Custom tuned exhaust

[Jprather]

Hello forum,

I have been toying with the idea of creating my own exhaust for my Baja bug. Since the rear is completely open I have space to be creative.

I have a stinger exhaust now and really like the look of the stinger but it is way too loud inside. I want an exhaust that is as quiet as possible.

So I have been researching resonators and suitcase style mufflers. I think that a suitcase muffler connecting the heat exchangers and the upper exhaust ports at the rear of the engine could work. Similar to the original bug exhaust just larger.

I can across literature discussing exhaust pulses and why it is important to match pulse for efficient filling and scavenging of the cylinders. Subaru has been using 'equal length' pipes to achieve this.

My concept is a rectangular box bolted directly to the rear of the engine matching right up with the heat exchangers and upper exhaust ports. Inside the box I'm envisioning perforated tubes going left and right directing the exhaust back and forth. Eventually exiting the top of the box through a stinger.

What cylinders would need to be matched on my bug? Is it as simple as measuring the length of the exhaust tube and making them match?

Any input would be appreciated. Thanks




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[Marc]

As you say, what you're contemplating is not unlike a stock muffler. I don't think it'd make much difference if you "paired" the cylinders but if you want to try incorporating that, pairing #1 & 3 and #2 & 4 (alternating in the firing order, 1-4-3-2) should have the most potential for quietness.
Back when we had a Formula Vee at the shop with 4 equal-length open pipes, I recall they were 42¾" long - right where the second heat ring developed on the tube when the little 1200 was held at the RPM where peak power was desired. Probably not applicable to a larger, street engine, but it's notably longer than what the stock pipes are - their length was more likely chosen for packaging rather than performance.

All that said, what I would probably do instead is to use a "sidewinder" header, the kind with the collector pointed off to one side, and fabricate a big U-bend coming up from the collector flange to go into a large muffler. With unlimited room you could even double it back to go through a second muffler - you'd just need to make some support for the weight so it wouldn't all be hanging off the heads. Start with something like this but run the muffler above rather than inside the fender.


Starting as it does with a "street" header, that setup doesn't offer a lot of ground clearance. If you'd settle for being more conventional, a system using a "Quiet Power" style muffler has a pretty mellow tone. EMPI makes this one:

IMO either one of these 4-into-1 systems would probably offer better performance than a "box". Directing the tailpipe back rather than up will cut down what you hear in the cab too.

[Ol'fogasaurus]

The term tuned exhaust is a US term, it is called other things in other parts of the world. Also, two stroke and four stroke terms and tuning seem to be different. Headers and tuning the exhaust were just being started to be produced, and tuning was starting to be understood as I was getting into cars.

This might be confusing so if your smart you will stop reading here!

As I remember tuning has to do with the pulses coming out of the cylinder with the length being measured from the valve. Without getting into too much detail, most of which I have forgotten anyway, the idea, as I remember it, was to end the pulse at the collector at just the right pulse length, a flairing of the tube end also was involved here, I forget the term but there is math to support the length. This is also where the flaired exhaust tips came from which was a fad that I don't think really did any good but almost everyone had to have them.

The right length causes the pulse to go back up to the valve which then goes back down the tube again after hitting the valve pulling the next pulse with it. The cylinder merges are designed to support each other but the ringer here is RPMs can change the tuned length. As I remember longer headers support torque and shorter headers support RPMs.

Merging into the collectors, e.g., 4 into 1 was just being understood but then there was the tri Y style header also. I don't remember the good side of that system but I think it is still around.

Usually, again as I remember, header manufactures work with dynos to come up with the best over all tuned length. I think mufflers also go through the same dyno design process.

If my memory is wrong I am sure I will be corrected.

More than you asked for but maybe there is a shread of something you can use here.

Lee

[Ol'fogasaurus]

Marc's photos are of the turbo style of muffler which came from the turbo charged Corvair Monza's. I run a turbo style muffler that allows me to surpass the 93 decible limit where I ride.

Merged exhaust gives you more power than the twin bazooka style of exhaust. I have had both and you can feel as well as hear the difference when you run the quieter turbo muffler. I think you get a better bottom end also.

Lee

[Marc]

"Tuned-length" independent pipes are peaky compared to a 4-into-1 system; to broaden their effective range they need to flare out towards the end, like four little stingers...that'd add to the level of difficulty in plumbing them into the "expansion chamber", and I still think it'd be down on power compared to the conventional setups.

The "dune-buggy-duals" AKA dual-cannon systems pair #1 & 2 and #3 & 4, so each side handles two consecutive exhaust pulses and then sits idle while the other side works. The uneven pulsations make for a boomier sound than you get when you pair alternate cylinders (those have been compared to the sound of two out-of-tune Harleys at idle)...they're made strictly for looks and packaging convenience, performance is not really any better than with a stock muffler - just sounds more aggressive.

The muffler on the Sidewinder system picture above is similar in design to the "turbo muffler" but I think the cheaper EMPI system uses what's called a "Quiet Power" - the first VW headers to market had glass-packs and were quite pipey, these were developed to produce a more civilized sound and to be thin enough to not drag on the ground when used with a Sedan header - they're just a simple conventional muffler with the addition of some fiberglass batting to kill the "boom" since the limited volume didn't leave room for much in the way of chambering.

[Ol'fogasaurus]

The word surpass in this case i meant quieter than the 93 decible limit imposed where I ride. By turbo style I meant conceptually similar. My turbo muffler is a bit larger than what Marc showed but the concept he was showing has a lot of merit for a fairly quiet or a quiet hide a way exhaust system.

Lee

[Jprather]

Thanks for this feedback. After looking closer at the sidewinder header I can see sort of equal length pipes mating 1 and 3, 2 and 4.

The pipe connections to the heat exchangers are flanged for bolts. Does anyone know where to get new exchangers with this flange welded on?


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[Marc]

Repop heat exchangers come from Denmark, "Dansk" brand. They're made differently than the stock ones, arguably better for exhaust flow but they provide less heat (at least as much airflow, just not as warm). The jackets are also a little shorter, distancing the rear from the muffler inlet junction - that reduces the incidence of exhaust leakage being drawn into the heat by the eductor effect caused where the airflow has to be faster to get around the pipe where it enters the box. All things considered they're a good trade-off unless it gets real cold where you live...and as a bonus, they're a little lighter. Almost all VW parts houses handle them. There's even a "lightweight" or "racing" model that has NO heat exchanger finnage inside, just a pipe in a jacket - not a lot of heat from those, but some...and the defroster airflow volume is ample.
Here's a comparison of the insides of the Dansk (left) to the OEM factory heat exchanger:
Originally they all had the same diameter exhaust inlet & outlet as the stock boxes, nowadays they're available with larger tubing - not really justifiable below 1800-1850cc IMO unless it's a high-RPM screamer. Same goes for 1⅝" or larger headers, the ubiquitous 1½" used for "street" machines with heater boxes is fine until you're running big displacement. Actual I.D. of the standard-sized boxes is <1¼", O.D. is ~1⅜", so they're slightly smaller than the 1½" O.D. headers. Flanged ones with fins inside are hard to find, most places only carry the big ones with no fins which are ~1⅜" I.D. so they're actually a better match to a 1½" system if you don't need much heat...small-tube ones with flanges exist, but again no fins and the flanges don't align with most available flanged systems so you have to modify the header to use them. In fact, I've bought the flanged big-bore boxes and 1⅝" headers from the same place and found the flanges to be misaligned, requiring reworking (if you don't need the big-bore boxes you may as well do that with a finned box and have heat IMO, read on). http://www2.cip1.com/ProductDetails.asp ... D2507%2D10

If NO heat is needed, J-tubes work or you can step up to a "competition" style system that typically has a better collector design (more "merged" than most basic headers' collectors).

For years before the flanged systems and heater boxes were available, folks would "roll their own" using flanges salvaged from scrap exhausts or purchased new. For a 1½" system, using stock flanges allows you to use stock gaskets too.
http://www.geneberg.com/cat.php?cPath=15_455
http://www.geneberg.com/cat.php?cPath=15_456

The stock-style slipjoint setup's not really that bad, though. Arguably it can be better than flanged - some prefer it since it'll usually pull off before bending the box or damaging the head.

Most VW header systems are 4-into-1 with no "pairing" - typically the pipes are arranged around the collector in the firing order (the theory being that it creates a swirling effect to improve the scavenging and discourage reversion). Because of the firing order and physical space available, V8 engine headers are by nature uneven-fire (adding an equalizer "H-pipe" downstream of the collectors helps) so nearly anything is an improvement, hence the popularity of Tri-Y headers for them. "180°" systems are made for V8s which snake the pipes across the rear of the engine compartment - not possible with the stock firewall - and they work and sound great, essentially like two 4-cylinders singing a duet. That's the trouble they have to go to to mimic the physics that almost every off-the-shelf VW header has. "Tri-Y" systems haven't really caught on in the VW world, probably because they've rarely been shown to offer any advantage - but they pair #1 & 3 and #2 & 4 unless built by a moron.
The stock system on `75-up FI Beetles is laid out to maximize heat, performance considerations be damned - it pairs #1 & 2 and #3 & 4, with grossly unequal-length primaries feeding into larger-bore heat exchangers (so each one gets the flow from two cylinders instead of just one as on the carb'ed engines). I DO NOT recommend using that setup on a carb'ed engine - BTDT and it wasn't worth the hassle. Heavy, and a PITA to install or remove. Also no provision for intake manifold heat so incompatible with single-carb applications.

180° circletrack header on a smallblock Ford. There are 180° "street" systems that will fit with the stock firewall, but they don't even come close to being equal-length so I don't see the point.

[Piledriver]

Interestingly the watercooled Vanagons (late at least) had tri-y exhaust.
One advantage of a 4-2-1 is that the individual runners don't have to be identical, its the combined length of each 180 degree runner pair that has the largest effect. This can allow for a pretty compact layout.

The wbx "header" runners were vastly different lengths, but it worked pretty well, well enough to make 112 HP out of a low compression 2110 with tiny valves, marginal port flow and basically a Web 86 cam. In European trim with 10.5:1 pistons it hauled butt pretty well.
(The chamber-in-piston Heron style chambers are pretty knock proof, 1mm deck from the factory)
.
The WBX is basically a water cooled T1, rotate the head flanges a bit and a T1 header bolts on.
The reverse is of course true as well.

I'll dig out a factory WBX header this afternoon for a pic. (2.1 MV)

[Ol'fogasaurus]

Some of the FE block headers had a tube from each side that crossed under the pan and into the other side collector; this was to even out the pulses and make the headers flow differently. Those of us who played with "Babbitt Beaters" (Chev in-line 6s. Chev in-lines used dip oiling of the rods but started to change to full pressure oiling in '51 and finished the conversion in '54) used the cross-over/balance tube in front of the mufflers to quiet the exhaust down from a loud racking noise to a mellow sport car sound even with illegal "Smitty" mufflers. The old SBC engines often needed the balance tube to, again, get the rack out of the exhaust. It kind of sounded like a bunch of kids running down a hill in tennis shoes trying to stop; you know... slap, slap, slap. Some of the Y-block Fords had a similar rack to them also and benefited by the balance tube also.

In the late 60s ford changed the firing order on their small block to stop the destroying of the number one main bearing or was it cracking the block (two cylinders fired too close together and put a lot of strain on the number one area... I forget which now but that changed things a lot exhaust wise. I seem to remember a HP gain also. Some of my friends had rebuilt a '68 Cougar and couldn't get it to run right. I asked them what year cam they put in but they didn't know. I told them to try the later firing order and wha-la, it ran good.

Try-Y headers. This explanation may help some: http://www.stans-headers.com/tri_y.htm

I forget now what the effect of bends were in the headers but in the tail pipes the longer lengths with several bends quieted down the exhaust noise by quite a bit.