Formula for Curtain Area @ Valve Lift ?

[SteveS]

Great stuff..........!!!! We are not realizing as much lift and curtain as we think we are.

[mike_belben]

is it possible to take the 30* backcut too far? for instance, if im using a .035" seat, lap it to find placement and i bring the backcut right up to it, say .015" from the lap marks.. can this be adverse in any way?

has anyone ever tried putting a second backcut angle behind the 30* ? or experimented with putting an angle on the margin, for instance, a 75 instead of a 90* edge ? i havent tried myself for lack of a flowbench.

[MadBill]

I tried a 34°/23° back cut on a 45° seat with a 12° underhead angle, on the theory that air will supposedly turn 10° or so without going turbulent. It seemed to flow a little better than a straight 30°, but I got distracted and didn't pursure it.

[mike_belben]

nice!

may i ask to what extent you evaluated it, or perhaps what you considered 'a little better' at the time? appreciate your input.

[MadBill]

It was about 8 years ago; I'll see if I can unearth the file. I suppose it couldn't have been too earth-shaking though, or I would have pursued it more vigorously.

[mike_belben]

mmm.. i suppose thats true. thanks for your time bill

[bill jones]

-regarding backcuts on valves only:

-I just did two flowtests of a Subaru 4 valve per cylinder head----no backcuts at all and then a single 33 on the intake and a single 37.5 on the exhaust.

-The only measurable or positive blip on the flow curves was the exhaust at .100" lift which was 5 cfm positive going up from 88 to 93 cfm.
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-Typically I will do two backcuts if there is a quite wide single back cut.

-I usually then split the difference and make both back cuts the same width.

-on the intakes with 45 seats I use 33 & 22 on a semi tulip shape----or 30 & 15 on a nail head.

-on the exhaust with 45 seats I always use a 37.5 and a 29.5.

-just never found any magic with anything else other than I have seen some sbc heads respond to a very narrow 30 (.030" wide) with a wider 15 as long as the demarcation lines are nice and defined.

[mike_belben]

thanks alot bill, subarus still treating you well i hope. i guess making power is rarely easy huh? i just spent 3 long, aggravating days making a centerless valve spinner that would work on my surface grinder. i certainly learned more than i bargained for about refacers. right now im just going to convince myself all that work is gonna make 5million more horsepower and be happy about finally finishing those backcuts.

[bill jones]

-got any photos of that centerless setup?

-I'd like to see that.

[MadBill]

How timely! I had picked up the December Car Craft before we got into the back cut/double back cut discuusion, but didn't get around to reading it until just now. Lo and behold, they have article on this very subject (as well as a nice one on wet flow) that shows low to mid lift gains as great as 25%!

[vic_dahn]

curtain area.jpg

Just found a page from my notes that I would like to share.
A nice visual description on curtain area.

[John Wallace]

Thanks for the pic!

[bill wilson]

Good day! Is it possible that Rick360 could indulge those of us that missed the original July 2005 post by reposting his curtain area spreadsheet? This would be much appreciated. Thanks, Bill

[F1Fever]

you can try this... just download it, extract and open it. It will calculate 13 values from .005" to .800"

[Rick360]

Attached is the spreadsheet zipped up for download. This uses the angle and width of the seat, top cut angle and widths and the valve face angle,width and backcut angle and width.

This pic shows pretty well why the actual curtain area is really less than normally calculated area. The one F1Fever posted uses the seat angle and width to calculate the inner diameter of the valve seat and calculates curtain area there but is still considerably larger than the real curtain area.

Using a 2.08 valve with 50º seat .060" wide shows area of .62"@ 0.100" lift on the one F1Fever posted. My spreadsheet shows .426" @ 0.100". The old standard formula (VlvDiam x PI x lift) = .653". You can see how far off these are in reality. Specs for VJ = 43º-.100",50ºseat .060" : valve backcut 35-.070"wide and 12º valve back angle.

Code: Select all

Lift      Old Formula          My Formula
 .100"      .653sqin             .426sqin
 .300"     1.960sqin            1.461sqin
 .600"     3.920sqin            3.258

This is directly related to the low-lift flow thread in the advanced section from a couple weeks ago.

Rick