200 CFM Research

[gofaster]

I like your idea of a rigid stand to steady the pitot!

To keep my valve heights repeatable, I use an "inner" spring to maintain a fairly stiff resistance on the retainer/keeper assembly, and before testing, I run the valve in to coilbind to seat the keepers into in the retainer. After that the zero point is stable for the duration of the test.

[larrycavan]

It's my understanding that boosted engines do no see increases in port velocities. Rather, the air density rises, resulting in greater efficiencies. As such, IMO, the rules don't change for port velocites with regard to boosted vs n/a but are adhered to based upon several factors. Among those are the angle of approach of the port to the cylinder bore and the valve axis angle to the port angle.

[jsa]

A change in air density results in a change in speed of sound, so choke flow happens at a different air velocity compared to N/A.

[200cfm]

Would it be correct to say more cam lift (like from a cam change or rocker change) can move a port into a choke conditon? I am seeing higher air speeds at the higher lifting points. So couldn't certain lifts be detrimental for a given port size? And when do you know your air speed is in the zone? It is simply based on the average formula for fps? In Pipemax the default is 300 fps and you can go high or low. But how would you know the best average speed? Quess it has to be calculated and then track tested. That could take a lot of back and forth research to really home in on where it should be. I've got 10 sets of heads laying around in my garage so I guess I have 10 chances at getting it right.

[larrycavan]

[200cfm]

Ok, my understanding is starting to improve. I think I was hung up on producing a specific air speed and now believe the port should be targeted for a certain velocity range (like 260 to 285 down the middle). One thing I notice on the 1.780" valve that flows pretty good (vs. the 1.840" head). The seat margin on the 1.780" is on the very end of the valve. I haven't checked the other head yet because it is still on another engine but I am sure the seat margin has some gap distance from the very end of the valve.

Just wondering if this is a plus for air flow? If I had a close up camera I could explain this better. I don't think there is a back cut on this valve. My reading indicates a back could help the flow.

[200cfm]

Have started rereading Bill Jones Porting Book again. He uses valve square inch area X 80% to find the throat square inch area and from that find the diameter to set the throat dimensions. I have been using a % of the valve diameter which apparently is incorrect. Are there two methods? And what do I do with the valve stem diameter (11/32"). It has area and will reduce the throat. So is that area added or subtracted from the 80% area? So much to learn, so little time. Thanks.

[200cfm]

Made a profile of the pushrod turn area (same port) at .450" lift (87% theory) using the pitot probe stand and see a reduction in speeds which is good.

177 Cfm @ .450" lift
left wall middle right wall
364 347 335 Roof
364 326 303 Mid section
356 316 290 Floor

The left wall still has pressures exceeding the test pressure but I am thinking with the manifold attached these speeds will fall down to around 325 on PRt (pushrod turn) and should be ok. The SSR apex came in the same at 323 fps which would drop down to 290 with the manifold ( an assumption here).

Now a question on theory math calculations. The math says the average speed at the PRt is 333.4 fps. And the math says the CSA to make this average speed would be: 177/333.4 X 2.4 = 1.274 sq. in. That's a small area. BUT when I measure this area (using a stock mold) the PRt is larger. It measures 117 mm around the mold which calculates to an area of 1.688 sq. in. This is on a stock port mold. And the above PRt has been opened up so it would be larger still.

Why the disagreement? I am wondering if my port volume cc is in error (by a few cc) and does the radius attachment to the port entrance become a factor in the overall port length? My entrance radius adds a good .750" to the port length.

[200cfm]

I found an error mistake. The port volume is around 140 cc and I was using 150 cc in the formulas. I measured the port again to double check and confirmed the error. That puts the avg CSA at around 1.700" if I don't have port center line error. Will take the nine readings again on the PRt (pushrod turn) and see how it looks. I am thinking perhaps I should not try to peak the pitot pressure on each of the 9 measuring points but rather stay in the physical zone on the port with the pitot for the best reading.

[larrycavan]

[200cfm]

yes, it shows I need more cam lift to make even the minimum hp estimate. I widen the pushrod turn a little more an got 307/290/283 fps across the middle region of the PRt on .350 lift. Went to 340/320/314 on .450" lift reference test. Best cfm so far is around 185 @ .500. I am using this port for experimenting and so far I believe things are going in the right direction.

[larrycavan]

[200cfm]

Ok, will revisit Pipemax and use your recommendations. Thanks.

[larrycavan]

[200cfm]

Well, I found another math mistake I was making. Above I posted the stock intake mold measured about 115 mm around the PRt. I converted that to a pure circle and back tracked the math to get area. Found out that proceudure is incorrect. What I should have done was convert 115 mm to cm (11.5 cm), then to inches, 11.5/2.54 = 4.528", then convert that circumference to a perfect square by dividing by 4, hence 1.132" per side. Now I can square the side for CSA of 1.281 square inch.

I have opened up the PRt more and current readings at PRt on a .350" lift reference came in at 302 fps avg. Took 9 readings and averaged them. CFM flow at .350 lift came in at 170. Sooo, csa = cfm/fps avg x 2.4 = 170/302 X 2.4 = 1.351 sq in CSA for the PRt. Those 4 corner radius well fool the math alot if you go by pure width by floor-roof measurements.

Hence the new wider PRt is now approximately 1.351 sq. inch. This is much closer and unless I am erroring again somewhere, appears to be what is really cooking in this port.

Larry I studied that Pipemax hard late last nite and got lost. For a moment a light bulb came on in my head, then I made a change and my understanding "blew out." Can you post up an example from a live engine for a correlation comparison to Pipemax? Or I can send you my specs. Perhaps even a new topic title devoted only to Pipemax 101.
Thanks.