3 Automotive Myths That Cost Vehicle Performance


hi everyone John and Shawn here and while
we aren’t official Mythbusters we are technical advisors and experienced
automotive enthusiasts here at Summit Racing through the years of restoring
tuning fabricating and helping others build their dream we’ve run across some
common myths and misconceptions which we felt would be beneficial to discuss for
this video we’re going to cover three myths that could cost your vehicle
additional power and performance myth number one engine airflow bigger is
better right there’s a tendency among well-intended enthusiasts to choose
bigger components in an effort to achieve bigger power and better
performance bigger carburetor bigger cylinder head ports bigger cam the dyno
sheets and the time slips are in and they’re clear that bigger is not always
better let’s start by looking at carburetors for example CFM or a cubic
feet per minute is the rating used to define the velocity of air which travels
through the air horns of the carburetor feeding your engine proper air to fuel
ratio is critical for optimal combustion fuel economy horsepower and torque CFM
ratings can make or break a great engine build if the CFM rating you’ve selected
is too high for your engine the larger air horn inlet will significant the air
velocity the loss of air velocity means the combustion chamber will not fill to
the capacity required for a proper air fuel ratio this is one reason why bigger
is not always better when selecting a proper carb it will actually cause your
engine to put out less torque and horsepower this as well as other factors
such as carb tuning can cause your street vehicle to have poor low-end
torque and if you drag race significantly lower 60-foot times the
bottom line is it’s important to find a happy medium when selecting a carburetor
if your CFM calculations have you between sizes we usually recommend that
you go with a smaller carburetor the smaller carburetor will have better
low-end response and low end power will make streetcars easier and more
enjoyable to drive not sure about your required CFM we supply a digital CFM
calculator you can use for free on our website at summitracing.com Shawn
what other areas CFM play a role in your engine build it’s great
question John let’s talk about CFM and cylinder heads for a moment as a rule of
thumb smaller intake port volumes will produce more low-end torque and crisper
throttle response while larger intake ports allow more flow at higher rpms
therefore in most street applications and for the occasional weekend racer
smaller intake ports often deliver better results while larger intake ports
would likely cause the torque numbers to fall off on the other hand larger intake
ports are beneficial if you are building an engine with a large displacement or
if you’re building a smaller displacement requiring additional CFM
for reaching higher rpm and a dedicated race only situation so we’ve covered
some concerns with the bigger-is-better motto when it comes to carbs and
cylinder heads but what about cams yes as you may have guessed the biggest
duration and lift numbers aren’t always the best choice here either this brings
us to myth number two in our bigger-is-better conversation advertised
cam duration is a good measurement of cam size but is it actually you’ll want
to downplay the role of advertised duration when choosing a camshaft and
here’s why when shopping for an aftermarket camshaft you’ll notice many
camshaft manufacturers will list two different duration values one is
advertised duration and the other is duration at 50 thousandths inch lift
let’s look at the differences advertised cam duration is the degree of crankshaft
rotation that the lifter is raised more than a predetermined amount in varies
between manufacturers so if we attempt to select the camshaft based on
advertised duration it’s not an apples-to-apples comparison what you
really want to pay attention to is the duration at 50 thousands inch lifts this
is an industry standard way of comparing duration among manufacturers it is a
better way to compare between the different camshafts Shawn what’s myth
number three the third and final myth that we are going to expose today deals
with the exhaust system you’ve probably heard something like
back pressure is always good or you must have back pressure for the best
performance now the main purpose of your exhaust system is
to get rid of spent gases as quickly and efficiently as possible however back
pressure makes it harder to do this so why do some people say back pressure is
good a lot of it comes down to confusion between back pressure and exhaust
scavenging exhaust scavenging occurs as a pulse of exhaust gas runs through the
exhaust pipe and a little area of vacuum or low pressure is created behind it
this vacuum or low-pressure area helps pull along the next exhaust pulse it
helps better evacuate the exhaust gases coming from your combustion chamber it
even helps hold the next intake charge into your combustion chamber allowing
your engine to operate with a cleaner intake charge during the combustion
process this is why equal length headers are the most preferred header design so
the exhaust pulses don’t crash into each other but the pulse is pulled the next
one away from the engine so where does the confusion between scavenging and
back pressure come in this confusion occurs due to the small diameter exhaust
components required to maximize the scavenging effect and its performance and
benefits particularly at low rpms so while this may be more restrictive it
does increase the exhaust velocity and that provides the benefit not the back
pressure keep in mind though that there is a compromise when choosing the
correct exhaust pipe diameter while a small pipe creates a greater exhaust
velocity at low rpm your engine could ideally use a larger diameter pipe at
higher rpm so it’s really a matter of choosing the correct pipe size that
matches the power and the purpose of the engine finally if your application is
running a supercharger or a turbocharger the incoming air is being forced into
the combustion chamber therefore once the crammed air-fuel mixture explodes
there’s a lot more exhaust gases that need to be removed and even quicker in
this case bigger is definitely better with the exhaust system so we are going
to officially label these myths but did that’s it for now but be sure to
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