Miata Turbo Forum - Boost cars, acquire cats. - K or BP Engine to BMW E36/E46 Transmission Upgrade

Copied and pasted from our blog post here:

It’s time to share some long-awaited news about the transmission project we’ve been working on. Due to the huge power potential of a boosted K series engine, a stronger transmission option has been a really common request over the last year.

The wait is almost over. We have just finished a bolt-in transmission upgrade for a BMW E30, E36, or E46 transmission (1982-2006 BMW 3 series) in an NA and NB Miata.

The project originally began for the K series swap community, but we know our turbo BP engine friends have transmission struggles. Because of that, we have designed a transmission setup for both the K series and BP engine platform!

https://cdn.shopify.com/s/files/1/11...G?v=1492013070

In fact, phase one of our testing has involved installation of an E36 5 speed trans and Getrag diff in our BP-powered test car.

We looked at many transmission candidates for this project and ultimately settled on the the BMW 3 Series family of transmissions for their strength, dimensions, availability, and gear ratios.

BMW uses transmissions built by Getrag and ZF, and all E36 (92-98) and E46 (99-06) transmissions have the same bellhousing bolt pattern. As a bonus, the E30 5 speeds also share the same bolt pattern, except rotated 10 degrees. The ratios will appeal to some customers so we'll have a specific adapter for that trans as well.

Although many transmissions will work with these new parts, we narrowed it down to the following three options that seem like the best fit:

E36 5 speed ZF trans (same trans for M3 and base models)
E30 5 speed Getrag 260 trans (available in 6 cylinder cars, longer gearing than the ZF)
E46 ZF 6 speed from 2003-2006 330i and Z4 (non-M3 6 speeds)

Strength

First and foremost, we needed a new gearbox to solve a problem: broken transmissions in high HP applications.

Lucky for us, these transmissions can be found holding 1000+whp on a drag strip. There is some debate as to how much each gearbox can reliably hold, but ZF and Getrag underrate them from the factory. 800+whp has become common for a boosted E36 or E46, so based on the number of high HP turbo BMWs out there, we’re well within the limits with our turbo 4 cylinders.

Just for fun, here's an E36 M3 street car with the stock transmission running an 8.94 quarter mile with 1051whp and 870wtq. The action starts 90 seconds in.

Size and Packaging

At first glance, these transmissions are quite small, particularly the 5 speeds. The entire trans is only 22” long, and the 6 speeds are about 4” longer. They are wider than a Miata trans, but both 5 and 6 speeds fit in the Miata trans tunnel without modification. The 5 speed requires a few smacks on the top of the tunnel to make clearance for an unneeded bracket, which could also just be cut off the gearbox.

A 5 speed weighs just 77 lbs, with the 6 speed coming in at 96 lbs.

From left to right, E30, E36, E46, Miata 6 speed:

https://cdn.shopify.com/s/files/1/11...G?v=1492013200

Price and Availability

These are available everywhere, as you can see from a quick search at car-part.com. Here are common price ranges we’ve seen, as of April 2017:

5 speed ZF $200-300 (we paid $275 shipped for ours)

5 speed Getrag 260 $200-400 (we paid $300 shipped)

6 speed ZF $500-750 (we paid $500 shipped)

Rear Transmission Mount, Driveshaft, and Differential

Since a stock block K20 can put down 600+whp when boosted, we knew that a rear end upgrade would be in order for many customers, although not required.

This transmission was also chosen because it pairs well with the GM Getrag ratios used for the V8 swaps (3.23, 3.42, 3.73, and 3.90). These diffs are dirt cheap, and are available with a strong clutch-type LSD.

For testing, we picked up a 3.42 LSD for $124 and a 3.73 LSD for $180.

Here's our 3.42 installed in our subframe, thanks to parts from V8 Roadsters:

https://cdn.shopify.com/s/files/1/11...G?v=1492014358

https://cdn.shopify.com/s/files/1/11...G?v=1492014348

Since this setup is designed for high HP applications, we have opted to delete the PPF like the V8 cars. Since the new mounts and frame rails add a good deal of rigidity to the chassis, the PPF isn’t missed. We used upgraded rear transmission mounts and differential mounts, so shifting is precise, even with stock BP engine mounts.

Here's our Driveshaft Shop axles and hubs from V8R, along with our custom driveshaft:

https://cdn.shopify.com/s/files/1/11...G?v=1492015437

Expect to see a rear Miata Torsen mount available that will allow a Miata rear end to be used with these transmissions as well.

Shifter

These transmissions operate with a selector rod that attaches to the back of the gearbox, so the shift lever distance from the bellhousing face can easily be adjusted just by changing the length of the selector rod. This is good news for us, as we’ve been able to fine-tune the shifter position for the same stock Miata shifter position.

We followed the lead of many BMW racers by deleting the upper shifter arm and have replaced it with a solid mounted plate that bolts to the top of the Miata trans tunnel and accepts the shift lever ball.

https://cdn.shopify.com/s/files/1/11...G?v=1492015235

We've already ordered a shorter, straight shifter for testing from a Z4 3.0 to replace the part you see pictured from an E62 M5. There are many versions of shift levers available from BMW, and we're honing in on the perfect candidate for the Miata.

https://cdn.shopify.com/s/files/1/11...G?v=1492015286

We’ve also designed an adjustable selector rod so the shifter position can be set in the right location regardless of which trans is used. The double sheer design of the rod also eliminates slop, resulting in a high level of precision and shifter feel. It will certainly be a nice upgrade for the current 6 speed Miata trans users.

Gear Ratios

Gearing in these transmissions shines with high HP cars, which works well for our purposes. The E36 and E46 gearboxes have short gearing, with the 1:1 gear being 5th in both the 5 and 6 speeds. Both transmission ratios are nearly identical, while the 6 speed adds an overdrive gear on top. The Getrag 260 trans from the E30 is a longer ratio 5 speed, which will be preferable for certain BP-powered cars. We only plan to make an adapter plate for this trans that mounts to the BP, not the K.

Since the E36 and E46 transmissions have shorter gears, they will do best when paired with a longer ratio rear end, especially for street cars. For many BP powered cars, we like the 3.23 and 3.42 Getrag ratios, or the Miata 3.63 and MFactory 3.30. BP cars for track use will do well with a 3.90, and K powered cars will shine with a 3.73 or 3.90.

The 5 speed ZF is a particularly great option for any track car that has no need for an overdrive gear. It’s by far the most popular transmission used in E36 and E46 BMW race cars, and also my personal favorite. Cheap, lightweight, very strong, and short gearing.

If we could change anything about these transmissions, we’d like to see a little longer 1st and 2nd gear, but some things are a trade-off. Track users will gear the car to optimize 3-4-5 top speeds, and street car users will probably prefer a longer ratio rear end to lengthen 1st and 2nd, and give a lower cruising RPM on the highway.

I’ve been (aggressively) commuting the last few days in a BP-powered car with a ZF 5 speed and the 3.42 Getrag and the gearing is great. A 3.73 or even 3.90 would probably be fun too.

To reduce the “what’s the best setup for my car” questions, we’ve gone ahead and added a gear ratio calculator to the website, under the Resources menu. It includes all NA and NB Miata transmissions, all three BMW transmission options, and all Miata and Getrag rear end options.

Clutch Options

Again, because these transmissions come from cars with high HP levels from the factory, clutch options are fantastic for our purposes.

Our flywheel is designed to work with any E46 or E39 (5 series) pressure plate. Yes, the E46 M3 and E39 M5 use the same pressure plate as the base model cars. Because this pressure plate is used for the 400 HP V8 in the M5, BMW guys are holding 400-500wtq with their stock clutches. The benefit to us is that we can run an inexpensive OEM replacement clutch kit that can handle most turbo 4-cylinder builds and still maintain nice street manners.

Here's our prototype BP-E36/E46 flywheel and adapter plate:

https://cdn.shopify.com/s/files/1/11...G?v=1492014595

Production adapters will of course will be ground smooth and zinc plated. The flywheel weighs 13.5 lbs, and we have a few tricks we may implement to get the weight down even further. The K series adapter and flywheel are built in the same manner. We'll post up pictures as soon as the final adapter plate design is here.

The OEM BMW flywheels are all dual-mass, so the clutch discs are very light and also unsprung. Most BMW enthusiasts replace the stock flywheel for a lightweight aluminum unit and still run the OEM unsprung disc.

https://cdn.shopify.com/s/files/1/11...G?v=1492014632

Since the disc surface is solid (no pucks), there is still smooth engagement, and we were pleasantly surprised by how easily we were able to modulate the clutch. The pedal feel is perfect with the Wilwood 13/16” master cylinder.

https://cdn.shopify.com/s/files/1/11...G?v=1492015066

The 5 speeds and 6 speed clutch discs have different spline counts, so it’s important to pair your transmission with the matching clutch disc. Our kit will include the appropriate clutch kit, so no worries there.

Thanks to the short length of the trans, installation is even easier than stock when lowering the engine and trans through the engine bay:

https://cdn.shopify.com/s/files/1/11...G?v=1492016106

The final transmission kit will include the following parts:

Steel 0.5” adapter plate, zinc plated (K series or BP)

Aluminum flywheel (K series or BP) that accepts an E46 clutch kit

Driveshaft specific to your application, rated at 500+hp

Rear transmission mount assembly (5 speed or 6 speed specific)

Performance poly transmission bushings

V8 Roadsters frame rails (optional for 5 speed, required for 6 speed)

Billet aluminum solid mount shifter plate

Adjustable length selector rod

BMW clutch kit with correct disc that matches your application

BMW shift lever

BMW shifter ball bushing

BMW clutch fork pivot pin

BMW clutch fork wire clip

BMW/Sachs E36 slave cylinder

Wilwood 13/16” Master cylinder kit with braided stainless clutch line

Reverse light switch jumper

GPS-based speedometer adapter

Additional options:

Rear mount for Miata diff due to PPF delete

Getrag diff upgrade from V8R (stage, 1, 2, and 3).

As mentioned above, the PPF (power plant frame) gets deleted. While I’m sure some transmission adapter would have been possible, it seemed silly to adapt a flimsy PPF to a different trans and diff, so we opted to design it from the ground up and add frame rails into the package like the LS and LFX swaps. The car feels perfect, nothing is lost, and the chassis seems to have gained some rigidity as well.

Release date:

2017 availability is the goal, but there is no exact estimate at this point. Parts will go into production after some very rigorous testing. Be sure to subscribe to our newsletter for the latest information and pricing.