I will tell you that IF you want to do a header like Calvin Elston builds, then you really need to start over with your entire program. That is because (this is as close as I can get to how he explains it but you can certainly go to speed talk and yellow bullet and search his stuff and read it so here goes…) almost all exhaust ports and exhaust valves are too big. I agree with that. So, if you can shrink the exhaust valve and port and then make the intake valve and port bigger, you can use a header like he builds (a tri-y design with…now I can’t think of it…a merge collector) and if you change you cam timing to match I believe there are some huge power gains to be made.
The problem is how many of us are going to do that? If I was younger and I was up for 40 plus hours of welding and all the R&D I’d do it. But I’m not young and I’m just not into it.
I will say this though. Cam timing, compression ratio and header cross section are very much intimately related. Trying to change one and not the others will not produce decent results.
I cant tell you how many times I’ve tested what is considered bigger diameter tube headers and not one single time have I lost power. Not once. BUT…I also picked the cam timing or the engine they were going on already had favorable cam timing. So what is “favorable” cam timing?? We have to talk about the heads.
All in-line wedge heads are deficient in overlap flow. And you NEED overlap flow. Without overlap flow, the engine will be what I call numb to almost any changes one might make. I know we’ve all seen engines like that. You can port the heads and the results suck. Change the intake manifold and nothing. Bigger headers and again nothing. BTDT. We also see it on dyno tests all the time. They also don’t respond to normal tuning changes. The engine is just numb.
When you see something like that, bet everything you have that the cam timing is wrong. It is the brain of the engine. And a numb brain means a numb engine.
Contrary to popular belief, lower compression engines usually need a bigger primary tube than the same engine with a larger primary tube. It’s all related.
So now we have a relatively high compression ratio and no overlap flow and you bolt on bigger headers and…nothing. And I’d bet when the big headers went on, you had to add jet. That’s your clue the header isn’t working. Big jets are SLOW.
Look at a Hemi. It has incredible overlap flow. So much so that you need to do things to reduce it or you throw power right out the pipes. The in-line wedge is the other end of the spectrum.
Speaking specifically about carb applications, you need to pull on the booster as hard as you can. Anything you can do to pull the booster harder will make the engine make more power.
The number one killer of overlap flow is wide LSA’s. It just kills overlap flow and makes the engine numb. The closer to correct you are on overlap, the more power you will make because you are using that overlap to pull on the booster. The same goes for head porting. If you port the heads and flow more, but it requires bigger jetting, you can almost bet it will lose power. You lost the pull on the booster. It seems counter intuitive but that’s how it works.
The current trend has been for years now to reduce seat to seat timing (not bad if you do it correctly) and then open the LSA up to extend the RPM range where they need it. Invariably that engine will be numb to tuning, header sizing and intake manifolding.
So the header gets blamed when the cam is the issue.
We just went from 1.625 to 1.875 headers on @lead69’s Dart. The engine was pretty close on tune before the headers. Just bolting on the bigger headers made the engine pig rich. And most would jump to the conclusion that the big header was killing it. In fact, the opposite was true.
The big header actually was pulling harder on the boosters and even on the idle circuit (which is exactly what you want the header to do) and made it pig rich.
I had to go up .008 on the idle air bleeds, but I really need to go in and reduce the idle feed restricters some and go back down in idle air bleed for drivability.
I went 7 sizes SMALLER on the primary jets and 8 sizes smaller on the secondary jets. It’s pretty close at a cruise but it’s still a bit rich at WOT.
And all that is because the compression, cam timing and header size are working at overlap to pull on the booster.
If you bolt on better header or a better intake and it does nothing or goes slower, look at your cam and what you have at overlap.
