BMW e3 Downpipes & Exhuast
We've been working hard on developing some good solutions for vintage BMW's, and not the least of that is a nice modular exhaust system that replaces the factory system while adding some much needed design improvements.
Like mandrel bent downpipes that can be clocked appropriately, hangers that don't fall off and require you to sacrifice a shoe lace to keep your exhaust from dragging, and a well designed flow and configurable sound.
We started with the downpipes, building a fixture of a very reasonably intact set of factory iron oxide encrusted examples. We then built a test set out of 1 5/8 mandrel u-bends and adjusted accordingly for slight deviations from the factory bend radius. Armed with the first prototypes as a guide, we then built a second set, fully welding each piece to the next to account for movement of the material as the welding process moved along. This gave us a perfectly fitting, radius adjusted design from which to mandrel bend the subsequent sets from a single piece of 1 5/8″ or 1 3/4″ tubing.
We're quite pleased with the results. Check 'em out:
You'll notice the V-Band flange and retainer. These promote a focused seal at the gasket and utilize a similar to factory gasket with a fire ring and multi layer metal design for a long term seal and full clocking capability of the downpipes for proper alignment and easy installation. In addition to the trick flanges, the downpipes feature the following materials and manufacturing techniques:
- All components are T304 Stainless Steel for longevity in even the most harsh conditions. Your e3 will melt before these corrode.
- Each joint is TIG welded with 308L rod for a high quality connection throughout. Prototype bends are welded, but final units will be custom mandrel bent for repeatable results and seamless design.
- V Band Flanges are CNC Machined for perfect flat surface (which is retained after welding).
- 3/16″ Retainer flanges are high precision CNC Waterjet cut from stainless plate.
- Optional O2 sensor bungs available in each downpipe, one for factory EFI use, the other for optional Wideband datalogging.
The exhaust we have developed behind these downpipes is still in flux, but we're eager to let our customers behind the scenes at the shop to show off the initial prototype and to discuss design considerations as we finalize the design process.
With performance interest in mind, we settled on a 2.5″ mandrel bent exhaust system with a single center resonator, optional high flow catalytic converter (nobody wants a stinky car) and rear muffler. Our initial design utilized the following components:
- Mandrel bent, matched and in-house fabricated Y pipe for smooth transition of gasses into exhaust system
- 12″ Long, 4″ diameter stainless center resonator.
- Center resonator locating bracket and rubber isolator to help isolate moment arm load from the exhaust to the factory clamshell downpipe mount. Attaches to factory center support bearing stud, replacing washer and nut with flange nut.
- Urethane exhaust hangers with bolt up mounting mechanisms utilizing factory holes in forward exhaust hangers. Standoffs shown in photos were for mockup only, but ran great and did the trick.
- Hushpower II stainless rear muffler.
- Slip Fit mandrel bent tight radius tailpipe in brushed stainless steel w/ beaded lip. Emulates factory rolled chrome tip with a race inspired straight pipe appearance.
A few of the discussion points on this design are as follows:
- Length of the tailpipe tip. We're of the opinion that a little longer is helpful to get the exhaust into the stream of air behind the car and out of the leaky trunk seals and cabin. Should we make this length configurable to taste? Maybe two options: Short and long?
- Clamshell Downpipe Mounting: Factory system is stamped steel and puts a nasty point load on the downpipes. a billet aluminum clamshell design attaching off of the lower right bellhousing bolt would allow any e3 running a either the zf or borg warner automatics as well as the zf or getrag manual transmissions to mount the downpipes rigidly using a design with more surface area for prevention of pinch point. Perhaps an optional addition with extra cost?
- Flexible couplings aft of the clamshell. This is something we discussed (and purchased 8 flexible couplings in anticipation of), and now that we have the system mounted and isolated using the new mounting technology, we're not convinced that they are necessary. Adding them will add roughly $75 to the cost of the downpipes, perhaps an option?
- Resonators and sound. We'll continue fiddling with tuning the exhaust note by swapping a few different resonators into the center location as well as testing moving the current 12″ resonator relative to the Y (a great suggestion by Mike Fields of the Senior Six Registry). We will potentially offer one or two options on resonators to suit our customers liking if there's that much of a difference to justify the extra inventory.
- Mild Steel – would a budget mild steel system be desired? Wouldn't be hard for us to do.
So that is where we're at on the development of our exhaust for the e3 Sedan. We'll be test fitting the downpipes on a few other cars like the 3.0CS as well as the e28 5 series to see if the upper sections work well in those chassis'.
Next up on the e3: Chassis bracing, braided brake and clutch lines as well as high performance radiator and fan options.
Jason Griffith, Engineering Director, Nameless Performance, Inc.
