Positive pressure flowbench

[Farrell]

The air flow definately goes up. What I found most interesting though was that, most times,the flow doesn't go up exponentialy.Twice the pressure doesn't get twice the flow.However there were times when you got more than the calculated value. Probing the valve head at a given lift while increasing the pressure was one of the most interesting exercises I've done with this bench. Though it may sound obvious,a maximized boosted head has very little in common with a maximized naturally aspirated head.Line of sight porting has some relevance here. Of course,since the port velocities are so much higher,the combustion chamber and piston dome/chamber interface has to be addressed as it relates to combustion.Unless ultra high quality fuel is used,certain steps to calm the combustion proccess may have to be employed, ie: squish manipulation to avoid flame front issues.Larry Widmer does this with his "Roller wave" pistons,by essentialy creating a "May" chamber.

[200cfm]

So under boost velocity increases at the curtain area throat discharge. So I should be able to increase the throat area more than in a NA engine. Thanks.

[Farrell]

The velocities around the seat are much higher since velocity is just a function of restriction.Remember Bernoulli.What is really interesting is to watch the main area of flow around the valve seat move as the pressure is raised. The SSR and that part of the valve are just bypassed as the air slides across the back of the valve toward the back wall. From here on,the chamber needs to be laid back as far as necessary without removing support from the column of air.It's all cut and try here,but the boosted bench gives me real world conditions to help me make the proper design changes.I remember a couple of years ago Darin Morgan saying the they were hooking up an additional air supply to their benches to be able to test @ 65" and were finding power in their IRL heads.Obviously this is atmo stuff,but they found it relevent.

[200cfm]

Here is my latest test chamber. I layed back the eyebrow to the quench line and it picked up mostly in the mid lift range. Any opinions on this?
Wet flow would also change behavior based on your research.

[Farrell]

For anyone interested,the bench in the pictures is for sale.I've been at this porting/engine building stuff for thirty years and no longer have the desire to beat my head against a wall.You can't do this stuff for a living in Canada.This is the most repeatable bench I've ever used.Since it can flow in both directions,it can be used for wet flow testing. I've got about $7K in parts alone, into it.Make me an offer.

[Farrell]

Is this a boosted application?Laying back the chamber as you have done will have a result similar to putting a back cut on the valve.The biggest gains will be in the .200"-.400" range.But you know this.If it is boosted,I'd recommend placing the head on the block and marking the bore on the head and relieving the chamber to the bore line.I do this on all but the most compression deficient motors.I try for at least .150" around the valve head if possible.If it is necessary for the application you can you can open the chamber and notch the cylinder wall for clearance.Another way is to offset the intake guide towards the centre of the bore.Neither of these options is particularly difficult,but it takes extra time is all and you have to ask yourself,do I really need this?Is that an Olds?

[Unkl Ian]

Why do those valves appear to be almost the same diameter ?

[200cfm]

Yes, I run boost from a turbocharger. Head is a cut up Studebaker section half. Exhaust is 1.625 and intake is 1.850. Thinking of applying this chamber test to my standard heads on the motor. Over on Speedtalk the discussion on port air speeds under boost conditions was it stayed the same as in a NA motor. But your test experiments show that is not correct. Thanks for the research sharing on this issue. I have always believed the throat discharge speed had to be higher since the pressure in the port / bowl would be higher but had no scientific proof. Your bench design shows that velocity speed increases under boost and that has to be a big deal in cylinder filling and cam choices.

[blaktopr]

Tom. Take a look at this. It discusses choked flow of gasses and its mass flow in regards to positive pressure and vaccuum.
http://www.therebreathersite.nl/04_Link ... Choked.pdf

[Farrell]

One thing that I haven't talked about here is when the port has reached its critical point.I've seen this as early as 100" on some wedge heads.Like the valve head velocity experiment, the lift is left at a set point and the pressure is increased. When critical flow has been reached,the port will start pulsate.As the pressure increases,the pulsation gets worse while flow inceases initially cease and then start going down with increased pressure.