An internal combustion engine must be viewed as a dynamic, living, breathing, self driven air pump.
In order for an engine to perform up to its potential, all aspects of the combination
must be thoroughly thought out and calculated for best results. From the air cleaner to the muffler, each and every component
of the build needs to be in sink and matched with each other for the desired rpm peaks and total output. If just one part
is off, it creates a "choke" or "mismatch" and kills off the true potential of the engine.
Velocity: One of the most used terms in engine building. While it can be very useful, it can also be the death
of an engine as well. A much better way to look at port sizing is plain and simple "air speed". Too little and the
engine feels lazy below torque peak. Too much and the engine will go into choke when the air speeds get too high. Air speeds
through your ports are calculated through displacement, rpm, and the cross section of your ports. For the engine to maximize
its potential, air speeds must be managed through proper port sizing. Not too fast and not too slow.
Torque: You will often hear "who cares how much horsepower, I just want a lot of torque". This statement
cannot be further from the truth. Torque is a function of displacement, compression ratio, and volumetric efficiency. While
making a lot of torque is paramount in any great build, it must be able to carry the torque into the rpm in order to make
some big horsepower. Lots of low speed torque is great for pulling off corners or leaving a stop light, but once any engine
speed gets involved, horsepower will win every time. Every acceleration/ET calculator in the world is based on weight and
horsepower. Torque without RPM has no capability to do work. And its ability to DO work is directly proportional to the RPM
at which it produces it. Do not be mistaken, Atwood Performance does believe in making big torque, as it makes for big
horsepower. Just remember, 10 ft/lbs at 2500 rpm equates to 4.76 hp, while at 5500 rpm, that same 10 ft/lbs produces 10.47
hp. This is why down shifting allows the bike to accelerate faster.
Horsepower is what wins races, period. If you like leaving your buddies eating your exhaust on strait aways, you must have
more horsepower or a lighter bike. Take your pick. All the torque down low is great, but if your engine can't rev (breath),
you can't make the horsepower. Producing horsepower is DIRECTLY proportional to how much air/cfm your
engine can flow through it. This is fact.
Compression Ratio: The static compression ration (actual) is chosen
mainly on two parameters. Camshaft duration and fuel octane. Please consult with Atwood Performance about proper static compression
and dynamic compression ratio as well.
Volumetric Efficiency: VE is a measurement
of cylinder filling. It is a percentage number that compares the actual volume/pressure in your cylinder at intake valve close
as compared to the ambient atmosphere that surrounds it. 100% would mean complete filling of the cylinder. Some engines produce
VE's in excess of 100%. Generally the higher performance of the engine, the higher the VE. Great cylinder heads have a
large part in producing high VE's. This, in effect, produces more tq, and at the end of the equation, means more hp, faster
acceleration and a more efficient engine.
The combination: The total combination
is what makes for a great build. Getting the big three right is a great start. Heads, cam, and compression. Once displacement
is chosen (the bigger the better for street), heads cam and compression come next. Getting these right is an absolute. Atwood
Performance actually prefers the customer to hold off with the cam(s) purchase until a thorough discussion has taken place
in order for the cam timing to be correct. Atwood Performance has a very strong understanding of cam timing and its influence
on the engine's tq curve and running characteristics.
This dyno chart
is very interesting. It is a simple before and after of the same engine about a week apart. Only change is the cylinder heads.
They went from stock (99 castings) to the RW 1900 set up. Notice the hp increase as the rpm climbs. This is a very mild build.
Andrews 26 cams, 9.4:1 CR. The improved tq is due to massive increases in Volumetric Efficiency.