Thread: isuzu induction system

Originally Posted by c.h.i.e.f

On a related issue i have done a bit of research on intake designing and an interesting fact i have found is that a rough surface flows better and has less resistance than a polished smooth intake....off to do some bead blasting now

Sorry, but that is wrong.

Here is a "Moody diagram" which is used for engineering pipe flow systems.


The right hand axis is roughness (k) divided by pipe diameter (D). The Left hand axis is friction loss factor.

You can see that for turbulent flow, friction losses increase linearly with k/D. reduce roughness (k) for the same pipe diameter and you reduce friction losses.

AIRFLOW:

I had a quick look at the appropriate CFM calculators, and it would seem that a 3.9L running @ 4000rpm top, would require over 413CFM (0.19 cubic_meters_per_second, 11.7 cubic_meters_per_minute) assuming a VE of 1.5 with a good turbo.

The V8 requires almost 500CFM.

So I reckon a V8 snorkel will do the job.

However, maybe somebody with the required maths can tell us what size pipe is required to flow 500CFM easily?

Originally Posted by jakeslouw

AIRFLOW:

I had a quick look at the appropriate CFM calculators, and it would seem that a 3.9L running @ 4000rpm top, would require over 413CFM (0.19 cubic_meters_per_second, 11.7 cubic_meters_per_minute) assuming a VE of 1.5 with a good turbo.

The V8 requires almost 500CFM.

So I reckon a V8 snorkel will do the job.

However, maybe somebody with the required maths can tell us what size pipe is required to flow 500CFM easily?

500CFM in sensible units is ~0.24 m3s-1

Re=rho*V*D / mu
Re = reynold's number (see Moody Diagram)
rho = density (of air)
V = velocity
D = diameter
mu = dynamic viscosity of air = 1.81e-05 at 20oC.

For a 3"/75 mm diameter inlet pipe, V=54 m/s and Re = 4.1e5.
That is a reasonable value, however could be slightly lower.

However, when designing an intake, it is much more important to ensure the pipes are as smooth as possible, bends are as large a radius as possible, and transitions are smooth. All of this will likely have much more effect than the diameter of pipe you use.

Originally Posted by isuzurover

500CFM in sensible units is ~0.24 m3s-1

Re=rho*V*D / mu
Re = reynold's number (see Moody Diagram)
rho = density (of air)
V = velocity
D = diameter
mu = dynamic viscosity of air = 1.81e-05 at 20oC.

For a 3"/75 mm diameter inlet pipe, V=54 m/s and Re = 4.1e5.
That is a reasonable value, however could be slightly lower.

However, when designing an intake, it is much more important to ensure the pipes are as smooth as possible, bends are as large a radius as possible, and transitions are smooth. All of this will likely have much more effect than the diameter of pipe you use.

Thanks, but surely there is a minimum diameter? Surely I cannot use a 25mm / 1-inch pipe?

Originally Posted by jakeslouw

Thanks, but surely there is a minimum diameter? Surely I cannot use a 25mm / 1-inch pipe?

Minimum diameter is when you hit the speed of sound. Until then it's just extra restriction.

Originally Posted by jakeslouw

Thanks, but surely there is a minimum diameter? Surely I cannot use a 25mm / 1-inch pipe?

The engine would still run (albeit smokily) if you did.

However, as I posted in another thread - all things being equal (pipe smoothness for example), then:
Pressure Drop = Velocity^2 / Diameter

Basically, the above equation says that bigger is (a lot) better.

The only caveat is what I said about smooth pipes/bends/transitions in the post above. A 4" pipe with tight bends will probably have more pressure drop than a straight 3" pipe the same length.

I think they only place you want a rough(ish) surface is when you have fuel and air mixed togther.... Like a intake manifold on a carbed petrol engine. Air is different to air/fuel mix. On 2 strokes engines it is common to have a rough finish on the intake and to polish the exhaust port.

I have also seen some stuff written about surface finish on hulls of outrigger canoes and racing skis....slightly rough is getting some good results, but again water is not air

i was hoping that you commented on this isuzurover i found it quite strange that a rough surface would flow better too but then it rambled on about the design of a golf ball etc etc but obviously i should not take notice of this info then....why i was looking into intakes was i was wondering if there was any gains to be had from a new intake design such as the "banks big hoss" but i cannot see it being worth the effort for the gains?