What is a stroker or stroked engine?

"Enjoy reading our guide to stroker engines."

An engine might be referred to as stroked, or a stroker engine. So what is a stroker engine?

So do stroker engines make more power? What are the advantages of stroker kits? Do you always increase the stroke or are there benefits in shorter stroke engines?

To many new to the world of car tuning this term can be confusing so what do we mean by a stroker or stroked engine?

The Pros and Cons of stroking an engine

Let's look at the pros and cons of stroker kits and how this will affect your enginesĀ  performance.

The stroke refers to the distance the piston crown travels inside the cylinder.

So if an engine is stroked or you fit a stroker kit you are effectively extending the travel of the piston. This will increase the engine capacity.

We generally assume the stroke is lengthened but it can also be shortened.

As the piston crowns travel changes so does the engines capacity and compression ratio.

By stroking the engine you can maximize your power output. Increasing the displacement offers obvious gains, and more fuel and air being burned will make more power.

If you want to add forced induction or increase the intake charge pressure you could reduce the compression ratio of the engine to reduce the risk of detonation or premature ignition.

You can't just fit longer rods to the engine, the piston will smash into the head or the valves and cause a lot of damage!

Commonly a reduced height piston crown is used to provide the required clearance and this is matched to a revised crankshaft.

Also the crank shape and profile will alter the throw of the pistons, so a typical stroker kit will use revised crankshaft, conrods and piston crowns.

Whilst you are increasing the stroke it is also common to alter the bore of the engine, effectively making the cylinders larger.

On a cast iron block this is a matter of machining out to the required size. When aluminium blocks are used it gets more challenging and in many cases you can swap the cast iron liners for thinner units, but on others you'll need to machine out the block and fit revised liners.

By altering the stroke of the engine you also alter the characteristics and power profile.

Generally speaking shorter stroke engines as found on many JDM Japanese engines will rev much higher and tolerate higher RPM. Torque and power peak higher up the RPM range. See the notes below on rod ratios to explain this.

If the stroke is long as is the case on many large V8 engines, you have a lower revving more torquey bottom end but a fairly short rev range.

The benefits of stroking an engine

With a longer stroke

Stroker kits have advantages but these depend if you are lengthening or shortening the stroke.

  • Increasing compression ratio
  • Increasing cylinder capacity
  • Gaining bottom end torque (and top end torque but most commonly it will be lower down the RPM range)
  • Engine response is usually slower
  • Engine wear is generally greater due to the extra forces

And if you shorten the stroke

  • Lower the compression ratio allowing for more forced induction
  • Increase the safe working RPM range and red line
  • Raise the top end power band

In some cases you may want a long throw crank, shorter rods and higher piston crown. This will really alter your engines characteristics and performance.

Generally speaking this is the route to go when you want to build a much stronger engine with forged components.

Rod ratios and angles

Getting more technically minded we can start to look at the angle of the rod. As the crank rotates you get sideways pressure on the rod and piston crown. Longer rods generally have a larger rod ratio.

To calculate your rod radio take the length of the rod and divide this my the stroke of the crank.

It is over simplifying things to think you can just throw any rod, crank and piston in your engine and that as long as it doesn't hit anything you'll always make more power.

You need to look at lots of factors to get the best out of this mod.

Questions to ask are

  • How near am I to detonation?
  • Is the engine designed for top end power or low down torque?
  • Where do I want the power band?
  • Do I need stronger engine parts to handle the power or do lighter ones work better for my aims?
  • Can I get pistons & rings to suit the new engine bore size?

Don't turn a long thrown engine into a short throw one or vice versa, you'll be asking for trouble but there is usually a fairly wide optimal range with all engine types.

By buying a stroker kit, you'll hopefully be buying all the parts that someone has sat down and done all the calculations for you.

A good reputable engine builder or motorsports tuner will happily sit down and do the calculations and ensure you get the end result you desire. There really is no substitution for experience when it comes to stroking an engine as there are so many factors and variables to be taken into account.

Engine oil choice will also become critical, the speed the piston moves against the cylinder wall will alter, as it covers a different distance for each revolution.

Expect to alter other components to get the most from a stroked engine, such as fuelling and your camshaft profile.

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