Stroker Crank

If you want to have a larger engine (and who doesn’t?).  The most effective way is with a stroker crank.  What that is, is just a crank with a longer stroke than what originally came with the engine.  Sure, you can bore the engine out, but that usually has limited displacement gains versus a stroker crank.

One thing to realize is that stroking an engine will not make more horsepower (at least not much of a difference) if you don’t make any other changes to the engine.  It will make more torque, and the horsepower will happen lower in the RPM range.  That is because the engine is moving more air at the same RPM versus the original stroke because it has more displacement.

That means that you can run the engine at lower RPM and get the same acceleration.  Or you can increase the airflow with a different cam, different heads etc and have the same operating range as before, just with more power.  Have you ever heard the saying, there is no such thing as too much cam, just not enough motor?  That is why it is true.

Of course, there are trade offs, just like anything else there is no such thing as a free lunch.  The first is, there is only so much room to fit that larger crank into the engine.  And even ones that will work, may require you to grind on the block to make sure there is enough clearance.  And depending on the engine, there may be other changes that need to be made also.

The other issue is that with the longer stroke, the piston is moving farther.  That means that you need to make sure it doesn’t go too far up the bore, or you will end up ramming the piston into the head, and that may cause a problem.  Actually, I would like to see the engine that doesn’t cause a problem for.

There are two ways to combat that.  The first is to move the pin higher in the piston, which shortens the compression height, which is the distance from the center of the piston pin to the top of the piston.  This works great, until you run out of room to move the pin higher and start hitting the rings.

It is possible to put the pin behind the oil ring.  A spacer piece usually goes over the pin to help keep the ring in place.  This can cause increased oil consumption, but how much depends on the engine (and the oil you use).  If it is a big deal depends on you, do you want to have to keep adding oil?  And it may be that there may be no increase in oil consumption either.  That would be something to check with who you are working with on building the engine, either an engine builder, or the supplier of the stroker kit.

The other way to make sure the piston stays inside the block, is to shorten the connecting rod.  Of course, there are drawbacks there too.  There is something known as rod ratio.  That is the ratio of the rod length to the stroke.  The first advantage to a higher rod ratio are that the piston pushes less on the side of the cylinder wall.

Check out the two drawings below.  Note where the top end of the connecting rod is pointing on both.

Stroker Crank Stroker Crank

Yes, my drawing skills need some work, but you can see that the piston is being pushed more to the side with the shorter rod that the longer one.

The other benefit to a longer rod is that the piston spends more time near top dead center than with a shorter one.  That means that if you measured for how many degrees of crankshaft movement the piston was within 0.001″ of TDC, the longer the rod, the higher that number would be.  I would like to have a nice animation of it, but I am still working on getting good enough to make them.  I keep hearing George Carlin with his Al Sleet character, “if you look at our weather map, you will see we don’t have one”.

These differences, as long as they are within reason, don’t make a huge difference in the average engine.  If you are talking an F1 engine, then sure, especially since they turn such high RPM that if the rod is too short the piston could end up moving down before the combustion flame reaches it.  But since I doubt you will be building an engine that will turn 18,000 RPM, it isn’t a huge concern.  Just run as long a rod as you can and call it good.

The beauty of stroker crank kits is that they can be as simple or as complex as you want it to be.  You can just put the crank from a larger engine into yours, and as long as everything fits, there you go.  That is how stroker engines started, with things like the 383 stroker Chevy, a crank from a 400 in a 350 bored 0.030″ over.

And with improved manufacturing that makes it more economical to make pretty much everything, it is possible to get a stroker crank kit for any popular engine, from your Honda, to your VW, to your Pontiac GTO to your lawn mower.

Actually, stroking the engine on your lawnmower could be pretty easy, since there are many variations of small engines as far as power and displacement.  The same with the old air cooled VW engines.

Muscle car enthusiasts didn’t invent stroking an engine for more displacement, I am sure that has been around for as long as there have been the parts and manufacturing abilities to do it.

And if you want to see the George Carlin bit I referenced before, here it is on YouTube.  There is no profanity, but it is still not exactly kid friendly.


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