IMHO, there's at least two problems here:digger wrote: ↑Wed Oct 25, 2017 2:54 ami've seen tests where a bigger primary makes more power than a smaller primary that would be classified as being a "more correct" size, translating to if you have a header that is considered too large then maybe making it the smaller size in itself might not produce much benefit on the dyno.
So does this inherently mean that something about the combination is "off" that you need alot more primary pipe than what you might get away with on a fully developed engine? what have people seen as steps that they needed to also make to get the smaller pipe to work for them? of course there will be no single answer as it depends where you start from....
1. How are you defining "correct" size? -This is a huge part of the issue. Are you sizing the initial pipe diameter based upon the area of the heads' exhaust port? The Valve? The HP level? --The different methods applied to different heads and engine combos will end up with different answers to "correct"; some of these old models are proving to be wrong, and some are proving to be right. I think basing the "correct" size upon the exhaust port area is probably destined to fail, unless you have a head with a "correct" sized exhaust port area to start with. -Should we instead base the correct header primary starting pipe diameter upon the valve or the HP or the view of both?!?? How SHOULD we define whether a head's exhaust port is "correctly sized?!?" (valve area to exhaust port area???) -I'd love a good answer to either of those questions, for sure.
2. Much of the old-school, stereotypical header primary diameter recommendations are based upon the assumption that you have a single pipe diameter (non-stepped design) for the entire length of the primary; many of the pros providing headers to pro racing teams are using stepped designs. Even the magazines have plenty of dyno evidence that to get max power potential out of an engine, you don't just start at the pipe diameter that you'd use if you were using a non-stepped design and then work your way up. As far as I can tell part of the benefit of going with a stepped design is that you can start off with a SMALLER initial pipe diameter at the head to improve velocity/blowdown length / scavenging AND maybe even gain a little bit of anti-reversion benefit for the first 7-12" of the primary pipes and THEN step up 1 or two more times and you get the torque and scavenging benefits of the smaller diameter pipe, and the HP benefits of the large pipe.
If I copied/pasted correctly, a direct quote from Calvin Elston on this subject: "If you hold a tube size for more than 10"-12", ie not stepping, you are giving up top end power". Note: This seems to line-up very well with the guidance from Larry way back at the beginning of this thread in terms of placement of the anti-reversion chamber in the primary pipes ("within 12 inches of the head"), and those devices also perform the step-up to the next diameter. Note2: Over email John Grudynski @ HyTech also stated that the anti-reversion step-up devices are most effective about 9" off of the head.
-Isn't it time more of this tech "trickled down" into affordable street headers? I certainly think so because maintaining low-to-mid range torque, a broad powerband, and decent fuel economy with a performance-oriented cam is so important in the street space. I'm honestly shocked and borderline appalled that this technology and know-how seems to be limited to the $4,000+ exhaust system racing team domain still. (Calvin started doing this stuff in the 90s and it still isn't available in the street perf bolt-on market?!?!?) I really hope if more consumers start asking for it, it'll finally trickle down... Or maybe it will take the modern models being democratized by being loaded into something like PipeMax v4 or Burns Stainless releasing X-design to enthusiasts to really drive awareness to the influential enthusiasts...???
Here's Calvin's guidance on "off-a-port tubing diameters and power levels supported" (the language of "off-a-port", seems to be intentional and hinting at stepped header design, and I'm of course assuming an "ideal" end-to-end designed header) -If 590 HP out of 1 5/8" tubing (outside diamter & off-the-port) is possible, the old-school guidance is just terribly outdated if we're talking about custom or DIY headers or even headers produced with modern understandings of the science, at all in the conversation:
- 1.5″ OD 18ga tubing off a port can handle 430hp in broad powerband, (BP) and 475 in tight powerband, (TP)
1.63″ tubing is good to 550 in BP and 590in TP.
1.75″ tubing is good to 700 in BP and 780 in TP.
1.88″ tubing is good to 875 in BP and 1000 in TP
2.00″ tubing is good to 1000 in BP and 1200 in TP
The header primary pipe diameter discussion seems to always go back to a "tyranny of "OR" situation (either you can go with a smaller primary diameter and get more torque, but limit upper HP, OR you can go with a larger primary diameter and sacrifice lower-end torque and gain upper RPM HP), but it can definitely be an "opportunity of "AND"" situation (you can start with a smaller primary and keep low end torque while also supporting higher RPM HP capability with a properly designed stepped header), if we maintain the complexity in the conversation that comes from the real option of stepped header designs.
-From what I'm gathering from the likes of Elston Headers & Burns Stainless, is that roughly the same situation exists with Tri-Y header designs: although people have long been framing the "Tri-Y" vs. "4:1" trade-off as one of low end torque vs. high RPM HP, superior understanding of the science, allows a properly designed Tri-Y (for the application) to balance things like the collector size / choke and pipe lengths to make this an "opportunity of AND" situation, too.