Headers

[Anonymous]

bwawuz02, what are you talking about? Runner length? We are talking about headers, not intakes... unless you mean primary length.

I know there is an equation floating around that will calculate the correct primary length, but I have yet to find it. I'm gonna check out the headerdesign site now.

[Anonymous]

Alright, just used the calculator on the headerdesign website... I used my car for an example (225hp @ 5800rpm)

The calculator says:

Primary Pipe Inside Diameter: 1.39 in.

Primary Pipe Length: 36.2 in.

Collector Inside Diameter: 2.17 in.

Collector Length: 13.7 in.

The one factor I didnt know was the exhaust port length in cylinder head. They say to use 0 if you dont know it. Does anyone know it? I also used a "performance factor" of 6.

http://www.headerdesign.com/login/header.asp

Feel free to login as me.

dave@dykz34.com
pw: headers

[SappySE107]

How thick are the tubes supposed to be? 16 gauge, 12 gauge? Whichever that is, what does gauge mean in terms of inches? The headersbyed sheet is based on outside length. I need to read over it again to see if it mentions the thickness of the tubes for their measurements.

[bszopi]

Gauge listing...

http://www.unc.edu/~rowlett/units/scales/sheetmetal.html

[Streetrodder83]

What they say on the site is 16 guage or 18 guage is used for primary construction. Depending on which one you choose you'll add two wall thicknesses to the ID and you'll to come up with the OD.

[SappySE107]

The charts I have go by OD, so I have to see what thickness those are derived from.

[SappySE107]

Ok, its based on .049 thickness, and gives the ID as well in the headersbyed papers. Seems to be based on 17-18 gauge tubes.

So, with 1 1/2" tubes, the ID is 1.402. 1 5/8 tubes it would be 1.527" ID. 1 3/4 is 1.652 ID. That program says I would want 1 1/2" tubes then for a stock DOHC. What I dont understand is if the HP basis is before or after headers. I would expect it to be with headers, otherwise you are basing output on any possible stock manifold setup...not good.

[Anonymous]

But there is a flaw in calculating the output of the motor "after" headers... we dont know how much they will make! Anyways, you can always just plug in different #s and try them out. I wouldnt do 1.5" primaries... I'd go with 1 5/8".

Do you happen to know the exhaust port length? Ive never heard of that one being needed before...

[Streetrodder83]

To me, I feel that this program can only make a general assumption. It has no idea what port velocity is, flow rate of the heads, or anything like that. Being that it is setup with 355 input values, one can only assume that it is based upon a SBC head. So I am alittle hesitant to believe the numbers it spits out. From past experience, I would start with 1 5/8". This is what I have commonly run on engines displacing under 400 CI. I have never flow benched one of these heads so I am unaware of what they are capable of flowing on the intake or exhaust. Not knowing this info has hampered my ability to design anything with any accuracy for my turbo manifolds. As soon as I get the chance to give my car alittle down time the heads are coming off to be flow benched.

[SappySE107]

Measured from the valve seat, its 2.25"

Those measurements aren't based on SBC chevy head unless you enter those specs. I believe they are a good starting point for the power range you say peak power is at for length, but as with everything else you can't just base facts on programs like that. It would require a few of us to make nearly identical sets minus diameter and length to dyno.

I dont know what flow numbers on the exhaust have to do with header design though.

[Streetrodder83]

It does factor into header design, but some of it has to with preparing the cylinder head. You want the the exhaust port to flow about 75% of the intake port on a NA engine. If it is flowing 70% the exhaust side needs alittle work, where as say the port is flowing 80% the intake needs working. Where as forced induction require a defferent flow bias, and what is needed depends on the application. For me since I am going to do turbos, I am very concerned with port flow the amount of flow dictates the power that can be created with it. When constructing a header you need one that is small enough to generate strong compression/suction waves, and that it is large enough to allow the exhaust gases to flow freely from the cylinder. Obviously you don't want to build a header that is going to be a restriction or one that isn't going to help draw the exhaust out of the tube or scavenge the cylinder properly. Knowing what you're working with allows you to tune your engine that much easier and better. By no means do I know everything as I am really learning a lot about this as I am designing mine.

[Anonymous]

That program isnt very accurate. For exhaust port length it only allows you to enter the number into a tenth, not hundredth. I also was playing with it and entered 300hp @ 5000rpm and it said the hp was unrealistically high. But if you enter it to 5800 (my peak hp) its fine. It also never says over 34" primaries. But it does say 1.51" primaries which agrees w/ Headers By Ed.

[SappySE107]

I know about pressure and sound waves and tuning primary length and diameter for powerband range...but no where in those formulas does exhaust flow have an impact. Also, I have see 70% 75& 80% 85%. I asked this before so Ill ask you, how did you come up with 75% as being ideal...for any application at all.

Headers for a turbo have 1 real goal...keep the pressure high. You will have too much backpressure under boost to really tune a set of headers much other than not too large which will drop pressure and cause more lag. It definetly doesnt follow the same principle as NA.

Length of primary pipe (in feet) P = (ASD^2)/1,400 d^2

A = exhaust valve opening period in degrees of crankshaft rotation
S = stroke length in inches
D = cylinder bore in inches
d = exhaust valve port diameter in inches

Thats just one formula that I wrote down onto the site from a book which has more math than I comprehend. Its also outdated so its used as a baseline like any other formula.

[Streetrodder83]

75% is a number given to me by several machinists, and one cylinder head specialist. None of them gave me a reason why, and I am not sure if any of them knew themselves. I think it was a number that was come upon quite a few years ago, and the reasoning has been lost along the way. Essentially the flow rate is a volume of air, gas, whatever. The diameter of the tubing has to be sufficient in handling the volume of spent, and so that you don't put too many bends or too tight of a radius of bend. Restrictions in the header are going to limit the volume of gas that can move through it at a given rate. But, having it too large will weaken the effect the pressure waves will have on each cylinder, so it has to be a balance between the two.

[SappySE107]

according to ed, the bends are less important than actually making the primaries equal length. interesting since that makes the flow if each pimary less important than the pressure ans sound wave pulse tuning. so again i say exhaust flow numbers mean little other than being a part of the factor for engine power output.