Does the 90 GTU use exhaust back pressure for the 6 port system or whatever? I have a friend who wants to delete the cat and straight pipe it to the mufflers. What kit do y'all use to do this? I have an FD so I've never looked into it for the FC.

Thanks guys

 

No. It is vacuum controlled to my knowledge on all cars 89-92 (Series 5). Exhaust back pressure is used only from 86-88 (Series 4). Though I don't know enough about the Series 5 system to give you any advice, there is a popular conversion which is to use an rpm switch to control an electric pump to drive the actuators at the correct rpm, there is a write-up here:

Emissions Removal

If the system works, I would just leave it be.

EDIT: The page linked above with the conversion write up is entitled emissions removal, presumably an error made by the author. The content is correct though.

 

The S5 aux ports are powered by pressurized air from the airpump.
No vacuum, no exhaust.

 

ok, thanks. Is there any part/model in particular y'all tend to go with deleting the cat? And I get "if it ain't broke", but it's not my decision, lol. I just promised to find the answers. I was looking at RX7.com which I think would be fine but they use presidencies and I think my pal was wanting a straight pipe instead. And is the exhaust set up similar to the FD? With a manifold/downpipe to a midpipe to a cat back? Or it it just two sections?

Thnaks

 

If the exhaust is still stock, there's a manifold> two precats> the main cat>y-pipe.

 

I should have noticed my error when I recommend replacing it with an air PUMP, lol.

 

the FC NA exhaust sleeve and manifold design are pretty neat. it has two functions, the first is to make the exhaust quieter, the second is to decouple any tuning effect downstream from the manifold (search for exhaust box).

the piping size Mazda chose happens to be on the small side of just right, also. the real problem with the S5 stock exhaust is that the flanges are welded on the INSIDE and not the outside so that while the pipe is ok, it necks down at the flanges.

the easy fix is a die grinder, the bucks up setup is some kind of downpipe/presilencer. or downpipe/cat.

after that the stock mufflers could probably be swapped for something straight through, and then you'd find that the intake is now the restrictive part