First gen rear spring perches
 

Can anyone offer advice in switching from 5"od springs to 2.5" springs? The upper spring search looks like a pain in the ass to remove.

I did this on my race car. I used a plasma cutter to remove the upper spring perch then replaced it with something I fab'd up to fit a 2.5" coil over. On the axle I built a new mount that includes an adjustable mount. It came from a circle track adjustable spring perch I bought a bunch of years ago.
My build thread
https://www.rx7club.com/build-thread...-992966/page2/
Post #35 shows the upper mount. Note that I added a 3/8 weld nut so that I could install a screw in bump stop.
Post #13 has shots of the axle mount

My car sits about 4 inches lower than stock and this does create issues getting a spring with the right free and compressed heights with the desired rate. Currently I am running a 2.5 x 7" 275lb spring. If I go over 300lbs I will probably have to go to a 6" spring. In hindsight, a 1 7/8" coil over would have been a better choice.

Wow wish I had the talent to build a car like that. Love the airbox, I have been trying to work something out. I ran 3.5" brake duct from each bell to a filter but vibration tore it up.

Thanks. I struggled with the air box design for quite awhile. It helps to be in a shop with some one that can TIG weld aluminum. I also have a short intake manifold which allowed for a taller box.

Another option would be to duct air from cowl opening at the base of the windshield like they do in NASCAR. I have seen this done on a really fast E Production car. Safe as long as the cowl is sealed from the interior. Only for a race car.

Here's what I have going on now

https://cimg3.ibsrv.net/gimg/www.rx7...55ee90a34.jpeg
https://cimg4.ibsrv.net/gimg/www.rx7...6dd1678fba.jpg

Nice!

What we have noticed on the street ported race cars in my home town is that there is column of air that pulses in and out of the throttle bores. So much so that one car cracked the aluminum lid on an air box and caused pop rivets to loosen. That is why my air box uses fairly thick material and is welded together. Even so, I am always looking for cracks.

So my conclusion is that you need an air box/chamber over the throttle bores on these cars. The bigger the better.