Turbos are more efficient than blowers because they are not parasitic to the engine, but since turbos cause a restriction in the exhaust while a free flowing true dual system can be used with blowers, does this weigh out the efficiency lost by the crank-driven blower? Its a bit of a brain twister, but it makes sense. Blowers are parasitic because they steal hp from the crank, but turbos are also parasitic because they restrict exhaust flow. Does this make the two equal?

 

if the turbo is sized correctly for the application, it will be more efficient than a blower, also correctly sized for the application. Todays centrifugal blowers are extremely efficient, but they still rob a lot of power from the crank.

That was the first reason why I decided to go with a turbo during this build. The second reason is that I've read where quite a few people have snapped off crank snouts and twisted off balancer hubs when they get into the big HP numbers. I'd rather deal with a little more tubing and pay a little more money than put all that stress on the snout of the crank.

 

You're absolutely correct. My turbo causes almost 50 psi backpressure. However, this reduces hp less than the power taken directly off the crank to drive a supercharger.

All things being equal in my setup, the turbo made 30 rwhp more than the supercharger at 13 psi boost.

Also to consider, I couldn't get over 15 psi out of the blower without belt slip. With the turbo, 22 psi is simply a turn of a screw away. . .

Mike

 

Yep turbos use wasted heat energy to drive the compressor but they also impose parasitic drag from the backpressure in the exhaust which is applied to the piston top. During some areas of the operating range a centrifugal belt driven supercharger will actually give back to engine rotation by assisting in crankshaft rotation by applying pressure to the piston top during the intake stroke. There is no free lunch however and all means of compressing the intake charge require power to do so. If you look at the brake specific fuel comsumption numbers from high output versions of both types of supercharging they aren't really that far from each other. Yes turbocharging is a bit more energy efficient but the difference isn't as dramatic as some would have you believe. I wonder if an honest direct comparison engine could be built for a test, using camshafts and exhaust optimized for both engines. It sure would be an interesting test.

 

I would say that the average combination using a 350 v-8 and a T-4 turbo will gain 30 - 50 hp over an equivalent supercharger. That's with the exhaust backpressure running 2.5x boost. However, it is possible to get backpressure less than boost with the large "thumper" T-6 turbo's. With 20 psi boost and 18 psi backpressure, there is no parasitic loss. The 20 psi boost on the intake stroke "produces" the same amount of power that the 18 psi exhaust pressure "absorbs" on the exhaust stroke. Consequently, spool-up suffers. With backpressure = boost, a turbo will blow a supercharger away on the order of hundreds of hp, but unfortunately most of us can't (choose not to) fit a T-6 turbo under the hood.

Mike

 

While I am tempted to throw up names of professional catagory racers and championships won with centrifugal sperchargers where turbos are allowed and virtually unrestricted I don't think that this is what the thread is about and the DEBATE will run for a long time as to which is actually quicker at the track. There have been amazing accomplishments using both types of supercharging and I like turbos, despite anybody's assumptions based on what I do. I just wanted to address the misconception that there is no parasitic loss from a turbo installation, but that a belt driven supercharger is somehow a huge power sponge. Different strokes for different folks as they say. I am willing however to debate about the installation, support, performance and system durability of a belt driven supercharger versus the still immature bolt on turbo system market, and right now I'm confident that we have that beat. The turbo systems will get better and the market is maturing and stepping away from the "black art" that many used to hold it back from the masses. This is good for everybody in the performance industry.
I will have a Buick GN someday just because I've always wanted one to play with.

Now Mike you've homebuilt your system and are obviously smarter than most, but as the turbo market stands right now you'll remain in the minority for quite a while. Most enthusiasts will not reap the gains you have developed for youself as its still a crapshoot out there. Who sells an affordable turbo manifold for a Gen 1-3 SBC? Banks has held them back for years.

Is a T6 housing streetable with a 350-400 cu in engine? I mean will it produce smooth power in a street engine?

 

You won't get an argument outta' me on that one. To buy a centrifugal kit and install it is super easy. 10 hours of wrench-turning, close the hood, done. I've had my turbo setup apart at least 5 times to fix various odd problems.

I've never had one, so I don't know. The boost isn't exactly instantaneous on my "too small" T-76, even when I had it on the 383.