peejay

More pics available. Not sure if the volume/shape is anywhere near "ideal" but it can't be any worse than what's on the car right now.

Hyper4mance2k

iTrader: (40)

Ther are now extensive pix of mine in my exhaust thread.
https://www.rx7club.com/1st-generation-specific-1979-1985-18/racing-beat-exhaust-dyno-compairisons-823229/page3/

Shainiac

iTrader: (7)

Can you post pics showing the other side of the transition?
Whats the max and min OD?

jgrewe

iTrader: (1)

I understand the expansion chamber theories a little and have seen that they work on our engines but what are effect are you trying to create? With the expansion rate you have built you will get a lot of turbulence right at the front end that may cut your flow. Air will expand smoothly at a 7 degree angle from the direction of flow so you might be better turning yours around to keep the flow smooth.

You get the same turbulence in the exhaust port at the top from how steep the roof slopes up from the hole.

peejay

Well if you look in that pic, you can see the abomination of wrong flanges and splices and cuts and re-splices that is the current post-header exhaust on my car.

So really anything is better at this point

Plus the current exhaust steps down in size about 1/2" immediately at the flange, which is kinda bad because there is virtually no collector volume at all. The new system at least expands out again right afterwards, looking like a crude venturi.

If I wanted to make it perfect, I'd never get it done. The whole car is an exercise in "perfect is the enemy of good enough" - sometimes good enough really is good enough. And some times it isn't... but this is how education happens.

This is the other side:



And as it will be installed in the vehicle:



I am using band style exhaust clamps to hold it all together. I love them - they always come back apart again and they don't leak like flanges tend to. And they're a HELL of a lot cheaper than V-band clamps.

rotarygod

iTrader: (1)

Even with an expansion chamber I don't think the collection point should be anywhere in area larger than a 2-1/2" pipe but possibly even smaller. I'm suspecting that the expansion chamber on the ISC setup isn't working like an expansion chamber but rather like an exhaust box. I'll have to go into more detail later.

peejay

I know exactly what you're talking about, and I was planning on making one of those at one point. The only problem at the time was, the rule of thumb was that the box had to be about 2 times the engine's displacement, and I was trying to figure if I should do something two and a half liters in size, or five liters...

Given that the idea probably was "make sure the box is twice as large as all exhaust pulses" then two and a half liters would make sense... but if you figure that theory was dependent on the pulses coming much futher apart, then 5 liters would make sense...

Either way it would be a lot of volume to try and fit next to the transmission.

You know, I wonder what the volume of a stock exhaust manifold is? I also wonder if you could sit one on, say, 12 inch stacks up next to the intake manifold like where a large turbo goes. Hmm.

I bet a one-into-two chambered muffler would make a spiffy exhaust box for a mid-engined car, now that I think of it... may have to slice it open and do some minor work to the internal tubes, but that's relatively easy compared to constructing one from scratch.

Kenku

Actually, kinda curious about some of the theory on the expansion chamber stuff; in the midst of building a header and had intended on doing something similar to the ISC idea of expanding out to a muffler core... but it would be nice if the bits after the collector were optimized in some sort of non-random-guess manner to start with.

diabolical1

iTrader: (3)

so you decided to go that way, eh? glad to see it - for curiosity's sake. why not just get some 3.0 or 3.5 inch tubing and do what you have to do? i get a 12-inch length of 3.0 inch tubing for roughly $2 locally. it can't be much more than that in Ohio ... Shallow Pockets.

on a separate note, i think you and/or Rotarygod are going to have to explain the "expansion chamber vs. exhaust box"-thing. i'd like to know ...

rotarygod

iTrader: (1)

An exhaust box would optimally be at least 15 times the displacement of the engine which is huge but you'll start seeing some effects at areas smaller than this.

Basically what an exhaust box is, is a way to make the exhaust "think" it's exhausting into open air. I like to say that a noncollected exhaust is collected at infinity but this isn't terribly accurate. The pulses do collect of course. They just do it in an infinitely large space behind the car however they have no effect on the other pipe. An exhaust box is meant to simulate this space. Let's say I had an exhaust box with 2-30" long exhaust primaries entering it. The box works as my collector. The back of the box has a single pipe coming off of it going through a muffler. Out of this whole system, the only important thing to tuning becomes the primaries. The exhaust box would effectively end the system. This would be no different than having those same 30" primaries being true duals that dumped to the ground. Obviously that would be very loud.

The exhaust box would have a small muffling effect but really just serves as a large space to diffuse the exhaust gasses. The pipe going out the back of the exhaust box suddenly serves no other purpose than to get rid of gasses. It's length has no effect on tuning. As long as it flows what is required, it's invisible to the rest of the system. You could have a 2-1/2" pipe or a 5" pipe. As long as they could each flow more than the engine needs, it doesn't matter and power won't change. The neat thing about the exhaust box and the lack of influence the rear pipe has is that you can muffle the hell out of it with no performance consequences so long as your muffler(s) flow enough. The problem with the box is the size that is needed to be really effective. It can be nearly any shape but does need a certain minimum area where the pipes enter it. I need to figure that out again.

An expansion chamber works very differently. I look at an expansion chamber as 2 separate parts although 3 would be more appropriate. The first is the divergent cone or the megaphone. Your pipes first must merge together in a collector. If the collector is too large, you'll get little scavenging on the other pipe. If it's too small it's a restriction. If we have 2-2" pipes, we do not need a 3" collector. Flow is not a constant. It pulses. We never need the sum of the pipe area due to this. We do need more than 2" though. Somewhere between 2-1/4" to 2-1/2" would be a good collector area on 2-2" pipes. The divergent cone must now be attached. The angle of divergence plays a role and consequently the length of the cone as well. Which is better a shallower angle, longer cone or a steeper angle shorter cone? That's a good question. The simple answer is that it depends. There is of course always practical limits to anything but there is no one right answer as different engines have different requirements. How large should it expand to? That's too is the same type of question and again it depends. If we wanted no muffler (and lots of hearing damage) I'd say use a megaphone that is long and expands to 5". However other things come into play when we want to add a converging cone to create a system.

I've got lots of work to do today so I'll continue to expand on this more later.

peejay

Yeah I guess what I am doing is more like a taper collector than an expansion... just with a venturi at the start.

The total cost to me was "free". The main cone is a Meg's tip scaveneged from a street rod we put together in '04, the front cone used to be part of the catalyst that was on my VW for a bit over 210k (finally failed for NOx), the flange was something I was going to need anyway to do ANY exhaust (paid $10 for the pair, including bolts/gaskets/chunks of pipe), and the exit piece was just a short piece of 2 1/2" scrap.

So, effective cost, free!