Does anyone know how much "power consumption" the turbo will add when you add a turbo to a 650cc 4-stroke single? In other words, at very low boost, the power is less than stock due to the exhaust restriction to run the turbo. As you increase the boost, eventually you will be making power beyond stock. How many psi of boost is a typical crossover point where you stop losing power and start making power over stock? At what level of psi to reach making power, would you conclude the size of the turbo is incorrect - 2 psi? 4 psi? 8psi? I'm concerned that I may be loosing 50% of stock power up front to run the turbo. If this is the case, does this mean the turbo is too big or too small?

 

You might get more info from a buggy forum. The buggy guys have been turbo charging for a long time.

I would think that there would be little drag at low end. Maybe not much more than a stock exhaust. The restriction would increase as boost pressure increases, but so would power from the boost.

I think the expression turbo-lag mostly has to do with cars that had underpowered motors that were turbo charged to compenstate. I read a quote about the '84 SVO Mustang that the turbo kicked in when passing, just about the time you decided you wouldn't make it and hit the brakes.

As for whether it is too big, a bigger turbo should have less restriction at low boost. The only thing it would effect is that higher RPMs would be required before you see boost pressure.

One thing I wonder about is valve overlap. Normally aspirated engines use an overlap between exhaust and intake to get more power. The exiting exhaust gasses help pull in the intake charge. Your stock motor is pretty high output so it probably has a good amount of overlap. Big overlap is not needed with turbo engines and can cause problems. There is an increase in cylinder pressure and more pressure is left in the cylinder when the intake vavle opens. If this pressure exceeds your boost pressure, the exhaust will go through the intake valve. You are better off with a cam designed for a turbo charged engine. Most engines have larger intake valves than exhaust valves. Some buggy guys are using valves that are all the same size. This lets the excess pressure out faster and probably helps spin the turbo more.

 

 

First I must say I am not a turbo expert. I would think that any positive pressure above atmospheric pressure would give you a positive HP gain over a standard motor. If the turbo is not spinning fast as at idle the piston on a turbo motor will create a larger vacuum than it would if it was a naturally aspirated engine. This is because when the turbo fins are not moving or moving slowly they actually hinder airflow into the motor. When the turbo spools up it forces more air in at a higher pressure and therefore more power.
I used to have a Trans-Am with a 4.9 litter turbo V8. Man it was fun to drive when the turbo came up to speed. It was also nice at low RPM being that it was still an 8 cylinder. Never should have sold that car. Oh well. Sorry for getting off the subject. You just got me thinking of those glory days. Hope this helps.