Tuning the Mitsubishi 6B31

"Comprehensive guide to tuning the Mitsubishi 6B31 engine!"

We are regularly getting posts asking about the things we recommend when it comes to 6B31 mods that work. So let us look into Mitsubishi 6B31 upgrades and outline the best upgrades on this great engine and point out some potential pitfalls along the way.

Let us look into 6B31 tuning and highlight the ultimate mods that work. Mitsubishi 6B31 are popular tuning projects and with carefully picked tuning mods like remapping, turbo upgrades and camshafts you will dramatically enhance your driving enjoyment.

When talking about the optimum parts for your 6B31 engine, we are going to focus on the parts that give the best power gain for you money.

The cam profile plays a big part in the engines power output so cam upgrades make quite a large difference. The intake and exhaust durations will alter depending on the chosen cam profile, so large bhp gains are on offer for cam upgrades.

NB: Fast road cams tend to push up the bhp and torque throughout the rpm range, you may sacrifice a little low down bhp but your high end rpm power will be higher.

Motorsport and race cams, push up the high end rpm power band but as a result the car will not idle smoothly and low end power nearly always suffers.

On a daily driver you need to optimize your power band to your driving style.

I'd never find a 6B31 Motorsport cam is a pleasure to live with when driving in heavy traffic.

Each engine responds better to mild camshaft durations so set your engine up on a rolling road.

The ECU mapping and injectors and fuel pump also have a large bearing on the power gains you'll hit.

A longer valve duration can alter the power band and on most engines the exhaust and intake durations do not need to match, although most cams and tuners use matched pairs there are some advantages to extending the intake or exhaust durations.

6B31 Tuning Stages

Stage 1 6B31 modifications: Intake headers, Fast road camshaft, Drilled & smoothed airbox, Panel air filters, Sports exhaust manifold, Remaps/piggy back ECU.

Stage 2 6B31 modifications: fuel pump upgrades, Ported and polished head, high flow fuel injectors, Sports catalyst & performance exhaust, induction kit, Fast road cam.

Stage 3 6B31 modifications: Competition cam, Twin charging conversions, Engine balancing & blueprinting, Adding or Upgrading forced induction (turbo/supercharger), Internal engine upgrades (head flowing porting/bigger valves), Crank and Piston upgrades to alter compression.

Carefully think through your options and then find your modifications and set yourself a power target to avoid disappointment.

ECU mapping will help to establish the full potential of all the modifications you've done to your 6B31.

It will usually give around 30% more power on turbocharged vehicles and you can expect to see around 15% on NASP engines, but figures achieved often differs on the modifications you've fitted and the condition of your engine.

Feeding air into the 6B31 engine is the main goal to any car tuning project.

Intake flow the air from the air cleaner and allow it to be sucked into the engine cylinders.

The size of bore and shape and flow rate of the Intake headers can make a noticeable difference to to fuel atomisation on the 6B31.

It's not uncommon that intake manifold are begging for motorsport parts, although some car makers provide well optimised intake manifold.

Adding a 6B31 larger valve kit, doing a bit of 6B31 port enlargement and head flowing will also increase bhp and torque, the fantastic side effect is it will afford you a better bhp and torque increase on other modifications.

Turbo upgrades for the 6B31

NASP engines need quite a lot of work when you add a turbo, so we have a separate guide to help you take into account the pros and cons of going this route on your 6B31

The more air you can get into an engine, the more fuel it can burn and uprating the induction with a turbocharger upgrade makes superb power gains.

If your car is fitted with a turbo parts are relatively easy and most turbocharged engines are built using uprated components.

There are weak spots for every engine, with some being very over engineered and some only able to handle stock power

See where you'll find these restrictions and install stronger pistons, crank and engine components to survive the power.

We see many guys spending a lot of money on turbo upgrades on the 6B31 only to have the whole thing catastrophically fail when it's first rolling road session.

Larger turbo chargers will usually experience low end lag, and smaller turbo chargers spool up much more quickly but won't have the top end engines power gains.

In recent times the selection of turbo chargers is always increasing and we now see variable vane turbo chargers, permitting the vane profile is altered according to speed to lower lag and increase top end power.

Twin scroll turbo chargers divert the exhaust gases into two channels and push these at differently angled vanes in the turbocharger. They also help the scavenging effect of the engine.

It is not unusual that there is a limitation in the air flow sensor (AFM/MAF/MAP) on the 6B31 when considerably more air is being pulled into the engine.

We note 4 bar air sensors coping with quite large power gains, whereas the OEM air sensor limited bhp at a much lower level.

Adding a supercharger or additional turbo will make large power gains, although more challenging to install. We have this in depth look at twinchargers if you want to read more.

Uprating 6B31 fuel injectors

When you increase the bhp you will need to look at to the fuelling.

More bhp needs more fuel. Don't forget to be generous with your injectors flow rate.

As a rule of thumb add 20% capacity when specifying an injector, which takes into account injector deterioration and allows you some spare capacity should the engine need more fuel.

We think this one is common sense, but you'll need to match your fuel injector to the type of fuel your car uses as well.

Best 6B31 performance exhausts

You may need to replace your exhaust if the current exhaust is actually causing a restriction.

On most factory exhausts you'll find your flow rate is still ok even on modest power gains, but when you start pushing up the power levels you will need to get a better flowing exhaust.

Do not go with the biggest exhaust you can get this will slow up the exhaust flow rate - the best exhausts for power gains are usually between 1.5 to 2.5 inches. It is the shape and material more than the bore size.

Common exhaust restrictions come around the catalyst and filters installed, so adding a higher flowing performance alternative will help avoid this restriction.

Problems and issues to look out for on the 6B31

The 6B31 engines, if regularly serviced and maintained, are generally very reliable and have few issues.

Regular oil changes are vital on the 6B31, particularly when the engine has been modified and is putting down more power than the manufacturer intended.

For more information on Tuning your 6B31 engine please join us in our friendly forum where you can discuss tuning options in more detail with our 6B31 owners. It would also be worth reading our unbiased tuning articles to get a full grasp of the benefits and drawbacks of each modification.

We need your help to complete and improve this page, so do give us your feedback in the comments box below.

We really like hearing from our readers, and hearing about which upgrades were the most effective for them, it helps us improve our recommendations and articles to reflect current trends in modifications and ensures that our 6B31 guides and tips are kept up to date.

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