Flapper Valve Mod
All Si and VTEC Preludes have a dual stage intake manifold, similar to what twin cam B16 and B18 motors have. Why the H23 Prelude motor has it is beyond me cuz the two stage manifold is usually part of the VTEC system. A normal manifold has 4 runners, one for each cylinder(on a 4cyl). The dual stage manifolds have 8 runners in the top two sections. At low rpm's, only 4 of the 8 runners are "open" and at a certain rpm, a set of flapper valves opens up to allow more airflow which results in more hp. Why don't they just have all 8 runners open full time? Simple, too much air will result in a low end power loss. So until ~4800rpm you have 4 runners open, then at 4800 a set of flapper valves opens and you have 8 runners all the way to redline. The runners have a diaphragm that controls the opening and closing. The diaphragm has a vacuum line running to it and at 4800 where there is enough vacuum pressure, the diaphragm opens the runners. Ever notice how your Si at around 4800 drastically changes sound and it sounds like the car is going to explode? Well that changeover in sound is the manifold opening up.
This is what the middle section of the manifold looks like with the flapper valves. The silver diaphragm on the left side is what controls them. Until 4800 they are closed, but after 4800 they open up as shown in the picture. All the small piping and fittings are for Nitrous so don't look at those.
So what's the mod? Simple, if you want more power sooner, unplug the vacuum line that connects to the diaphragm. Now the flapper valves will always be open at all rpm's. Yes, you will lose a little bit of low end power, but you will gain power sooner(above 3700) as a result of them being open. 4800 is a good rpm, but it's possible to take advantage of the 8 runner design before 4800 and by unplugging the vacuum tube, that's what you're doing. Since racing always takes place above 3000 or so rpm, you won't have to worry about losing low end power. After all, why else would you want your car to be faster if you're not racing it?? ;)
This is where the flapper valve is located on the engine. You can see it's between the brake master cylinder and the left side of the upper intake manifold(attached to the manifold actually). There is a line connected to the top of it, just unplug it so it hangs or cap it off. I have anal retentive people asking me all the time if stuff will get into the vacuum tube if it isn't capped off. Let's be realistic, the opening is smaller than 1/8" wide, do you think that with the hood shut, something will bounce around in the engine bay and land inside the tube which is tucked under the manifold?? Hell no. Unless your car sinks to the bottom of a deep lake, you're not going to get any water or dirt in there. Besides, that tube BLOWS are out when the engine is running, how is something going to get sucked inside there if there's 40lbs of air pressure blowing out of it constantly?