BMW e36 318 tds - modding thread

[AndrzejM]

First fillup and I'm impressed with the FE of Berta. I just wanted to know the baseline so I wasn't hypermilling too much. But still 52MPG it's a good starting point. I think I know my new goal I want to go for 1000 miles/tank.
Berta has 62 litres tank so I need to go below 3.85 l/100km (61.9 MPG US), and I'm almost sure I can achieve that. Keep your fingers crossed for that

[MetroMPG]

Great early results!

I always admired the 318t because it was the only hatchback BMW availablein this market. (Of course no diesel version here.) I still see one occasionally on the road.

I'm looking forward to seeing what you do with it.

[AndrzejM]

Unfortunately here in Poland we have very strict rules and I can't go for a boat tail or to delete mirrors. But for sure I'll try kamback, wheel spats, smooth belly pan, grill block. So there's a lot of work ahead of me.

To be honest I'm still thinking how far I can go for FE. Should I stip everything, delete as much as possible to save weight. But on the other hand I'm rarely driving in the city, and on the highway weight is not such a big issue. Any suggestions?

[tjts1]

I wouldn't do the rear batman thing... whatever that is on the back window. You already have a fairly steeply raked back window. With that thing added you're actually increasing the size of the wake behind the car.

[MetroMPG]

tjts - Unfortunately, you can't make that assertion without knowing whether flow reattaches on the stubby "decklid" portion of the hatchback.

If flow separates at/near the top of the rear window, and does NOT reattach downstream, then a properly designed "batman thing" will definitely help.

I think the profile of the rear of the car falls well outside of the optimum. My money's on Batman to the rescue.

[tjts1]

BMW put a lot work in the E36 aerodynamics. They got the angle right at the factory. Either way you are increasing the size of the low pressure zone behind the vehicle which is counter productive. The batman thing or whatever is only useful on cars that have a blunt rear starting at the top of the rear windshield.

[straight5]

I wouldn't strip it. If you stripped it what would you gain? 1 or 2 mpg? You can have a nice car and get 51mpg instead of 52 - not at all worth it.

[MetroMPG]

Originally Posted by tjts1

They got the angle right at the factory.

Not quite. The "right" rear roofline would look a lot like a Prius, or the latest Mercedes E Class coupe (if you prefer a trunk), or like the Chevy Volt (if you prefer a "notched" hatchback style like the 318t).

The effective angle of the rear of the 318t (end of roof to tip of decklid) is ~20 degrees. The glass alone is closer to ~23 degrees.

The ideal angle, per the diagram you posted, is ~13 degrees. Not coincidentally, that's much closer to what you'll find on the Prius, E Class coupe or Volt.

Either way you are increasing the size of the low pressure zone behind the vehicle which is counter productive.

Tuft testing of the 318t would probably show that there turbulence & flow reversal on the rear deck lid. That means flow separation (low pressure wake) starts at a point on the rear glass, not at the trailing edge of the "deck lid". It may seen counterintuitive, but a properly designed batman thing will permit controlled pressure recovery further aft and lower down, and would result in a smaller overall wake.

Given the angle of its rear glass, the 318t needed either a taller rear deck lid, a longer one, or a big-ass spoiler for better aerodynamics from the factory.



(Chevy Volt - GM image)

[AndrzejM]

I think that Metro has a point, and as you can see below the "ideal" aerodynamic profile is way different from the back of my 318 Compact.

[AndrzejM]

Hi again!

It was a long time since my last visit here, but haven't forgot about you guys. Unfortunately there's many things that I have to deal with right now so my modding plans for Berta are those of low priority Anyway I just got back from Prague and it was for now the longest trip with Berta. But before about FE I wanted to tell you that there's a problem with Berta's injection pump, cause it's sweating with fuel. I presume 0.5 litre per 100km or something about that. I have to deal with that asap. Just need to but a gasket set take out the pump and fix it once for all. Anyway I've managed to drive Berta for 1195km (~743 miles) till the yellow fuel light went on. After filling up I've counted FE and it showed up that consumption was ~4.33l/100km (54.32 mpg US) I thing that's amazing with the leaking fuel pump. In a month I'm planning next trip (over 3000km), hopefully I'll be able to fix Berta till then, and maybe I'll have time and $ for some eco-upgrades. So keep your fingers crossed.