I'm thinking of creating a website dedicated to modifying 504s and 505s as well as other RWD pugs. Steve Palocz (NSW club president) will probably be working on this with me. There are heaps of websites dedicated to 504s and 505s but very little have anything to do with modifications. This is unfortunate because they have well designed bodies and suspension which are well suited to much higher levels of power.
Here's a list of some of the high power conversions I have seen or heard of:
Richard Markin in Queensland had a turbo charged 504 which produced about 240hp in it's final configuration. It used the 504 2 litre engine with lowered compression and had a suck through carburettored, non-intercooled system. He had a 4 branch exhaust which went over the head to the turbo which blew directly into the inlet tract and sucked through a 45mm weber. I think he said he was using about 12psi boost and the head was modified.
Ivan Washington in Victoria used to have a 504 running 9psi boost, with no other mods, which had 105hp at the wheels at 4000rpm. Thats the same as what a N/A 2 litre Pulsar produces at about 6000rpm.
A guy in Dapto (near wollongong) has a 505 with a GMH 3.8 litre V6.
Michael Richards in Sydney had a nice lowered 504 with a P76 V8. It had extractors and apparently produced about 200hp.
There are a few 504s running around with Rover V8s.
One of my friends who had a modified 504 sold it a year ago and bought a BMW 323i. He is now sick of the BMW and wants to go back to a 504 and eventually stick a modified Rover or possibly Lexus V8 in it.
I've seen quite a few 504s with PRV V6s in them. I drove one with a standard carburettored 2.7 PRV V6, but I found that it didn't feel much faster than a 4 cylinder 504 and it certainly didn't rev as well. I drove another modified V6 owned by Chris Lynch in Newcastle. It was quite impressive. It was a 2.7 with extractors, a single 2.5 inch exhaust, modified camshafts, modified head and a volvo injection manifold breathing through a Gas-research LPG mixer. It revved quite nicely and sounded like a Ferrari Dino. It felt nothing like the stock V6. I wouldn't have a 504 with a stock V6, because for the same money you could turbo or supercharge the 2 litre and have a much faster car.
I've got a turbocharged 504 engine sitting in my father's garage, but I don't have the time at the moment to sort it out well enough to put in a car which I use daily and need to keep as reliable as possible.