embergloP51":3m13ziq6 said:
I really don't see any disadvantage to twincharging as long as you can get it set up correctly and tuned well. I was hoping that somebody here had played with this type of system and had some insights about it.
The roots supercharger and turbocharger combination works out well.
The supercharger adds low-end torque that replaces turbo lag and helps turbo spool time
while the turbo adds to the upper rpm supercharger boost and efficiency limitations. It is a good partnership.
Most DIY supercharger projects utilize the wrecking yard variety of used or rebuilt Eaton superchargers.
They are good for boost levels up to 8 to 10 lbs. Much past that they run too hot and detonation becomes a problem.
These units also do not reach their max volumetric efficiency until a certain rpm.
If a lot of boost is needed at very low rpm then the rule is to use a smaller supercharger and spin it faster to achieve desired boost levels. The problem here is, at high engine rpm, the supercharger rpm is too high and it runs hot once again.
Let’s say the goal is 15 lbs of boost
One plan is to run the supercharger at a comfortable 6 to 7 lbs of boost and have the turbo feed the throttle body at the inlet of the charger to add the additional boost. The turbo can easily utilize an intercooler between it and the charger which reduces the heat, increases efficiency and reduces detonation.
At low rpm, WOT will give you 6 to 7 lbs instantly and the turbo will quickly spool the extra 8 lbs where the wastegate will regulate at that point. You still reference the wastgate controller off the intake manifold for total boost which is 15 in this case.
The turbo is sized according to the supercharger flow rate.
At 7 lbs of boost the charger is pumping about 45% more air by weight than the engine so the turbo will be larger to match
Plan 2 is more complicated.
Most of the chargers have a bypass valve that is used during no boost conditions like cruising where it opens the charger inlet to the outlet letting the charger recirculate air through itself instead of pumping into the engine.
This drops the drive power to very little and the recirculating air cools the charger.
When the throttle is snapped to wide open and the turbo is just idling the charger supplies boost and there is a vacuum at the inlet of the charger. As the turbo spools the charger inlet transitions from a vacuum to pressure. At that point the turbo is pumping more than the charger. Once the inlet side sees a pressure, you can use the pressure to control the bypass valve and begin opening.
Once the bypass is fully opened, the charger is bypassed and the turbo is supplying all of the pressure.
The charger only needs a small amount of power to drive it and the air is not being re-heated by the charger after being intercooled.
In this second case the turbo will be sized according the engine airflow rate since the charger is bypassed at upper boost levels and will be smaller than the previous case.
The alternative to all of this is to hurt your wallet and use one of the following instead without the turbocharger.
Eaton TVS, Whipple or Kenne Bell superchargers.