Dang thanks for the good info. Talking about the heat and reliability topic, I too have always understood that aftermarket turbo setups are very deceptible to engine heat and were not so reliable. But I think now they are a thing of the past, but for what I do not know. I guess I am going to have to do some research and find out how turbo quality has advanced througout the years. Also I know that you guys were talking about a turbo taking a dump, but the way I see the turbo itself runs pretty cheap anyways. I mean one could replace a turbo for about $800 and be up and running.

 

I didn't take the time to read through all the other posts here, so maybe this has already been said..

Turbos and superchargers do not compare to each other when talking about boost pressures and power levels. There is the obvious factor of the power needed to drive the supercharger coming from the crank versus the reclaimed energy in the exhaust used to power the turbo, but there is more. Given two engines configured exactly the same, one with supercharger (air/water aftercooled roots for example) versus turbo (air/water aftercooled), both programmed for proper air/fuel ratios, the supercharged engine will take more boost without detonation. Detonation occurs because cyclinder temperature is too high. Pressure increases temperature. The supercharged engine has less backpressure and therefore flows exhaust better from the heads than the same motor would with turbo(s). The extra exhaust left in the cylinder of the turbo motor is hot and takes up space in the combustion chamber. On the intake stroke less air and fuel can be taken in than the same conditions on a supercharged engine. So, given that example, the turbo motor might handle 5psi without detonation and the supercharged one 6psi. Which one would put more power to the rear wheels? The turbo. It's a trade off. Power isn't about boost, it's about getting combustion chamber pressures to the brink of detonation as efficiently as possible. Turbos win there because they use wasted energy, not useful torque from the crankshaft.
Another interesting point, in terms of combustion chamber pressure, is that freeing up the intake with larger throttle body, valves, more lift, bigger ports, etc. and the same for the exhaust will detonate at lower boost levels than the stock configuration because more air and fuel can get in with less effort (boost.) Overall power increases because the energy needed to make that lesser boost is also less.

 

On this URL

http://www.automotive.eaton.com/prod...rgers/M112.asp

are some useful graphs for the Eaton M112, used by Magnacharger in their RADIX product.

 

You'll always have my vote for most perceverance(sp?) bro!

You're kit and mine are not so disimilar. A high HP STS kit requires a higher stall just as much as a centrifugal IMO. I know you'll see some excellent HP with your setup mate. You've worked too hard not to.

 

I know this is well over a month old... but I didn't see this mentioned.

When people talk about a supercharger, they mention parasitic drag and the associated loss horsepower.

I do not believe turbos use free or wasted energy. When I think of a turbo, I picture bolting a little kazoo, backwards, on the end of my exhaust. The small exhaust outlet HAS to give up some power over a nice free flowing (superchargers) exhaust system. Maybe not as much as a SC, but something.

Does anyone have any facts about backpressure in exhaust systems and turbo systems?

 

Exhaust backpressure is much higher. Cams for superchargers and turbos should be shorter duration than naturally aspirated cams. With the superchargers minimal overlap is preferred so that high pressure boosted air and fuel doesn't end up getting pushed into the exhaust before the exhaust valve closes - especially at full throttle. It seems to me like duration becomes more critical with a turbo motor to get cylinder pressure minimal by the start of the intake stroke.

When it comes to exhaust for superchargers the bigger and more flowing the better - no need for backpressure. I would be curious to see how many pounds of exhaust pressure a turbo needs to make , say 10psi of intake pressure.

 

That's EXACTLY what I was thinking.

I thought I read somewhere (this is going to sound nuts), but I thought a thread mentioned that a Lightning of unknown model-year and unknown modification, was running a cat-back system that kept popping apart at the clamps when running down the quarter. The person ended up doing some testing and found 7psi in the exhaust, with either that cat-back, or the stock system - I forget which.

7psi sounds like a whole lot, but I'm not sure if it's that-much when you're pushing a boatload of air through the motor.

 

I've heard a 2-1 ratio is good to shoot for.(10 psi boost, 20 psi exhaust pressure) I think Parish is around 3-1 right now.

 

my set up is not ideal. a log style turbo header wasn't made for a 408 and t76 trying to push 20psi. in the corky bell book is says you can achive more intake presure than exhaust presure at least in some rpms if you have a perfect set up but most people seem to say a 2-1 is acceptabe. i see over 50psi in my cross over tube at 15psi. but like i said this kit was made for a 5.3 pushing maybe 10psi and for that it is great. at lower boost levels i see the 2-1 ratio or less.

more of the power you make gets to the driveline with a turbo. how much more i dont know but i have seen refreneces to 20% loss with a blower vs 5% loss with a turbo. sounds reasonable.

 

A little off topic here, but anyways:

The biggest reason you don't know of any 100k+ turbo lsX style engines around IS because of how easy it is to increase boost. The heat generated by normal daily driving in a vehicle with a low boost application of about 5 psi isn't near the amount generated when at wot and producing 10 psi. Since the numbers aren't so incredibly high, the increases in temp can be dealt with accordingly by the use of oil coolers, heat sheilds, and intercoolers; yes there is more heat created with the use of a turbo than the use of a supercharger. Now, imagine...

You have been ridding around all day, so everything is good and hot. You decide to turn the boost up, not much, only about 2 psi. Ohh, no. Now that impeller housing is glowing red, your timing and fuel curves are way off, the engine is detonating because you are now out of the efficiency range of the compressor and the output temps are outrageous. This is a major factor in the dependability of your turbo and engine. This horrible event doesn't happen with a s/c because boost isn't adjustable.

There are many factory turbo vehicles that see 150k+ with no damage to turbo or engine; it all depends on the operator.

And one other thing: Turbo setups generally get better gas mileage since on of the forms of energy driving it is the wasted enery from the engine in the form of heat.