[cobrashoch]

Bare with me a bit as its sunday morning and I just got up. I have been playing with dynos for years and I think some of the issues brought out in the above posts about dynos show there is a lot of folklore about these machines.
First off, the two most common is types of dynos are as listed above, a chassis dyno and a water brake engine stand type of dyno. There is a third type, a engine stand type that uses a type of torque converter setup.
Next, in spite of what you have been told none of them are accurate. Period! By that I mean no two dynos are exactly the same. I've never seen any dyno that could give you exactly the same numbers even on identical pulldown setups, on the same dyno, let alone the same numbers from a seperate dyno. So where does this leave you if the numbers you geting are only as good as the last pulldown you referance against? WELLLLLL that depends,,,,,,,,on the type of dyno you are using and what you are trying to do.

Engine stand type dynos are good for tuning engines and/or changing setups as eluted to above on the engines only. This is the hot setup for engine developement work, or if you just want a quick break in on a engine with some relative numbers. But I can't stress this enough,,,,,,, don't stay there long, as these types of pulldowns do not reflect real world engine uses. And these pulldowns are VERY hard on new engines. In fact I know one engine builder guy that will not warranty his engines if they are broke in on a dyno first! This type of dyno is the most usefull one for finding extreme problems or tuning problems because they don't mask problems with other factors associated in a car.

The second type of dyno is a chassis dyno or as most commonly called, a rear wheel dyno. It's the least accurate type of dyno but is a real world type of dyno because you test your engine in the car it's intended for. Some things to remember here are rear wheel dynos need to be rated at about twice the horsepower you are shooting for, to be anything like accurate. For example my engine puts out about 700 horsepower and in Indy I only know of one R.W. dyno that is rated to 1500 horsepower. If you don't care about accuracy of numbers, and this is where I am, then you can use underpower rated R.W. dynos quite effectively to tune a engine/car. That brings me to another point. This type of dyno can be used to measure power gains made by chassis tuning of your car. So if you install low friction brakes or a different rear end, or whatever, this is the guy that will measure gains made in this department. To me, this factor alone makes it the winner of the dyno wars.
About exaust gas temp. probes (EGT) vs. oxygen (O2) probes. Depends on the fuel you are using. If you are using any fuel that has oxygen in the formula such as alcohol or nitromethane then a O2 probe will read lean even when you are running rich. Thats why the top fuel guys use temp sensors and not O2 sensors. A temp. sensor is slow to respond though, unlike a O2 sensor and those same top fuel guys use temp sensors for telemetry data only, never for tuning/control on the fly.
Getting back to the real world and gas use now. I found that EGT probe readouts and power numbers are allmost identical to O2 probe readouts and power numbers on the same engines when using gas. The only differences are at the extreme edges of the bell curves and weird things happen there anyhow. For the example above of a EGT setup showing rich but actually running lean your engine would lean surge or backfire or something. Point is, on the extremes other things usially happen that tells you something is wrong. Its not brain surgury here guys. Where the techlology is right now though, O2 sensors are the only way to fly when using gas and tuning, esp. when using O2 sensors for tuning on the fly.
One other thing, If ypu are using O2 sensors to tune your engine on dynos make sure that the fuel/gas you are using to tune with does not have alcohol in it. It will skew your tuning results. It most likely will be O.K. to use acohol added fuels on the street after you set your engine up though. But boy, do I hate alcohol for a engines fuel, and for a bunch of reasons.
cobrashock