Cold air intake design

[sbrats]

1) Looking at aftermarket stuff, air boxes are removed and a less restrictive tube and filter are put on - smoothing out the flow (in theory). However these are subject to water pickup if the intake tube is low to the ground and under hood temp because most of the time the filter is in a convent space around the engine. I look at the factory snorkel picking up air at the leading edge of the hood between the top of the radiator and the hood – but the cross sectional area is small and the path is quite restrictive. Assuming that I could make something that would fit straightening out the flow what is the math to determine the cross sectional area required at the leading edge vs length of the runner between this point and the filter and how does this relate to TB dia. and engine size?

2) How would one determine the max size of TB for a given engine size?

3) How would I determine opening rate vs practicality with regard to the TB – if I modify the TB size (increase dia) the amount of flow (and performance) will increase at a given open angle resulting in quicker response – however there must be a point of impracticality. If the calculation in 2 was used at various speeds giving resultant values of cfm at various speeds – could open area at these points be used to plate opening rate? (I guess that’s why progressive 4 bbl carbs where used – how could this be done for a single TB)


Is there are a couple of good resource books around – let me know please


Thx
Steve

 

[NormPeterson]

(1) You could pick a value for pressure drop between the air inlet and the throttle body that you're willing to live with. From that, plus the length (including any bend effects) and the airflow requirement you could iterate for a cross-sectional area solution.

(3) As you note, response becomes nonlinear as TB size increases. What you can do with a single TB to make it progressive is to fit the throttle cable to a sector whose radius from the throttle shaft centerline is not constant. The big radius goes where the cable leaves the sector at idle, so you get fewer degrees of throttle motion per inch of cable travel at the beginning.

Norm

 

[sbrats]

What would be a suitable pressure drop value?

 

[GregLocock]

0.1 psi. Even a production system with an air filter will meet that.

It is more important to pick up from a high pressure, low temperature, point than to worry overmuch about pressure loss in the duct. You should lso be wary of heating in the duct.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Warpspeed]

The main factor is air density, and some measurement and calculation will quickly confirm that unnecessary temperature rise is usually a far greater evil than unwanted pressure drop.

Heat soak in the engine compartment is your worst enemy. Finding a source of reasonably cold air to start off with is usually not that difficult. Keeping it cold on it's way through the entire induction system is going to be far more of a challenge.

 

[sbrats]

Thanks...
Remote temp measurements I can do, any ideas about acceptable temp rise – closer to ambient the better. I had though about trying to pull from the base of the windshield – but this would involve some cutting and hacking. A new hood with some form of blister would also do that.
I though the easiest thing would be to pick up at the leading edge of the hood (cool and high pressure) running between the engine and the underside of the hood – isolate for vibration and possibly made of carbon (or GRP), insulate and assume that a cooling effect would take place at motion. Heat soak would be an issue in stop and go traffic but wouldn’t be any worse than stock

S

 

[Warpspeed]

I suggest you go out and buy one of those cheapy indoor/outdoor digital thermometers. The outdoor sensor will come attached to about a six foot wire.

That is better because you can easily figure out temperature rise above whatever the ambient is on the day, and that is a more useful thing to know rather than the actual temperature.

Measure the temperatures along the whole induction system, that is actual air temperature, not the temperature of the surrounding pipe.

Measure pressure drops at full throttle with a water manometer at various places too.

You will very quickly figure out where the worst pressure drops, and temperature increases are.

Realise that a 10% overall pressure drop is highly unlikely. But a 10% increase in temperature (above absolute zero) is quite possible, even highly likely !

Just monitor things for a few days under different driving conditions and try to understand what is actually going on thermally.

Idling in heavy traffic, the induction temperatures are going to soar, and there is absolutely nothing you can really do about that. It probably does not matter at part throttle anyway. It is when you floor it, you want those temperatures to come right down as quickly as possible.

A cold air pickup, thermal insulation, and low thermal mass is the key. Phenolic spacers at strategic points may insulate cylinder head conducted heat from the runners and plenum.

A skinny standard plastic induction system may actually pick up less heat than a monster aftermarket polished aluminium tube. Or maybe not. Make the measurements, do the density calculations and do whatever is needed.

It could be the cheapest horsepower increase per dollar spent you will ever get.

 

[GregLocock]

Typical MAT is 60 deg C, with an ambient of 20-30.

So that is almost a 10% reduction in true volumetric efficiency.

Coo



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.