In message , David Lang
writes
Hi Peter
Blame it all on Daniel Bernoulli in 1738 and Giovanni Battista
Venturi (1746-1822).

The detergent is drawn into the lance by the Venturi effect, the
lance design on most if not all pressure washers means this only
works at low pressure settings. You can have foam or high pressure
but not both at the same time.

Spot on Peter! Physics & history!

DIY pressure washers either have a pick up tube on the machine or a bottle
attached to the lance. The bottle does give more of a 'foam' effect.

But why do you want foam? The detergent you have is unlikely to remove the
road film on your car however long you leave it on. You need a traffic film
remover - check out;

http://www.chemiclean.co.uk/gt.html

http://www.jjsmith.f9.co.uk/turbo.html

I use these people

http://tankstorage.co.uk/index2.html

It's very difficult to price up TFR without knowing the concentration
and recommended dilutions

--
geoff