The exhaust port above flowed on the SF and PTS. The SF used the original calibration markings. The PTS used a 2.054 plate with a .605 cd and all factory settings.
Do 3 heads on both benches back to back if possible. Use sf man with your corrected cal. Post results in one post. I have an idea. Need 3 data sets to conclude anything. I/E would be preferable.
Calculated the change amounts from lift to lift as another way to look at this. As is, a sharp edge orifice is being compared to an orifice with some depth. While not exactly apples to oranges, it's not apples to apples either.
1 Sharp Edge Orifice with Digital Manometer using 16" DP as full scale.
2 Square Edge Orifices with Analog Manometer using 6" DP as full scale.
The charts show the trend for behavior.
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What I've done here is to track rate of change in cfm per lift check point with average change per lift, per manometer with standard deviations and charted the results. The reason....... IMO, we need to look at orifice behavior trends and stop putting so much reliance in coefficient of discharge as the final say in orifice trim calculations. There is another way IF it is in fact needed. Emphasis on "IF"!!
1st - STDEV across reveals that value as it relates to the 3 benches on a per lift basis.
2nd - STDEV down reveals the full range value for each bench individually, through the lift cycle.
While method 2 is in some ways meaningless, it's not totally meaningless. When that STDEV is subtracted from that benches AVG, it shows another trend...or behavior if you please.
Several interesting things are revealed through this type of analysis. What I see is the PTS seems to hold full range deviation better than the rest. That may be a good or a bad thing. It's good if you want the lineraity that you think you should have but maybe not so good if you believe the orifice's flow dynamics do in fact lean toward having a sweet spot. Perhaps the sharp edge orifice supplied by PTS is less prone to the "sweet spot" than the square edge SF orifice. It's arguable and wothy of it's own discussion.
1. It's important to consider that the PTSDM was the only Digital unit in this comparison.
2. It's important to consider that the .200 lift readings taken on Chad's SF600 were taken using a range larger than would be optimal for the amount of flow available at that particular lift. The manometer was below 50% of scale for that life. Well out of the "sweet spot of >60%".
Now, even though the .200 values are considerably different on the PTS bench with PTSDM [previously explained], It is interesting to see the trend for rate of change shows considerably less deviation than the straight CFM numbers show from .200 to .300 where the first change takes place! Also note that initial CFM value was considerably different to start with at .200 but the measured rate of change is remarkably quite similar in all the tests.
Cd of any of the particular orifices is what we tend to run to when we want to match things up. That's fine but it's a number that may not be "fully" representative of an orifice's tendency to have a sweet spot. The Cd affects the full range of the orifice. It cannot be a 100% accurate tuning tool for the orifices tendencies if a "sweet spot does in fact exist.
Another key thing I see from .200 through .500 is that area is the one with the greatest rates of change taking place. Remember, the rate of change is the pressure differentials taking place inside the cabinet's chambers. What we read are the gauges abilitiy to measure that change.
Next we see the .600 lift "door slam affect" as the curtain area stops yeilding available flow area. That actually takes place at about .507" lift but the test wasn't stopped there, specifically meaured and then finely incremented in say... .025" increments to reveal any interesting characteristics taking place. We don't tend to do that when flow testing a head anyway...but for this purpose it might show a better picture of the trend. The STDEV yields the greatest differences at that lift for this test while the rate of change has drastically fallen off.
What's happening from .600 to .800 range is the least amout of pressure differential across the orifices. Flow yields are at their smallest increases. I feel it's that scenario that may cause a manometer to pehaps "struggle" with internal cabinet turbulence affects...don't know for sure...just a gut feeling on that. The SF units show a flow loss while the PTS shows a gain. Seemingly BOTH cannot be correct....But, then again, for their particular orifice, in their particular cabinet....maybe they are both correct.
Almost Done.... As I see it, if a guy was to trend some cylinder heads via further comparisons with other cylinderheads and maintaining strict test conditions...and if those trends were in fact directly proportional for all the heads tested and revealed the same tendencies at the same lift check points....... The solution is not a matter of Cd adjustment but a finer correction formula for the lifts. That's easily handled with software formulation. I could put that inot FSE every easily. [Chad's the anal guy here and the one with the equipment to do it]
Lastly consider this. The flow bench does not duplicate the engine. It can and will mislead us if we put to much faith in it. That's been proven over and over and over again. Case in point... .600 lift. We have 3 decreases and one increase. What if the PTS is correct? What if you developed the port on the SF bench and were running a .600 lift cam? Would you in fact be working at increasing flow in the port in a region where it was not reversing? Would you in fact be on a wild goose chase. What I'd like to know is who's orifice is Chad using in the bench he got dialed closely with Darin's. I know it was the FP manometer but don't now the orifice...
JMO.... Fire back with whatever you see...feel...agree....disagree with.
Larry C
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Last edited by larrycavan on Mon Dec 26, 2011 2:34 pm, edited 3 times in total.
Reason:tidy up the cut and past formatting disaster
