This is about Steve Dulcich's article in the Winter Engine Masters. I rebuild heads and do some porting. My question is about step headers. First, they work in some combinations, why, what is the reason for that ? It seems to me that there must be expansion going on in the primary, that is raw gas in the exhaust flow. If so, is there some point at which a certain amount of camshaft overlap would indicate step headers might be worth trying ? Is there a rule of thumb ?I have recommendations for regular headers, but there is growing interest in step headers. Thanks, Greg
Greg Step headers work on a large Cube Engines beit a Small Block or big block. You will need the cam specs. to see how long the overlap cycle is. It helps the bottom end for torque some. We ran them on the dyno some time ago and really for the gain a Long Tube Header works the same. If you have good flowing heads a long tube or TRY-Y's. Believe it or not. I ran a 402 BBC with a 3.5 exhaust header tube with a 20" extension. The header scavanged very well. It cut down on reversion because the exhaust pulses carried the exhaust gas out. It was only L88 cam. So a bigger cam will work even better. Exhaust Pulses are sound waves). I'm sure you know what the overlap cycle is. But for our younger gear heads I'll just repeat it. The overlap cycle is also known as the 5th cycle. Back in the day Ed Iskanderian named it the 5th cycle.The overlap cycle is when both the intake and exhaust valves are off their seats at the same time. The bigger the cam the more overlap. Every cam has overlap. Actually this is the time when the pistons will hit the valves if your Valve to Piston clearance was not set properly.In the overlap cycle because both valves are off their seat the exhaust will flow into the intake and contaminate the next intake cycle. If you look in the intake manifold, and you see a black sooty substance in there. That is the exhaust gas backing up into the intake. This is known as reversion. Back in the day and some guys still use them we installed tubes inside the exhaust tube of the header to help stop reversion.Greg I would use a big long tube header. Put an extension on the header. You can even use a down spout type extension. Lets see what some of the other guys think.Bob
check out headers by ed website, they give good tech on how to start off creating a tuned header exhaust. I remember the reversion cones, too the idea of the header is to keep hot air flowing out of the head, so it doesn't back up into the chamber. in a perfect world, the only expansion of gases should be expansion down the tailpipe. the less efficient the port, the more efficient the primary needs to be to create a drop in air pressure so the exhaust comes out to fill it in and stablize pressure in a system that, of course, has a hole at the end of it if overlap or a good three angle valve seat cut (which creates a little effect like overlap, in that the flow from when the valve is on the seat to, say, .03 inches of lift increases, so as the intake closes and the exhaust opens, both flow better) helps to clear out the chamber, rather than back gasses up the intake port, then the exhaust primary better keep up. if the flow is bad out the head, then a bad flowing exhaust may not penalize only b/c there's little left over to penalize after the head and cam choice has already screwed up the flow.
Just because you can't do it...doesn't mean its impossible