Stroker engines explained

What exactly is a "stroker engine" ? What are the advantages and disadvantages? And 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 with moving the pin around and lengthening the rod. This is where the deck height of the block comes into play. The block's deck height is the distance from center line of the mains to the deck surface where the head gasket lives. For example, a Chevy small block has a 9.025" deck height and a Ford 302 has a 8.206" deck height. Guess which one can accept more stroke. Never fear, Ford fans. The 351W has a 9.500" deck height to out-stroke those Chevy guys. And the cam in the 351W doesn't get in the way!

    You have heard some people talk about a "zero deck engine". This refers to the deck clearance of the engine being zero (i.e. the pistons comes all the way up to the top of the deck suface with no clearance). Typically from the factory, engines will have about .020" of deck clearance. Different manufacturers and different engines will vary, of course.

    Here is your next handy formula:


    deck height = (stroke / 2) + rod length + piston pin height + deck clearance

Some people like to think of the first part as the "assembly height", so here it is like that:

    assembly height = (stroke / 2) + rod length + piston pin height

    deck height = assembly height + deck clearance

    I guess it just makes more sense to some folks like that. Either way, that equation must balance for the engine to work on paper. I say "on paper" because you have to keep the limitations of the parts in mind. For instance, piston pin height can only be so small before it is impossible to manufacture. Usually, pin heights less than 1.000" are very difficult unless you use a really small pin. But a really small pin might not be strong enough.... You start to see the games we play. Also, cranks need to balance. If component weights go up, and strokes go up, counterweights need to be bigger to compensate. This requires longer rods, which moves the pin again.... more games.

    So, block clearancing, deck height, what else can there be?

    Not much really.

    But there are other things that occasionally come into play, so I'll mention them briefly. Since compression ratio is determined by the volume of air in the cylinder at bottom dead center (a.k.a "swept volume") divided by the volume of air at top dead center, if you increase the stroke, you increase the "swept volume" and increase the compression ratio. Increasing the bore size does the same thing although only slightly. For instance, compare a pump-gas friendly flat top 327 Chevy with 9.7:1 compression with 4.000" bore and 64cc heads. now build that block with a 4.000" stroke (yes, it will fit. It's not easy, but it will fit!) and flat top pistons. It suddenly becomes a race-gas only 11.7:1 compression! This means dished pistons or bigger chamber volume heads. Different heads might not be an option, and as the dish gets deeper on the piston, the piston face and pin might try an occupy the same space! More limitations...

    You might have also heard people say "you can't rev a stroker motor as high as a non-stroker motor". Technically, no. Realistically, yes.


     OK, here's what I mean. There are two main factors at work here. One is maximum piston speed and the other is rod ratio. Maximum piston speed is the highest speed the piston (and more importantly, the rings) will see. This is directly related to the stroke of the engine and RPM (obviously the maximum piston speed will occur at maximum RPM). The limiting factor here is actually the rings. At some speed, the rings will be sliding along the bore faster than they can handle and will wear out or fail altogether. The "old school" rule of thumb is not to exceed 140 feet per second. Before this scares you, realize that a Ford 302 would have to rev to 10,700 rpm to get to 140 ft/sec. A stroker 347 would see 140 ft/sec at 9450 RPM. So technically there is a difference. Realistically, is anyone really going to be revving a 302 or 347 that high? OK, sure someone out there might be, but most of won't. For the few who will, that is the reason I brought it up at all. Modern rings and bore finishing may allow for higher maximum piston speeds. If you plan on building a high RPM engine, talk to your preferred ring manufacturer about what the rings can handle.

    Another factor is rod ratio. This is perhaps the most obscure topic of all. Rod ratio is the length of the rod divided by the stroke of the crank. Rod ratio has several effects. The most important one related to stroker motors is side loading. Yes, I know there are other factors like piston acceleration and dwell time, but neither is much of a limiting factor. They just change the engine characteristics which is a topic for a different article. Side loading is increased with lower rod ratios. Likewise, it is decreased with higher rod ratios. The best way to understand side loading is to try and imagine that the rod is trying to shove the piston through the side of the clyinder wall instead of up the cylinder where it belongs. Some people will talk about this subject and refer to "maximum rod angle". "Maximum rod angle" and "rod ratio" are directly related. They are basically two ways of saying the same thing. Higher maximum rod angle / higher side loading / lower rod ratio all describe the same problem - increased friction due to side loading. This leads to higher piston skirt wear rate, more drag and more heat. All of these things will result is some loss of power compared to the same size engine with a higher rod ratio. Higher rod ratios / lower maximum rod angles / lower side loading will have less friction, lower piston skirt wear rates, etc. So, where is the "limit" you don't want to cross? This is where it gets "fuzzy" and opinions step in. Smokey Yunick was a "put the longest rod you can find in there" kind of guy. While there is certainly a case for this argument, at some point increasing the rod ratio  more just doesn't realize any more gains. It's a diminishing return sort of thing.

    I think it's time to throw out some practical numbers to think about so here are some common rod ratios.

    Ford 302 (5.090" / 3.000") - 1.70

    Chevy 350 (5.700" / 3.48") - 1.64

    Ford 351W (5.956" / 3.500") - 1.70

    Chrysler 360 (6.123" / 3.580") - 1.71

    Chevy 400 (5.565" / 3.750") - 1.48

    Ford 460 (6.605" / 3.850") - 1.72

    Chevy 454 (6.135" / 4.000") - 1.53

    Chrysler 440 (6.760" / 3.750") - 1.81

    You can see the typical range is from about 1.50 - 1.80 with few exceptions. While there are a lot of people that believe the longer the rod the better, period. The other side of the coin is a lower rod ratio motor can have better throttle response, might be less timing-sensitive on forced induction engines, and other deeply theoretical topics. Before you worry too much about rod ratio, also understand that smaller bore engines and engines with less cylinder wall clearance tend to resist piston skirt wear better. And with better cylinder boring technology and skirt coatings, skirt wear is not as much of an issue as it used to be. Do Chevy 400s have a reputation for wearing out quickly? Do Chrysler 440s have a reputation for lasting forever? Put aside your brand bias and admit to a "no" in either case. Rod ratio, while neat to talk about and consider is not everything. There are a lot of people who will say "never give up a single cubic inch for better rod ratio". For most street engines or common racing engines, I believe in striking a happy medium. So here is your rule of thumb for rod ratio: try to stay above 1.45 or so for a street engine with modern pistons and boring technology. Racing engines that will be rebuilt often can go lower. It is generally accepted that past 1.72 you won't realize any significant gains. Diminishing returns, remember? The improvement from 1.40 to 1.50 is significant. Going from 1.75 to 1.85, not so much. Also keep in mind that this rule of thumb is just a general guide for typical V8 engines. Like I said before, you will find a lot of opinions on this subject. Part of the reason for that is that tangible results are difficult to find. Like I said, within reason, it is just not that big of a deal. Now, don't go build a 15,000 RPM bike engine with a 1.40 rod ratio and wonder why it flies apart on you. On the same note, don't hesitate to turn your Chevy 327 into a 383 with 5.7" rods because it will drop the rod ratio from 1.75 to 1.52. That is, unless you plan to rev it to 10,000 rpm. Stroker engines will usually have a lower rod ratio. How low is too low is a matter of great opinion and it also depends greatly on what the engine will be used for. Yes. There are a lot of things to consider and an engine builder's experience and 1st hand knowledge of particular applications goes a long way. If it were cut-and-dry everyone would build the same motor the same way and that would be no fun.

    I want to take an opportunity at this point to clear up a common misconception on a specific application. It is commonly thought that a Ford 347 uses oil. This comes from the unfortunate fact that early Ford 347 pistons were not made correctly. When a piston pin height gets smaller, at some point the pin will intersect the oil ring. When the piston is designed with a steel oil rail support or something similiar to support the oil ring properly this does not create a problem. Some early Ford 347 pistons were not designed this way and the oil rings "fluttered" and the engines used oil. Most every piston manufacturer now uses oil rail supports and it is not a problem. Many other engines also have pins that intersect the oil ring and do not have the same reputation. The following engines also use oil rail supports: Chevy 383 with 6.000" rods (3.750" stroke 350), Chevy 496 with 6.385" rods (4.250" stroke 454), even a Ford 331 (3.250" stroke 302), and others.

    So there you have it. Bigger is usually better. Make sure it will fit without exceeding your comfort level for clearancing. Make sure the math makes sense and parts are available. And keep your compression ratio, rev range, and rod ratio in consideration.

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