What is a chassis dyno?
The vehicle is parked on the “drive roller”, the vehicle is driven, and the output is measured by the dynamometer. The dyno measures torque and power delivered by the power train of a vehicle delivered to the surface of the "drive roller" directly from the drive wheel or wheels (without removing the engine from the frame of the vehicle).
Horsepower gains and drive-ability improvements can be made on the dyno by custom tuning computer chips and/or flash tuners such as the SCT XCalibrator 3, and by tuning fuel pressure and ignition timing.
How long does it take to tune a car? How many passes do you make?
The short answer is “As long as it takes." Regardless of the combination, we tune the car until it is right. There is no pre-set limit on how many pulls we will do. Some cars are very simple to dial in, with straightforward combinations of parts, MAF sensors that are calibrated perfectly, fuel systems that are working right, etc. Those cars are done in about three hours with as little as three or four full-throttle pulls and a few "miles" of part throttle tuning on the dyno. Other cars have more complicated setups (high stall converters, MAF transfer functions that need a lot of work, injectors or meters that are past capacity ["pegged"], FMUs that have to be recalibrated, fuel pressure that needs to be adjusted, supercharger belts that are slipping, spark plugs that need to be re-gapped, etc). A really complicated car could take all day and 15 or more passes. The best thing to do is to arrange to drop the car off for the day. We take our time and will give you the best results.
How important is dyno tuning?
Given a choice between spending $500 for a day on the dyno and upgrading to a trick set of cylinder heads, most racers on a budget would opt for the heads. But a different view should be considered. A good set of heads on a poorly tuned engine is simply a poor set of heads. "The goal should be to maximize the stack of parts you have."
What do I need to know before the car is strapped on the dyno?
Be realistic about the time needed to test different cams, cylinder heads or headers. Be sure cams or high-ratio rocker arms have been mocked up in the engine to test for piston-valve clearance before going to the dyno.
What should I bring?
Spare parts, especially anything related to fuel and ignition. Don't forget gaskets, pushrods, rocker arms and other items that could break or fail during a run. Many common components for example, caps, rotors, spark plugs are available in the Speed Shop, but if you know you have an unusual combination having extra components will ensure a productive session.
Is the dyno hard on the vehicle? What are the risks involved?
Running the vehicle on the dyno puts no more load on the engine than going full throttle on the street (or track). In many ways running a vehicle on the dyno is actually safer than running full throttle on the street (or track), because conditions such as air/fuel ratio, coolant temperature and ignition timing are monitored closely.
Any time an engine is operated at full throttle, whether on the street, racetrack or on the dyno, there is a small risk of something going wrong or a component failing. While this risk is actually somewhat smaller in the closely monitored environment of a dyno session, the risk still exists none-the-less. Cessna MotorSports, its employees and owners assume no liability for engine or driveline damage that may occur while the vehicle is operated on the dyno. All dyno work is strictly and expressly AT THE CUSTOMERS'S OWN RISK. All dyno services require a signed liability release form.
How could two dynos give conflicting data for the same engine?
People say dynos don't lie. Well they don't but the people entering the acquisition data DO. If they fudge the numbers here or there (barometric pressure, relative humidity, air density, air temperature, etc.) then the dyno can only compute what it is told to compute. So in reality, the dyno won't lie, but it will give bogus readings if the data isn't accurately fed into the machine.
Let me tell you a quick little story. I got this call from a potential new customer. He runs a car on the open track and it's a 383 Chevy with mildly ported iron heads. He said it dyno'd at 538HP with 9:1 compression and ran on 91 octane fuel. I said that was hard to believe so he sent me his dyno sheets and sure enough, it said 538HP BUT when I looked at the atmospheric pressure reading, it was at 20 psi !!! Let me explain something; on any given day we have 14.7 psi of atmospheric pressure. Engines don't actually "suck-in" fuel and air. It is actually blown-in by the atmospheric pressure we have around us. Just like you wouldn't get sucked out of an airplane if the fuselage got a hole ripped in it you'd get blown out. There's no vacuum in the air at 35,000 feet! There might be a vacuum in space but planes don't fly in outer space, they fly INSIDE our atmosphere. The pressure is very low way up there (because there's no air), so to protect your ears and so you can breathe they pressurize the cabins with air pressure and oxygen. Well, if a window pops out of the plane, all of the air (about 8psi worth) all wants to go out, and it "blows you out" with it. OK, back to engines. So the atmospheric pressure actually blows air into the engine. If you increase the atmospheric reading on the dyno, it will calculate (correct) the power number. Now let me tell you a quick little tidbit bit about blowers so you'll understand what the hell I'm talking about. Blowers INCREASE the amount of pressure being forced into an engine. If you have a 350 cubic inch engine and you ADD 14.7 psi of blower boost, you just DOUBLED the cubic inch displacement of that engine by 100% simply because you doubled the atmospheric pressure. Now that 350 cubic inch engine has 700 cubic inches of "volume" inside making the power. Well, it's pretty easy for a 700 cubic inch engine to make 700 HP, just as it is easy for a 350 cubic inch engine with 15 psi of blower boost to make 700HP. It's all the same thing, just done different ways. Pressure is pressure and pressure makes volume! Anyway, my point here is, the atmospheric pressure reading on this guy's dyno sheet was at 20 psi. Well that is equal to having a blower on that engine forcing more than 5 psi of blower boost into it! Figure an average, well- built 383 engine with 200 cc heads and a cam profile like this guy had (which was fairly radical), that engine would have surely made an easy 435-440HP. Now add that 5.3 psi of boost and it would have easily tipped the dyno at about 535 - 540 or so HP. How do I know that you ask? Because on any "typical" performance engine, you get "about" 18 -20 HP per pound of blower boost, so 5.3 psi of added boost into that engine comes out to an extra 95 -105 HP ON TOP of what it was actually making. So let's say he had 425HP. Now add the equivalency of 5.3 psi of blower boost (95 - 105 HP) and you end-up with 535-540HP, which is exactly what his dyno sheets said he was making. Go figure.... This ain't my first rodeo :-) So the dyno didn't actually lie but the data fed into the machine certainly did. This is how magazine articles inflate numbers on dyno's, how a cheapie engine can look good on the dyno and how you can "fudge the numbers" on a dyno to make it look like the engine makes more power than it actually does. Now for DeskTop Dynos. You can calculate just about anything with math. All Desk Top Dyno's do is crunch numbers. What they don't do is take into consideration things like "velocity", port matching, exhaust physics, air / fuel ratios, hot day air as opposed to cold day air, humidity and a zillion other factors that need to be part of the equation. The problem is there is no place to add such numbers in a desk top system. You can put in a 283 engine with a 600 CFM carb into a desk top system and it will give you a power number. You can take that same 283 configuration and bump-up that carb to 10,000 CFM and guess what? That DeskTop system will show that it makes much more power. The reality of that is... its bullshit! Those little systems are ok for calculating valve timing figures and "approximate' power numbers within a given scale, but they certainly cannot replace the real thing or else all of the major engine building shops would sell their dynos and buy the $40 systems on a CD ROM and call it a day. It doesn't work that way and big shops still use VERY expensive dynos for a reason. Someone once asked me, "Bob how come your 400 HP 350 eats-up my buddies 450 HP engine at the race track." I just answer, "it's because my dyno measures horsepower with horses and those other guys measure their engines with Ponies!"
By Bob Cessna
Why should I choose Cessna Motorsports for tuning my vehicle?
Why use a chassis dyno instead of an engine dyno?
The Chassis Dyno is "real world", it tells you what horsepower you have where the wheels meet the track.
What's with the numbers?
Customers are given corrected horsepower and torque numbers that are computed from the raw numbers to reflect changes in temperature, atmospheric pressure and altitude. This correction factor provides a standard upon which testing can be equally compared under different weather conditions or from different locations.
Note: During this correction process, unsavory or ego-driven engine builders can fudge the numbers given to customers and magazines. What this tells you is be realistic about the truth in advertising. For more information, see How could two dynos give conflicting data for the same engine?
How do I pick the right carburetor size for my car & engine combination?
There are 3 main parameters that correctly determine your baseline selection. From there you need additional information to fine tune that selection. Let's start with the big 3.
A) Cubic Inches: Simple parameter for total cubic inches including any additional bore & stroke.
B) Total RPM Range Both Minimum and Maximum:
C) Power Level: The third major consideration is horsepower. Your engine needs X amount of airflow to reach a certain power level efficiently. The formula for CFM consumed is (CFM = CID x RPM x VE ÷ 3456). Here CID = Cubic Inches & VE = Volumetric Efficiency. This is just a rough estimate as VE number is the basic efficiency or cylinder filling of the engine. Example: A typical small block Chevy 383ci stroker engine at 6500 RPM assuming 100% VE. Plug in the numbers & you get a CFM requirement of 720. Street motors might be 90% & a good race engine might be 125%. Save yourself some time; learn about airflow numbers! Real Carburetor Airflow Numbers. Years ago the Society of Automotive Engineers provided a standard for Airflow that's still used today. For 4-barrel carburetors that number is measured airflow at 1.5" pressure drop, or used on a flowbench is 20.4" water. For example a 750 cfm carburetor should flow 750 (cubic feet per minute) at this pressure drop. A carburetor has fuel flowing through it which actually displaces some airflow (usually around 8%), so this should actually be flowed with fuel or wetflowed. The problem comes when some carburetor folks do not use SAE numbers. We've had many carburetors through the years claiming to be one number & some would be well over 100 CFM less. Some carb folks would use 28" of water to flow carburetors & hence the totally unrealistic claims. Now as far as airflow goes we've found that it takes roughly 1 CFM to make between .7 & .8 horsepower. If you take the SAE numbers of 1.5" of depression to get you're CFM then on our previous example with the 383 Chevy then you need a 720 CFM carburetor roughly without considering the other factors. We have found that you actually would be better off using a carburetor that flows this amount of air at a lower pressure drop (less restriction) like 1.2" to 1.3" as this usually makes more power.
If we can be of any help we have a great tech department that can supply the correct Performance carburetor for your specific application.