Calvin's post talks about an engine I'm intimiately familiar with.exhausted wrote:Ok, Erland, as I would deal with all customers, do you know the area of your exhaust port at the flange? If this 4 banger makes about 220hp? then primary size should be no bigger than 1.5" OD or 38mm OD with a .050" wall. That is all you need max. So right off the bat I would tell you the header is too big and if the port is that big then I would recomend starting there, ... why is it so big, (probably from the manufacturer,,sigh), do the rules allow making port smaller and would you consider that, and how big is the valve and can it be made smaller? and would that allow a larger intake?
1.75 primary can handle 400 hp so I don't need to go much further to tell you probably the only thing you are ever going to get your header to change anything is the overall length. Almost any motor will respond to that.
I just recently did a Triumph TR4 2.1, (or maybe 2.2?) litre tractor motor, made 194+ wheel hp and the header was 1.375" off the head, which was actually 2.5% smaller in area than the port. Going from his old header of 1.625 off head. Engine went from 180 to 195 hp.The choke was 2.0" with a taper out to 2.5 but after 8" the tailpipe went back down to 2.25 for the rest of the car. It is a 4into1 header with AR valves in the primaries about 10 inches out. If we had done a 421 the choke would have been even smaller. 1.75" Secondary would be getting too big but there is a much larger area difference in smaller tubing going from one gauge to the next than in larger tubing. I would probably be inclined to step the secondary from 1.63 to 1.75 and 2.0" max choke, or stay at 1.63 with a 1.88 choke.
It can be hard to imagine this but that is an example of an engine that was dealing with contaminated reversion the entire power band. If I remember it topped out at 8k but usually only run to 7500. The big header could not keep the burnt gases away from the cylinders. If you hear what I am saying, we did not make this engine pull close to 10% more air for the entire power curve with the header, the header was able to keep burnt gases away from the cylinders and we had more available oxygen to make more power the entire power curve. It is amazing and he is very happy.
Can we agree that if you have a beautiful depression across overlap from a "tuned" header, that does not mean that the chamber is pulling from only the intake.
The total lengths are effectively the same and the collector is the same. The only change is diameters change, the construction itself (bend radii, etc) and the A/R chambers. The engine also required a full main jet less and an air corrector change. It was pulling on the booster MUCH harder.