Try to keep any overbore to a minimum to preserve cylinder-wall integrity, and investing in sonic checking isn't a bad idea if you have doubts about the thickness of the cylinder walls. Two-bolt mains are adequate on the street, although main studs are a wise addition. If four-bolt main blocks are available for your application, they're advisable, and for more than 10 pounds of boost, they should be added to any block if it's feasible.
Cast crankshafts will suffice in low-boost, low-rpm situations, but forged cranks are generally preferred when boost goes much past 8 pounds or the engine will exceed 6,000 rpm.
When building a blown engine with boost 6 pounds or higher, performance connecting rods ar
Most factory steel rods are good to 8 to 10 pounds of boost, but as a precaution, they should be fitted with aftermarket bolts. However, keep in mind many aftermarket rods are now available for not much more that the cost of rebuilding a set of stockers. Full-floating piston pins are recommended.
While factory cast pistons will work at low-boost levels, forged pistons are a better choice due to their strength and the ability to handle elevated combustion temperatures (which happens with more mixture in the cylinders). An alternative with moderately boosted engines are hypereutectic pistons; they split the difference between cast and forged pistons in both price and strength categories. But the fact is if you're building a blown motor the pistons aren't the place to try and save money.
Because the blower is doing the work in terms of filling the cylinders, the heads don't normally require significant modifications for low boost levels. Moderate boost levels call for ports and bowls to be cleaned up and large ports and valves will be of benefit with big boost numbers.
The real difference in cams for blower motors has to do with lobe centers, or the separation between the intake and exhaust lobes. Wide-lobe centers, 112 to 114 degrees, spread the valve events apart, which means less overlap (during the period when the exhaust valve is not quite closed and the intake is just beginning to open, so both valves are in fact open) and results in more cylinder pressure. Of course, that's just another way of saying there's more stuff in the cylinder to burn, and that makes more heat. So what happens is this: Wider lobe centers equal more power and more heat; closer lobe centers equal less power, but a cooler-running engine with less chance of detonation.
Another area of cam design for blown engines that is often discussed is duration. Many engine builders favor a split-pattern cam, which is another way of saying a cam with more duration on one valve event than the other. For blown engines, there is often more duration on the exhaust because the blower is doing the work filling the cylinders on the intake stroke more effectively, so more duration on the exhaust just gives the piston more time to clear the spent gasses from the cylinder.
Dave Rios opted for twin Garrett turbochargers feeding electronic fuel injection on this 4
Finally, a blower has a tendency to calm a radical cam, so you can get away with more duration on the street than you could with a naturally aspirated engine. The bottom line is a blower makes up for a lot of an engine's shortcomings and even a relatively stock cam will work; more lift and longer duration just give the blower more opportunity to fill the cylinders.
Vibration Dampers Versus Hubs
A vibration damper does just that: It cancels out the harmonics that occur in the crankshaft as the power produced in the cylinders is applied to it. In most cases, the vibration damper also mounts a pulley that drives the engine's accessories, such as the water pump, alternator, etc. The problem is that most stock vibration dampers aren't strong enough to withstand the stress of a Roots-style blower.
One way to deal with that issue is to replace the vibration damper with a solid steel hub. In most cases, the blower and belt tend to do the same thing as a vibration damper, just not as well. We prefer to use a vibration damper that is designed for the task of mounting pulleys and driving a blower.
No matter what kind of system is in use, carburetors or fuel injection, a blown engine has the capacity to use lots of fuel and the last thing you want to do is run it too lean. And while there are lots of variables due to engine displacement, type of induction, and so on, a high-capacity fuel pump (or in some cases, pumps) and large lines should always be part of the package.
For low boost levels a stock bottom end will suffice, however, main studs are advisable.
All performance engines need a strong spark; those with blowers are no different. However, blown engines often have unique needs in terms of the advance curve.
It stands to reason if a blower puts more air into the engine, there's more that has to get out. That means, in most cases, 2½ inches is the minimum exhaust system size, and 3 inches is better when the boost gets into double digits.
More power equals more heat. Cram in as much radiator as possible, run the biggest fan that will fit, use a shroud and a recovery tank.
Stick around and we'll show how to put your truck under pressure, make it haul, and keep it together.
At a given pressure, the hotter the air, the lower its density, so the mass of intake air
These are impellers found in centrifugal superchargers. On the left is a turbocharger-styl
Pistons are often the weak link in a supercharged engine. Cast pistons should only be used