QuestionDear Sir, I have some questions for you to answer and hopefully,
you will be able to tell me the steps needed to acheive what I ask
for.
Is the compression ratio purely determined by the ECU or by the
pistons and con rods? Can I increase the compression ratio by
pushing more fuel and air into the compression chamber or do I
have to change the conrods and pistons? If I have to change the
pistons and con rods, wouldn't it change the capacity of my
engine? (note, question pertaining to N.A engines)
How can I change the powerband of an engine into a
narrower,higher RPM range?
Can the maximum RPM speed of an engine be changed at all?
Must the gear ratios be changed too the the maximum RPM
speed is tweaked higher?
What is engine blueprinting?
What is the disadvantages and advantages of using DOT 5 brake
fluid as opposed to 'normal' grade brake fluids?
Will a soft top car have more drag and less body rigidity
compared to a hard top one?
How does a different cam profile affect the engine performance?
As a general rule, how do a person judge which type of profiles
affect which area of performance gains?
AnswerJoseph-
Well...since you asked the question in an order, I'll answer them in that order.
An engine's compression is mostly determined by the piston/head/rod combination. You can change the pistons for more compression or you can buy heads with smaller combustion chambers to get the same effect. The dynamic compression could be changed by forcing more air and fuel into the cylinder (this is essentially what happens in forced induction motors), but you'd have to be careful to tune for the changes. It is, however, much easier to change the static compression (the comp. ratio when there isn't fuel and air in the cylinder) since it doesn't require quite as much tuning. Chaning this doesn't necessarily change the displacement of your motor, but when installing new pistons it's a good idea to go with a .030" overbore which will produce a slight displacement increase.
The powerband of an engine is fairly easy to change by simply swapping the cam. A cam with more duration and a lower lobe seperation angle will perform better in high rpm operation than one with less duration and higher lobe seperation angle. Things like your torque converter (in an automatic tranny setup) can also effect the powerband for a given motor.
Yes, the maximum rpm limit of an engine can be altered. Most of it has to do with the balancing of the internal reciprocating parts (pistons, rods, crank, etc). The better an assembly is balanced, the more rpms your motor can turn. Ironically, decreasing the engine's displacement with the proper crank/rod/piston setup can also allow the engine to rev higher. I'd seen purpose-built motors that started off as a 302 and ended up as a 260 that could turn up to 10,000rpm. You'll also have to take into account the camshaft profile to make sure it can provide air and fuel at higher rpms as well as the valvetrain so valve float isn't an issue up high. The gear ratios don't necessarily need to be changed, but a numerically higher gear ratio will allow the engine to reach it's powerband quicker.
Engine blueprinting is a practice that generally refers to having a builder take careful inventory of parts and make precise measurements of the build of the engine. Any engine worth buying will be "blueprinted", but aren't always advertised as such.
Using DOT5 is really only worth it in a car that sees significant hard braking. It's main benefits are a higher temperature tolerance and it's ability to withstand higher pressures than "normal" fluid.
A soft top car will not have any extra drag, but it will cause the car to have less rigidity than a hard top car. With the loss or the metal over the passenger area, you lose structural integrity since the metal there will keep the frame from deforming. A soft top car can be reinforced, but that adds weight and generally doesn't provide the same rigidity as a hard to model.
As for a cam's effect on the engine, it depends on how much you change the profile of the cam. Since the cam is essentially the brain of the engine, a small change in the profile would result in small change in the engine and a big change in profile would result in a big change in engine charatceristics. As for reading cams...there are so many groups of the it's impossible to cover them all. However, there are a few things that you can always pick out. A higher lobe seperation angle will mean a good idle and lower powerband while a lower angle will mean the opposite. An acceptable duration for a street cam is around 225 degrees ( @.050"). Any more and you'll start losing vacuum for things like brakes. For higher rpm applications, a split duration cam (one with more duration on the exhaust side) is preferable since leaving the exhaust valve open a bit longer will create an overlap when the intake and exhaust valves are both open creating a vacuum that will suck more air into the cylinder.
If there's a specific setup you're building for (road race, drag, etc.), let me know and I can be more specific. This is all pretty general stuff. Hope it helps.
Steve
