- 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.
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