Off throttle blown diffusers question.

[Tom550]

So I've searched a bit and can't find what I'm looking for. I probably won't ever be on this forum but I thought you guys would be the best to ask.

I've pretty much understand how blown diffusers work but how big of a difference in downforce is changed when no gas is flowing out?
I've read saying downforce is lost. Wouldn't when nothings flowing out of the exhaust make it work just like any other normal diffuser?

Or is the diffuser designed differently where it only works with the exhaust?

This has just been bugging me and I can't figure it out.

Thank you.

[Richard]

You'd want to maximise the downforce so that would lead to a huge diffuser with high exhaust flow.

However when the exhaust flow slows or stops, then the diffuser would be too big and inefficient. It might even stall.

So you'd end up with a smaller diffuser that works over a range of exhaust conditions.

[flynfrog]

You can be more aggressive with the outlet angle if you are planning on blowing the diffuser. This may result in a non blown diffuser that doesn't work so good.

[Tom550]

Ok I think I get it.
Thanks guys

[raymondu999]

Also Tom; you need to remember that when you're going full blast down the straight; you'll be producing a lot of gasses from the exhaust. When you're coasting in a slow corner with no throttle; your engine will be running at a much slower RPM and producing much fewer gasses. So this creates the added complication of having more downforce when you're full throttle; less when you're on less throttle. It's an added confusion for the drivers and it becomes slightly more unpredictable to drive the car because the rear grip changes as you go along.

[olefud]

I expect that there are few circumstances under which no hot gas would be flowing. Most throttle-lifts occur at the end of straights with the engine wound tight and high heat in the combustion area. Rather than closing the throttle and cutting fuel flow, I expect that engine management systems cut the ignition while allowing fuel and air to be pumped through the engine. This diminishes the piston assembly pumping load and, with evaporating fuel, cools vital parts. And incidentally the fuel mixture may ignite in the hot exhaust to produce energy and gas through the diffuser