700 SERIES INTAKE RESTRICTIONS
Why Buy A K&N Filter?
By Bill Watson
Restrictions to airflow cost you power and efficiency. When air is on its way
into the engine of a Volvo 700 series, there are a number of items that cause
restriction, thereby reducing airflow. Obviously, the first efforts for the
tuner should be directed to the highest restrictions, but as we'll see below,
many people just guess where the restrictions lie and throw money towards fixing
what are in fact the smallest restrictions (the excellent OEM paper filter,
or the OEM throttle body).
Many may not care how this testing was conducted, so let's cut to the chase.
Here, following the natural flowpath that air takes to get to the intake manifold,
are the pressure losses on my 1987 B230FT 740 at 5500 rpm and 10 psi of boost:
- Airbox snorkel (ie, from the "outside" to the "dirty"
side of your filter) 5.0 inches of water
- Paper air filter (just the loss across the filter element) 1.0 inches of
- Air filter lid & AMM elbow (ie, from the "dirty side of the filter
to the AMM) 9.0 inches of water
- AMM (loss across the Air Mass Meter) 5.5 inches of water
- Pipes to compressor, and from compressor to intercooler 0.0 inches of water
- Intercooler 38.0 inches of water
- Pipe from intercooler to throttle body, including 90 degree bend 0.0 inches
- Throttle body 0.5 inches of water
In a search for more airflow (more power), where would you concentrate your
efforts? Obviously the intercooler is the first place but it is an expensive
proposition. Short of buying an aftermarket intercooler, one option that Turbobricks
founding member Philip Bradley has considered is stacking two factory intercooler
cores and building custom end-tanks for the pair. The resulting double thickness
I/C is a promising, moderate-cost idea. This would also increase the thermal
efficiency as airflow would be halved per intercooler. For details of the Volvo
factory intercooler thermal and pressure efficiencies and a general technical
discussion, click here. If you're accustomed
to pressure in psi rather than "inches of water", then click here.
WHAT NOT TO DO:
Where is the LAST place I'd recommend you spend your money? Replacing
the piping, the paper air filter or the throttle body. And remember, your replacement
filter or TB would not have 0.0 inches of water pressure across them, so you're
looking at one of the most expensive bang-for-the-buck mods that I can see.
Below, you'll notice I haven't replaced any of these items on my car. In case
you think I'm simply afraid of fabricating hardware, you can click here
to see some of my homebrew intake and exhaust manifolds, fuel tank, turbo systems,
and exhaust systems. This is where my time is presently invested. Someday I'll
address these and update this article with results. OK, rant over.
WHAT CAN BE DONE:
Besides an expensive I/C upgrade, the next place to hunt would be the air
filter lid and Air Mass Meter (AMM) elbow, (some fabrication in order here)
followed by the AMM. Looking at the AMM, I'd have to estimate that the two screens
are 90% of the pressure loss, so I'd consider removing the downstream screen
as that should be on the safe side in terms of hurting the delicate hot-wire
element. Philip Bradley mentioned that replacement AMMs don't even have a downstream
screen if this makes you more comfortable with the thought of removal. I'd estimate
its removal would drop the restriction from 5.5 to around 3.0 inches of water.
Even though this is a 700 article, longtime Turbobricks member and Volvo racer
Anthony Hyde did some pressure drop measurements on his
200 series car that has the earlier "mechanical flapper" CIS Bosch
K-Jetronic mass air meter. These are significantly more restrictive and I believe
he found the pressure drop to exceed 30 inches of water!
The next modification in line is the 5.0" loss from the factory inlet
snorkel. This loss is commonly alleviated by adding additional inlet holes to
the 'dirty' side of the air filter. I've seen a number of sites that show how
they made their modifications (try here
and here for starters).
Keep in mind that cold air is your goal, so obtain your additional inlet paths
from the front of the car. In theory, every ten degree F rise in inlet air temp
is the same as adding FOUR inches of pressure restriction! As you can tell,
cold air is a very high priority.
All that's left is the previously-mentioned air filter and throttle body. I
would suggest that modifying these components are very expensive HP gains in
- The advantage to a cone-shaped K&N filter here is that if you don't
like fabricating, this would allow you to remove the fist three restrictions
listed in the table above, which total 15 inches of water!! If you design
a shield so that your new setup does not start breathing underhood air, this
should show a nice seat-of-the-pants gain. I'm not a believer that the K&N
actually filters out dirt better though than Volvo's OEM filter, (separate
rant and my personal opinion) hence I will try to build something that utilizes
a factory paper filter.
- Why do people who install larger throttle bodies say their car feels faster?
Because the larger plate, opened at the same throttle position (angle) is
a larger leakpath for air to get in the engine. So it feels faster for the
same motion of your right foot. But it's the exact same as just moving your
foot more with the factory parts - and wide open, we've shown above that the
factory TB is fine. One other reason I don't like the larger TB as a solution
on an automatic car; you've now changed the relationship between engine output
and throttle plate angle, and your tranny shift points and internal pressures
are all programmed off the factory relationship. You increase your opportunity
to have too low of main pressures in the transmission during shifts, possibly
increasing slip during clutch pack engagement and earlier failure of the clutches.
"Adjust your tranny cable accordingly" is all I can say - as your
motion ratios will always be wrong now but I'm sure some additional preload
will get you by.
METHODS OF OBTAINING THE ABOVE RESULTS:
These are small pressure drops relative to the pressures being measured. The
tool of choice here is a "Delta P" gauge, which has two pressure nipples.
You hook up pressure lines from both sides of the element you want to test,
and the gauge shows the difference between the two pressures. Here are some
the delta-P gauge
Snorkel-loss pressure tap
Air filter loss taps
Hopefully this information will help direct efforts and monies in the most
- Bill Watson