Header Power

It is often said that a header is the beast bang for the buck among all bolt- ons. Along with the Place Racing CAI, the Hotshot header is one of the most popular modifications on an SR20DE engine. On dynos the results have varied from 5-15 horsepower depending on other modifications on the car. One question that has not been answered to my satisfaction is how a header generates power? After reading several articles and several posts on the SE-R mailing list, this is what I have learned.

The first difference that I noticed between the stock header on my SE-R and the HS header is theway the primaries of the stock header are bent at almost a right angle. The image to the left shows this. This design does not allow for the smooth expulsion of exhaust gases. Hotshot header primaries, on the other hand, are bent at a much wider angle. This allows for a the expulsion of the exhaust gases with less turbulence.Also note the difference in the primary runners between the Hotshot and stock headers. In the stock ones the runners are short and convoluted while the Hotshot runners are straight and longer. Again, this allows for smoother flow of the exhaust gases and more power generation.

Both the Hotshot and stock primary runners pair the number 1 and number 4 and the number 2 and 3 runners before they enter the secondary pipe. The pairing is crucial since these runners correspondto cylinders that are in opposite firing order. As Mike Kojima puts it..."Adjacent cylinders in the firing order are kept separate so the exiting pulse of one cylinder cannot contaminate the next cylinder that maybe on the overlap part of the power stroke." This is crucial in avoiding turbulence in the primary runners. As you can see the Hotshot runners connect to each other in a straight and non-convoluted way before entering the secondary. This smoothes the flow of exhaust gases and reduces turbulence in the runners.

Header design should also take advantage of the overlap period (the period when both the exhaust and intake valves are open when the cylinder is at or approaching TDC). When combustion takes place within the combustion chamber the cylinder is pushed all the way back to the bottom of the chamber. On the way back up the exhaust valves are open and as they begin to close are followed by the intake valves opening. This is a crucial moment for a properly designed header. A good header must be designed to not only evacuate as much of the exhaust gases from the cylinder, but to create vacuum behind the exhaust gases that will suck fuel and air mixture into the cylinder when both the exhaust and intake valves are open. It is here that the length and the diameter of the primary pipes are crucial. A well designed primary will accelerate the evacuation of the hot high-speed high-pressure gases from the cylinder. These gases will generate a low- pressure pulse immediately behind them. A properly designed header will take advantage of the low pressure pulse to not only suck out most of the burnt gas out of the cylinder but also to suck in fresh fuel and air mixtures.

I am sure that there are other ways that the header generates power but this is the best summary as I (a lay person) understand it. I hope it helps.

Installation

Hotshot headers come without any instructions. If you want instructions the best one are on SE-R.net. These header install notes are excellent. Just make sure that you follow all of them. I made the mistake of not using Permatex Copper Goo on the secondary copper gasket and it kept leaking. You must use the goo along with the copper gasket to avoid any leaks. Also make sure to re-torque the headers every now and then.

This is a top image of my Hotshot header. Note that you cannot install the old heat shield on the new header. This might be a problem in passing the visual smog check here in California. To the right is a close up image of the oxygen sensor. The Hotshot header fits all SR20DE equipped cars. As such it has fittings for the oxygen sensor, the EGR valve and the AIV valve. The AIV hole is for the 91-94 SE-Rs. This hole is plugged for the 1995-98 SE-Rs and the 1998+ Sentra SEs.

Since I installed the headers I have registered the following impressions:

The noise level in my car has increased. The tinny noise that was caused by the CAI is now more pronounced. But at idle unless your really close to the engine compartment you cannot tell the difference. At WOT it is a different story. The CAI and the header work off each other to produce the most beautiful symphony.

Most of the power of the header comes at top end. Below 4000 rpm there is some change but not that much. Above 4000 rpm is when the real magic comes into play. The car seems to lunge forward and reaches redline very quickly. This is in line with Sport Compact Car's findings. Between 2200-4300 rpm the average horse power and torque gain was 0.1. Between 4400-7400 the average horse power gain was 3.1 hp and torque gain was 2.6 lb-ft.