The exhaust port area of a stock 13B is equivalent to a 1-5/8" round hole. I see no reason why you'd want a larger exhaust pipe. Even a large ported exhaust only adds up in area to about equal to a 2" hole. At that point I could see it but for street use a thickwall RB header pipe isn't a problem.

 

You can use the diameter to manipulate the length. If packaging a longer length primary is an issue, use a larger primary.

gd

 

have you looked at the MFR system on the IMSA SA22? its TEENY, easily the smallest pipes i've ever seen on a rotary 45mm OD

 

The old race housings from the 80's that didn't have exhaust sleeves kept the exhaust area pretty constant. This made them look very small. The reality is that the street versions are less than optimal and the exhaust sleeves makes the header pipes too large.

 

lol, be sure to post up all of that zero overlap reciprocating engine tuning information

 

Still can't get over that lack of overlap can you? You can tune without overlap. Overlap is not the key to making power. If it was, the Renesis would still be a 160 HP n/a 13B. The Renesis engine's exhaust problems come from flow through the ports which is terrible. That's the restriction on that engine. Anything you do after that point is after the worst flow area so gains will be small. If you've found any power through any exhaust design, and I know you have, it was completely due to tuning only and had nothing to do with flow. The stock exhaust system physically flows more than enough for the engine's power level.

 

It's nothing you don't already know Gordon.

 

From a theoretical standpoint, how would one make the casting better? If possible.

I'm not familiar with the renesis. I know they have side port exhaust to achieve less overlap than the 13b. But, is it just a bad design from Mazda? Just the whole having to leave through the side instead if the periphery? And do the exhaust ports in the center plate stay divided or do they share the same hole through the plate?

 

let me take a stab at this, and if i'm wrong, i'm sure RG will correct me accordingly ...

to correct, or rather improve, the flow through the Renesis exhaust ports requires re-transitioning/re-contouring the port bowl and aperture. however, any really significant work in that vein usually leads to breaking into the water jacket. so far, i've only seen one person do a job (in photos, not in person) with the exhausts that has truly impressed me, and that is BDC.

to make the casting better, i would imagine if they reconfigured the water jacket in such a way that it gives you enough material to work. at least, i'm pretty sure this would work for the secondary exhaust ports. i'd imagine working the primary exhaust ports would tend to be a little more tricky since they share a runner. even though it would change the dimensions of the housing, perhaps if they made it thicker (similar to the thick 20B side housing), so that they could be separated, maybe that could make better flow out the primaries.

 

It's very hard to explain what is wrong with those housings without actually showing you one. The exhaust sleeves are much larger than the exhaust ports. When looking at it, it would appear that you could just hog out the ports to match the size of the sleeves. However if you try this you will hit the water jacket. Mazda cast these engines in such a way that there is a very small amount of water jacket behind the entire port and runners. It gets very thin and runs right near the port opening. You can't really enlarge the opening without fear of hitting it. All you can really do is add a slight radius to it. There is no way to make them larger without cutting into the water jacket and then filling it back in with weld. I tried this and ended up cracking the cast iron housings. It's not easy. Lots of people kept telling me to do certain things to the housings so they wouldn't crack but they always did. I actually had one of our best certified welders try it here at work. No luck. I'm not saying it's impossible but I haven't found a way to do it.

On top of this, the ports travel straight back into the runnersby about 3/8" or so. When they hit the exhaust runners, they don't turn gently. There is a 90 deg angle with a sharp edge. It is terrible for flow. Although the port timing of the side exhaust port is more condusive to making power over the peripheral exhaust port, it's flow characteristics are ultimately what holds is back. Due to the shape and angle that the exhaust runners leaves the housings, there is no way to make a new sleeve that helps the transition without severely choking the exhaust path. You also can't enlarge the port to match. We are pretty much stuck.

As a result of this, there is huge turbulence at the exhaust ports. This is the worst flowing part of the system.

 

This topic is really good, but I'd just want to say that this isn't always the case.. NA race engines with usually a quite narrow powerbands use a plenum with is fed through a pipe with a predifined length.. the aim for this is take make the air resonate there so that for a given rpm, the pressure in the plenum 'peaks' when a cilinder draws air in, if you want in a 4 cilinder engine to use this effect to happen at the same time with primary intake runner length tuning the frequency should, offcourse be 4 times greater, this effect is similar to what happens at the exh side just after the collector

now for a rotary, what harmonic is realistic to use to calculate primaries? also when would one want to have the lowest pressure at the port? when it opens I guess?

 

The key word here is Race, yes the feed pipe will change the tuning of the intake but the amount of change is not enough to give two ***** about unless you are trying to shave a few 1/10s of a second off you lap time.

 

I'm not sure you fully understand how the exhaust box works.

 

Probaly not, I'm always keen to learn..

I tought the piece just after the collector, whether be it a pipe or megaphone, it's function is to create a low pressure, like in minimum of a sine wave, at the collector end, on the right moment..

do you mean something different by exh box?

 

Heh, just dipping into this a little... I think I'm with RG here in reference to how often things go around in circles with people reinventing the wheel. I think I'm with t-von for ideas upstream of the collector, subject to testing of course.

Regarding the exhaust box... yeah, well, skipping exact numbers because testing and variables, but very roughly... there was some mention of the ISC expansion thingie possibly operating as an exhaust box, and it looks like that's something like 5-6 times the volume of an individual exhaust pulse. Having not played with that idea in specific, is that within an order of magnitude or so of a decent starting point? I'd prefer not to have to figure out how to package a 55 gallon drum as part of an exhaust system...

 

So RG... why don't you think the expansion chamber actually works, and what is the smallest exhaust box that we could use to get it to work? After grinding my brain for a long time I'm thinking the expansion chamber might be doing a little of both. where the exhaust expands it has to be creating a low pressure zone and maybe it's pulling the next pulse out. I figure that's why my TQ peaked in 3 spots. What else would cause that?

 

Something interesting.

I put a 1 7/8" restrictor in the exhaust tip. Really cut down on the droning on the highway. Zero effect in 1/4 mi times or engine tuning.

I have a chunk of 2.5" exhaust pipe as my tail pipe, so my restrictor was just a 1 7/8" ID to 2 1/4" ID adaptor hammered into the pipe. (2.5" OD pipe is 2 3/8" ID, 2 1/4" ID is 2 3/8" OD) The key thing is that the small diameter section is facing inwards, so it is reflecting some exhaust energy back into the pipe. Kind of an anti- anti-reversionary device. But it's at the very end of the system...

With the half bridge, the droning came back at 70mph. Need more throttle to go that speed so more airflow and thus more noise. I removed my restrictor, cut the small-diameter section of pipe out leaving just the inwards-facing ring, then welded a second ring to the very end of the tip. So there are two 1 7/8" rings instead of just one, spaced about two inches apart, and no large diameter section of pipe at the end. Problem solved.

And, still no effect on the top end.

 

I never said an expansion chamber can't work.

 

I'm didn't say you said it can't work, but you suggested it might be acting as an exhaust box. I'm just trying to figure out way the smallest an exhaust box could be and maybe ill try that next.

 

Looks like I'm doing an expansion chamber in my 3" system. My reasoning is because RB's 2" and 2.5" presilencers have what looks like an anti-reversion chamber which also acts as an expansion chamber, but their 3" universal presilencers don't have it - they're supposedly meant for turbo applications. My 3" system is NA so no turbo to quiet things down. Gotta do something.

 

what kind of NA do you have the needs a 3" exhaust? I hope you're name is a hint to this as a 3" on a 13b NA is WAY over kill and is more than likely making you lose power.

 

Hi Guys, I have a question.

Please take a look at this picture and let me know if this can blow my engine due to back pressure being put to the engine? This is how the exhaust was collected on the 12a that I swapped to my starlet (damn mechanics, I didnt know crap at that time about exhaust or rotary at all). Now I know after reading this thread and the forum that this setup is fucked and was doing more harm than good!

Thanks

 

thats the weirdest way to collect, Its not going to harm the engine, but its not doing it much good neither.

 

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

 

I see nothing in that thread that says I'm wrong
You changed a lot more than just the size of the exhaust, proving ONLY that the total package you ended with is better than the one you started with and the size of the tube used is only a part of that, this thread was even about informing people that the exhaust is a compleat system * to recap, the compleat system you ended with was better that the compleat system you started with and there is no data proving that a compleat system with a smaller diameter tube would not be even better / worse*