AFR 210cc SBC Eliminator ported numbers update

[David Vizard]

nitro2 stated exhaust flow does not exceed supersonic.

It does in NHRA Pro Stock.

Really? That is something I should dig deeper into.
DV

[Ron E]

I think Clint was referring to velocity in the port. Across the seat at low lifts, the pressure diff's show sonic velocity across the valve seat at crack-open. How long it remains so depends on various factors. .300" or higher lift isn't uncommon before it drops to turbulent speeds where the bench can be very helpful . The big throat is good for sonic flow as it reacts to the size above shape of the restriction. Turbulent velocities are sensitive to the shape of the valve-job and a bigger throat can calm a port during the mid/high lifts while providing a bigger orifice for improved low lift (sonic)flow. Most, not all, but most don't really need an increase in port size at the header flange.

[n2xlr8n]

Excellent fellas- thanks for the clarification.

<back to lurking where I belong>

[RevTheory]

I know Tony Mamo doesn't think that flow that backs up hurts power even if you lift the valve up into that area but many completely disagree. Does a porter have to jump through hoops to get AFRs to level off instead of backing up?

[digger]

The flow definitely reaches sonic in the Throatfor quite sometime, I don't know if it exceeds it though I'm rusty on what is needed to achieve supersonic speeds. My recollection was that it couldn't exceed it in Throat

[gruntguru]

The pressure ratio (ratio of absolute pressures) across a pipe element will determine whether flow is supersonic. As a rule of thumb, the critical pressure ratio (where sonic velocity and choked flow begins) is approximately 0.5 At low lifts the seat area is the "pinch" point and the flow there will be sonic until the cylinder pressure blows down to less than double the pressure in the port.

Supersonic flow can only occur in the divergent section after the seat and only at pressure ratios less than the critical pressure ratio. (Flow at the "pinch" point will be sonic (Mach 1) and cannot exceed Mach 1)

[digger]

The pressure ratio (ratio of absolute pressures) across a pipe element will determine whether flow is supersonic. As a rule of thumb, the critical pressure ratio (where sonic velocity and choked flow begins) is approximately 0.5 At low lifts the seat area is the "pinch" point and the flow there will be sonic until the cylinder pressure blows down to less than double the pressure in the port.

Supersonic flow can only occur in the divergent section after the seat and only at pressure ratios less than the critical pressure ratio. (Flow at the "pinch" point will be sonic (Mach 1) and cannot exceed Mach 1)

That was my understanding to so you could theoretically get supersonic in port but not the throat

If the throat is sonic under What conditions will the port become super sonic downstream is it just with a specific geometry after the throat?

[MadBill]

I don't think that's possible. It requires a very carefully contoured converging/diverging (Google de Laval) nozzle to create supersonic gas flow in the divergent section. Sonic flow through the seat itself is one thing, but It seems unlikely that it could be supersonic in the actual port, or even in the immediate downstream blend angles region, given the relatively sudden area changes below the seat which when converted from an annular to an equivalent cylindrical nozzle would be far from optimally shaped. AIR, the diverging half angle is usually in single digit territory for discharge into atmospheric pressure.

Maybe Clint will weigh in with some fact-backed data...

[digger]

I guess I'm curious as to what geometric guidelines you'd need to follow to have that kind of divergent nozzle needed to generate it. I see the curve of the port as problematic.

I seem to recall Larry widmer saying he developed supersonic ports

[WeingartnerRacing]

Eric,

What are the real port volumes here?
DV

I just poured the head 230cc

[randy331]

Eric, did you figure the port energy before and after your port work ?

Randy

[WeingartnerRacing]

Neither

[David Vizard]

Eric,

What are the real port volumes here?
DV

I just poured the head 230cc

Eric,
thanks - but could I trouble you with the valve sizes and the port volume of the before head as well of two ex port vols please?

I am doing some graphs here which I will send to you via a PM. You can publish or not as you may choose.
thanks
DV

[randy331]

Eric, did you figure the port energy before and after your port work ?

Randy

Neither

Then what criteria do you use to base an improvement on ?

Randy

[WeingartnerRacing]

Eric, did you figure the port energy before and after your port work ?

Randy

Neither

I look at the engine needs (size and rpm band) then figure out the size the port should be then go about making the head that size or as best as the casting will allow. Then it’s better.

On heads that are close to the right size I try to get the most air out of them without altering size dramatically.

I guess size and cfm is my answer in short. No port energy formula.

Then what criteria do you use to base an improvement on ?

Randy