Engine mounts: The Complete Guide

Engine mounts: The Complete Guide


This is an engine mount.
And we’re about to use it to mount the engine
to the subframe.
Now I hear you – “Alex, that’s just a
block of rubber.”
And yes, you’d be right.
It is a block of rubber – but of course I’ve
got a bit carried away and made five pages
of notes so… let’s get stuck in and learn
absolutely everything there is to know about
engine mounts.
So, what does an engine mount do?
Well, it does two things: first it attaches
the engine to the rest of the car.
Now, the simple way to hold the engine would
be to just grab a couple of big bolts and
bolt the engine down to the body.
But the engine is a hive of mechanical activity.
There are things spinning around, moving up
and down.
There’s about 50 gas detonations every second
at high revs – and inevitably… no matter
how perfectly balanced the engine is – that
creates a lot of vibration.
And if the engine was just directly bolted
to the vehicle body it would transmit those
vibrations straight into the rest of the car
and driving would be a bone hammering experience.
You see what we go through for you guys??
You can see the vibrations from the engine
by putting a bowl of water on the roof of
a car.
And the vibrations are most noticeable at
idle and they tend to smooth out as the revs
increase.
So engine mounts have a second purpose in
addition to just holding the engine in place
– they dampen these vibrations and stop them
travelling into the rest of the car.
Not only do we have the vibration from the
engine out into the rest of the car, but there’s
also other things that cause engine movement.
Bumps in the road, accelerating and decelerating,
and the torque of the engine itself.
Now, there’s a lot of inertia inside the
motor – and when the engine speeds up or slows
down, or the revs go up very quickly, or go
donw, the engine wants to twist on its mounts.
You can see this when you have the hood open
and hit the throttle – there’s a rocking
movement in the engine.
So the engine mounts need to restrain this
movement.
Big, modified, high-power muscle cars used
to have a chain that would tie the top of
the engine block to some strong, rigid sturdy
part of the engine bay.
An actual metal chain to restrain the engine.
Dealing with this vibration is part of a field
of automotive engineering called NVH, or Noise,
Vibration and Harshness.
We’ll have a whole episode on that later
but briefly noise is… well, noise.
From the engine, the road, airflow, whatever.
Vibration is obvious.
But harshness is something else – it’s a
measure of how noise and vibration actually
feel.
Sometimes a certain frequency of vibration
or noise feels good – a purring engine, a
smooth road, gently swinging in a hammock.
Other frequencies feel uncomfortable – whining,
scraping, or knocking.
And this is what harshness refers to.
Let’s get back to engine mounts.
What you might not realize is that the drivetrain
in a vehicle is completely isolated from the
rest of the bodywork.
There’s no solid metal connection between
any parts of the drivetrain and the rest of
the car – the drivetrain being the engine,
transmission, differential, all the way out
to the wheels.
And at every connection point, there is some
form of soft damper.
On this car, we have only four points where
the drivetrain is connected to the body – two
rubber engine mounts, and two rubber mounts
at the back on the differential.
And since the exhaust is bolted directly onto
the engine, it’s also isolated from the
bodywork using rubber exhaust hangers.
Not only does this isolation make for a more
comfortable experience, it also gives the
engine an easier life because there’s somewhere
for this vibration energy to go… it’s
not just shaking itself to pieces.
So engines are fitted to the body, or in our
case – the subframe, using engine mounts,
or motor mounts.
The mounts for this engine are as simple as
they get – it’s just a block of rubber.
This car has only two engine mounts – one
on each side of the block.
A typical rear-wheel drive car like this one
will have a mount on either side of the engine,
and often a third one that supports the transmission
further back – although not on this car because
it has a backbone called the Power Plant Frame
that supports the transmission.
Front wheel drive cars tend to have three
mounts for the engine in a triangular pattern.
By the way, transmission mounts and engine
mounts are exactly the same thing.
Occasionally an engine will include an upper
mount which prevents rocking of the engine
by tying it to the bodywork inside the engine
bay – that’s a more refined version of the
chain that used to be used in muscle cars.
Normally these upper mounts are a tie bar
with rubber bushings – again to isolate the
vibration from the bodywork.
And a bushing, incidentally, is a cylinder
of rubber that acts as a vibration absorber
where two parts are bolted together.
We’re going to come across them mainly on
the suspension and steering systems.
Simple mounts like these are made up of two
separate metal plates laminated together by
a chunk of solid rubber.
These are blocks, but they can also be disc
shaped.
Studs are welded onto the plates – and note
that these studs don’t run through the rubber.
One plate bolts on to the subframe, and the
other onto this support bracket which attaches
to the engine block.
As usual – the design of engine mounts is
a trade-off – soft mounts will isolate more
of the vibration, but they’ll allow more
engine movement.
Firmer mounts restrict movement more, but
transmit much more vibration.
To get technical – engine mounts should be
frequency responsive.
There are basically two types of shaking that
this engine mount should handle: to use everyday
terms, these are vibration and bouncing.
Now intuitively, you know the difference between
vibration and bouncing even if you can’t
explain it in scientific terms.
Bouncing is low-frequency, high amplitude.
Vibration is high frequency, low-amplitude.
Vibration comes from inside the engine, from
unbalanced engine forces.
Imbalances in the rotating and reciprocating
parts, and from firing pulses.
Bouncing comes from driving along on uneven
road surfaces, and accelerating and decelerating
and the torque from the engine.
We want the mount stiff to prevent bouncing
– but soft to isolate and absorb vibrations.
So the stiffness of an engine mount would
ideally be frequency dependent.
Stiff at low frequencies and softer at high
frequencies.
This block of rubber has been engineered as
far as possible to meet these two goals.
And you can see that there is a part where
one of the metal plates curves around, leaving
two ears that help to dissipate vibrations
at a certain frequency – it’s a bit like
a tuning fork.
In about the 80s, manufacturers began introducing
hydraulic engine mounts, or hydromounts – these
are hollow rubber cushions filled with a liquid.
These absorbed more vibration without allowing
too much movement.
But they are expensive to make and they are
less durable than the solid rubber mounts
like this.
More recently active engine mounts have been
developed and these respond to the specific
RPM of the engine – a bit like noise-canceling
headphones.
Commonly these are vacuum controlled, where
the computer uses a vacuum to control the
stiffness of a mount.
More fancy types of active mount are controlled
with electromagnets, or even electromagnetic
fluids, or a motor with a counterweight inside
the mount itself that moves in an opposing
pattern to the engine’s vibration.
OK, so what’s the failure mode of an engine
mount?
Well, like all rubber parts, they’ll deteriorate
with age.
Some mounts will last the lifetime of the
vehicle, others will need replacing at some
point.
But basically the rubber will break down,
leading to cracks.
The mount may lose its dampening capacity
– so you’ll get excessive vibration inside
the car.
Or the mount becomes weak and the engine will
start rocking excessively – particularly when
you change revs, or accelerate or decelerate
quickly.
Knocking noises from the engine bay should
have you looking at the engine mounts.
And if one mount goes then the rest are probably
on their way out too.
So it’s generally best to replace mounts
as a complete set since they’ll all tend
to fail around the same time.
The process of replacement is to take the
weight off the mounts with a jack under the
engine, and… erm, replace the mounts.
I’ll fit these engine mounts….
I’ll fit these engine mounts to the engine,
get it onto the subframe and then we can move
on to something a bit more exciting.
But who am i kidding, these ARE exciting!
The mount fits onto the supporting bracket
with a square hole that locates there, and
it’s easiest to tighten this nut up in a
vice.
This mount should actually be covered in rubber
all the way, but I’m reusing an old one
because I don’t have any new ones yet.
That’s good and tight, we can now fit this
to the engine.
And that fits to three holes on the block
and it can only fit to three particular holes
so there’s no danger of putting it in the
wrong place.
There’s a plate that goes underneath, and
we have two water drain holes here.
These need to be at the bottom.
You can fit it either way, but these need
to be at the bottom.
Like that.
And hopefully this locates and holds itself
as we crane this back onto the subframe.
On one side of the subframe we have a hole
and that’s the one we should fit in first,
and on the other, we have a slot and we should
use the slot second, and that guides the mount
into position but it’s always a bit tricky
getting these engines onto the subframe.
And the mount is secured to the subframe with
a washer and a 14mm nut through the turret.
I feel like this is a bit of a milestone.
We’ve got the engine essentially finished
and mounted in the car.
Well, I think we’re almost finished..
We’ve just got the suspension, the rest
of the subframe, the electrical systems, the
transmission… basically everything else
to do.
However, this is a milestone, and I feel happy
about this.
And I think moving this around on this little
trolley, and assembling it on this kind of
stand – it’s going to be absolutely brilliant.
So join us next time.
I’m not sure what we’ll do… maybe the
serpentine belts?
We’ll see.

44 Replies to “Engine mounts: The Complete Guide”

  1. Absolutely super professional video. This answered all my questions. Purchased a 1996 Miata with the whole engine apart. putting it into the car in two days. Thanks again.
    KB

  2. excellent vid. for my 2013 Audi TTRS i just changed my dogbone mount insert and my NVH went up but i LOVE the way it shifts now. ideas of what i can do to reduce NVH without screwing my shift feel?

  3. Wao Man, I am making Engine Mounts for last 10 years, this is best basis education of EM Presentation I have ever seen

  4. Great informative video ¦:¬D Really well explained – the best I've seen on You Tube. More videos please!!

  5. Got to ask, exactly what class of cars have chains over the engines? I have followed developments in performance cars since 1958, and have never seen any such thing. Perhaps you are thinking of the straps that pass over the top of the supercharger on fuel-burning drag race cars. Those straps are mandated to restrain the supercharger from breaking off of the engine and blasting to altitudes and distances that endanger everybody in the area following a fuel explosion inside the intake system. Such incidents were common enough to make restraints mandatory.

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