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I first had this question while playing a racing game - it was a touring race with BMW 320i's, and I noticed that I could rev up to 8,500 RPM before shifting gears. Now my car (an older 320i), certainly can't hit 8,500 RPM - it can do a bit over 6,500 RPM before I need to shift (the rev limiter kicks in shortly after that).

I'd expect that one of the modifications would be adjusting/removing the rev limiter, but I also expect that if I did that to my engine as is, it would suffer. So what engine modifications are needed to enable safe higher RPMs?

Back to racing games again, in Forza Motorsport 2 there's modification which enables this - "racing cams and valves". Might this be one of them?

By the way, by "safe", I don't mean zero negative impact (I'd be surprised if an engine lasted just as long after such modifications), but perhaps minimal negative impact. Sorry for the subjectivity of this, but hopefully we can get some useful answers!

PS there really should be a "modifications" tag.

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4 Answers 4

up vote 3 down vote accepted

The rev limiter would have to be removed. This would be involved in a custom DME. With the limiter removed then you'd be able to blow it up if you wanted to.

To support the higher revs, the vehicle would need the engine internals worked on. Stronger camshaft(s), valves, valve springs. (Head work). Depending on the strength of the rotating assembly at the core of the engine, you could even be looking at crank/piston/rod work as well.

Then you'd have to start looking at the transmission. If it will be able to support the higher revs. What I mean by this is if the clutches, orbitals, front pump (if applicable), input/output shafts, will be able to withstand the rise in pressures and temperatures.

By the way, by "safe", I don't mean zero negative impact (I'd be surprised if an engine lasted just as long after such modifications), but perhaps minimal negative impact. Sorry for the subjectivity of this, but hopefully we can get some useful answers!

This is NEVER the case. If you've ever noticed, mechanics and hobbyists that build track cars are usually working on them more than they are racing them or even driving them.

The way I look at it.

Build it to blow it up, and then build it again

The competitive aspect of racing and the learning that knowledge that you get from building to me is absolutely priceless. Anyone can play a game/pass a test. But, it's in the field where you learn.

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Thanks for a thorough answer, I didn't think about the impact on transmission! You make a good point at the latter end of your answer - perhaps the only "safe" way is to completely replace the engine with a performance one, like an M3's. –  andrewb Mar 15 '13 at 3:55
    
The only "safe" way is to just drive it as it came from the dealer. They've built it the way they ship it for a reason. But if you're going to play. Then play hard, and expect to spend a lot of time and money with the project. I'm not wealthy by any means, but I've always got a car ready for the track, or being built to for to track. It's an addiction. Take your car to the track stock and try it out. Don't think you're going to look dumb being "stock". How else are you suppose to get a base line figure on how your car runs. Then you can see real gains from upgrades. Not what the website says. –  cinelli Mar 15 '13 at 4:27
    
I suppose so then. A track day would be a good plan. I'm fascinated by the engineering of modifying cars, but I'm still not sure if I want to be spending my money that way. You're the second person to tell me that it's addicting so I'll have to think about it carefully! –  andrewb Mar 15 '13 at 6:29
    
If you're a competitive person. Then even a simple game of kick-ball turns into a self made mission to win.. (A little off the topic. But you get the point). Good luck if you go to the track. –  cinelli Mar 15 '13 at 9:19
    
Oh I am... and I'm very much a big problem solver/engineer and have enjoyed athleticism. All a good recipe for a very successful and possibly obsessed racing driver! And that's why I tread carefully! –  andrewb Mar 15 '13 at 12:52

There are 3 key factors to work on when uprating for higher revs:

  • Moving parts

Spinning a rod, cam or flywheel faster than it is rated for will result in it destroying itself, often spectacularly, as the stresses become too much. Moving linkages and pistons back and forth also takes a lot of energy - Kinetic energy = 1/2 m v squared, and it is that squared which causes the problem. What they do in formula 1 to enable higher speeds is reduce the mass of each part, but this requires very strong materials.

  • High pressures

Fuel must be pumped faster, air needs to enter and exit the engine faster, the compression of the engine will typically be higher etc - so the pressure in pipes and cylinders will be higher. To cope with this, all these areas need to be made of higher strength materials, gaskets need to be more robust, pipes mustn't split etc.

  • High temperature

As you are running everything faster, temperatures will be higher in all areas of the engine and exhaust. Using materials with a higher failure temperature point is essential. As with pressure, this includes pipes, gaskets etc., but also the types of metal used in the cylinder head, pistons themselves and the exhaust.

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1  
Moving parts => steady state motion plus torques plus accelerations plus jerk (the first derivative of acceleration). Everything gets much worse much faster as rpm increases. The conversion of linear to rotational energy and back again, the stresses... urgh. The mind boggles. –  Bob Cross Mar 15 '13 at 12:21
    
Was an excellent documentary just on the stresses in F1 engines at 18000rpm. Astonishing that they don't just rip themselves to pieces, as they are so close to the limit of manufacturing capability. –  Rory Alsop Mar 15 '13 at 15:06

Probably the most critical part for higher RPM's is the valvetrain. So you want better valve springs and retainers and a custom ground cam that is designed for the RPM you are trying to acheive. This is all to ensure that you don't encounter a situation where the valves cannot close fast enough and the piston head crashes into the valve. The higher the RPM the faster the valves have to close and the greater chance of a valve crash if the valve springs cannot close the valve quicker than the piston travels upwards. So having strong enough valve springs and a cam that will close valves at a safe time for the rpm is the most crucial thing.

Raising the RPM of an engine generally increases the peak HP of the engine so you need to make sure that other components in the engine and drivetrain can handle that added HP and stress. This is where upgraded rotating assemblys rods and pistons come in and also may require transmission and clutch components with better HP handling.

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As Cinelli said, you'll have to do extensive work to the header and possibly the rods,pistons and bolts too.

But here's a little secret: manufacturers always leave a wide and comfortable margin of error in their mass-produced engines to make them more durable. In the case of your 320i, it is entirely within safe limits to increase almost any performance-related property by 10% and still see very little extra wear. For instance, the rev limiter can be tuned to only engage at around 7000RPM (assuming it's at 6500 from the factory) or the turbo boost pressure can be increased from 0.3 bar to 0.35 bar (or add about 10 - 20% to whatever that is in PSI). I would recommend you buy a colder running thermostat and install a larger, flow-matched intercooler when you do this because what you DON'T want is a 10% increase in operating temperature. The one thing that ISN'T very lenient is the difference between safe operating temperature and "blown headgasket" temperature. Especially in turbo cars. Turbo cars usually run at 90 to 100 degrees celsius and at 112 degrees celsius the heat will warp your aluminium header. And it's very easy to reach that temperature in hot weather while sitting in traffic.

Warning: you can do exactly ONE of those things and still be safe. If you increase boost pressure AND the RPM limit, your car will break in a spectacular fashion.

Or if you REALLY want to rev to 8500 safely, buy a Mazda RX8.

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