quote:
Originally posted by Bosswrench:
... shouldn't aftermarket heads with so-called 'corrected' exhaust ports be able to use symmetric cam durations for intake & exhausts? Or do you think this is a matter of 'one-size-fits-all' with regard to cam grinders ...
It’s not the performance of the exhaust port that defines the need for the exhaust lobe’s duration, the duration of the exhaust lobe is defined by the valve events. The opening and closing of the exhaust valve in a street engine is influenced by three factors: (1) the requisite to exhaust into a muffled exhaust system, (2) the requisite for high rpm performance extending to 6500 rpm or higher, AND (3) the requisite of satisfactory engine performance at low rpm.
All of the “important” exhaust action MUST occur during the first 100° to 110° of crankshaft rotation after the exhaust valve opens; any duration beyond that shouldn’t accomplish much if the exhaust gases have been properly scavenged from the cylinder by the time the piston begins moving upward during the exhaust stroke (this occurs at approximately 30° ABDC). It’s important to conceive that the “bulk” of exhaust gas scavenging occurs during the last half of the power stroke and during the piston’s dwell period at BDC! Properly scavenging the exhaust gases while the piston is moving downward during the power stroke requires opening the exhaust valve early enough that there is some pressure in the cylinder to overcome the exhaust system’s back pressure. If the pressure in the cylinder is too low, then the piston’s downward motion could draw exhaust gases from the exhaust system into the cylinder, which is just the opposite of what we want to achieve.
I have good luck with most applications getting 351C street engines to rev freely to 7000 rpm by opening the exhaust valve around 80° BBDC, these are engines equipped with headers and decent mufflers. But in most applications if the exhaust valve is opened just 5° later, around 75° BBDC, the engine will tend to lose steam at 6000 rpm or earlier.
Piston motion provides the energy during the overlap period that causes gases to flow in directions we don't want them to flow. There is a window however at top dead center in which piston motion virtually stops, it’s called the "dwell period". I've learned empirically that in regards to the 351C this window is about 60° wide, extending 30° on either side of TDC. So the exhaust valve should close by 30° ATDC in street applications where low rpm power and drivability are desired characteristics.
Opening the exhaust valve at 80° BBDC and closing it no later than 30° ATDC encompasses 290° duration, with an exhaust lobe centerline of 115° BTDC. That’s why the exhaust lobe tends to have about 285° to 290° advertised duration (measured at 0.006), which in the case of street cams is often more duration than the intake lobe. A 351C with 4V heads will tend to make peak BHP at 6000 rpm and doesn't need a long duration intake lobe. Small port heads and stroker engines will tend to make peak BHP at lower rpm, and will require a longer duration intake lobe in order to raise the rpm at which peak BHP occurs.