12a Exhaust Design
 

I've had search and read through a lot of posts about exhaust design for turbo and N/A engines but I still have a few questions and gaps in my understanding that I would like to fill before I continue. (hopefully you won't mind giving me some of your expert advice)

A bit of background first.

I live in the UK and after much searching for an uprated exhaust for my MK3 1984 RX7 I think I have finally settled on the idea of having my own system custom made by a specialist.

My car is 1984 UK spec RX7 - This means it has slightly larger ports than a US 12a and of course I have a steering box on the same side as the exhaust manifold so space is more limited than a LHD car.

The conclusion I have arrived at is that I would like a long primary system kind of inbetween the RB streetport system and their base system. This is how I would descirbe the system I have laid out in my mind.

From the exhaust port its straight out of the port for at least 3" 's (if possible) in 2" <?> 16 gauge stainless pipe. The 2" pipes would then merge at around the centre of the car 36" ? and into 2 1/4" pipe and then onto a straight through silencer and then onto a baffled rear box. All would be stainless packed.

What I am aiming for is a quality exhast system that does not resonate like a cheap thin walled system and is not too much louder than the stock system. But is also not going to limit the performace (so basically a nice compramise between performance and noise)

Ca someone please point me in the right direction and give me some advice so I can make the right decission

Cliff notes

I live in the UK so RB is V. Expensive
I'm getting a custom made exhaust
I want power and low'ish noise ... a compramise
its a 12a but with bigger ports and its RHD so space limited

Thanks,
Mike

(P.S appologies for the spelling I'm an engineer I can't help it :p: )

The long primaries will make it louder than short primaries. The thicker the exhaust walls are the less of the tinging there will be. The best exhuast system I have experienced, would be headers straight to a flowmaster. It is loud though, the reason being is the flowmaster is made of very thin material. Most of the noise comes from the flowmaster being so thin and all of the noise comes through the walls of the thin metal. If you could wrap it again with thicker metal walls and pack in between with silencer packing, I would think It would quite it enough.

So you are saying you like the flowmaster exhaust because it was tinging? I hate it when my car does that, i'd rather get the thick exhaust.

Will the Racing Beat "Street Port" exhaust system fit? I get excellent performance from this exhaust with my stock-block 84 12A with 79 carb, and the noise level is close to stock. Many of my friends complain that my exhaust is too quiet.

If you are going to make your own exhaust, I recommend using two staggered pre-silencers like the Racing Beat exhaust, and don't go cheap on the muffler. For a race-worthy custom long primary system, Racing Beat recommends 1.7-1.8" ID primaries collecting with a 30-40degree convergence point 120-125" from the engine flange to the crotch of the collection point.

IMO the Flowmaster muffler is not suited for a non-turbo rotary engine. If the Flowmaster is tinging a lot, it is usually because the rotary engine blew one of the cheap baffles off its weld. Two out of two of my friends who tried the Flowmaster blew out the baffles with their NA 13B in a matter of weeks.

No, I dont like the tinging. Never had any problems with flowmasters. Iam currently running the Racing Beat Street port exhaust on my 12a street port with Holley 600. Feels, to me like more restriction than the Flowmaster. When I finish breaking in my engine I will run it on the track and see if it runs better with straight headers. If I get better numbers with out the muffler connected I will be converting back to the Flowmaster. Oh and by the way, I would stay away from the pre-silencers, RB says it gives slightly less performance than with out? Keep with thick walled exhaust!

My assumption was right, on a slick track and crap 185 tires I was getting 10.0 secs in an 1/8th mile. I took out my mid pipe and ran straight headers I brought my time down to 9.6 spinning the tires off the line and through second. I will be looking into converting back to a Flowmaster!

That's pretty cool you had so much difference with a straight header exhaust, 0.4 sec in 1/4 mile is noticeble.

MikeMR : I had the same problem as you,seeing that my car is also RHD.Firstly,let me ask 2 questions : Is it a stock port ? Daily driver ?

I decided to design my own header,and it was made with me standing at the exhaust shop.Mine will be a bit too large for your application though.I can however try to point out some noticeable points.

I`ll use mine as an example : Its made of 57mm (2.25 ") Pipes,mandrel bends.Merging into 2.25",the header is flanged,and can be removed without the rest of the exhaust.

For your application : 45-50 mm pipes(1.77-2.0").Thick walled steel.2-4mm (0.07-0.14")With a resonator in each pipe.Merging into 1 behind the drivers seat. a 2.24" pipe will do nicelly ,maybe even a 2.5 ".The length should be 120 ",it will yield good results

Running into a turbo rear silencer (Semi free flow)

The import things are : The pipe should come out at least 25mm (1") before starting to turn from the flange at the engine.the pipe should be as close to equall length as possible ,6mm (1/4 " ) if your serious about performance.The bends are important ,they shouldnt get smaller as they bend ,going a bit square around the bends is ok,but not smaller.

The collector should be equal ,and preferably big.
So that there is no interference between the 2 pipes comming together.

Hope this is a pointer to you ?

Sorry i dont have any pics to post

Karis

1/8 mile, not 1/4. I will be back at the track soon and should have a better time than 9.6 in an 1/8th NOT 1/4. I have ported my intake and will have a better fuel pump, so my fuel pressure will stay at 6psi. Hopefully low 9's?

on my 82 i am going to go from a road race header, to dual 2" pipes down to right after where the main cat was, them im going to collect into a nice sized pre-silencer, and then split back into the dual 2" that the stock system uses to the muffler

hehe - mine will just be directed straight out of the bootlid. No collector, jus a load of noise and (high-end) power!!

PS: this is in my beetle hybrid, not an RX7