Performance tuning 101 for a naturally aspirated engine: There is no shortcuts

Lately, there has been a trend among experienced tuners to turn their eyes back to naturally aspirated engines, thanks to their ever challenging and unforgiving nature. Their is just something fascinating in a high revving naturally aspirated engine, especially in a V8.

Let’s admit: If you are interested in maximizing horsepower: buy a turbocharger. It is the easiest way to add power to your engine. But if you are like me, you are not interested in max hp. You may be allergic to turbolag, and interested in faster-than-light throttle response, in order to maximize the traction and driveability. Mate, this is the right blog for you. It is not about maximum hoos pauwo. It is about maximizing your fun, while you are driving along 101 on a west coast, or where ever you are enjoying your E30. In these situations, 300 to 400 engine horsepower and steady 400-500 nm of engine torque is much better than a galaxy exploding power curve. On rear wheels with 4th gear it means approx. 1500-1900nm of torque, if you are using a 5 speed ZF S5D310 and 3.15:1 ratio differential. You can consider 2000nm of rear wheel torque the upper safe limit for twisty (and both dry and clean) roads with 225 semi slicks. Just put the fourth gear on, and start enjoying.

The general idea is, that you may seek for more speed, but you may not be able to put any extra torque to rear wheels, or otherwise you may loose traction. This seems to be pretty tricky for folks to understand. It is the torque on rear tires that makes your driving either fun, or scary and difficult. And that’s why an electric car can be very fast on a track: they are providing steady torque, without any interruptions whatsoever. It means they are easy and logical to drive up to the limit. It is the same reason, why modern BMW diesels have even four turbochargers.

If you are living in the U.S, its pretty straightforward to install a LS engine into your E30, and start enjoying. While in Europe, M6x is popular swap due to a high number of potential engine donors available. And that makes it interesting tuning project for E30. With a basic M6x swap, you can get up to 286 hp for your E30. So your motives to build an engine swap may vary where you live at. M6x is very capable engine, and will response into a tune like any other engine.

Why not M5x then? the V8 has both more displacement and valve area than its little sister – making it the better choice for a NA build. There is no replacement for displacement – the V8 is basically a two four pot 16-valve 318is engines in the same. But if you are going to turbocharge your engine, the six pot can be better starting point, thus its much cheaper to rebuild with forced internals. Turbocharging M6x is more expensive, requires high demand from powertrain, and all these will add tens of kilograms of unnecessary weight on a front axle.

If you are about to keep your engine NA – go for V8 – and keep it as light as possible, thus it is one of the keys for good handling. Even among NA M6x builds, you can save up to 27 kilograms from a front axle, only by making smart decisions for your build. But V8 is still so heavy, you may say. Well, it can be either a true or false. A well built M6x swap is equal in weight to a stock E30 325i. I know this, because I’m a guy with two engineering degrees, and being taught to measure, before start making any hypothesis. In full driving condition, including all fluids and 3/4 of gasoline E30 weights 1180kg with M60 engine. It is in the par with the stock E30 325i. It is a bit of a surprise, but M6x cylinder block is lighter, weighing only 28 kilograms, than a good old four pot M10 block. The name of the game is aluminum.

A good reminder if you are first-timer in performance tuning, please repeat with me: A naturally aspirated engine is as strong as its weakest link. So tuning a NA engine is like drinking a fine vine: there is no shortcuts in the making. Repeat with me: there is no single shortcut. Not only horsepower figures, but also the quality of your build through lightweight parts has to be taken into account. Many times the information from public bimmer forums is limited what comes to NA M6x performance tuning. You must face it: Tuning a naturally aspirated engine is an expensive task. If you have extra penny to spend for performance tuning, then go for it. If you don’t have, it may be advisable to stick with a stock engine.

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