Moving from the ITB MR-2 to EP RX-7.
I’m new to the RX7 and would like some information about the exhaust system. I’m building an first gen EP 12a or 13b race motor and it will only be used for the race track. I have been trying to figure out what exhaust set up I need. Some say long primary, some say short, no collector, yes to collector, muffler, no muffler, etc….. So here is what I was thinking, let me know what you think, remember I’m new.

12a or 13b street port
2.5 headers going to dual 2.5 pipe to a 2.5-4x12” long collector on each primary, then to a 2.5 Y to 3” over the axle to a muffler and a short 3” tail pipe.

Please let me know if this will work or let me know what I have to change.

Thanks
Bill
Ashland VA

 

How competitive are you looking at being? The guys at the runoffs are using variants of short primaries (like, collecting around the firewall) and with SDJ just sort of disappearing, there's a lot of guys just making their own headers. KC Raceware does one of them.

Also, for the first generation RX-7 running EProd, talk to someone about non-stock front hubs; KC raceware does a setup and there's other options too. The stock brake rotors, under the thermal loads of an EP car, crack, and with the rotor being the same casting as the hub, can lead to the front wheel parting company.

 

I'm running an FC 13B in EP, and I have the same question about the exhaust. Right now I run one pipe for each rotor out the back, never joining the two. One muffler on each pipe. I have no clue how much power I'm giving up, but I figure it's probably not the best setup. I'm weighing my options for a new exhaust.

And while I'm off topic, if you're doing a new EP build with the FB, keep in mind that with the new radial slicks you might not have to flare the fenders. I don't know how much room FBs have in the fenders, but the Hoosier 215/580R15 might save you money in bodywork. Those are what I run on my stock-fendered FC.

 

Can opened. Worms everywhere.

Exhaust has such a huge effect on the performance of these engines and everybody has their favorite set up. I'll just say search, read and experiment. You'll want the system collected. Short primary or long depends on your porting, intake system and where you want power. You'll also NEED muffler, maybe two to make the car quiet enough and to keep you from going deaf.

I've tried just about every length primary on different cars over the years. You're probably chasing the last 7-10 hp when you get beyond a nice flowing system. By the time you're done they will cost you about $200 for each one,lol.

 

robert at rotaryshack makes a wicked one, look on his fb, sick *** welds and great power

 

Look at what the other EP cars are running. The 2 most popular options in my opinion are the Mazdaspeed/ISC and SDJ designs. Both are NLA but Rotaryshack is making an excellent copy of the SDJ. You might be able to find a used Mazdaspeed/ISC header and can fabricate the mid-section yourself. There are pictures and details on this forum I posted before. You will absolutely need a muffler, these are very difficult to keep quiet and they also melt most mufflers in just minutes. Consider a Burns muffler with SS scrubble you can repack or be prepared to change your muffler often. Found a pic that may help, that's a Rotaryshack 13b on the left and SDJ 12a on the right.

 

This is helping and again thanks for the info.

 

if i can throw a suggestion out there, make the system modular, and then go to the dyno.

just to use my car as an example, i've got an RB header on there, and i was going to buy three 4 foot lengths of pipe from RB, and cut one in half. i then am just going to order some clamps and go dyno.

this give me, header + collector, ~26"

header + 2' of pipe + collector ~50"

header + 4' of pipe +collector ~70"

header + 2' of pipe + 4' of pipe + collector ~90"

and i can go dyno all four and see what the engine likes. after i get close its a small matter to try 89" or 95" or whatever.

then once i know the primary length, i can pick mufflers that actually muffle.

i'm not playing with diameter, the 44mm ID of the RB pipes is about right, for me.

so i would suggest trying something like that, it would be a day on the dyno, but it does take the guesswork out.

 

Just happen to have an ISC full exhaust for sale if you are interested for a 13b

 

The best power we've seen is from short primaries, into a long expansion, a short reversed expansion back to 3" to the back of the car. The length of expansion really depends on the port work of the engine.
As far as muffler, forget about not running muffler. You will need them and at least two or three of them. The more money you spend on the muffler, the less you need. Mazdaspeed has some which are heavy, but work well due to lava in them.