Stroker Engines

Posted on: Monday, November 30th -0001

What exactly is a "stroker engine"? What are the advantages and disadvantages? What is involved in building one?

What exactly is a "Stroker engine" or a "stroker kit", what are the advantages and disadvantages, and what is involved in building one? This is a commonly heard question. Here are the answers!

A stroker engine is basically just an engine that has had the stroke increased to more than was originally from the factory. A stroker kit is a collection of components that make building a stroker engine possible. Sounds easy, right? Well, there are limitations to what can be done depending on which engine you are building. More on that in a minute. Why you would want to build a stroker engine is a question that first must be answered.

To understand the benefits is simple enough: Power and torque comes from burning fuel. While squirting more fuel into the engine is simple enough, it won't do any good if you don't provide more air also. By increasing the stroke of the engine (and also the bore, too) you can increase the size of the engine. No, I'm not talking about the physical outside dimensions of the engine, but the "breathing capacity" of the engine. For example, a 302 cubic inch engine simply means that in 2 engine revolutions, the engine will theoretically ingest and exhaust 302 cubic inches of air. More air, more fuel, more power! A 347 cubic inch engine will ingest and exhaust 347 cubic inches of air every 2 revolutions. For you mathematically challenged folks out there, that's 45 cubic inches more air every 2 revolutions! Big deal, you say. Well, that is a 14.9% bigger engine. 14.9% more air. 14.9% more fuel, and theoretically 14.9% more power! So that 350 hp 302 you built would theoretically be 402 hp as a 347!

I know, I said "theoretically" way too much and here is why: Making more power in an engine is not only about increasing the breathing capacity. There are other things to take into account like camshaft specs, head flow, and a myriad of other things. Without getting too detailed, you need to understand that not all components that work well for a 302 cubic inch engine work best for a 331 cubic inch engine or a 347, or a 354, etc.

It's been said before that "it's all in the combination". This is certainly true. Everything must be taken into consideration, and increasing the breathing capacity of an engine is one of them. Almost never is increasing the breathing capacity a bad thing. Yeah, I said "almost". This has to do with the limitations of the engine block and factors like that. We'll discuss that in a minute.

Here is a handy formula for finding the engine size of a V8 engine:

engine size = bore x bore x stroke x 6.2838

All dimensions should be in inches.

Notice the bore is counted twice? You might think that increasing the bore would have more of an effect than increasing the stroke if you wanted to increase the engine size. You would be right except for one problem: You can't make huge increases in bore size, the block usually won't allow it. Darn those limitations. For example, most small block Ford blocks can only be bored .060" oversized (4.060" vs. a stock size of 4.000"), It is quite easy to increase the stroke by .400" (from 3.000" to 3.400"). In addition, if a block is bored to the limit, it cannot be bored again after that! Most people will want to save a little room for freshen ups or rebuilds.

Now that we are on the subject of limitations, let's explore some of them so you'll know what you're up against. The engineers that designed each block never intended for stuff like this, but they did leave some room for improvements. Some engines more than others. Not all engines are created equal. The longer stroke crankshaft and the rods have to be able to rotate in the block without running into things like the crankcase walls, bottom of the cylinders, oil pump bosses or pickups, oil pans, windage trays, main cap girdles, or even the camshaft!


The camshaft!? Really?

It's relatively easy to get out the grinder and grind away parts of the crankcase, notch out the bottom of the cylinder walls, buy a different oil pan or pickup, or buy a different main cap girdle. But you can't exactly clearance the camshaft now can you? Fortunately the only engine that has this problem is a Chevy small block. Unfortunately that is the most common engine people build stroker engines out of! For the Chevy small block, that is the single biggest obstacle to overcome when building a stroker. Design changes in the connecting rod and/or camshaft (smaller base circle) can help with this, but they can only go so far. You will have to consult different component manufacturers to see just how far you can go.

OK. Block clearancing. That's one. It's also the most common. The rest are a little most obscure and might require a bit more thought to understand.

Another limitation involves the entire crank/ rod/ piston combination. Imagine a single cylinder worth of crank, rod and piston combination all assembled at bottom dead center. Now, if you increase the stroke on the crank, it will pull the pistons down into the counterweight of the crankshaft! Not good. Remember, the original designers like to keep things tight in there. Never fear, this is remedied by increasing the rod length. Increasing the rod length moves the piston away from the counterweight and has the added effect of making the rod ratio more acceptable (more on that later). Now rotate the comination to top dead center with the increased stroke and longer rod. Now everything sticks out of the top of the block! This is solved by changing the pin location in the piston. This dimension is called the "compression height" of the piston.

I hate that term.

While changing the compression height by itself would alter the compression, it is not the "ideal" way to change the compression if that is what you wanted to do. The location of the pin in the piston is really dictated by the stroke and rod combination you are attempting to build instead of the compression ratio you want. Compression ratio should be changed by altering the face of the piston and/or combustion chamber of the cylinder head. If I had my way, I'd call this dimension "pin height". That way it's effect on compression ratio wouldn't be implied. Since I am writing this, and this is my world, I'm calling it "pin height".

Imagining all of this, you should start to understand the limitations associated.