OK, if there has one thing that being a forum surfer has taught me over my brief tenure of being such, is there are a lot of things that affect, generate, hinder and enhance engine performance that are not understood or simply just ignored.
Most of this stuff is simply just not shared, be it that they are considered speed secrets or the lay person simply does not understand it, whatever.
I wanted to start a thread to discuss some factors that affect engine performance and the parameters that performance engines live within. Read, argue, whine or cry, this is the truth. And as Jack Nicholson said in "A Few Good Men"- "you can't handle the truth". Well, maybe you can, let's find out.
Get your calculators ready, over the next few weeks, you will need them.
Now, with all of that out of the way, let's start by nailing down the first thing we need to consider when wanting to build a new engine and get the maximum performance out of it. Knowing what we want to use it for will deliver the best possible results. Building a high strung engine to take the family to picnics on Sundays will probably not deliver the experience you are after. Well, maybe the experience you are after, but probably not everyone else in the car.
The best place to build a starting point is to determine intended purpose of our engine and then correlate this to a range of piston speeds.
Piston speed will have a crazy-huge effect on how the engine performs.
I am going to break this down into different categories. For those of you playing along at home, I will give you several examples to experiment with, I am not going to lead you down a primrose path to a result that I have predetermined. Your rules, build what you want without spending a dime.
OK, the categories are:
Street engines (stock)
Mild performance or street/strip
Endurance/NASCAR/Road Race
Maximum effort (Mileage limited/drag race)
AYFKM effort (Component failure limited)
Now, the classification for each of these categories are split by how fast we fling the piston. But first, I guess we need to know how to determine piston speed so we can then determine which category we belong in, or vice versa.
The formula for Mean Piston Speed (MPS) is MPS in ft per minute=(inches of crankshaft stroke x (engine RPM/6))
Depending on the stroke of the crankshaft, and the RPM you expect to see, we can categorize our engine. This will affect our pocketbook, because it will directly affect the type and quality of parts we need to buy to support our performance objectives.
Now let's put these categories in a manner where piston speed can help us determine what we are building. Or, what we need to build to fit into a certain performance category.
Street engines (stock) 2000 to 3000 ft/min
Mild performance or street/strip 3000 to 4000 ft/min
Endurance/NASCAR/Road Race 4000 to 5000 ft/min
Maximum effort (Mileage limited/pro class drag race) 5000 to 6000 ft/min
AYFKM effort (Component failure limited) 6000 to 7500+ ft/min
OK, so if you tried a few different combinations, you probably realize we can change the category by solely changing the stroke, changing our RPM, or both. So great, wow, what a speed secret. Thanks a whole pantload Parker, I read through all of this for that? You're an asshole. Yeah, I know I am, thanks. But the thing that we need to understand about the formula is you need to understand engine speed (RPM) is power. Let's take a look at two theoretical engines and another formula, this one you have probably seen.
Horsepower = (Torque x RPM)/5252
By solely changing RPM, I can make one of these engines a grocery getter and the other, an F1 contender.
Two little six cylinders; both make an eyeball popping 246 pound feet of torque :lol
In the first one, my maximum usable RPM is 4500, and when plugged into the above formula (go ahead and do it, doofus), we see the engine is capable of making about 210 horsepower. :facepalm:
Now we will leave everything the same as far as torque, but let's give the engine an F1 race car caliber RPM of 19,250. After you do the math (I don't have to call you doofus this time, do I?) we see that though my torque stayed 246 lb/ft, my horsepower climbed to a panty-dropping 910! :willy
RPM is power folks, RPM is the shit. RPM can also stand for "Ruins People's Motors" if you don't know how to handle your newfound knowledge.
Piston action has a dramatic effect in the way air and fuel enter, and exhaust leaves the engine. Going back to our categories, we are going to relate the effectiveness of this piston action with our engine and piston speed categories. We do this using something called Volumetric Efficiency, or VE. VE simply put is; if I have a 100 cubic inch engine, and that engine can breathe in and expel 100 cubic inches of air for every camshaft (two crankshaft) revolution(s), it is 100 percent volumetric efficient. If the design of the heads, intake and exhaust only allow 85 cubic inches of airflow through the engine, it is 85% volumetric efficient. If it can move 110 cubic inches of air flow, then it is 110% volumetric efficient. Are engines over 100% VE possible? Why yes, yes they are, and I am going to show you how to build one. Wow, a 350 cubic inch engine that can move 385 cubic inches worth of airflow? Oh yes, yes indeed.
But that will wait until the next installment, for now, let this digest.
Street engines (stock) 2000 to 3000 ft/min 80-90% VE
Mild performance or street/strip 3000 to 4000 ft/min 90-110% VE
Endurance/NASCAR/Road Race 4000 to 5000 ft/min 110-120% VE
Maximum effort (Mileage limited/pro class drag race) 5000 to 6000 ft/min 120 to 127% VE
AYFKM effort (Component failure limited) 6000 to 7500+ ft/min 125 to 128% VE
So we have learned that how fast the piston moves has a great affect on the performance I can expect. And we also learned that piston speed is primarily controlled by the stroke of the crankshaft and the RPM of the engine. We also see how changing piston speed makes my engine fall into different performance categories.
Next time I will give you some real world examples I have tuned on the engine dyno and tested on the drag strip, as well as begin to talk about hw we get the air into the engine to take advantage of all of this piston speed we just talked about.
Cheers, Parker
