- Long-tube headers will improve torque.
- An increase in compression will improve torque.
- Increases in breathing (in the r.p.m. area of concern), improve torque.
- Camshaft choice can dictate at which r.p.m. torque will peak and can encourage torque production. They can also kill low r.p.m. torque in favor of higher horsepower- TO A POINT.
- More Ignition advance can create torque.
- Increases in displacement create torque (as can a longer than stock "stroker" crankshaft). *This is not an inexpensive option.
IF a "high peak horsepower" cam profile is used, it requires r.p.m. to produce the horsepower. Low r.p.m. torque drops off though, which is NOT really what you need in a heavy truck. So between torque and horsepower you have a see saw: Low-end torque can go up but higher r.p.m. horsepower will likely drop off and vice-versa. In a naturally aspirated engine, that is how it is.
Forced induction (turbos/superchargers) fill the cylinders early (producing torque) but they are at the mercy of the rest of the engine to see how much power can be produced at a higher r.p.m. IF you install a forced induction system on an engine that can also breathe, it will fly!
Look at Dom's truck: Striker-R cylinder heads (big breathers properly designed) PLUS forced induction= 180+ m.p.h. 1/4 mile times (with very few passes on it). That same forced induction setup with a stock set of Viper Gen III heads and the performance numbers would be CONSIDERABLY lower. Again, breathing is a huge factor for horsepower production.
I picked up 143 wheel horsepower by changing cylinder heads and a complimenting (mild) cam profile on my personal SRT-10. Unfortunately, the heads I am are harder to come by and are (arguably) expensive. It depends how badly you want to add 200 flywheel horsepower without using any power adders!! It also shows you how much difference breathing (and proper combustion) makes.
You don't hear about semis needing to shift at high r.p.m. in order to make their power as diesels build torque down low. They produce gobs of cylinder pressure at low r.p.m. (where it's needed) and not a great deal of horsepower, up top, in comparison.
Actually, there is no such thing as horsepower, only torque.
Reciprocating engines create a twisting force (torque).
That twisting force is created by cylinder pressure.
Horsepower is "Torque measured over time". Once torque starts to fall off as r.p.m. climbs, horsepower "takes over".
If the torque being generated can be carried into a relatively high r.p.m. (a function of breathing), horsepower increases will also follow.
Diesels are big on torque and less so in the horsepower arena because of breathing (or lack of it). Their whole induction setup relies on small sized components (valves and ports) to encourage early cylinder-filling. That small size also prevents big horsepower numbers because small can only do so much and eventually becomes a restriction to breathing, limiting horsepower.
Take a straw and blow: Lots of pressure out of the end.
Now, take a 2" pipe and do the same thing (almost no pressure out the other end) Same straw, same guy, same volume of air but a vastly different result. A crude example but it might help to illustrate the concept of an engine breathing.
Naturally aspirate engines rely on atmospheric pressure to fill the cylinder once the intake valve opens. Smaller sizes encourage velocity and early (low r.p.m.) cylinder filling which equals early (low r.p.m. torque production).
Continuing on the diesel example:
IF you spent a lot of money on diesel cylinder heads, horsepower can rise substantially but so does the r.p.m. required to obtain that horsepower. BUT diesels also have the burden of HEAVY internal engine components, and to build one properly takes considerable money.
Viper engines make good torque, fortunately for us. They don't need to rev overly high to produce power.
Man, there is a LOT to this and I'm just touching on very few highlights and some information may even seem contradictory because it's difficult to provide information in absolutes without looking at the bigger picture.
If you are changing flow through the engine (exhaust will do this), you should have it tuned.
Most of these guys will give you a tune based on your current setup and then send you others if you change more components (within reason). If you are planning some major mods, do those first then get a tune for that (on a dyno is best).
It have tried the guys on this site and my personal preference is Chris Jensen out of Los Angeles.
Guys here have also had good results from Torrie.
Hope some of this helps.
If you have questions, fire away.
Season's Greetings
Ronnie