We all know that the single-best performance upgrade you can make to a car is to reduce weight. Whether its sprung or unsprung weight that you shed, it positively affects just about everything: acceleration, braking, handling through corners, MPG, and the list goes on...

Last year, when the gearbox in my classic Impreza blew up, I took the opportunity to refresh both the clutch and the flywheel while easy access was available.

Lightweight Flywheels - Pros & Cons

Before we talk about the results, we'd probably answer the question of why you'd want to make this change. While we're at it, we may as well discuss the pros and cons of such a modification because as we all know - there's no such thing as a free lunch.

So, we all know that a flywheel is a thing that exists. Some of us know what it does, but some of us don't, so for the latter, allow me to explain:

In it's simplest form, a flywheel is a "heavy" wheel that stores rotational (kinetic) energy and smooths out the delivery of power from an engine to the transmission.

The reason I put quotes around "heavy" is that it doesn't have to be heavy heavy like your mum, it just has to have an appropriate amount of weight to it to do its job - which is to smooth out power delivery from the engine's crank to the transmission. The only way it can smooth that delivery of power is by having some weight to it to create inertia.

We could go into the weeds and go into the scientific data, lots of charts and whatnot but I don't want to do that, so I won't. A better solution is giving you the pros & cons of reducing the weight of your flywheel, so here goes:

The Pros

• Revs will climb significantly faster, allowing for more powerful acceleration
• Acceleration will feel a lot more responsive as there is less inertia to overcome when putting the power down
• It causes less wear on the engine because the rotational mass reduces significantly
• You lighten the car, which in itself is an upgrade

The cons

• It's a royal pain in the butt to install
• Revs climb faster, but they also fall faster, meaning your gear shifts will have to become quicker
• It takes slightly more effort to get the car moving from a standstill as there is less inertia in the flywheel
• There is a slightly-higher chance of stalling the engine due to the reduced weight and inertia
• You will feel more vibrations through the car from the engine as there is less mass to dampen those vibrations
• Your engine may idle roughly as there is less inertia/mass in the wheel

On the face of it, listing more cons than pros doesn't get us off to a very good start - but as with so many things in life, the facts only tell you part of the story, and this is where experience comes in.

What's it like to live with a light flywheel?

Totally worth it. I can't help but feel like a lot of the negatives are overblown compared to the benefits, let me explain:

Firstly, the responsiveness is night and day. You really feel the delay between the accelerator pedal and the engine response evaporate. Revs climb a lot quicker and the car feels a lot more punchy.

If you enjoy heel-and-toe driving, a lightened flywheel gives you access to much more precise blipping. It seriously only takes a quick tap of the pedal to send the revs soaring, which is perfect.

What about the negatives, though? I will concede that it takes slightly more finesse to get the car rolling slowly. Because of the lack of mass/inertia, you need slightly more power to get the car moving if you try to roll slowly. I mention 'slowly' because if your preferred approach is to launch, you won't notice the difference. Launches definitely feel better with the lightweight flywheel as you're able to get into that all-important boost range much, much quicker. Anecdotally, in nearly two years of driving with the lightweight flywheel I've probably stalled the car twice. It really isn't an issue.

What about those vibrations? Well a classic Impreza wasn't, isn't and will never be a Rolls Royce. Yes, you get slightly more vibration when you first start the engine but once you're moving, it's no different at all. Honestly, I like it. Once again, very overblown.

I have noticed no difference in idling either. Smooth as butter.

So, to wrap up...

Would I recommend a lightweight flywheel? Yes. Absolutely. It's one of those transformative modifications that you can do to a car that really improves the experience day-to-day and lives up to the hype. The biggest drawback in my mind, undoubtably, is the hassle of installing it. You have to drop the gearbox and remove the clutch. It's a pain of a job, not because it's difficult, but because it's time-consuming, hard to access and a bit finicky overall. If you have to replace/upgrade your clutch, you may as well do the flywheel while you're there. This is without a doubt the best time to do it.

A quick note: Don't overdo it

You know the saying: Too much of a good thing...

One thing you'll hear a lot around the internet is that you should lighten the flywheel or the crank pulley or the propshaft, but never all together. Reducing the mass too much removes too much dampening and too much inertia and makes the car much harder to live with.

When people mention that it's easier to stall the car with a lightened flywheel, they only typically mean when you pull away. If you lighten the entire assembly it becomes much easier to stall the car when decelerating. The reason being that the lack of mass on rotation means the revs will drop really, really fast. Like, so fast that it drops below the stall point of the engine as you're coming to a stop. You'll also introduce a fair amount more vibration and likely face that rough idle situation people mention as a negative.

Keep it sensible. A lightweight crank pulley may reduce rotational mass, sure, but it'll also remove dampening of vibrations which could be detrimental to the health of your engine.

If it were my car, I'd do the flywheel first as it will without doubt have the biggest positive impact. Then, if I really felt the urge, I'd do the crank pulley as its relatively cheap and very easy to install (and remove if you went too far) and finally the prop shaft, as its the most expensive part and will have the lowest ROI.

Just my two cents.