions above………………
    1.Fuel is a major factor in a cars performance! And more so on a tuned car, it is very important to have the correct octane rated fuel in your car as a lower spec fuel will reduce the cars performance.
    As a safe guard make sure you have had at least two complete fills of your chosen fuel before the dyno runs as lower grade fuel will dilute the high octane fuel.
    If the car detects knock it will reduce timing and so power and on a 400hp car one degree of timing is worth 4-5hp so a loss of 4 deg of timing is around 16-20hp loss in power.
    2. This one is easy to understand, your car needs to be in its best state of tune if you want to get the best results. The dyno is a very harsh place for a car and it gets put under a lot of strain, so it is imperative to check over everything as a car engine seizing on a dyno is not a good idea........
    3. To get an accurate result from a traditional rolling road the contact point of the tyres on the rollers needs to be consistent i.e. even wear and even tyre pressures. The tyres form one of the highest losses from the engine to the road, on average 37hp loss!
    This is where up to date dyno's are a must as the tying down of your car is very important, too loose and the figures can be over calculated, and strapped down to tight (guys sitting in the boot!!!!!) and the figures will be to low.........
    This is where the Hub dyno systems come into there own as the dyno bolts to the hubs of the car after the removal of the wheels so there is no problems with slippage or overheating......
    4. Simple one again......On a HUB Dyno if you can’t get the wheels off you can’t get on the dyno and it is time to go home....
    5. Easy to say hard to do sometimes.....Getting to the dyno early means more time for the car to cool down and more time for you to make sure you have completed all the other tasks on you list.
    6. This is another simple one...... make sure your air intake is clear before you get on the dyno as any obstruction (leave’s/dirt/rabbits) will mean a reduced air flow and loss of power.
    7. When you talk to the operator before the runs you can discuss how he wants to run your car and you can explain what you want from the data collected. A dyno has many different modes to calculate power and the "Power run" is just one of them.........
    A simple power run means the car is fitted to the dyno and the operator performs a full throttle run from a low RPM in one gear to the red line, this will give you a plot showing how much Torque your car makes at all points along the curve and from that a BHP figure is calculated.
    You could also ask for a run where the operator holds a fixed RPM then accelerates hard in one gear to simulate overtaking (this is a good bit of info for turbo cars to show where best to change gear to maximise boost)
    Five minutes spent at the start of the day can save you time and money......................
    8. Almost all modern cars suffer from heat soak into the engine and ancillaries, this is due to the amount of stuff that is packed into the engine bay and the limitations the body shape has on space.
    As your car will be standing still it will heat up very fast and the engine bay will get hot along with everything else in the engine bay. Heat is the single biggest cause of a poor dyno result as hot air reduces power and sensors reading to hot will make the engine management reduce performance to save the engine.
    9. I cannot stress enough the fact that a hot car will under perform but if you look at it this way a Monaro at the red line in 4th gear is pulling around 130mph and I know of no dyno that has fans that can produce that much air flow. At 130 mph it would be a wind tunnel and people would be blown over. Also a 5.7ltr V8 moves 34,200ltr of air at 6000rpm (or the size of a large petrol tanker!!!) so again a couple of fans are not going to get close to supplying enough air flow!!!!!!
    The best dynos cells have a temp control environment to keep air temps low this way they don’t need to produce large amounts of air flow to keep the car cool.......
    When an engine management system detects an inlet air temp in excess of its safe level it reduces ignition timing to reduce power and thus save the engine from damage. On the road this is happening all the time (after being stuck in traffic you put your foot down and you think the car feels slow, that is because it is slow!!!!) but it soon recovers, on a dyno this is not the case and you just get less and less power.
    While on the engine management front, to properly dyno a modern car you need to have full control of the cars ECU/PCM this is because you must see what changes the car is making during the dyno run, is it reducing spark due to heat, is it invoking some form of torque management due to Throttle position/RPM/Torque, is the traction control system still having some effect (off is not off on most modern cars!!)
    Without this level of info your results could appear poor but in fact were totally
    normal given the operating conditions the car was dyno’d under.........
    BMW have just about produce a UN Dynoable car in the new M5 as it has so many systems to monitor wheel speed, Throttle etc it just puts the car into a safe mode and that is the end of your fun.
    When doing some dyno work with the “VXR Performance Centre”, "Triple Eight" and "Vauxhall" on their new Astra VXR we logged air temps of 77deg at the throttle body and we were pumping in air at 16deg and the car just reduced power. It was also not possible to dyno the car without removing relays to stop the traction control cutting in.......
    10. As above "Heat is bad"
    11.The first run will loosen up the car, second run will probably make the best power and the third will come close to the second and verify the power your car was making on that day!
    12. your evidence of the day is a print out but it is only any good if you understand it.......
    (fig 1)
    A good dyno print out should show Torque and BHP on the same sheet and use the same scale. One thing to do when checking a dyno sheet is to look at the 5252rpm figure as it should be the same number on Torque and BHP because BHP is calculated from torque......HORSEPOWER = (TORQUE x RPM) / 5252
    Also make sure your sheet is produced using a DIN standard and is taken from the wheels and is not a calculated flywheel figure.........
    13. A very good starting point for more power is Air Fuel Ratio and getting a printout showing your current AFR will give you a heads up on current/future performance. (fig 2)
    FIG 1

    FIG 2

    As with all test equipment the set up of the Dyno is very important to maintain accurate results, so far I have assumed the dyno is correctly calibrated and the operator has ensured the car is correctly mounted on the Dyno.
    But there are some things that can affect your results which you need to know about.
    1. The intake air temperature probe should be mounted in a position which accurately monitors the actual air entering the car. If this is not done the dyno will make a correction to the results in an attempt to allow for different air density and therefore more or less power.
    The most common problem I have seen is when an operator puts the temp probe to close to the radiator or lays the probe on a surface which has a large degree of heat soak and this then makes the reading to high. The result of this over reading means the dyno adjusts the final figure in an upward direction.
    On the dyno sheet you will see and ambient temp number printed and an air intake number. You should be happy that the ambient temp was accurate based on the weather on the dyno day and before you trust the results the IAT number should be within 5deg or so of the ambient temp. (fig 3)
    As a rough guide a 20+deg difference in the two numbers could equate to a 20BHP gain (a cheap way to get more power is to put the probe in a sweaty arm pit !!!!)

    2. The gear the car is dynoed in make a difference to your results as you gain mechanical advantage by using a lower gear and thus more Torque.
    The ideal situation is to dyno the car in a gear which is as close as possible to 1:1
    3. On dynos using rollers it is important to make sure the contact point is the same each time the car is tested, as when the car pulls up onto the front roller the further up the roller the wheels move the higher the torque figure can be............
    4. When your car is run on the dyno the operator selects a "Ramp Rate" which in simple terms is the steepness of the hill the car is climbing during the run and the speed of that accent.
    A supercharged car, a Turbo car and naturally aspirated cars should have different ramp rates based on how they produce there power and using the wrong rate will affect the final figures...........
    I have glossed over many problems involved in dynoing your car in an attempt to give you an overview of the process. In conclusion if you get within 5% of the figure you expect you have probably got what you expect.....
    Insignia VXR, quite nippy!