Modified Jennings Port Time-Area
Gordon Jennings presented formulas for
determining the peak rpm of the intake, transfers, and exhaust ports. (read full pdf) They
were originated by Yamaha engineers working with their road racers. The main idea behind them, which still holds true, is that to produce maximum power all the ports need to be targeting the same peak rpm. The formulas seemed to of been a good method for them to use but are not really
all-encompassing for other types of engines.
After a failure using the Jennings formula I decided to modify and
harmonize it with successful port arrangements. That is to say that if a
cylinder is "successful" of having a decent powerband and good power then
it must have port time-areas tuned for that powerband and that correct
formulas would agree that those time-areas indeed do match the power peak
rpm's it acheived.
Shortcomings of the Jennings
Its formula for the exhaust port does not take into
consideration how many degrees it has to let spent gases out before the
transfers open. In reality the only time that is important for it is that
"blowdown time". That is because if cylinder pressure is still substantial
when the transfers open then the entry of intake charge is delayed which
limits the dynamic time-area of the transfers which keeps engine rpms from
going any higher.
Its formula for the transfers and intake (for piston
port engines) only considers the open area for the first half of its
opening (by degrees). But in real life the whole port is "effective area"
as it is in full use when the piston has fully uncovered it. And if the
ports are not as wide at the bottom as they are at the top then the
formula misrepresents the true average area.
My modifications make up
for these shortcomings. For the exhaust only the blowdown time-area is
used. For piston port intakes the same Jennings mean time-area is used but
only in addition to 1/2 the area that is the most limiting (restrictive)
when the port is fully opened. If the carb throat diameter is smaller than
the fully open area of the intake port at the cylinder then half of that
area is what is used. If the intake manifold is smaller than the fully
open area of the intake port then half of that area is what is used. For
transfer ports 100% of the limiting area is used in the formula. If the
transfer channel opening at the base of the cylinder is smaller than the
transfer port opening then that is what is used.
Reed valved intakes
aren't considered because of their very long intake duration that makes up
for any lack of area. The engine tuner just needs to make sure there are
no obstructions to flow and that the piston windows are
"Blowdown area" refers to the port area in square centimeters (square mm/100) uncovered when the piston edge is at the top edge of the transfer port.
"Limiting area" is the smallest
area (in square cm) along charge flow path when the transfer port is fully open. (Only use the area of the transfer ports on one side of the cylinder.)
"Mean area" refers to the port
area in square centimeters uncovered when the piston is
half way through the first half of the port opening. If the intake port begins to open at 60°BTDC then the mean area is the uncovered area when the piston is at 30° (half way
between 60° and TDC). The transfer areas for the formula are the total
amounts of only one side of the cylinder. 1/2 the cross-sectional boost port
channel area when the piston is at BDC should be added.
refers to the total port duration of opening in degrees. If the transfers
open at 125°ATDC then their duration is 110° ((180-125)x2). If the intake opens at
60° then its duration is 120° (60x2).
is engine displacement in cc which can be calculated at
Peak Exhaust Port rpm=(blowdown area x duration)/(.0004 x
Peak Transfer Ports rpm=((limiting area)x
duration)/(.0004 x displacement)
Peak Intake Port rpm=((mean area + 1/2
limiting area)x duration)/(.00085 x displacement)
An easier way to figure the intake port rpm, if the port is very conventional (old style) in shape, is to just use the limiting area and use this formula: rpm=(limiting area x duration)/(.000455 x displacement). Use an online calculator to find the area of a known diameter of carburetor throat if it's area is the most limiting.
Refining the formulas for determining ports top rpm
I ported one of my 55cc cylinders to rev much higher, using my revised
formulas for determining peak rpm for each port. I ported it and tried
it out and got just a little higher rpm. I was shooting for 9500rpm but
only got around 7500. Looking at the porting the most obvious aspect of
it that was contrary to high peak rpms was the lack of blowdown degrees
(exhaust opening to transfer opening). A normal low for this type engine
is 30 degrees but mine was 23 degrees. Thinking about it on a long bus
ride I figured that any formula could only get away with using degrees
instead of the actual port open time if all the ports to be calculated
were to have nearly the same peak rpm. But that if you want the formula
to work on any peak rpm range that it has to use time and not degrees.
The original formula was formulated just in that way, to only be applied
to road racing engines. But port open time, with the same amount of
degrees, at 7500 rpm is 33% more than at 10,000 rpm. And I also figured
that the amount of pressure (from combustion or crankcase pressure)
should be part of the formula because higher pressures cause higher gas
exit speeds given the same size port. So the new formula for exhaust
ports takes into account the port height so that an opening at 80*ATDC
assumes a blowdown speed 30% faster than that of a port opening at
100*ATDC. That is because peak pressure occurs slightly after TDC and
lessens as the piston lowers (and expands the available space for the
hot gases). For the transfer ports I figure the crankcase peak pressure
into the formula so that twice the pressure gives 50% more intake
entrance speed. These are both only educated guesses that no one except
the best engineers at the best motorcycle factories know precisely, but
this attempt is better than not figuring them in at all. I am now in the
process of evaluating 4 different cylinders running at different peak
rpms and trying to tweak the formulas to match them all, to be