The Cars | Avid4D

The AVID4D GARAGE: MODS and TECH

A long term passion I have is cars, sports cars in particular, and my current garage consists of a BMW Z4 E85 3.0i and an Alfa Romeo 4C Spider.

I do not like the over-the-top styling you see many car enthusiasts deploy, but I believe subtle styling modifications can improve the visual appeal. Mechanical and electronic mods are also welcome.

Brief history of my cars

Some other cars I have driven

Z4 Chassis

Z4 Suspension and chassis regidity upgrade

Changing the suspension system was a game changer in terms of traction. The factory system was probably worn out given its age, but I didn't really know how bad it was until I put the new system in. Add to this an increase in chassis rigidity thanks to a front strut tower bar, and my Z4 now feels flat in the corners (ie. very little body roll) and there is much more grip.

Z4 Brakes

Z4 Brake system upgrade

There is no point chasing speed and power gains if your car can't stop because you will eventually need to turn into a corner! Providing substantial braking performance over the stock BMW system, I upgraded my Z4 with 6 piston front brake calipers and 356mm drilled and slotted discs, and 4 piston rear calipers with 330mm drilled and slotted discs. This moves more brake fluid which in turn softens the feel of the brake pedal, so to firm it back up I replaced the stock brake master cylinder with one from an E46 M3. Braking now feels firm and very capable.

Z4 Differential upgrade

Z4 Diff

There are a few different final drive gear ratios in various E85 Z4 models but my 3.0i had a 3.46:1 with an open differential from the factory. The one positive about the factory differential, designated 188K, is that it has fins for cooling which helps with more aggressive driving.

I'm not going to go into the science of gear ratios and LSD here because that is a very long discussion, but I chose to keep the factory 188K differential housing to maintain the stock look and upgrade the internals with a 4.1:1 final drive ratio plus a helical LSD. The effect is quicker acceleration due to the 4.1 drive ratio and better grip in cornering under acceleration thanks to LSD. The side affect of better acceleration in this method is a lower top speed, but as I have never maxed out my Z4 at 250kph I don't really find this to be an issue.

Z4 Entertainment system upgrade

Z4 ICE

The factory stereo and sat-nav system was impressive in 2003 but had definitely seen better days. The audio quality from the stereo head deck, amplifier, multi speakers and twin sub woofers still sound clear and capable of good volume however, but it was the low resolution screen and DVD sat nav system that let it down. To replace it I selected a 10.25 inch Android touchscreen that integrates with the factory stereo head unit. The screen was from Avin USA and was designed for an E83 X3 but fits perfect in the E85 Z4. If your car does not have the factory sat nav screen you will need to cut a hole in your dash however. The stereo head deck needs to be on AUX input, and then the touchscreen manages all bluetooth, MP3, video, app and streaming tasks. Doing it this way means the steering wheel volume control still works, although the skip/previous track buttons do not. Overall the screen lifts the interior appearance of the E85 Z4.

Q: Theoretically, how high can my engine rev?

Engine RPM is limited by various factors. One problem with high RPM in a DOHC engine is valve lift, where the springs literally cannot pull the valve closed fast enough and the piston hits it. This is only ever a bad thing and leads to a severe engine failure!

Another limitation is determined by the bore and stroke of your cylinders, in particular the stroke. Engines can only move the cylinders at a max speed of approx 25 m/s. Doesn't matter if it is a road car or a Formula 1 engine, this limit is constant. With this in mind, and knowing your cylinder stroke in mm, the cylinder speed can be calculated for various RPM.

Piston Speed = 2 x stroke length x RPM / 60

My BMW Z4 E85 3.0i

Bore 84mm Stroke 89.6mm

Piston Speed

2 x 0.0896 x 6500 / 60 = 19.4 m/s

So in theory if the M54B30 engine could reach 8,000rpm

2 x 0.0896 x 8000 / 60 = 23.9 m/s

This is quite close to 25 m/s so for reliability allowance let's assume a red line of 7,500

2 x 0.0896 x 7500 / 60 = 22.4 m/s

My Alfa Romeo 4C Spider

Bore 83mm Stroke 80.5mm

Piston Speed

2 x 0.0805 x 6500 / 60 = 17.4 m/s

So in theory if the 1750 TBi engine could reach 8,000rpm

2 x 0.0805 x 8000 / 60 = 21.5 m/s

This is still well below 25 m/s so let's try raising the RPM even further to 9,000

2 x 0.0805 x 9000 / 60 = 24.2 m/s