Simple Question, Difficult Answer | Connecting Rods 101 [#TECHTALK]
- One of the common upgrades we'll make when building a performance engine is to replace the factory connecting rods with something a little bit beefier and a little bit stronger and fortunately in the aftermarket, there are a wide range of options to choose from. This is something we're commonly going to do when we're building an engine to support more power, more torque, a higher RPM limit or potentially all three of those things simultaneously. When you are trawling through the internet looking at the options available for your particular build, it can be a little bit baffling and one of the obvious questions you might be asking is how much power can this particular set of connecting rods handle? Unfortunately the answer to that question is a little bit tricky and not as simple as you may think. So here we're going to go through the forces that a conrod has to support and what you need to consider when choosing rods for your particular application. Let's start by having a look at a factory connecting rod and an aftermarket connecting rod for a Subaru FA20 and we'll have a look at some of the features of those two options.
We'll start by taking a look at the factory connecting rod out of the FA20 and this isn't really so much a case of focusing on the FA20 rod as some of the manufacturing techniques that are commonly found in a factory connecting rod and some of the shortcomings that result. However one specific aspect I do need to touch on with the FA20 rod because it is peculiar to the FA20 is that the parting line for the cap is essentially offset, it's not perpendicular to the centreline of the rod and this allows the rod cap to be removed in the FA20. The rod itself is manufactured using a powdered metal technique which is essentially, for all intents and purposes, where a mould of the connecting rod is packed full of powdered metal and then the mould and that powdered metal is heated up to a point where all of the powdered metal melts and then fuses together. The result of this is it's cheap to manufacture, it does the job for the intended purpose but there is no real grain structure to speak of within the rod so this results in limited strength. Obviously this becomes an issue when we start modifying the engine for more power.
Now another aspect with the powdered metal connecting rod manufacturing technique, the way the caps are broken away from the body of the rod. I've got another part of the rod here, or another part of a rod which is the cap and essentially this is called a cracked cap and basically the manufacturer, once the rod is made in one piece, to separate the body of the rod from the cap, the cap is literally cracked or broken away from the body. So this results in a broken appearance to the mating surfaces. Now there's pros and cons with this.
First of all it means that it's essentially going to be impossible for us to install that cap around the wrong way, it's just not going to mate nicely if we put it around the wrong way. The coarse structure on the mating surface as well also tends to locate the cap to the body of the connecting rod reasonably well. Downside for us in the aftermarket is that if this connecting rod ever needed to be resized then it's very difficult to do so. The resizing process relies on the machinist taking a small amount of material off the parting line of the cap and body and then the cap and body are bolted back together and the big end of the connecting rod is resized to get it back to the factory size and the correct distance between the big end and the small end centres. Now let's look at a replacement aftermarket connecting rod for the FA20.
And the one we've got here comes from Bryan Crower. This is a H beam design and in this video we're not really going to be focusing too much on the difference between H beam and I beam, that's a different topic for another day. However there are a few features that we do want to focus on here.
Obviously for a start to be compatible with the FA20, the key elements, the big end bore, the pin bore, the distance between the centres is obviously still going to need to match. Likewise, it still has the offset cap to allow the rod to be assembled and removed with the crankshaft and the connecting rods in place, just like the factory FA20. One of the key elements with this aftermarket rod though is the material that is used. This is a forged material. And essentially what this means is that we've got a big piece of material that's going to be used as the blank to manufacture the connecting rod out of and this is forced under high temperature and pressure into the rough shape of the connecting rod. There's an advantage in doing this which is why forging is used for conrods, it's also popular for manufacturing pistons.
By forcing the blank of material into the shape of the connecting rod under high temperature and pressure, it creates a grain structure that essentially flows around the shape of the connecting rod. That grain structure is really important, that's what doesn't exist in our factory powdered metal connecting rod and that makes the conrod much stronger. We've also got a superior material being used that is much more pure, it doesn't have inclusions in the material that could be a potential weak point that could result in failure. The other element to discuss here is our connecting rod bolts and as with a lot of aftermarket connecting rods, these use the popular 3/8th inch ARP 2000 rod bolts. And there are a range of different rod bolts available from the likes of ARP with different strength and different characteristics, we'll dive into that in a little bit more detail but essentially what we need to know here, when compared to the stock rod bolts, they are much stronger and they can provide better clamping force.
So the upsides here, first of all, better material, we've also got a better design in terms of where the material through that connecting rod is located. This will be designed using finite element stress analysis to know that when that conrod is placed under high levels of stress it's not going to bend and buckle and then we've also got those stronger rod bolts so all good stuff. Begs the question though, how much power can these rods handle? Now the problem with that specific question and this is why it's very difficult to get rock solid answers if you Google this on the internet or even if you are on a conrod manufacturer's website, is that it depends. Now I know that sounds like I'm dodging the question but it really comes down to how the connecting rod operates inside the engine and the stresses and strains that are applied to the connecting rod.
To get a better understanding of this, let's go through the stresses and strains that a conrod needs to endure. Let's start with the first force that the connecting rod needs to withstand and probably the one that most people think of when they consider rod failures which is compressive strength. So that's where we've got the connecting rod attached to the crankshaft at the bottom, we've obviously got the piston on the small end of the connecting rod and during the combustion process we've got a huge amount of cylinder pressure acting down on the top of the piston then being transferred down through the beam of the connecting rod into the crankshaft. This is what creates torque at the crankshaft and in turn power.
So of course the beam design in the material need to be spec'd to withstand the amount of cylinder pressure that is being applied to the top of the piston. However, cylinder pressure does not directly relate to our engine power. Yes there is a link but there's a little bit more going on.
What I mean by this is the amount of cylinder pressure that would be generated in maybe a high boost, relatively low RPM four cylinder turbocharged engine, will be much much greater than the likes of a naturally aspirated Formula 1 engine that runs to let's say 17,000 or 18,000 RPM. The actual cylinder pressure in the F1 engine is much lower. However it's getting that cylinder pressure much higher in the RPM which is how the F1 engine makes such high power levels from such a small capacity. Without trying to get off track here too far, the relationship between power and torque is the formula power, in horsepower, equals our torque in pound foot, multiplied by our engine RPM and then divided by a constant which is 5252. So what this means, all things being considered is that if we make X amount of torque at low RPM, our engine power will be lower than if we make that same amount of torque at very high RPM because that RPM factor is a multiplier in that power equation.
So just getting to the point here that it doesn't really matter what the power the engine is as far as the connecting rod's concerned, it's the cylinder pressure that it needs to be able to withstand. Let's move onto the second force involved in the operation of the connecting rod and that's the tensile strength. And this is essentially the strength of the connecting rod in tension where it's trying to be pulled apart. Now you might not think that that's a really big consideration in the design of the connecting rod however it really is.
And it's easy to overlook this. This is what happens every time the piston goes past top dead centre on the exhaust stroke, basically the connecting rod and piston assembly is moving up on the exhaust stroke, forcing the spent combustion gases out of the exhaust valves. Now when the piston gets to top dead centre, the connecting rod has to slow that piston down, stop it at top dead centre and then accelerate it down again on the intake stroke. And as the piston passes through top dead centre on that exhaust stroke, it no longer has the compressive force that it has during the compression stroke to actually help slow the piston down.
It's all 100% up to the connecting rod in order to make the piston slow down, stop and then accelerate away from top dead centre. So that places a huge amount of tensile force into the connecting rod. Considerations here, the beam of the rod does play a part in this but actually the cap is a really big factor here. Particularly the strength of the cap or the rigidity of the cap is key. If there isn't sufficient rigidity in the cap, it can deform.
The other aspect which is probably the key here is the strength of the connecting rod bolts, it's those rod bolts that hold the cap to the body of the connecting rod. If they're not up to the task, very high RPM can easily end up seeing our connecting rod bolts fail. The other thing we need to consider here with respect to RPM is that the forces involved actually increase with the square of RPM. So without getting too deep into the maths here, what this simply means is that even a small increase in our RPM ceiling can actually have a really significant impact on the amount of stress and strain applied to the connecting rod and the conrod bolt so give that some thought when you're thinking about increasing your rev limit from 7000 to 8000 RPM without addressing the components inside your engine. So at this point we've looked at the different forces involved in the operation of the connecting rod and hopefully at this point you've got a better understanding of why we can't easily give a horsepower rating to a connecting rord.
Remember under the compressive forces, it's the cylinder pressure that matters and the cylinder pressure doesn't directly relate to our engine power unless we also take into account the RPM range at which peak cylinder pressure is occurring. Now this is where certain manufacturers can start to give you at least some guidance on the specific power supporting capability of a rod. If we're talking about one specific engine, an engine where the rod manufacturer has gone through the design process for that engine and has a reasonably good idea of the RPM range that that engine is going to see as well as the cylinder pressures that are likely to occur within that RPM operating range, then they can start to extrapolate this out into a horsepower value and give this to us as a range that we should be able to work within. Now why do we even do this? I think this really comes down to the fact that the aftermarket industry has kind of become used to working in horsepower values.
Even though it really doesn't actually make any sense when we're specifying a connecting rod for our build, people want to know what sort of horsepower values these rods can support and this is why the manufacturers are kind of forced to give some information that actually isn't that easy to extrapolate so that we've got an idea whether the rod is going to be suitable for our build. If instead the rod manufacturer gave us a cylinder pressure value or a compressive strength value for the rod, that for us is probably not going to be that meaningful. Alright let's have a look at another couple of aspects with our connecting rods that we also need to consider. Now obviously not all connecting rods are made equal and here we've got two options for the popular Nissan SR20DET. The top rod here is a K1 Technologies rod. This is an H beam design, forged material for the construction and uses the ARP 2000 rod bolt.
We have personally seen these rods support upwards of 700 horsepower and 8000 RPM in the SR20 platform. Below is the BoostLine rod. Now BoostLine do actually give some information on expected power handling capability.
In this case a slightly ambiguous 1000+ horsepower. These rods are able to be sped'd with either the ARP 2000 rod bolt or as we'll talk about in a moment, if you're expecting very high tensile forces in the rod then you can spec these up to the custom age 625+ fastener from ARP. Just looking at the two rods side by side, we probably don't need to use finite element stress analysis or be a mechanical engineer to just visually see that the beam of the rod on the BoostLine design is quite a lot beefier. So by eye, we could reasonably expect that the BoostLine rod, obviously not withstanding differences in metallurgy is likely to be the stronger option. I've mentioned rod bolts a couple of times so we'll just go into a bit more detail on that. As I mentioned, the rod bolt is really the key to holding the cap on the body, particularly as the rod goes past top dead centre on the exhaust stroke and has to slow down and then accelerate the piston past top dead centre.
So the whole time that's happening, the cap is trying to tear the rod bolts in two. So this only gets worse as our RPM increases so particularly with engines where we're intending to significantly increase the RPM limit, we do really need to give some thought to the rod bolt design or the material that the rod bolts are made from. And one of the common options here is to move from the popular ARP2000 material which is popular in a lot of aftermarket connecting rods, up to the custom age 625+ material.
So that material is significantly stronger, provides a lot more clamping force between the cap of the rod and the body of the rod and it's less likely to fail under very high tensile forces involved in high RPM operation. This obviously comes at a price though. The custom age 625+ material is more expensive so you can expect to pay more for these rod bolts.
You also do need to be mindful of the fact that special materials like this can suffer from what's referred to as hydrogen embrittlement. In particular when we are handling these rod bolts, it's important to make sure that you are using gloves so that you don't transfer the oils and acids from your skin onto the rod bolts. This is essential if you want the reliability and potential out of these bolts that their price tag suggests. The other thing I want to touch on as part of this video is balancing the connecting rods. So this is a little aside from the strength but bear with me because there's a reason I want to discuss it.
So this is a Proform conrod balancing fixture and the idea behind it is that, as you can see, it can support the small end of the connecting rod and this particular little hanger here is supported on ball bearings, meaning there's very little friction. Likewise the big end of the connecting rod is supported on this stand which sits ontop of our scales. So the idea with the conrod balancing fixture is that it can separate the weight of the big end of the connecting rod from the small end. That's really important because of the way the connecting rod operates in our engine. When the connecting rod's fitted to our engine, part of the mass of the rod is considered to be rotating, moving around the engine with the crankshaft, the other part of the mass is considered to be reciprocating, moving up and down the bore attached to the piston.
So this is why we can't simply take the connecting rod, weigh it on a set of scales and then just remove weight from anywhere on the heavier conrods until they all weigh the same, we need to make sure that all of the big ends weigh the same and all of the small ends weigh the same. And this does take a little bit more effort than just overall balancing the likes of a piston for example. Now why I wanted to raise this is that one of the common mistakes I see even professional engine builders make when it comes to balancing connecting rods is where they remove material from the connecting rod.
Let's say we've got this BoostLine rod and we find that this particular rod is a little bit heavy so we need to decide where abouts we can remove that weight. One of the most obvious places to remove weight is the two ribs on the back of the cap. By simply running these on a linisher and flattening them down, it's very easy to remove a significant amount of weight relatively easily.
Problem with doing this, and why we definitely don't want to do this is that those ribs are there to make that cap much stronger and more rigid and remember I said that particularly under high tensile forces that we're going to see at high RPM, the cap can tend to distort. If it does this, we're going to potentially run into problems with bearing reliability and this could end up damaging our expensive engine so the ribs, they're there for a reason, do not touch them. They are key to the rigidity of the connecting rod cap. Instead, let's have a look at where we can safely remove weight.
Now obviously every connecting rod's different so it's about applying a little bit of common sense. What I like to do is remove material down the edge of the cap and the body here and there's actually a nice little pad here which would lend itself really well to linishing some of that material away and we've got the same on the other side. Essentially though, even without the little pad there, we can see that we can actually basically chamfer the edge of the rod here and we're not going to end up adversely removing material and affecting the strength of the rod cap.
The other thing that should go without saying is that when we are removing material from the rod to balance it, we want to remove as little material as we possibly can so that we're not affecting the strength and we want to remove this equally from each part of the rod so if we are going to be chamfering the sides of the rod along the bolt like I've just shown you, instead of doing all of this just on one part, we've got four areas we can remove material from in the big end of the rod and we want to remove a small amount of material from all four rather than one large amount of material just from one area. So hopefully at this point you've got a better understanding of the stresses and strains that a connecting rod needs to put up with. While obviously I haven't been able to answer that question of how much power can my connecting rod handle, you've also hopefully now got a better idea of why that's actually not such an easy question to answer. If you are doing something pretty unique with your particular engine and you're expecting to run significantly more power, more cylinder pressure or much higher RPM than what could be considered typical use with that engine, it's always a good idea to consult with your conrod manufacturer, they'll have a tech support line and they'll be able to give you some more specific advice as to whether the rod that you've selected is going to be up to the task for your application.
This can be cheap insurance to protect your expensive new engine. If you liked that video make sure you give it a thumbs up and if you're not already a subscriber, make sure you're subscribed. We release a new video every week.
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