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Aluminium Conrod Pros and Cons | GRP Connecting Rods [TECH TALK]

September 3, 2019


– Alloy conrods have long been a mainstay of the drag racing fraternity. They’ve got some great advantages in a drag racing application but for wider use in a more conventional motorsport application, maybe circuit racing, or even your everyday road car, there are some downsides. We’re here with Woody from GRP who specialise in making aluminium conrods to get the lowdown about the pros and cons of an alloy rod. So first of all obviously when we’re making a connecting rod out of an aluminium material, it must be a lot lighter than a conventional steel conrod, is that the case? – Typically in a big block Chevrolet rod you see sometimes up to 200 grams weight savings. – So when everything’s spinning around in that engine at several thousand RPM, any weight that you can remove from the rotating components has gotta be an advantage, is that fair to say? – Absolutely, it’s gonna be easier on crankshaft, easier on the main webs on the block. – OK so why is it that we see aluminium material conrods being so popular in high end drag racing applications but less so in a circuit or maybe even a road car application? – Because in the drag race world when you go to a power adder application, you need that shock absorber which is what the aluminium rod does. It doesn’t transfer all the hit to the bearing. Whereas in a road race application, because of the engine deceleration, the aluminium rod’s just not well suited for it. – OK I wanna focus on both of those topics individually so let’s just talk about the shock loading capability. Are you talking about power adders there, and particularly although we obviously don’t wanna run our engines into knock or detonation, when we’re talking about very high powered engines and we’re tuning right on the edge, that can certainly be a potential downside. So the aluminium conrod helps protect the rest of the components such as the crankshaft by absorbing that shock of detonation, is that correct? – That is correct but the inverse of that is because it’s soaking up some of that shock, you’re constantly flexing the material which is part of the reason why you’re not gonna see the cycle life in an aluminium rod you do a steel rod. – So that’s really the big thing that comes down to engines which are set to see long periods of service life, the aluminium material, no matter how strong that material is, how well designed that connecting rod is, the aluminium conrod is going to have a fatigue life? – Correct, aluminium’s got a far shorter cycle life than steel does, it’s just inherent in the material. – OK we’re gonna come back to that fatigue life, but you also talked about the forces in deceleration. So can you talk about how that affects the alloy rod. – Well when the piston and rod assembly gets to top dead centre and it has to change direction, you’ve gotta pull that entire weight down, back down the bore. And that again, that’s just what fatigues everything. You’re trying to wrap the cap of the rod around the crankshaft and it’s flexing everything and that’s again part of why you don’t get the cycle life in an aluminium rod that you would a steel rod. – Now this is actually made worse though in a road racing application compared to a drag racing application because in drag racing, while we do have to decelerate the piston, or change its direction as it goes past top dead centre transitioning from the power stroke to the exhaust stroke, at the end of the drag run, that engine’s shut down. So how does that differ when the engine is constantly seeing periods of deceleration on a racetrack for example? – Right you know you get to the end of the track on a drag strip and you wanna shut the engine off clean, and that’s why we don’t recommend an aluminium rod in a road race engine because you’ve got that constant engine deceleration and the vacuum above the piston, when you drop the butterfly shut. – So the vacuum actually makes things slightly worse, yes? – It does. – OK so when we’re talking about choosing an aluminium conrod and also factoring in the life expectancy of that rod, what sort of things do we need to keep in mind to decide how long the rod will last? – Component weight, crankshaft stroke, engine RPM, they’re all a factor. And also the fastener you have in the connecting rod, they all come into play. – OK so I noticed there that you didn’t actually mention power which is interesting. Can you tell us why the power is actually less of a consideration to the component weight, RPM, and stroke? – Typically the compressive load on a connecting rod is not what’s wearing it out. Again it’s that piston and rod assembly getting to top dead centre and having to change direction. That’s usually what’s wearing things out. Unless you get into a situation where you over horsepower a connecting rod and physically s bend it, compressive load’s not hurting it. – So when it comes to the life expectancy of the rod, this is an area where we hear a lot of confusion, misconception, misunderstanding out in the greater industry. And obviously it’s impossible for you to say that a rod will last for let’s say 200 runs down the drag strip, there’s all those factors that you’ve just talked about. But how can a customer potentially decide how many runs they can safely get out of the rod? – The best thing to do is contact the manufacturer. We’re gonna wanna know what the crankshaft stroke is, what kind of piston weight you’ve got, what RPM you’re turning it. Given those things, then we can give you a safe ballpark number of how many runs to put on ’em. – There’s some other factors with the alloy rod design, in particular when we look at the big end bearings, there’s a pin that locates the big end bearing shell. Why’s that necessary in an alloy conrod, we don’t see that technology applied to a conventional steel rod. – Because an aluminium connecting rod grows more than a steel rod does thermally from temperature, you need that bearing locating pin. ‘Cause you start to lose crush on the bearing. – OK so this is just the rate that the aluminium material expands with heat compared to steel? – That’s correct. – OK so does this also affect, if that bearing journal is growing in size as the rod heats up to operating temperature, does this also affect our ideal cold oil clearances? Obviously that’s gonna become greater as the rod heats up compared to a steel rod? – It does become greater as the rod warms up, but we do not recommend compensating that clearance, we don’t like to see people tighten up the clearance cold. It’s just typically better to run your standard oil bearing clearance knowing that it is gonna get a little bigger and if you see a drop in oil pressure, I would recommend going to a heavier viscosity oil rather than tightening up the clearance. – The design of these alloy rods as well, we conventionally see that the alloy rods are an I-beam design, when we’re looking at steel rods, these are both I-beam, or H-beam, why is the I-beam design so popular with the aluminium conrod? – Manufacturing for one, also space limitation. With the H-beam rod the beam gets so wide that now with big stroke combinations, the rod runs into the camshaft, or in other areas you run into clearance issues. – And even with the I-beam design as it is, these are already wider through that beam than an H-beam steel rod? – That’s correct. In aluminium you just have to have more material than you do steel to get the strength. – So when it comes to design and manufacture of these rods, do you just have off the shelf products, or do you also custom manufacture to a user’s specific requirements? – We stock very few things. Pretty much everything we do is custom built to order. – And what’s the sort of typical turn around time once you receive an order? – Two to three weeks. – Once of the other aspects we often hear about is that aluminium conrods stretch in operation, particularly at high RPM and thus they require additional clearance between the piston and the cylinder head. Can you talk to us about what actually happens there and how much of a real problem stretch is in the aluminium material. – You’re not really seeing stretch in the aluminium material I think that’s kind of a wive’s tale that goes around the industry. The material grows thermally from expansion. Aluminium just grows a lot more than steel does, that’s what you’re seeing. And you do have to compensate, you’re gonna need more piston to head clearance with an aluminium rod than you do a steel rod. – So could you give us an example of how much additional clearance we should be looking at? Obviously that’s gonna factor in the length of the conrod itself but can you give us some kind of sort of rule of thumb there? – Typically say a six inch long small block Chevrolet rod’s gonna grow right at 10000ths of an inch at temperature. – OK so it’s actually not as much as most people are probably recommending. I’ve heard recommendations of sort of 40 to 60 thou of additional clearance, that’s just not necessary? – That’s correct. – Look Woody thanks for giving us some insight into those alloy rods there. If people wanna find out more about GRP, how can they get in touch? – Really the best thing to do is call us. Telephone number’s 3039357565 and we’d just love to discuss your combination, get you set up with what you need. – How can they find you on the web? – They can, grpconrods.com – Alright thanks a lot Woody, enjoy the rest of the show. – Thank you much. – 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. And if you like free stuff, we’ve got a great deal for you. Click the link in the description to claim your free spot to our next live lesson. You’ll learn about performance engine building and EFI tuning and you’ll also have the chance to ask questions which I’ll be answering live. Remember it’s 100% free, so follow the link to claim your spot.

46 Comments

  • Reply Tumdéaux February 19, 2018 at 8:02 pm

    Aluminum conrods. Alloy ≠ Aluminum.

  • Reply Absolute Power Components February 19, 2018 at 9:19 pm

    Would be nice to hear facts on titanium rods

  • Reply Jim Stanley February 19, 2018 at 10:16 pm

    Very little power gain to be had with alloy or titanium rods; that’s why alloy rod manufacturers rarely give specifications in that context.

    As the guy says, it’s really just the alloy rod’s malleability that suits them for such a high powered environment. However, the greatest load on a conrod is directly after the exhaust stroke, as then their ductility is strained, and alloy rods have far less ductility than steel conrods.

    So, alloy rods (and to some extent Titanium) rods give very little HP benefit for drag racing applications as the engines don’t rev into very high numbers like F1 engines do.

    Another way to say it is; for almost all (including some performance/racing) applications (and within reason) the overall static (translational) weight of the crankshaft and con-rods don't matter much.

    This applies to alloy con-rods as well as the majority of their measured (translational) weight actually becomes rotational energy/weight when the engine is running.

    This means that very little of those sections of the lighter conrods that actually made them slightly lighter than the other steel conrods will be near the piston’s wrist pin; and since those conrod sections (near the wrist pin) do not constitute and/or meaningfully contribute to rotational mass/energy even when the engine is running (as they are translational mass) there really is very little real (translational) weight saving.

    And, it is translational weight saving that matters the most with these components.

    Put simply the majority of the mass of a conrod is always at the big end section – so a translational weight saving of a few grams across the whole conrod (when it is not in motion when the engine is running) may not mean so much in rotational mass/energy terms.

  • Reply JT Heglin February 19, 2018 at 11:49 pm

    I really like your videos. You go so much more in depth about the specifics than other people you make it easy to learn quickly. Your name definitely fits I get educated with every video I watch. Keep up the good work! I know it's a little broad for many of your topics but personally I would really like a video on building torque in small displacement engines (think rally cars)

  • Reply Martin Hulse February 19, 2018 at 11:52 pm

    Triumphs used to get 10's of thousand miles out of there aluminum conrods

  • Reply Aaron _z33 February 20, 2018 at 1:56 am

    Would be cool to see you guys interview someone for bearings. There's so many different kinds to choose from.

  • Reply King Of Show Bullies February 20, 2018 at 3:18 am

    Great vid as always

  • Reply jesse chen February 20, 2018 at 4:38 am

    Design everything below endurance limit of the steel 😉

  • Reply Joshua Bauer February 20, 2018 at 5:30 am

    Question: Long, insightful, and detailed requiring an equally detailed answer
    Answer: "That's correct"

  • Reply V C February 20, 2018 at 9:24 am

    Absolutely awesome interviews every time Andre. Credit to you mate. You ask all the the right questions and leave nothing left needing to ask 👍👍

  • Reply Daniel Brealey February 20, 2018 at 10:26 am

    Good vid, as usual… It's interesting though, you see quiet a few guys in the States running alloy rods in 1200 – 1400hp 2JZ's that are running on the street. It's scary hearing rods rated in "runs". I want alloy rods like these guys use in their 2JZ's for my RB30. The only question I think wasn't answered well was- at what point should I start to think about using alloy rods in my engine. Going to sign up for the listen in, was a little disappointed when I signed up for one of your tuning listen in's and it was live to air in the bloody middle of the day. Dude, most people are at work on the eastern seaboard during the day- and these are the people with the finances to be able to pay for one of your actual courses. It's these people you want to show a little teaser to in the form of a listen in. What's the point in putting it on at a time convenient for dole bludgers and children? Most workshops (like mine) close at 5p.m.

  • Reply Kevin Bodman February 20, 2018 at 3:56 pm

    Andrea,you really are a outstanding teacher.Thanks

  • Reply John Mc Donald February 20, 2018 at 5:04 pm

    He just keeps repeating what the guy is saying, way too much bla bla for info given, the guy likes to hear himself talk.

  • Reply lacossa nostra February 20, 2018 at 5:53 pm

    why not make titanium conrods?

  • Reply GordoWG1 WG1 February 20, 2018 at 7:01 pm

    Very good Q&A session – the only thing I'd've liked to have been asked was the affect of temperature as, back in the day, it was believed that rev'ing the rods when cold was much more likely to cause failure than when the engine was warmed up. Don't know if that was an old wives' tale or fact, though, as there isn't a lot of warming up for top fuel engines… Maybe down to different alloys being developed?

  • Reply eeeen321 February 20, 2018 at 8:03 pm

    Best interviews out there. Always informative and interesting.

  • Reply Fotis Manelides February 20, 2018 at 9:28 pm

    Well…That's correct

  • Reply Jess Stuart February 20, 2018 at 9:59 pm

    To a rough approximation, the peak force on a connecting rod will be during the intake stroke, and consists of the force needed to accelerate the piston downwards, and the force needed to pull the piston down against the vacuum at the intake manifold.

    f_peak = (piston mass)*(stroke_length/2)*(2*pi*RPM/60) + (vacuum)*(pi*(bore/2)^2)

    The actual force calculation is a lot more complicated due to the non-sinusoidal motion of the piston and friction forces. Be sure to use consistent units (Use SI units and everything will work out).

  • Reply Colorado Shooter February 21, 2018 at 3:44 am

    I work right across from Woody in the same motorsports building making a different part for race engines. Woody is big into Nostalgia drag racing and I’ve been meaning to hit him up at the track. He’s the one you talk to when ordering.

  • Reply OneFastDuster February 21, 2018 at 11:51 pm

    That added "5th" syllable drives me nuts!

  • Reply PPR Darryl February 22, 2018 at 11:31 am

    Very good questions in depth covering all parameters.

  • Reply Richard Fergusson February 22, 2018 at 1:40 pm

    Awesome video as always, cheers Andre!

  • Reply Mattias Bromark February 24, 2018 at 4:29 pm

    Great stuff man. Love this channel

  • Reply mr2paddy February 25, 2018 at 1:34 am

    would love to find out more info on titanium rods andre. great vid!

  • Reply Fridgemusa February 25, 2018 at 11:53 pm

    TL;DR, Alloy rods are good for drag racing, Steel rods are better for street and track cars 🙂

  • Reply Godfrey Poon February 26, 2018 at 11:47 am

    GRP Conrods?
    Glass Reinforced Plastic???

  • Reply j0nesy February 27, 2018 at 9:30 pm

    You need a job on one of the top motorsport channels as an interviewer, fucking brilliant questions every video

  • Reply Montana Native March 16, 2018 at 4:26 am

    the biggest load on a rod is during hydro-lock in a fuel car and under detonation in any engine 😉

  • Reply Edward Ratliff March 17, 2018 at 8:08 pm

    I had aluminum rods in my 97'Dodge Neon Sport. It made it through a full drag season but as soon as I took it out on the street one of the rods stretched and hit the valves ever so slightly. Ruined the whole build. It revved really quick and made it up to many 9000 RPM many times. I had to modify the block to allow clearance for the much thicker rods. The engine was a 12:1 compression rev monster but at that time had a stock flowing head with rather large Crane cams. Made close to 200whp and matched well to the low weight Neon chassis. Ran low 13s on drag radials. Long story short for a dedicated drag car aluminum rods work well but don't expect them to make it through many seasons.

  • Reply Marcus Taber April 6, 2018 at 2:38 am

    Aluminum, Aluminium, Condominium and this guy is the Ted Cruz of motorheads.

  • Reply Mowdown April 10, 2018 at 4:40 pm

    That guy knows his shit, lol

  • Reply Lime April 11, 2018 at 3:49 am

    6:10, heavier viscosity oil than tightening up clearance.

  • Reply Nicholaus Straach April 15, 2018 at 9:10 am

    American – Aluminum
    Everyone else – Aluminime

  • Reply TYRE MARKS MEDIA August 16, 2018 at 4:34 pm

    would the rods be recommended for a daily running usage on a stroked engine?

  • Reply Dylan September 17, 2018 at 1:29 am

    You didn't mention how the rods do in street applications I would like to see how they would do in a high performance street engine

  • Reply Real Deal News November 23, 2018 at 1:10 am

    BS- iron last longer than aluminum is so nobody would switch to aluminum blocks and heads, and so forth pitch your opinion I know better people are flocking to aluminum billet blocks, heads they can be made of a special blend of multiple alloys that are super light stronger than steel/iron all around better? if it was not people would not want the Chevy LS engines!!!!!!!!!!!!!!!!!!!!!!!!!

  • Reply John Creed November 24, 2018 at 9:56 am

    So purely weight savings.
    Makes sense for pro teams.
    On the street stay steel.
    Dealers choice 😎

  • Reply high life 1320 November 28, 2018 at 4:01 am

    Now that is the type of rep I want during my phone call.

  • Reply magnesium cast December 31, 2018 at 12:28 am

    hey guys tell me why i didn't find no one magnesium pistons and conrods?
    in fact mag. is stronger and easier than aluminium

  • Reply Stephen Srouji January 1, 2019 at 2:01 pm

    With regards to aluminum rods, how suitable do you feel they would be in drifting, for example? Not quite drag racing and not quite a full road course.

  • Reply felix crab February 14, 2019 at 9:54 am

    How do alloy rods especially titanium ir something do in road cars, is the life much shorter?

  • Reply Thermionic Tube February 18, 2019 at 5:38 am

    Aluminium rods can run into the block not a cam..

  • Reply RudyRay AAW March 25, 2019 at 10:35 pm

    He was a better interviewee than the Carrillo connecting rod representative, by far.

  • Reply Glen Brannon April 5, 2019 at 3:49 am

    How do I claim my spot?

  • Reply Jaydee Brickey April 9, 2019 at 7:07 pm

    R these rods all felons

  • Reply Jaydee Brickey April 9, 2019 at 7:10 pm

    Someone please say,Dwell

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