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Use of ribbed hoses and its adverse effects on the intake air flow

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I'd like to clarify this issue once for all.

Comparing corrugated (dryer) hose with smooth duct of the same diameter, the latter creates less flow restriction.

See results here:


Scroll down to the heading title "Phase 3" and you see the results.


Explanation of the empirical results may be explained in the following manner:

Head Loss in a Pipe

Overall head loss in a pipe is affected by a number of factors which include the viscosity of the fluid, the size of the internal pipe diameter, the internal roughness of the inner surface of the pipe, the change in elevation between the ends of the pipe and the length of the pipe along which the fluid travels.

Valves and fittings on a pipe also contribute to the overall head loss that occurs, however these must be calculated separately to the pipe wall friction loss, using a method of modeling pipe fitting losses with k factors.

Darcy Weisbach Formula

The Darcy formula or the Darcy-Weisbach equation as it tends to be referred to, is now accepted as the most accurate pipe friction loss formula, and although more difficult to calculate and use than other friction loss formula, with the introduction of computers, it has now become the standard equation for hydraulic engineers.

Weisbach first proposed the relationship that we now know as the Darcy-Weisbach equation for calculating friction loss in a pipe.

Darcy-Weisbach equation:

hf = f (L/D) x (v^2/2g)

hf = head loss (m)
f = friction factor
L = length of pipe work (m)
d = inner diameter of pipe work (m)
v = velocity of fluid (m/s)
g = acceleration due to gravity (m/s²)


hf = head loss (ft)
f = friction factor
L = length of pipe work (ft)
d = inner diameter of pipe work (ft)
v = velocity of fluid (ft/s)
g = acceleration due to gravity (ft/s²)

The establishment of the friction factors was however still unresolved, and indeed was an issue that needed further work to develop a solution such as that produced by the Colebrook-White formula and the data presented in the Moody chart.

The Moody Chart

The Moody Chart finally provided a method of finding an accurate friction factor and this encouraged use of the Darcy-Weisbach equation, which quickly became the method of choice for hydraulic engineers.

Friction Factor Chart / Moody Chart

The friction factor or Moody chart is the plot of the relative roughness (e/D) of a pipe against the Reynold's number. The blue lines plot the friction factor for flow in the wholly turbulent region of the chart, while the straight black line plots the friction factor for flow in the wholly laminar region of the chart.

In 1944, LF Moody plotted the data from the Colebrook equation and the resulting chart became known as The Moody Chart or sometimes the Friction Factor Chart. It was this chart which first enabled the user to obtain a reasonably accurate friction factor for turbulent flow conditions, based on the Reynolds number and the Relative Roughness of the pipe.

Friction Factor for Laminar Flow

The friction factor for laminar flow is calculated by dividing 64 by the Reynold's number.
Friction factor (for laminar flow) = 64 / Re






Polyethylene PE - Corrugated with corrugated inner walls 0.018 - 0.025

Polyvinyl Chloride PVC - with smooth inner walls 0.009 - 0.011





e/D of the corrugated surface > e/D of the smooth surface

Corrugated duct creates larger layer of the turbulent flow close to the wall effectively reducing the  laminar (fast) flow cross section area. Any bends make conditions worse.


Therefore, replacing corrugated hoses with smooth bore ducting improves the turbo compressor efficiency.

Edited by MrDangerUS
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  • 3 weeks later...

My Friend is a power train engineer, and says it makes no difference. As he designs engines for a living I trust his opinion (trained at Lotus). The turbo will suck in the air it requires.


Others may disagree, but that is what I have been told.

Amateurs built the Ark

Professionals built the Titanic

"I haven't ridden in cars pulled by cows before" "Bullocks, Mr.Belcher" "No, I haven't, honestly"

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After reading this, I've called a friend who have tuned engines rather seriously for many years, and he told me, that in theory the smooth hose can transfer more air than a ribbed one. But the difference is smaller than the effect that say bending a tube because of for example Space problems, or to create a watertrap, or similar, have.

He have been on some seminars on induction and tuning, new systems etc. in Germany, and have seen the effect at Mercedes engines with a venturi effect shaped like two trunphets against eachother to increase airspeed and thereby filling the plenum more, more air to turbo's etc. There an 80mm diameter at the most narrow point, could feed two turbos, producing over 500 HP's. The size of the venturi-system was more than a beerbox (in Danish 30 bottles of beer). Other versions had gunbarreled shapes (spiraling) Airways, also to increase airspeed and capacity and at the same time reducing induction noise.

Anyway it was also pointed out, that the more "clean" (non-ribed), (contrary to Lotus perpendicular ribbed ones) the less strain on the turbo relating to suction on the intake.


Just my two-pence,


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Nobody does it better - than Lotus ;)

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  • 4 months later...

The maximum centrifugal compressor volume flow rate is normally limited by the cross-section at the compressor inlet.

Reducing the pressure gradient across the compressor increases its volumetric efficiency and decreases

discharge air temperature.

Smooth bore duct ( larger than compressor inlet dia), increases turbocharger thermal efficiency.


In addition, Esprit CAI inlet area (scoop opening) can be easily opened up by 40-50%



Oil at Compressor Outlet
A dry type filter, as it accumulates dirt, becomes increasingly restricted and causes a pressure drop across it. A partial vacuum at the compressor inlet will result. This will not affect the engine under load because a positive pressure exists at the seal behind the compressor impeller. During engine idle or extremely low load, a partial vacuum occurs at the compressor inlet and behind the compressor impeller simultaneously. If this condition continues for any length of time, it will cause oil to be sucked from the bearing housing, through the seal, and into the compressor housing and eventually into the pipe between the compressor and intake manifold. Too small of an air filter will cause the same problem. The solution is simple: a commercially available gauge mounted between the air cleaner and the turbocharger will signal when the inlet restriction is too great. To prevent the air intake restriction from becoming too excessive, service the unit on a regular basis. If the air filter capacity is too small, install a larger unit

Edited by MrDangerUS


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I totally agree with mr D .  Air flow management , no matter how slight, will give results.

The standard ribbed twisting and turning system offers little in the way of flow efficiency.

The ram air mod people do helps, but still leaves a lot of room for improvements.

The standard air filter is very restrictive in comparison to what is now available and should

be the first thing to address. After that the ducting can have an effect..

The less bends and smoothest transition within the system, along with high flow air filter is

the way to go..  

As mr D. said , the less work the compressor has to do sucking, the cooler and more efficient

the turbo will be...better VE  leading to BHP..

I initially used a variant based on Dermot's system , with great results.. later studies have shown

even greater benefits can be made by reducing the bends and using smooth wall ducting as mr D. said.

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Duct flow restrictions effect on the compressor inlet pressure could be categorized together with the filtration losses.

Why would anyone install K&N/Pipercross/Green air filter and leave corrugated inlet duct intact?



See page 154 of the above article turning vanes inside of the 90 degree elbow! (If you have any 90 deg turns, of course).  http://www.autospeed.com/cms/article.html?&A=110475



Air Intake Restriction

( Source:  Caterpillar service manual)
Excessive vacuum on the inlet side of the turbocharger (or the air inlet on naturally aspirated engines) can
result in reduced engine power capability and degrade engine performance.
Air intake restriction is also an emissions critical parameter declared to obtain EPA non-road certification.
Therefore, the air intake system’s total restriction (including dirty filters, duct work, vents, silencers, etc.) is limited depending on engine model, rating and air configuration. The air intake restriction limits for Cat engines can be found in the Technical Information Appendix or TMI.
In order to maximize air filter life, it is important to keep total duct restriction below 0.5 kPa (2 in. H2O).


Also, every additional restriction caused by the air inlet system subtracts from air filter life. Maximum filter life is partially dependent on the absolute pressure differential between the turbocharger compressor inlet and atmosphere.
Inlet air restriction includes the pressure losses between the air cleaner and the engine air inlet. For remote mounted air cleaners, the following formulas can be used to calculate duct restriction.
P(kPa) = L x S x Q^2 x 3.6 x 10^6

P(in. H20) = L x S x Q^2
                     187 x D^5
P = Restriction (kPa), (in. H2O)
psi = 0.0361 x in. water column
kPa = 6.3246 x mm water column
L = Total equivalent length of pipe, measured in (m), (ft)
Q = Inlet air flow, measured in (m3/min), (cfm). - (found in TMI or performance book, and corrected for site
conditions when necessary)
D = Inside diameter of pipe, measured in (mm), (inches)




As you will see in this article, when you test a truck under real-world conditions, power improvements from most aftermarket intakes will diminish due to the effects of heat under the hood.

Paper versus gauze air filters and dust out conditions.



Edited by MrDangerUS


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  • 2 months later...

Here is an example of a smooth bore duct which fits under the shelf panel (material 3.25" PVC piping).


3" dia aluminium tube from the air box to the turbo works well. 15-20 cfm gain.




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Final product.

Perhaps not pretty, but it is functional, improves engine breathing and shelf panel will hide it from the public eye.


(Well, everybody likes sausage, in spite the making of it is not pretty)


Edited by MrDangerUS


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How's it sound?


I once helped a friend make a K&N filter intake tube out of thin wall PVC pipe similar to what you used.  It actually sounded pretty good on his '91 CRX!  If you like a loud snarly induction growl.


Vulcan Grey 89SE


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  • Gold FFM

Now there's a lot of really confusing formula up there ^^^

What difference does it make on a rolling road? That's the acid test.

Mine sounds loud and throaty with the direct ribbed and shortened pipe - with an immense induction suck

Only here once

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The smooth wall system I illustrated on on page 15 post 289 of the 412 thread in Chat, was a

definite part of the power improvements seen on the rolling road.

The induction noise I am now experiencing is high with the window or roof open .  

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My car is LHD, so the noise coming from the right scoop does not get me.


since implementation of my cabin ventilation improvements,

I'm getting a nice fresh air flow without a need to keep windows open.


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  • 9 months later...

Let me first say your car looks awesome and shoe-horning an Audi V8 in there is a monumental achievement.  But in your case,with a normally aspirated engine a smooth induction intake tube is even more important than on a turbocharged powerplant for air flow.  It is all about turbulence.  Time to get some mandrel bent pipe and custom fab something to replace that ghetto billy-bob dryer hose.It kind of makes it look like you gave up at the very end because you just wanted to get it on the road. B-)  Its the finishing touches man! No need to carry-over Lotus' short-cut that was obviously just to save money.

Do you see dryer hoses on this?



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No its all good , attached to the end of that flexi pipe is a great stonking ITG filter, The pipe is thin enough to dicipate heat properly, where a large solid pipe will warm up from engine/engine  bay heat and retain it more and as we all know lower air induction tempratures are vital :)   The pipe diameter is also much larger than it needs to be so for what would be lost in this Turbulance effect is minimal if not nothing. It is all very well having a small front door 6ft x 4 ft and having a driveway full of stampedeing elephants trying to get in ! it just makes no difference you are limited by throttle body size.:)

I know where you are coming from Paul but sometimes you have to take praticality over looks, that pipe does exactly what it says on the tin :P


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On ‎4‎/‎16‎/‎2016 at 13:05, silverfrost said:

I know where you are coming from Paul but sometimes you have to take praticality over looks, that pipe does exactly what it says on the tin :P


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