Ok, so I believe I've narrowed down my equation with help from people here for what my stock valve lift is. Based on the valve lift formula (lobe lift x roller or rocker arm ratio) for the exhaust side which I've tried to be conservative with (.295" x 1.8 = .531" valve lift).. I've come to the conclusion that if I use .531" of valve lift for the Header Primary Tube Size formula: √(valve lift x valve diameter) x 2, it will have given me a very close size of primary tube size for my headers. So here goes:
√(.531" x 1.417") x 2 =
√.752427" = .687425501"
.687425501 x 2 = 1.73485----" or basically 1.75" primary tubes. I was taught that you always go to the nearest size whether up or down, but if using a power adder, it's always better to go to the larger size or even one size larger than that. So with a 50-100 shot of NOS, I figure that 1.75" (1 3/4) primaries should be accurate right? Anyone know if this is a good size to run? I was set on 1 5/8" primaries but now I'm not so sure. My reason for being set on 1 5/8" primaries was due to the following math:
Let's say we take 1 5/8" primaries (1.625").. this is the outer diameter (OD) of the pipe. Let's assume a wall thickness of 0.040" for most common pipe, this means we have to take off 0.080" total when calculating cross sectioal area of the pipe. So 1.625" - 0.080" = 1.545". To find the cross sectional area of a circle we use this equation:
diameter squared x Pi /4 or 1.545" ^2 x 3.1416 / 4
2.387025 x 3.1416 / 4
7.49907774 / 4 = 1.874769435" or basically 1.875"
To find peak torque RPM with these primaries, we use the following equation:
Primary Pipe area x 88,200 / displacement of one cyl or 35.125 so..
1.875 x 88200 / 35.125 = 4708 RPM
Using the same math for a 1 3/4 (or 1.75") primary gives us a peak torque RPM of 5520 RPM. I've read that stock Rev limit is right around 5750 +/- 50 RPM and using the shiftpoint formula:
1265 x intake CFM / one cyl. CID we get the following for max Redline:
1265 x 165 CFM / 35/125 = 5942 RPM
So here are my questions: Which primary tube size on a set of full length long tubes would you use or have you used and what were the results? 1 5/8 or 1 3/4?
I should be able to keep my stock exhaust pipe diameter of 2.5", however I will be using an O/R H pipe and then mufflers. I have (estimated 50 shot but could be 75??) of NOS when the switch and MSD RPM box and all that works. I have MSD coil over plug Coils, NGK Iridium 9 spark plugs, and a K&N FIPK cold air kit. I also run Amsoil Signature Series 0W-20 synthetic oil and use 91 Oct fuel. When I get the exhaust on my car, I also plan on going with an SCT tuner with a custom tune. At some point as well, 3.73 gears are in the works and a centerforce clutch but that's another thread.. Anyways, help with this would be great!!! Thanks for making it through this if you're still reading, and any advice/experience would be great!!
---------- Post added at 05:06 PM ---------- Previous post was at 04:39 PM ----------
Just got an update.. looks to be like lobe lift is closer to being .2594" taken from a Haynes manual, but not sure the year of the 4.6L it was mentioning or if it was from the truck or mustang or whatever. But using that info brings valve lift down to around
.466" to .470" using a rocker ratio of 1.80:1 to 1.81:1. This would be much closer to needing 1 5/8 primaries according to the math. Still could use some help deciding though!!