Race Engine Forum UK

Hi!

I've begin to read a little at a time on air velocity.
This drawing is completely theorical and doesn't look really good - but the question is if the air only or air/fuel mixture can gain velocity if we create a vacuum which will create lower pressure than standard atmospheric pressure?



The diameter of the component are only to show where velocity is gained and lost due to diameter of each inlet ports section.

I've put a vacuum line at the entry of the inlet trumpet to a small vacuum pump in order to lower the pressure even on "open" throttle - so as to reduce air resistance - and maybe increase velocity.

I've also put a vacuum line at the stock injectors location situated right at the entry of the head port on my particular "test" head... I was thinking that the velocity will increase at an important part of the port.

I've not really studied the pressure drop "required" to make the air or air/liquid enter at higher velocity. I'm really not sure about this theory - hence asking around to see if this idea can help in certain places where inlet porting can't not really give more velocity?

Thanks
Hugo

OK, Hugo, my take on it.

First, nice dwg, well done. It is laid out nice and clear.

1. You do actually want a higher relative pressure - atmospheric (or higher, eg: by turbo/blower boosting) at the rampipe mouth, not lower. It is the pressure differential between cylinder and rampipe at atmospheric pressure that causes the air to move down the inlet tract into the cylinder.

2. As far as reducing the pressure between the throttle and the valve, actual depression generated by the cylinder on the intake stroke can be huge, especially at low throttle opening thus you would need a big pump to make a difference there, and naturally - if the pressure in the vacuum pipe was lower than that in the port on any throttle plate condition, open/part closed etc - all it would do is suck the air and fuel down the vacuum pipe - away from where you are trying to get it to go. Lowering the pressure at any region by pumping might encourage the fuel in, but it may not go where you want it and it will be achieved at the expense of air massflow.

Excuse me if I have missed the point, time is tight today, happy to look again if need be.

Hi Mr.Croft

No you've not missed the point my question is perfectly answered thank a lot
1-For the vacumm at the rampipe mouth I was thinking that a lower pressure at the entry will "help" the air to enter faster in the trumpet but you're totally right if I the pressure is lowered at the entry we have less pressure difference between rampipe and cylinder so the air will go slowly to the important place, the combustion chamber.

2-I was pretty sure that the air/fuel mixture can go down the vacuum pipe if the pressure in it is lower that the port but I've thinking a higher velocity can reduce "the problem" because the mixture have less volume and is far from the port walls.
But it seem I was wrong!
The air/fuel mixture will go automaticly at the "lowest" pressure point so right in the vacuum pump if this one is big enough to create a pressure difference.

So what I understand is adding X pressure drop in vacuum line present in the inlet port will just "cancel" in part the effect of depression create by the piston.

So thanks a lot for encouraging testing and your good answers on beginner questions!

Hugo

Hi

I dont really want to restart this old topic but my goal stay the same found ways to get velocity increased...
I have think about another solution which can be "strange"... so im looking like always at everyones opinions.

Im working now on my 4age 16v motor.
The early desing of the head(86.5-87) is a large port desing and late model(88-91) is a small port desing...

At the end the small port head increase power in all powerband due to I think velocity increased.
But what can happen if the port is reduced even more?
I have done some "theorical calculation" just for fun... And it look like that a 25-30% port reduction(At the "choke" point so the tightest point in the port) matched to the port entry can be interrested to play with or at least compare the results.

So here the main idea in pictures:



The roof of the port will remain un-touched.
The port bowl near the valve seat will be matched(4age heads have a lot of metal chips near int/ext seats I suppose newly designied heads are cleaned in this area)

For now this is only theory but I will do model test with clay if you seem it can have a certain potential even if the flow of the port will be less important!(For now every tech documentation Ive read are against me so its a kind of bad start but If I dont try I will never know!) In fact I dont really belive in flow=more power because the flow added by porting is higher but the velocity can be considerably reduce so my personal opinion(Im really not a professional! just an amateur) is there no advantages to got more mixture which cannot get in the cylinders at the right time.

Also I have some doubt about raising the velocity too much.
Because theoricaly I think the mixture will enter "too fast" in the intake phase and "stay" too much longer in the cylinder before the compression cycle to keep molecules "mixed" since fuel particles are heavier than air. I suppose this will create rich and lean spot in the mixture present in the cylinder....So that why I will begin with a 25% reduction on a spare head first just to see what happen! My calculation are only based on the port size/shape giving X velocity Vs stock valve diameter... So the reason why I mesure from "choke" point its to get the diameter mesurement of the valve somewhere in the port... And reduce this point from 1/4 diameter and getting I suppose a velocity increase right in the port bowl.

In a near future I will take precise mesurements and make a model with clay... testing will be another part of the game but for now I will try to get more fun with it with only theoric modifications.

Comments are more than welcome!
Thanks
Hugo