peejay

I haven't been able to find any back to back dyno comparisons for this system vs. other exhaust layouts. Anybody?

j9fd3s

iTrader: (3)

https://www.rx7club.com/1st-generation-specific-1979-1985-18/racing-beat-exhaust-dyno-compairisons-823229/

peejay

Yes, I'm familiar with that thread, but the only RB system in there is the short primary system.

j9fd3s

iTrader: (3)

lmao! i can't believe i missed that... no wonder he picked up 50hp....

peejay

Well, the exhaust is on.

It's hard to do any back to back datalog testing because TunerStudio is a douche and scribbled all over my map, so I had to start from scratch.

But I will say this: The car doesn't really like cruising anymore, there's a cruise stumble that feels like a lean misfire but I can't make it go away with more fuel. This may be a coincidence, like the plugs or wires died or something. That stuff needs replaced anyway. Cruise drivability was always an issue, the engine would miss if the AFR was leaner than about 13:1 unless it was loaded down. My maps would look goofy, rich in the 60kpa range and then leaning out to 14:1 at 80kpa before richening up again at WOT. Cruise on the highway is 50-70kpa. Anyway, I've tried richening out to 12:1 and the roughness/miss is still there. It'd be a weird ignition issue that would only show up at part throttle, but cruise has always seemed like the most difficult time to get the fire reliably lit.

And I did have to take out "some" fuel at WOT, how much I can't say because my old map is gone.

But it is WAY quieter. Enough that I'm thinking, next engine will probably be a (grits teeth) street port engine, albeit one with an uncorked intake side. I have a Holley manifold now and enough carbs that I can sacrifice one into a throttle body.

There's a dyno day next month, if I get the cruise miss sorted by then, I'll run it on the rollers and we'll see how much power it lost

rotarygod

iTrader: (1)

I had a long primary, a short primary, and a stock setup on my old GSL-SE. Obviously not all at the same time. I loved the long primary. It is my favorite system. I didn't have any cruise issues with it at all. In fact I felt more power in that range than with the other systems. If I was just cruising and stepped on the gas, the long primary had the most power at that location. The one thing I did notice was that when I went from the stock exhaust to the long primary, I had hesitation issues that I didn't have before. I ended up replacing the coils and going direct fire with the MSD and all issues were gone. For some reason the stock exhaust masked these issues where the long primary didn't. Later on I switched to a short primary and hated it. I lost all kinds of power.

peejay

I have an MSD driving leading and trailing.

I also have ~35x9mm bridge eyebrows. Not a stock port

j9fd3s

iTrader: (3)

kind of a bummer it lost power. oh well the muffler hunt continues!

peejay

I think I have the drivability issues mostly ironed out. The issue is that it wants to cruise at 60mph at 3000rpm/60kpa, and this is right at the precipice of the brap zone (or "combustion instability" in Mazda Engineering terms). Taller gears to load the engine down more would be nicer... or driving faster works, but traffic flows at 60 in my county so I'm kinda stuck there.

On the bright side, richening the mixture to 12.75-13:1 at 60kpa lets me use less throttle, and even though it wants as much as 12:1 (at 3000/50kpa) to run acceptably smooth, it appears I'm getting 22-23mpg now, or about the same as my VW gets. So there's no fuel economy penalty for driving the RX-7, which is nice. Just a noise penalty because now I can hear the transmission and the diff. Holy crap the diff. It makes all sorts of weird noises - gear whine on accel, bearing whine on decel, and a weird rattling rumble at just slightly more load than neutral. Oh well, I'm the doofus who set the gears up deliberately tight without adjusting the pinion depth.

Running rich all the time does mean I have to change the oil frequently, though. Tradeoffs.

I do wonder if a more early opening exhaust port would help get the exhaust moving with more energy enough to balance out the higher restriction. The midpipe to muffler junction is pretty bogus, too, part of me wants to redo it with a larger diameter Y-piece but the rest of me doesn't want to screw around with it because it's pretty quiet. (Understatement of the year)

I can't stress this enough. THIS EXHAUST SYSTEM IS EXTREMELY QUIET. It has to be heard to be believed. Even my co-worker tells me every morning, "I didn't hear you come in!"

j9fd3s

iTrader: (3)

lmao! my old black gsl-se was like that, street port system + broken diff... i chipped a couple teeth off the ring gear, and it made some odd noises!

i do think it would help if you redid the header length, and y pipe to muffler pipe, but how much? not much i bet...

peejay

I did an oil change on the 22nd, it's been 2,000 miles and the oil's a quart and a half over. Nothing new, just something interesting to point out - the perils of running rich all the time.

Had it at dyno yesterday, will post the sheet after I extrapolate the torque plot and lay it over the HP plot. HP plot has 5hp per point and torque plot is 1 ft-lb per point, so the torque curve looks very exaggerated.

But, on the bright side, I don't think I lost any power. Torque peak is 6000, HP is almost a straight line that curves and goes asymptotic to peak HP for the last 1000rpm or so to the 8000rpm cutoff. I want more power, but this'll do for now. Interestingly, torque is about 20 wheel ft-lb lower across the board compared to the 4-port engines I've done. 6-ports: They close too late at any RPM.

j9fd3s

iTrader: (3)

that's good news! even a 2-5hp loss would probably be acceptable, but zero is better.

i think the problem with the 6 ports are two fold, and its not fixable.
1. is that they close late. late closing = high rpm HP peak.
2. the port doesn't flow that well. the size is ok, but there are all kinds of corners, and walls and dividers and rods in them.

if you combine the two, you get a flat midrange, and weak top end, which is what we see....

peejay

What we see is this:



Which, really, isn't all that terrible. There's no real peak, the only thing is that it's soggy under 1500rpm. Torque falls off near max RPM which is actually kind of a bonus for me, since it helps keep the engine from "running away" when the tires are spinning. If the traction is just enough that the tires spin a little, it sits on the downslope of the torque curve instead of just requiring more driver attention.

What is a little odd is that the curves end before 8000rpm. The MSD has an 8000rpm limiter chip, and he was hitting the limiter on both runs.

John Huijben

iTrader: (1)

That's looking good, nice job!
I see the curve stopping early on our chassis dyno all the time. I'm guessing you didn't use an RPM pickup sensor connected to the ecu or the coils? What we usually do with a quick dyno run is run the car on the dyno at an x amount of rpm, usually 3000rpm in the same gear as we do the dyno run. When the car is running we tell the dyno software that this speed is 3000rpm. It calculates a roller rpm / engine rpm ratio and uses that for the dyno plot. At higher loads a little bit of wheelspin occurs, so the engine rpm / roller rpm ratio isn't correct anymore, and the rpm's will come out a little bit low on the dyno plot, usually it's about 3-5% depending on power output. horsepower is still accurate, but the torque plot is affected a little bit by this.

84stock

iTrader: (5)

Once i switched over to the streetport long primary exhaust my car made its best power ever. 227 rwhp with a streetported 13b and a camden.

And nothing has sounded better either, love this exhaust!!

Jeff20B

iTrader: (13)

The RB "streetport" exhaust is excellent. I built an exhaust for my REPU based on it. The results are superb. Actual useable low end torque for once! And I didn't even need a Camden to notice the abrupt increase in power.

I first tested this exhaust with a Nikki on an S4 NA engine. It had as much power as the old single 2" system did with a 7" Camden on a 4 port 13B. Wow! Then I tried a 74 ported 13B and a 5" Camden. Wow again! It feels kinda like it's got a V8 now.

I tried various intakes and exhausts on this truck over the years. It always ended up slower than I thought it could be. Now I know why.

The next exhaust I built was for the GLC. It is a short primary system collected into 2.5" based on an FC header from RB. This car is smaller and lighter but still feels like it lacks something. You guessed it: the exhaust is where it's lacking. It's not for lack of size or flow; I just lacked the space to go long primary, otherwise I would have. Now I know.

So for the next exhaust I build, I'm looking at ways to go long primary. I've got an MG Midget with a streetported 13B and an RB Holley 600 that needs an exhaust. I already pieced together some parts including an RB 2.5" presilencer and a couple of magnaflow mufflers with flanges, gaskets and u-bends. It all would fit ok and sound all right based on similar setups I've done. But this thread and my recent findings are making me yearn for a long primary even if such a tiny overpowered car doesn't need one. Why not go for it? I'll get about 86 inches of primary length which is similar to what's in the REPU. I'll have to go check what RB uses for primary length...

peejay

So, the exhaust got loud on the way to Coletti's. It's been getting louder. Well, I figured, it's probably the front flange leaking again. It's only held in by five bolts and, not having high-temp silicone to seal it after I unbolted it for the trans removal, I'd used Right Stuff as a kind of last-resort measure/science experiment, so I figured that it was leaking there.

Nope. The frontmost muffler was rusted through at the seam weld to the adjacent pipe. Trying to weld it shut, naturally, kept opening up the paper-thin metal, so where it was originally a hole the size of a pencil eraser grew out to a 4" long gash, and there's a nice huge blob where I used several yards of MIG wire.

Huh. I thought these things weren't supposed to rust out.

Weird thing is, there didn't appear to actually be anything in there. But, it's back to its absurdly quiet self. Literally can't hear the exhaust at all when cruising down the highway. Full throttle, or even just accelerating in city traffic, just gives intake moan more than anything else. It's surreal.

Jeff20B

iTrader: (13)

peejay, you were probably dealing with the expansion chamber section of the presilencer. They are empty for about 4" and then hits a 3/8" plate which holds the perforated core in a slip-fit design. Behind the plate is the pot-scrubber sound reducing material. The pipe entering the expansion chamber ends about an inch away from the 3/8" plate to allow the supersonic blast to dissapate a little before it enters the perforated core.