| Primary tube math |
Subject: Primary tube math by anesthes on 2008/8/6 16:40:41 I was going to post this on CF, but I don't want to get 12 responses from guys who said "xxx brand worked for me" yet did no research before their purchase. Heads are trickflow 23. They flow 177cfm at .500 lift, and 183cfm at .550 lift (exhaust). I'm running a .528 exhaust lift cam, so say around 180cfm at my running lift. Exhaust ports are "D" shape, and use a Felpro 1404 gasket. (1.5 x 1.5") Looking at headers, options are Hedman 1 5/8 coated for $411 to my door or Hooker 1 3/4" coated for $710 to my door. Hedmans are 14 gauge and have an inside diameter of 1.459" Hookers are 18 gauge and have an inside diameter of 1.642" Both headers are coated on the outside only, not on the inside. The Hedman headers, considering the following formula of 80CFM per square inch, and 15 gauge will flow 133.74 CFM. 1 5/8" - .166 = 1.459 ID Area = 1.6718621478590436 * 80 cfm = 133.74 CFM. The hooker headers using the same formula flow 171.47 CFM. 1 3/4" - .098 = 1.652 ID. Area = 2.1434332693206293 * 80 cfm = 171.47 CFM. I made a math error previously, so this post is corrected. It appears from these numbers that, a 1 5/8" header will support a stock application. However pretty much any aftermarket head flows 160+ CFM.. It seems as if, for good aftermarket heads you really want at least a 1 13/16" header. I'm not sure such a thing is available for C4's, but it makes sense why a lot of C5 guys are running 1 7/8" headers. Now the wrench thrown into this mix is forced induction. I'm running between 6-8psi of boost. My camshaft has 2* of overlap at .050". So I think we can throw out any sort of scavaging since the intake is pressurized. The question is, will the spent charge exceed the 183cfm of the cylinder head thus requiring a larger primary tube? Pretty much all primary sizing math appears to be based on naturally aspirated setups, especially setups with larger camshafts with increased duration. Thoughts? -- Joe |
