I am currently building a turbo evo so maybe the footwork on mine can be of some help here
A little lesson in boost
Superchargers
Superchargers are way more streetable as the power is more linear than a turbocharger. they usually feel like the faster you go the faster you can go because as the engine starts to rev up the pressures start to increase dependent on the rpm and at a constant rate of increase. When the maximum pressure is reached it will just circulate the extra air back through itself to limit the presure it makes. When the rpms fall the boost does as well again at a constant rate dependant on rpm. keep in mind though that superchargers are parasitic or use power to turn making your 135hp engine have to make 160hp to do it. One other drawback is that the supercharger is always working and will use more fuel forever after.
Turbochargers
Turbochargers use exhaust to spin a turbine wheel that is on a common shaft with a second turbine wheel that drives the compressor. To control the boost pressures the exhaust is by-passed around the turbo sometimes within the turbo or externally depending on design.
With this setup the power is more exponential in the way it builds power. When the throttle is opened there is a significant delay as it "spools" up to about 100,000rpm causing the effect that just as you would want to shift the power is finally there.
Once the throttle body is closed again there is another delay before the turbo slows down and with nowhere for the air to go it suddenly over pressurizes and can actually back up forcing its way out the turbo backwards quickly slowing the turbo or even spinning it in reverse.
Now when you hit the gas it has to start all over again. to solve this a valve is placed within the charged air that releases the pressure and keeps the turbo spinning so it doesn't have to spool up again for the next gear this also controls the overboost that can do serious damage to an engine between gears.
The nice thing about turbos though is that when your not hard on the throttle they are not making boost and are not rpm dependent like the supercharger so for highway use there is little change in driving characteristics. Turbos also do not use power to make it like superchargers.
Inter-cooling (charge air coolers)
With most of the supercharged setups that i have built we have generally experienced hotter intake temperatures than with a turbo.
Reason for this being that most conventional superchargers are roots style superchargers that generally bolt right to the intake of the engine forcing hot air directly from the supercharger to the intake of the engine
To get around this there are two common mothods used
1.) An air to Water inter-cooler can be installed thats basically a radiator at the front of the engine and one in the air charge with a liquid medium being circulated, pretty simple and efficient. The only problem being that it usually lacks the surface area in the intake to be efficient.
2.) the air charge from the supercharger can be re-directed through piping to a larger intercooler as is common with centrifugal superchargers and turbo chargers.
Usually an air to air style cooler is used with this method. This is essentially a radiator that the charged air flows through and placed at the front in the ambient airflow it is less efficient than an air to water style but simplifies the build and a large one can be used making up for it.
But thats not without problems either, For one thing it will have more plumbing that a toilet at this point and the extra surface area of the inter-cooler can cause the gas vapour to sublimate or pool within the intake at low RPM causing a rough coughing idle and hard tuning.
To get around this that carb can be placed after the supercharger / turbocharger in the pressurized air charge. This is not for the amatuer though carbs are basically pressure differential devices and you just totally changed the pressures it was designed to work in, carb modifications will be required and you will start drinking heavily if you try to tune it at this point.
for this reason it is better suited to a injected engine
Okay so now you have the Supercharger / Turbocharger installed but its not over yet.
The high comression that these engines make combined with the hot pressurized air coming into the engine will for one make the engine run quite a bit hotter. this extra heat will start to get your engine dangerously close to the self ignition or detonation point and thats pretty well the death of a connecting rod when that happens. This is when the fuel mixture explodes uncontrollable before it should as the piston is still on the compression cycle. This causes dangerous cylinder pressures and tries to spin the engine in reverse. three things can happen here the first is the rods break, not good. another is the cylinder walls will expand and the pressure blows past the piston relieving the pressure. After this happens it will snap back usually scoring the cylinder from contacting the moving piston. Best case scenario the head gasket blows.
As a rule of thumb 7:1 is good for pump gas 8:1 to 9:1 run the highest octane you can get and any higher than that is race gas or alcohol injection.
12.5:1 A/F ratios will also be required instead of 14.7:1 that N/A engines usually run
I hope this sheds some light on the topic for anyone thinking about running boost. I hope its not discouraging just know what your getting into before you attempt something of this magnitude. Boost is addictive and fun as hell when it all goes together.
Linear Mode
