NewbVetteGuy wrote: ↑Tue Jun 26, 2018 1:55 pm
Ok, I've done some more homework and have some more questions now.
Rick, I used your XLS to generate the "minimum window area" @ 0.100" increments with a typical 45 degree 3 valve angle and a 50 degree valve angle, I compared it to the standard "Diameter*Pi*Lift" calc and the "Diameter*0.98*Pi*Lift" calculation, for giggles, I used the Wallace Racing curtain area calculation, too and it's yet another, different #. The traditional formulas obviously have a pretty big difference with the new XLS.
I wanted to calculate the point at which my Profiler 195cc heads with a 2.15" min CSA would see the actual min CSA / choke point move from the valve to intake tract, the simple Diameter*Pi*Lift calculation estimates @ 0.339", but the new formula says you have to lift the valve 0.429" with a 45 degree valve and 0.431" with the 50 degree seat angle profile(and my 2.02" intake valves), assuming I did it right.
Question: When calculating Discharge Coefficients, shouldn't we be using the more accurate curtain area calculations as the basis?
Next, I want to use your calculator and the FLOW #'s from the HotRod 45 degree vs. 50 degree article to calculate the discharge coefficient at each 0.100" of lift between the 45 and 50 degree seats; I should just use the formula: window area * 146 = theoretical maximum flow for that area, but use the calculator's, more accurate valve curtain area calculation, right?
Adam
Without the valve back-cut angle and width, and in some cases the valve back angle, you can't calculate the real curtain area right. The Wallace calculator doesn't ask for those inputs so it is doing a simpler method that is less accurate.Looking at curtain area at low lifts with different back-cut widths and angles will tell you a lot about why the low lift flow improves with valve back-cuts.
As far as discharge coefficients, I prefer to use cfm/sqin (cfm of airflow per square inch of flow area). Making them into coefficients just complicates things because different people use different "max" flow for the DC calc, and even 146cfm/sqin is not a flow limit for airflow with 28" dp across a port. But to answer your question, yes using the most accurate area calculation will obviously give you the best information for comparisons. The curtain area is not the most important area when the valve is open fairly high in the lift curve. The valve throat cfm/sqin is a more important number i like to look at when the valve is open a lot.
Using real curtain area cfm/sqin at all lifts you flow would be useful and certainly better than the simple curtain method. Throat cfm/sqin should also be looked at and for all out power is probably more important. (make sure to subtract the valve stem area from your area)
Rick
I am reasonably certain that the primary reason valve back cuts improve low lift through convergence lift airflow is because it narrows the valve seat cut width which results in an increase in the valve curtain area. Here is an estimate of change based off a early 90"s Evolution Harley 80CI. This is not to say that you can't also improve the valve area CD with them, but the primary reason for the flow increase at lift is from increased flow area.
The Harley is a special case with a huge valve seat area on a tulip valve. I don't typically see this much suggested improvement by narrowing the valve cut. The head seat cut has relatively little effect in the model and it's area improvement by narrowing appears confined to the seat transition lift zone.
Harley 80 with backcut.png
Harley intake valve after 2 angle backcut (30-37.5-45)
20180626_184624.jpg
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Ok, I used the info in the HotRod article Rick360's XLS and then compared the curtain areas, CFM flow, and lifts to generate the CFM / Sq Inch between the 45 degree seat angle and 50 degree seat angle on the Vortec heads used in the article.
CFM/ Curtain Area Vortec Heads Hot Rod Article 45 degree Seat Angle:
lift Curtain area Flow CFM/sq Inch
.100 .441 63 CFM 142.9
.200 .934 129 CFM 138.1
.300 1.442 187 CFM 129.7
.400 1.989 224 CFM 112.6
.500 2.55 227 CFM 89.0
.550 2.834 229 CFM 80.1
CFM/ Curtain Area Vortec Heads Hot Rod Article 50 degree Seat angle:
Lift Curtain Area Flow CFM/Sq Inch
.100 .397 62 CFM 156.1713
.200 .820 126 CFM 153.6585
.300 1.358 183 CFM 134.757
.400 1.901 226 CFM 118.8848
.500 2.461 235 CFM 95.4896
.550 2.744 234 CFM 85.277
Then I took the results of CFM/Sq Inch of Curtain Area and compared the differences and generated % Diff:
Lift 45 degree 50 degree Difference %Difference
.100 142.9 156.2 13.3 9.3%
.200 138.1 153.7 15.6 11.3%
.300 129.7 134.8 5.1 3.9%
.400 112.6 118.9 6.3 5.6%
.500 89 95.5 6.5 7.3%
.550 80.1 85.3 5.2 6.5%
Average CFM/ Sq Inch from Moving from 45 degree to 50 degree seats: 8.8 CFM/ Sq Inch
Average % Increase: 7.5%
So, although the move from 45 to 50 degree seats on the flow bench only showed a 2-7 CFM increase on the Vortec Head used for the test, the average CFM per sq inch of area increased by 7.5%.
The dyno results showed an average torque difference between the two valve angles on these heads at a 0.28 Lift to Diameter Ratio of 3.5%, a Peak Torque Improvement of 5.4%, an Average HP Difference of 3.6%, and a Peak HP Difference of 4.2%.
I think the comparison of average CFM/ Sq Inch %Increase and the Average % Torque increase is the most interesting thing to me. A 7.5% average CFM / sq inche increase = a 3.5% Average Torque increase for this engine.
HUGE Caveat: I think I used Rick360's XLS wrong for the 50 degree seat angle as I used the default 50 degree seat angles in the tool; I probably need to update all the numbers from there and I'm happy too. The tool default has a 43 degree "Seat Top (Chamber Side) Angle", and a 35 degree Valve Back Cut angle for the 50 degree seat profile.
Looking at the CFM / in2 data for my heads and also understanding the lift at which the choke point moves from the valve curtain area to the intake pinch point, makes me ask another probably dumb newb question:
Does opening the intake valve significantly past the point at which the valve curtain area is no longer the choke point, actually hurt power / peak torque?
Real Example behind my question: My Profiler 195cc (as cast) heads with a 2.02" intake valve, with Rick360's detailed Curtain Area calculation will have the curtain area exactly match the intake port's min CSA at 0.429" valve lift. @.400" lift my CFM / inch2 is 122.4; at .500" lift it's 102.6 CFM/ in2, and at my Intake Valve's max lift of exactly 0.600" I'm at only 82.2 CFM/ Inch2. -There's a much lower air velocity entering the cylinder at the higher lifts.
-My understanding is that if you couple increased velocity with a later intake close you can stuff more air into the cylinder and increase your max torque peak - a lower max lift would increase my AVERAGE CFM/in2 / velocity past the valve and into the cylinder, right?
I will guaranty that just narrowing the seat width is not all which happens here with using a back-cut.
Several times I have made the same test on different heads having 50 degree valve jobs using the same valve and seat width during flow testing. Grinding it to a 40 degree back-cut, then a 35 degree back-cut, then 30 degree back-cut and finally a 25 degree back-cut and now the flow down low moves around and changes EVERY time yet the SEAT on the valve is the same angle and width.
Sometimes it even changes the high lift flow curve a bit.
NewbVetteGuy wrote: ↑Sun Jul 01, 2018 12:03 pm
Looking at the CFM / in2 data for my heads and also understanding the lift at which the choke point moves from the valve curtain area to the intake pinch point, makes me ask another probably dumb newb question:
Does opening the intake valve significantly past the point at which the valve curtain area is no longer the choke point, actually hurt power / peak torque?
Real Example behind my question: My Profiler 195cc (as cast) heads with a 2.02" intake valve, with Rick360's detailed Curtain Area calculation will have the curtain area exactly match the intake port's min CSA at 0.429" valve lift. @.400" lift my CFM / inch2 is 122.4; at .500" lift it's 102.6 CFM/ in2, and at my Intake Valve's max lift of exactly 0.600" I'm at only 82.2 CFM/ Inch2. -There's a much lower air velocity entering the cylinder at the higher lifts.
-My understanding is that if you couple increased velocity with a later intake close you can stuff more air into the cylinder and increase your max torque peak - a lower max lift would increase my AVERAGE CFM/in2 / velocity past the valve and into the cylinder, right?
Adam
Have you looked at how much lift you will have at and after bdc?
NewbVetteGuy wrote: ↑Sun Jul 01, 2018 12:03 pm
Looking at the CFM / in2 data for my heads and also understanding the lift at which the choke point moves from the valve curtain area to the intake pinch point, makes me ask another probably dumb newb question:
Does opening the intake valve significantly past the point at which the valve curtain area is no longer the choke point, actually hurt power / peak torque?
No, the valve needs to be opened much more than the convergence point for most power. The throat cfm/in² can be used as a yardstick for similar heads keeping in mind the effect of other choke points in the intake runner.
NewbVetteGuy wrote: ↑Sun Jul 01, 2018 12:03 pm
Real Example behind my question: My Profiler 195cc (as cast) heads with a 2.02" intake valve, with Rick360's detailed Curtain Area calculation will have the curtain area exactly match the intake port's min CSA at 0.429" valve lift. @.400" lift my CFM / inch2 is 122.4; at .500" lift it's 102.6 CFM/ in2, and at my Intake Valve's max lift of exactly 0.600" I'm at only 82.2 CFM/ Inch2. -There's a much lower air velocity entering the cylinder at the higher lifts.
-My understanding is that if you couple increased velocity with a later intake close you can stuff more air into the cylinder and increase your max torque peak - a lower max lift would increase my AVERAGE CFM/in2 / velocity past the valve and into the cylinder, right?
Adam
You need the higher velocity in the entire runner to be high to get max ve. Having the high cfm/in² at lower lifts should help too.
I had a few days of vacation and managed to get through the full thread twice finally. Feel like I'm finally starting to get it.
Blown away by all the gold nuggets shared and I'm going to try to distill them all down in the next week or so into some super condensed notes, but it'll take some time and now that I'm back from vacation work is pretty busy.
Having said that I wanted to try to apply some of what I think I'm learning via a thought experiment. Specifically the earlier comment than an AFR 195cc SBC head, even the Race variant might not be a great candidate to move from 45 degree seats to 50 degree seats and supporting angles. If I'm remembering the comment was that the race head already had 2.08" intake valves which were plenty large for a 4"ish bore. -My thought is also that the AFR heads already have such a small min CSA and are already such fast ports that making them even faster with a 50 degree seat profile probably wouldn't be helpful except in some weird towing low RPM scenario or some other weird edge case. I started looking at the CFM flow per min CSA of a bunch of heads and it seems to really show 1. how great the AFRs really are 2. how the AFR heads have slightly undersized min CSAs / slightly faster ports than typical sizing guidelines.
A few of the really good examples posted of getting good results with the move to 50 degree seats focused on heads that were oversized for the displacement / RPMs to begin with. Putting a 50 degree seat on a AFR 195cc head on a 6,000 RPM 383 seems like an opposite situation where you'd be making an already borderline fast port even faster, no?
"The correct formula for determining how big of cylinder head we need:
MIN CSA = (bore x bore x stroke x RPM x .00353) / 613.8 (.55 MACH x 1116 fps)"
SBC 350 @ 6,000 RPM* HP Peak= 1.92" Min CSA
SBC 383 @ 6,000 RPM* HP Peak= 2.1 Min CSA
*Yes, I know plenty of people on here who would say "7,000 is the new 6,000" for the street but I'm using a more conservative/traditional "Street" definition here.
AFR 180cc Street Head Min CSA: 1.81"
AFR 195cc Street Head Min CSA: 1.905"
AFR 210cc "Race-Ready" Head Min CSA: 2.1"
Profiler 195cc Head Min CSA: 2.15"
Profiler 210cc Head Min CSA: 2.19"
It seems like one of the more ideal situations for a move to a 50 degree seat would be when someone has bought one size too big of a head and one size too big of a CAM for the street / their displacement / desired RPM range -at rebuild time replace the 45 degree valves / seats with 50s and bring the average port speed up and support the bottom end better with the smaller behaving cam and reduced reversion. So, one of the LEAST ideal situations would be when the head / min and average CSAs /port speed is already too high for the combos / goals (seems like some common AFR combos would meet the bad candidate list).
I'm almost afraid to reference the other "rule of thumb" thread, but I do want to ask: Are there any "rules of thumb" about how a cam recommendation changes with the move from a "typical" 45 degree set of valve angles to a "typical" 50-degree profile?
The other thread started looking at discharge coefficients and from this thread, we know moving to 50 degree profiles makes the discharge coefficients increase fairly significantly.
-Is there any sort of generalization that can be taken away between what's ideal for the cam for two otherwise identical motors with only the valve angles changed from a typical 45 to a typical 50? -I thought I saw some discussion that with improving discharge coefficients, the recommended LSA widened, but the other thread was just too painful to read for me to spend much time in it... -I could see you wanting to actually DECREASE the LSA with 50 degree seats to get more overlap to take advantage of the decreased reversion so I'm all sorts of confused...
NewbVetteGuy wrote: ↑Tue Jul 17, 2018 5:40 pm
I'm almost afraid to reference the other "rule of thumb" thread, but I do want to ask: Are there any "rules of thumb" about how a cam recommendation changes with the move from a "typical" 45 degree set of valve angles to a "typical" 50-degree profile?
The other thread started looking at discharge coefficients and from this thread, we know moving to 50 degree profiles makes the discharge coefficients increase fairly significantly.
-Is there any sort of generalization that can be taken away between what's ideal for the cam for two otherwise identical motors with only the valve angles changed from a typical 45 to a typical 50? -I thought I saw some discussion that with improving discharge coefficients, the recommended LSA widened, but the other thread was just too painful to read for me to spend much time in it... -I could see you wanting to actually DECREASE the LSA with 50 degree seats to get more overlap to take advantage of the decreased reversion so I'm all sorts of confused...
Adam
In theory a 50 would make the engine see a smaller valve so the LSA would be tighter. For example Creasons EMC engine on a 45 would want a 108 to 109 but if you refigure it for a 50 it puts it in the 105 or 106 range. In theory.
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