I find I am trying to argue why it might be advantageous to have technically too small a turbo or turbos on a street car. This I think explains things quite clearly much better than I could.

"VOLUMETRIC EFFICIENCY AND BACKPRESSURE
Although the turbine recovers wasted exhaust gas energy from the expansion of the hot exhaust gas, the kinetic energy of the flowing exhaust gas and the acoustic energy of the exhaust gas, the working turbine also causes an increase in exhaust gas backpressure. This increase in backpressure can reduce the engine's volumetric efficiency. A typical, streetable turbo system has more exhaust backpressure than boost pressure and the power gains from such systems are due to the increase in the density of the intake charger, not due to increases in volumetric efficiency. (Volumetric efficiency, if you don't remember, is the volume of intake charge inhaled during the intake stroke vs. the actual displacement of the cylinder. VE is expressed as a percentage; the larger the VE, the better.) Backpressure is higher than boost pressure because the smaller turbine housings and turbine wheels used to ensure a quick spool-up time also, by nature, restrict the exhaust flow. We will explain the mechanics of this in more detail a little later. Racing turbos, the latest generation of medium-sized turbos and turbochargers for engines where throttle response is not much of an issue (like fixed industrial engines, long haul trucks and aircraft), have free-flowing turbines that have less exhaust pressure than intake pressure. Engines using these turbos often do have improved volumetric efficiency. This condition, where boost is higher than backpressure, is called crossover and crossover is what ever turbo system designer strives for. In crossover, VE percentages as high as 110 percent are not unheard of. Unfortunately, some of the design features that can create a free-flowing turbo can also contribute to turbo lag, something that is not desirable in a street-driven car that needs a wide dynamic power band.
Excessive backpressure is hard to manage in a boosted four-stroke engine. Excess backpressure causes what is known as reversion. Reversion is when hot exhaust gas gets pumped backwards into the engine during the overlap period. Reversion can cause the engines internals to get excessively hot as cross flow of the cool intake charge during overlap is one of the ways an engine cools its self internally. Hot internal parts can trigger uncontrolled combustion and engine-destroying detonation. Because of this, it is sometimes not a good idea to really crank the boost on an engine that has a small, high-backpressure turbo - in other words, the kind of turbo that usually comes on a factory turbo car.
This is a good reason not to go crazy with a boost controller on a factory-equipped turbo car. A little more boost, perhaps 4 to 5 psi might be tolerated, but trying for 20psi could be flirting with disaster. On small turbo cars with a lot of backpressure, camshaft overlap should be kept to a minimum. This means that the stock cam usually will work best. To deal with the problems associated with backpressure and reversion, the engine's tuning must also be compromised with richer mixtures and more retarded timing than what would normally be optimal for the best power. Even on full race turbocharged cars with low backpressure turbos, camshaft overlap should be several degrees less with more lobe separation angle than on an equivalent naturally aspirated engine, unless physical measurements indicate that the engine is in crossover in the engine's operational range.
Because of backpressure and VE issues, the correct turbo size for the application is very important when designing a turbo system. A small, quick-spooling turbo can be restrictive, causing a great deal of backpressure and reducing VE at higher rpm. This means that small turbos should be limited to lower boost levels. A big, free-flowing turbo can be laggy and unresponsive, making it unpleasant for street driving but producing awesome power at higher rpm. To combat high backpressure and possible reversion, the compromised tuning needed to prevent destruction with an overboosted small turbo will also reduce power. If a small turbocharger is running backpressure to boost ratio of more than about 1.8:1, a supercharger has a good chance of performing better. Fortunately, it is easy to design a reasonable responsive, powerful turbo system with a ratio of less than this.
In future installments, we will delve deeper into turbo tech, discuss some more advantages and disadvantages of turbo vs. superchargers and give some guidelines and parameters so you can figure out what it takes to build a system that meets your needs. "

 

Thx man its hard to find good info on this, I would like to learn more

 

Garbage if you ask me, thats very, very general and not even that 'correct' as there are allot more factors than that.

You can easily have a fast spooling, low rpm turbo .......... not one or the other. Good for newbie's I guess.

 

That really makes me think twice about turning the boost up on my 67mm. Im at 13psi right now but im not sure what my backpressure is. I was thinking of going up to 17-18psi. This is on a 347CI.

 

If it makes you feel any better my friend had his shaft snap when they turned up the boost on the 67 turbo the car was running. You can even see the turbine shoot out the turbo and spark on the track during the pass.

 

great write up....good info in here +1

 

I would bet your backpressure is probably close to 3:1.

 

We had one on the dyno do the same, it was a sts kit with 7psi and a brand new turbo from STS.

 

I think that article has some very good info that will help many people get "up to speed" on turbo theory. Who wrote it?

However. . . it only scratches the surface. Many, many other things affect backpressure other than just the turbine size. For instance, if you're running your compressor in an inefficient place on it's map (like anything 76mm or smaller on 325+ cid), then it will take much more power to spin the compressor. This means the turbine wheel has to do more work and will take more exhaust pressure to get there. You could say the same for piping, intercooler, muffler, throttle body, MAF, etc. . . losses.

Mike

 

interesting read

 

Please explain this a bit, why?

 

If you want a compressor to flow 80 lb/min of air at a 2/1 Pr and 80% efficiency, it make take 100 hp to spin it.

If your compressor at 80 lb/min and 2/1 Pr is only 50% efficient, then it will take 160 hp to spin.

The turbine has to produce this power. A turbine developing 160 hp will take more Pr (backpressure) than one that only has to produce 100 hp. The wastegate has to pinch off flow to force more through the turbine.

Mike

 

Turbo should be rated with a big * beside the HP #'s. It all really depends on the setup what level of HP the turbo will actually put out.

Best example is look at the 2JZ supra's with a GT47-88 making 1400rwhp on boost only, then slap that same turbo on a big 400ci LSX motor and your lucky to hit 1000rwhp if that.

 

The torque is the trade off.

 

You are telling me, I had a turbo eclipse and let me tell you the torque from that thing was unbelievable

 

Wow, that's annoying... Is that because of compressor efficiency?

 

more from the turbine size/efficiency I'd think...

 

island efficiency is high....small cube motor+big turbo you can get high pressure ratios that you see on Pupras while still being efficient.

 

Where can I see compressor maps for 76 or 80mm turbos we use on V8?

 

Old thread but good info plus I just got done talking to Jose@FI about this yesterday. I am running a S91 with a log setup. Running 1 1/2 tubes out of the flange with my main log 2 1/2 in. It will feed from the other side via the crossover.

Now to backpressure. I am told this is measured as close to the flange as you can. I have room for this so how much is too much or less? I was told you use a boost/vac gauge to get this reading.