Cams with no duration and nearly no overlap will always leave some of the spent gasses in the chamber.

This will scavage the chamber of most all of the spent fuel, and even draw some of the fresh air/fuel mixture through the chamber into the exhaust stream. This crap gets totally out of hand when you start theorizing scavanging effects of 'overlap' (being when the exhaust valve is open at the same time the intake valve starts to open). So the trick is to find a lift that takes advantage of the port design and flow capabilities, and match it with a duration that compliments the cylinder pressure and rpm range for the particular engine and it's use. Increasing lift on a very restrictive port like this can cause crazy pressure drops and pulses in the exhaust that can actually impeed flow even more. The port, no matter how big of a valve, or how much it is open, can just lay over and simply won't flow more. Then too you have what we call a 'knee' in the flow. So we increase the duration the valve is open, AND increase lift with the hopes it will flow the magic number. If the exhaust port is a big restriction, opening the exhaust valve wider generally doesn't help much, some, but not much. You can increase lift on the exhaust side in hopes to bring the exhaust flow up to 80cfm, or you can increase duration of the exhaust valve opening. You can do one of two things to bandaid things. Let's say your exhaust port is lazy and only flows 65cfm at. Good performance could be expected if the exhaust port flowed 80cfm. Just for simplistic numbers, let's say you have a intake port that flows 100cfm. That's why forced induction like blowers and turbos make such a DRAMATIC increase of power with only 5 and 6psi of boost. There is a BIG difference between the vacuum intake side, and the forced exhaust stroke. Just keep in mind the intake charge is drawn into the clinder by the pressure drop (vacuum) created as the piston decends the bore. The exhaust can be less because the exhaust is forced from the chamber by the piston rising in the cylinder. You want the exhaust to flow 80% of the CFM number the intake will flow. There a lot of things involved in intake and exhaust flow, but the magic number is around 78-80% (depending on which engine builder you takl to). What would happen if you used the 1.5 rockers on the exhaust and the 1.6 on the intake?Īnswer Courtsey of our own moderator Marv Davis So if this is the case, here are some hair brained thoughts on the idea. I read in the above mentioned thread that you have to 'subtract' the 'gain' from the exhaust side. Question: why do folks not add the 1.6 rockers to existing motors to increase cam lift?

350 small block oovdoo cam