February: Dyno Myths and Truths
Now while I certainly dont claim to be an expert on dynamometer theory, I do have quite a bit of experience using different types of dynos. Even the most simplistic hydraulic dyno can yield very valuable information if that information is gathered and analyzed correctly. A simple hydraulic pump with a pressure gauge can be used to measure engine output directly. Your kart mounted Digatron gauge will record the engines rpms for you. At least you will have a number for measurement comparison. But without a baseline for comparison, this newfound number is absolutely worthless. The #1 problem with dyno numbers is not the dyno itself, but rather, the user. Just like in most airliner crashes, almost always the fault will be that of the pilot over something mechanical. You see, a machine doesnt think, or over-think. That is the biggest hurdle most newbies in the dyno room have. The most important part of keeping useful information from your dyno is that it is all recorded consistently. Thats right, consistent! If you tested one day when the hydraulic oil was 140^, then always test at that temperature. If the ambient air temperature was 70^ when you tested, then make ALL tests at 70^! If the barometric pressure was 29.90 when you tested previously, then you must always test at 29.90! Now, obviously it is difficult, at best, to match every circumstance, but you honestly have to be as close as possible. Then, and only then, do you want to run calculations that correct to the previous test session standards. To make corrections on a test session is essential to most closely simulate the previous testing that you have done. That way you know you are comparing apples to apples.
But how do you correct everything? First try to eliminate as many variables as you can. In other words, build yourself a dyno cell or chamber so that the room conditions can be monitored very easily. A small room is easily heated or cooled, so ambient air temperature will be the easiest to keep repetitive. A simple thermostat in the room will keep the room within a degree Fahrenheit. Humidity is somewhat more difficult to change through humidifiers and dehumidifiers
honestly, we dont try to make the humidity level stable in our test cell, although we do keep very accurate and timely records of the humidity levels in the room at all times. Barometric pressure would be the most difficult to create without pressurizing the room, etc
This would be a very expensive venture, and I seriously doubt that any kart engine builder has this capability. We will undoubtedly have to correct for these variances from test to test. Be certain to record actual aneroid barometric pressure measured in the room, and check it periodically throughout your testing, as it does change slightly. You will also need to know your exact elevation in feet above sea level. While The Weather Channel is a great source for most families, you have to realize that much of the information given there is standardized, or corrected already. Keep in mind that it is corrected for the elevation, etc, of the location in which the broadcast is being made, not necessarily the location that the information was reported, or for your shops location either. Often times a call to your local airport can yield some helpful information. Pilots are always interested in elevation and aneroid (actual, not corrected) barometric pressure, as it gives them a more reliable form of air density. Air density is important to a pilot because the horsepower generated by his planes engines is dependent upon how much oxygen it can swallow, and how to most efficiently match the fuel curve of that particular engine to maximize horsepower and fuel economy. Remember, that it requires horsepower to move a heavier object, and extra fuel load on a plane is just extra weight it must power around in the sky. While thousands of dollars are spent on titanium and lightweight composites in a planes construction, it would be useless to just top off the tanks before every flight. A plane will only carry the amount of fuel necessary to make that particular flight in most cases. This is particularly true in the ever competitive commercial airliner industry where an extra mile per gallon can mean more profit. For corrections for atmospheric conditions, we will utilize a Society of Automotive Engineers, or SAE, chart that will correct for atmospheric changes, and we can follow this chart religiously every time we record results.
Ok, so now we have the room and atmosphere accounted for, but how about the dyno itself? If you are using a hydraulic dyno, this is pretty easy to monitor and influence.
Fluid temps are handled quite easily by a heater and cooler (heat exchanger or radiator).. if you are into making your own stuff...consider a 4 core radiator out of a Ford E350 or pick-up, that has a transmission line run internally through the radiator. Fittings are already made onto it, and it is a pretty straight forward idea...blow a box fan on it when the temp hits your dyno's hydraulic fluid temp number, or if you are creative enough with your time, put an electric solenoid or servo on it to switch it on and off at a given temperature. Biggest thing is to keep your fluid temps stable, and always make your power runs at the exact same temperature. Use high quality fluid as well, and keep it fresh and clean. Hydraulic oil does wear out over time.
Funny side story...
We had a customer that was very happy with our engines and was running top three every night out. Another local builder used the "inflated numbers" sales pitch and lured the customer to take our engine to this engine builders shop to have it dyno'd. This "other" engine builder uses a hydraulic dyno with a pressure gauge and Digatron tachometer gauge. He bolts our engine down to it, and blasts a couple quick runs on it. While our engine is sitting there idling, he claims his engines make "tons" more power. He then places his engine on the dyno, and "wha-la" -- more power..."See", he says. It measured almost 2 HP more - and that's a BUNCH folks! Hmmm.... this other engine builder talked the customer into buying an engine from him right on the spot. With numbers like that - can't go wrong, right? The customer takes this new engine to the track, all excited to have this extra-extra power, only to find out he's only a fifth place kart at best now. The customer brings the engine down to me to ask what might be wrong. First I told him that if there is something "wrong" with the engine, to talk to the "builder" of it...but I think at this point of the game he's realizing he was snookered. Seems our engine was only used as an oil "pre-heater" for this engine builder's hydraulic dyno in his unheated garage. Thats right, it takes less power to move thinner warm hydraulic fluid than it does to move cold thick fluid. Oh yea, I forgot to mention that no corrections were ever used! At least a calculator and SAE chart were never even mentioned.
So...you can't really "fool" a dyno...but you can have a "foolish" operator and "fool" a naive customer with a hydraulic dyno.
Oh yea...some flowbenches work similarly. ;)
My take on inertia dynos. Inertia dynos seemed to be all the rave a few years back. Inertia dynos are similar to chassis dynos in that they provide some "interesting" data. Clutch engage, etc. The biggest advantage of them, that I see anyhow, is that they are fairly simplistic by design...have very few variables, or parts to go wrong, and are CHEAP to build. Comparably speaking anyhow. :) Just the load cell for our dyno runs in the $1200+ neighborhood. I am certainly not an expert on inertia dynos so I can't vouch for their assets. I have my doubts, for sure, about some of the claims of being superior to a well designed hydraulic or water break dyno with decent data acquisition software. My biggest skepticism lies in the fact that we are working with an engine that does not throttle smoothly to begin with. A Briggs engine, in WKA stocker form, has a serious lull, or flat spot in the fuel curve at part throttle. This is most noticeable at about 4000 rpm. Interestingly enough, that is exactly where we are stalling our clutches at with the newer style cams today. This hesitation, or lull, can be audibly heard, and definitely shows up on an acceleration dyno. The bad thing is, that the rpm lull is never consistent from one run to the next. So you have to wonder just how much this little stumble at part throttle is affecting the total acceleration run. Obviously it will take longer to accelerate to the same target rpm if the engine stumbles worse around 4000 during one test than another. It is my personal opinion, and shared by some other talented engine builders, that this is the sole reason you see the use of offset throttle shafts so heavily today. The offset throttle shaft itself does not make any horsepower, nor does it flow any better than the standard one that came in the carb from the factory. In fact, it is often worse! What it does do, however, is create more pulse pressure, or vacuum signal, at the short stem of the carburetor. How does this help? Well, at full throttle it simply does not help, in fact, it often hurts. However, at part throttle, around 4000 rpm, the offset shaft will manage to pull a little extra fuel into the carb bore and it helps reduce the lean condition, or lull at part throttle problem that all Briggs have. This is especially true on a small .425 restrictor plate engine. Another point to consider, is that an inertia dyno is simple in design by spinning a known fixed weight to a target rpm. What rpm you start making pulls on the engine makes an enormous difference. Even 50 rpm one way or another can make a significant change. Ever tried to hold the rpm on your Briggs at exactly the same rpm for any length of time at all? While inertia dynos might be a great idea for translating horsepower from an electric engine or another source that goes through its rpm band smoothly and repetitively every time, it is not, in my opinion very valuable for accurately measuring the output of our little alcohol stumbling Briggs racer. Keep in mind that you are swinging about a heavy flywheel on the end of a shaft hooked to the business end of your Briggs race engine. Most inertia dynos have a flywheel that is huge and heavy. That flywheel has to be precision ground on all edges, not only to make it safer, but to make it as accurate as possible. If the flywheel is even a few thousandths out of round it will be worthless. Remember that steel is easily effected by temperature variances as well, and that under cooler temperatures, steel contracts, and under warmer temps it will expand. Albeit only thousandths, you have to take that into consideration when using a flywheel type dyno as well. Bearing drag, brake drag, shaft flex, and any other outside influences, including air density or turbulence created by the flywheel itself, can and will effect your results. As far as problems with any dyno...again, I think the biggest variable is the operator!
Now, if you have seen accurate dyno numbers on your engine, you are probably wondering why in the world all your competitors are turning all these unbelievable high rpms! When looking at the dyno sheet of a typical stocker, it is not uncommon to see peak horsepower to be around 53 or 5400 rpm even using the newest cam designs. Whats this? Outrageous! I told the cam grinder / engine builder to give me an engine that would turn upwards of 6500 rpms! And I get this pretty little print-out that says peak hp is at 1000 rpm less than what I expected. Well, welcome to reality. In oval track racing it is widely accepted that you will need to turn your little Briggs engine at 1000 or more rpm above peak hp that it produces. Why, you ask? Because of drag, loss of rpm in the corners, creating a wider power band, etc, etc. Theres lots of reasons why we all do this, but to make it seem clearer, consider this. If your chassis AND driver were to not scrub any engine rpm whatsoever in the corners. The tires, the bearings in the kart, etc, rolled without any friction or drag whatsoever, if there were no wind resistance at all, then you would theoretically be able to run the same engine rpm the entire way around the track. With this being the case, you would now want to run a much lower rpm and much higher mph around the track. But what about the starts and restarts, and that guy that got into the side of you down in turn two? You got it, you have to compensate for all these by turning your engine harder. In reality, were not too overly concerned with the peak horsepower that a particular engine makes. In fact I look more closely at average horsepower over the rpm band that the customer will be running the engine. I try to build an engine that suits his/her driving style and the track that it will be competing on. This is why so many engine builders would prefer to custom build your engine, rather than have them sitting on the shelf for the next person who walks in the door. You, as an educated racer, by now should expect this treatment as well. While anyone can take an engine off the shelf and sell it, it takes a knowledgeable engine builder to build your engine specifically for your needs. This is even more critical when looking at restrictor plate engines. While we dont concern ourselves with peak HP numbers, isnt it strange that nearly every engine builder touts or claims big / high horsepower numbers. When in fact, it is not difficult to build a high HP number creating engine. That same engine has a very narrow power band and is about worthless to 95% of the racers out there today. In fact, a good consistently high average horsepower throughout the engines upper part of its power band is what we measure an engine against. Of course everyone has their own standards of expectation, and we are no different. To hear an engine builder claim 10+ horsepower on his WKA legal stocker is simply absurd. Well, on our dyno it is anyhow. While on his dyno, he may reach those elevated numbers, there is no way mathematically that you can achieve such a high efficiency of horsepower on a flathead engine that is 13 cubic engines, with a stock intake flowing at less than 30 cfm at 15! Physically it is simply impossible. Thats not to say that this other engine builders claims are false, simply that he is not comparing apples to apples. Take that same engine builders pride and joy and put it on our dyno, and you are likely to see a 8.25 HP engine, NOT 10 HP! Likewise, Im sure that if that engine builder took one of our engines and put it on his dyno, he would likely see similar numbers to that shown of his own engines, 10+. Typically, a WKA legal stocker will make 8.2-8.5 hp between 4900 and 5400 rpm. We can always move the power curve up and down slightly with exhaust configuration, etc, but the number will be predictably similar.
Don't buy numbers...buy performance!