Okay, so I have a 2000 GT. I'm fairly certain that the cam and internals are stock. Anyways, I haven't been able to find any super reliable info on the net about stock specs. I'm trying to figure out Valve Lift for the exhaust valves. So far here's what I've found. Stock cam lift is either 504/531, or 505/534. The stock rocker arm ratio is 1.80:1 or 1.81:1. Using the Valve Lift formula to try to figure out valve lift is giving me this for valve lift:
Lobe lift (exhaust) x rocker arm ratio = valve lift so...

.531 x 1.80 = .9558" valve lift?? I'm seriously doubting this and I would like some clarification. Am i messing up somewhere here? Any help would be great.. basically the whole point of this is I'm trying to calculate proper header primary tube size as close as possible, and for that equation i need valve lift. Thanks again.

 

Specs at the valve are :
Intake: 186 deg @ .050/ .504”
Exhaust 194 deg @ .050/ .531”

lobe lift is:
Int: .295"
Exh: .280"

Using lift as a determing factor to come up with the primary tube diameter is not the correct way to go about it. Duration, RPM, Gears, RPM range, car's weight, all play a role in calculating the optimum diameter. With that, if using the stock cams stick with a 1 5/8" primary.

 

I understand there are many factors in determining size and type of header. RPM, Gear ratio, vehicle weight, altitude, CFM of my heads, and compression ratio all add up to telling me I definately need long tubes. I understand that. But when it comes to calculating very close to a precise measurement for primary tube size, this was a quick formula for determining proper primary tube size. I took it a step further with the following thread when I was still trying to figure out lift and all that:

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 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!!


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!!

 

I understand there are many factors in determining size and type of header. RPM, Gear ratio, vehicle weight, altitude, CFM of my heads, and compression ratio all add up to telling me I definately need long tubes. I understand that. But when it comes to calculating very close to a precise measurement for primary tube size, this was a quick formula for determining proper primary tube size. I took it a step further with the following thread when I was still trying to figure out lift and all that:

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 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!!


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!!

 

1 3/4" primaries won't offer any disadvantages overa 1 5/8" primaries and will offer room for growth. In your case a 1 5/8" combo will make no less power than the larger tubes.

Trucks had the same cams as Mustangs, in fact all PI engines had the same cams. Furthermore, the specs I provided are correct for PI cams, NPI had much lower lift.

 

I agree with the thinking that in the long run it would help with future upgrades.. however I have none planned for a long time at least. I was taught that too big of an exhaust is the same as having too small... not enough velocity and your molecular flight time of the exhaust increases. So that's why I'm trying to get the exact size or as close to it as I need. Unfortunately, I have no idea if the info I got from the picture another guy took from a haynes manual related to PI or NPI heads as there were no years shown on the page.. Would the info I stated above that came from the Haynes manual seem to be more relevant to a NPI 4.6L? And just out of curiousity, where do the numbers you gave me come from? I'm seriously not trying to be rude or an a** or anything.. I'm just very passionate about this. Thanks!!

 

Here's the picture I was sent that I'm referring to.

 

The specs I provided are what I got when I checked the specs myself. Crower also list those same exact numbers as the stock PI cams.

A large primary tube diameter will not hurt power in a fuel injected combo like it would if the engine had a carb. if you're worried about going too small, go with 1 3/4".

 

I appreciate you're continued help, but I'm still not quite getting what you're saying. Could you please explain to me how a carb versus being fuel injected has anything to do with how your exhaust flows through a smaller or larger diameter pipe? As I stated earlier, the molecular flight time of your exhaust increases with either a too small, or a too large pipe. So how does being fuel injected or carbed affect that? I'm not worried about too small of a diameter, or too large. My goal here is to find the right size for what the engine needs at this point without anything planned in the future that would affect header size. I can recalculate that at a later time and redo my exhaust later if I go that route. I'm just really curious where you're getting your info from and how you're going to back it up. I appreciate the help with the cam and valve lift though. I didn't know that Crower would have anything to do with stock cams, but then again, I haven't really ever talked to them. Thanks again!!