I have a question regarding engine dyno's. If you were to run an engine on an engine dyno with *all* it's accessories, its own carb, its own headers, and its own exhaust with mufflers, at the water temperatures it will run on the street, at the oil temp it will run on the street, (ie, not the shops carb, or some stepped headers, or a velocity stack, etc), how much of a difference do you think this would make in dyno readings?

Any thoughts on this?

I've done testing like this.

It makes a big difference. Of course, it depends on the type of muffler used, type of accessories, etc. I can't give you exact numbers, but 10% power loss isn't far off. So on a 400HP engine, all the accessories can bring it down to 360-375. But keep in mind that this is at higher RPMs when accessory drag is significant. At low RPMs, it doesn't loose too much power. Exhaust restrictions reduce power at all RPMs however.

Another big factor is intake air temperature. I did a test on a SB Chevy that made about 360HP if I remember correctly, with 68* degree intake air temp. I raised the temperature to 108-110* (somewhat typical to what an engine sees with an open element filter), power dropped to 335HP. That's a 25HP difference! My lesson learned: Cold air WORKS!

If you COULD run all of your stuff on the dyno, keep the oil temps fairly low, and the air temp at approx. 100 degrees it would be pretty close to what you get in the car.

When comparing the typical published dyno run numbers to real world, you would see at least a 10% drop measured at the crank by the time you ran your headers, exhaust, typical fresh out of the box carb, normal air/water/oil temps.

I doubt that the oil would get over 230 degrees on the street, but on a long dyno MBT/LBT run, it might get to 265-270 deg F., and at that point they would start to control it. Hot oil creates a lot less drag.

Mike