Exhaust
the 2 1/4" exhaust system will scavenge the combustion chamber not only faster but more complete, doing this greatly reduces the chances of contaminating the fresh fuel mixture entering into the combustion chamber.
Um I don't think so.
>only the headers scavenge and only when operating at or near zero back-pressure.
The exhaust system into which the headers work, at whatever size they might be, will immediately affect scavenging.
>Scavenging is happening during the overlap period, when both valves are open just a small amount. If the header is working right, then;
at low rpm it will be trying to yank the Fresh fuel charge out of the plenum and straight across the piston, and into the primary pipe. And it will succeed, if the headpipes are adequate.
This is normal at low rpm, and is what helps to make hi-rpm power. If you don't need hi-rpm power, then you can either; give up the headers, or buy a different cam.
The step header is a good solution for a street car.
> Reversion is a function of when the intake valves close. During "reversion" the A/F charge is always polluted. Reversion ends about the time that the vacuum peaks. For typical street cams, this will be between 1800 and 2200. It is best to chose a cruise rpm that is above this rpm.
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IMO, the 3inch front pipes on my car serve a second function. They have 33% more heat-radiating area per foot than 2.25s. Those pipes are about 5 or so ft long, before they enter the mufflers. This helps the exhaust cool before it gets there, where the honking big 3"in/3"out 3-pass muffs cool it even more. Cooler exhaust is denser exhaust, occupying less space, and it is slowing down. It loses it's harsh tone and at Part Throttle, trickles out the 3" tailpipes nonchalantly....lol. The turn-downs finish the job, directing the sound downwards; and nobody complains about my exhaust volume.
At full-song, while the headers are doing their thing, it does get louder; but I have never been pulled over for a loud exhaust.
>I think of it this way;
>on every two revolutions, my engine has the capacity to fire, at WOT, 367 cubic inches of very hot exhaust, times whatever the expansion ratio might be, into the dual 3"system. At say 5:1, that is ~1.1 cubic foot@ 2rpm .... At 5000 rpm, and 90%VE, that maths to ~2400 cubic feet of expanded gasses, per minute. My engine has to cram all of that into a total of 14 square inches of dual 3" pipes.
>Or how about this; My throttle blade area might total 8.95 square inches. And they are passing air at ambient temp, and at 1 atmosphere. The exhaust at 14sq inches, is about 56% bigger.
Your pair of 2.25 headpipes total 7.95 square inches; already smaller than the 4bbl; and that air is now about 5 times expanded. So your pistons are gonna have to pump some to all of that hot air down to the end. That is work that could have been used to propel the vehicle.
I'm not saying that you have to have dual 3s.
What I am saying, is what my thinking was that led to my car having dual 3s. All-in-all, I gotta say, the dual 3" mandrel-bent system is the best that I have ever had. I got no complaints.
