Dyno Exhaust Extraction size

What size pipe are people using for there exhaust extraction from the Vehicle to extraction motor the section screen shotted below in there dyno setups.

The unit I have ordered has a 250mm inlet and outlet on the motor but can't see ill need that size piping to the vehicle but thought I would ask as was thinking of reducing down but keep exhaust from motor to outside 250mm

As a general rule, the bigger the better - within practicality for handling. At a minimum it, and the extraction fan, need to have the airflow capacity to exceed the exhaust gas production of anything that is going to be used on the dyno', in order to prevent the gases 'backing up' into the work area. IIRC, the volume will be approximately proportional to the temperature in kelvin (plus a little for the fuel to gas during oxidisation), so whatever the CFM of air entering the engine may be several times as much, hot from the engine's exhaust. On that, what extraction fan are you using?

Having the larger diameter will also potentially allow cooling surrounding air to be drawn in with the exhaust, lowering the gas temperature which should aid in the service life of the hose(s) and the fan components - some engines have VERY hot exhaust gas temp's, and you need to allow for that, possibly even having a slip-in metal section that can be added as required.

Oh, on temperatures, what is that ducting made from - most conventional air extraction systems are intended for ambient, or relatively close to it, temperatures, and will simply melt from exhaust gas temperatures - even with the cooler air being drawn in.

As a general rule, the bigger the better - within practicality for handling. At a minimum it, and the extraction fan, need to have the airflow capacity to exceed the exhaust gas production of anything that is going to be used on the dyno', in order to prevent the gases 'backing up' into the work area. IIRC, the volume will be approximately proportional to the temperature in kelvin (plus a little for the fuel to gas during oxidisation), so whatever the CFM of air entering the engine may be several times as much, hot from the engine's exhaust. On that, what extraction fan are you using?

Having the larger diameter will also potentially allow cooling surrounding air to be drawn in with the exhaust, lowering the gas temperature which should aid in the service life of the hose(s) and the fan components - some engines have VERY hot exhaust gas temp's, and you need to allow for that, possibly even having a slip-in metal section that can be added as required.

Oh, on temperatures, what is that ducting made from - most conventional air extraction systems are intended for ambient, or relatively close to it, temperatures, and will simply melt from exhaust gas temperatures - even with the cooler air being drawn in.