Twincharging Cut Off?

[Dave1600]

Hi guys,

I'm a long time lurker here just looking for parts for my Datsun 180B SSS project. Since it will be supercharged via an Eaton M45, I thought I'd add my 2c here.

I'm using the bypass valve off the Mini Cooper S. Its reasonably priced (~$125 from your local BMW dealer) and is specifically designed for the job of acting as a supercharger bypass valve. Not sure about whether it'll be suitable in a twincharge setup ??

It may also possibly be a bit small for an RB, as I'm only using a little L18 engine.It has a bore of 35mm.

Just thought it may be worth looking at.

Dave

Edited March 3, 2006 by Dave1600

[ynhrgt]

Anything specifically from Turbonetics?

I was looking at the HKS GT 2 60mm wastegate.. but I guess that's too big.

I very strongly suggest you get it all working and sorted out before spending big money on cosmetics. Once something is ceramic coated, it is not possible to cut and weld it, or modify it in any way.

Any external wastegate will work provided the control diaphragm is AT LEAST twice the diameter of the wastegate poppet valve. Garret, Teal, or Turbonetics wastegates are very good. The Chinese ones you need to be a bit careful.

Nothing you can buy will work straight off out of the box, you need to find a suitable much lighter wastegate spring. That will take a bit of experimentation to get it exactly how you want it.

[Warpspeed]

Dave, I am not sure exactly how the system in the Mini functions. But the Supercharged 3.8 Commodore also uses an Eaton supercharger with a rather similar looking bypass built right into the blower casing. Now the Commodore uses a diaphragm actuator that looks just like the one in that picture. The whole thing is operated by the electronics in the engine management unit.

For instance in the Commodore, the ECU restricts boost until the engine is up to full operating temperature, and it NEVER allows any boost in reverse gear. But otherwise, the ECU fully controls the bypass system under all modes of driving. Unfortunately that system needs the "brains" of the ECU to operate the supercharger bypass actuator. You cannot just connect a hose from the actuator to somewhere on the engine and expect it to operate properly.

I have tried a bypass throttle body and actuator myself, and my experience with it was that it would just flop open, and flop shut. There was no proportionality at all, it was either open or shut with nothing inbetween. A microprocessor based system using PWM to progressively bleed air pressure into and out of the actuator with some sort of feedback may be possible, but it is hardly a do it yourself project. It would probably work something like one of those very expensive electronic boost controllers, but the software would need to be very different. I am an electronic engineer, and although possible, it is just all too complicated.

I have also experimented with various home made bypass valves, but the best system is still a commercial external wastegate modified with a lighter spring.

The main requirement is for the control diaphragm to be at least twice the diameter of the poppet valve.

Yes ynhrgt, a 60mm wastegate would work fine, but is the control diaphragm LARGER than 120mm in diameter ? Probably not. I have never seen any wastegate with a control diaphragm larger than 80mm diameter. They may exist but I have never seen one, 120mm would be enormous.

[ynhrgt]

Well, I was hoping for a product you could point out specifically to do the job but that's okay i'll find one. The control diaphram is supposed to be double the size right?

Dave, I am not sure exactly how the system in the Mini functions. But the Supercharged 3.8 Commodore also uses an Eaton supercharger with a rather similar looking bypass built right into the blower casing. Now the Commodore uses a diaphragm actuator that looks just like the one in that picture. The whole thing is operated by the electronics in the engine management unit.

For instance in the Commodore, the ECU restricts boost until the engine is up to full operating temperature, and it NEVER allows any boost in reverse gear. But otherwise, the ECU fully controls the bypass system under all modes of driving. Unfortunately that system needs the "brains" of the ECU to operate the supercharger bypass actuator. You cannot just connect a hose from the actuator to somewhere on the engine and expect it to operate properly.

I have tried a bypass throttle body and actuator myself, and my experience with it was that it would just flop open, and flop shut. There was no proportionality at all, it was either open or shut with nothing inbetween. A microprocessor based system using PWM to progressively bleed air pressure into and out of the actuator with some sort of feedback may be possible, but it is hardly a do it yourself project. It would probably work something like one of those very expensive electronic boost controllers, but the software would need to be very different. I am an electronic engineer, and although possible, it is just all too complicated.

I have also experimented with various home made bypass valves, but the best system is still a commercial external wastegate modified with a lighter spring.

The main requirement is for the control diaphragm to be at least twice the diameter of the poppet valve.

Yes ynhrgt, a 60mm wastegate would work fine, but is the control diaphragm LARGER than 120mm in diameter ? Probably not. I have never seen any wastegate with a control diaphragm larger than 80mm diameter. They may exist but I have never seen one, 120mm would be enormous.

[ynhrgt]

Hey Stocky or Warpspeed.. have you figured out how to retain the factory A/C or power steering with the supercharger?

[StockyMcStock]

i can't speak for a skyline engine bay, but in my VL i still have power steering and could run A/C in the factory location if i felt the need. removing or lowering the power steering pump down the block would give a bit more room if i needed it.

as far as wastegates go, i'd generally get the smallest one you can, so that your control diaphragm to poppet valve area ratio is the highest it can be. a 32mm turbonetics or something would be good i suspect.

[mitchy_]

sorry if its already been said,

but when using a wastegate as a bypass valve for the supercharger, is it placed before the S/C, and then the outlet pipe plumbs back in after the S/C?

[Warpspeed]

sorry if its already been said,

but when using a wastegate as a bypass valve for the supercharger, is it placed before the S/C, and then the outlet pipe plumbs back in after the S/C?

Well yes, I suppose you could look at it that way. But actually the wastegate flows from the supercharger outlet, back to the inlet. After all, it is a bypass system. There are basically four requirements.

1/ The supercharger boost pressure absolutely MUST be in a direction to force the poppet valve off its seat if there is a boost pressure spike, so the wastegate MUST flow in the same direction as it originally did when used as a turbo wastegate. Mounting it reverse way around so it flows in the wrong direction it is not going to work smoothly.

2/ The diameter of the control diaphragm absolutely must be at least twice the rated flow diameter of the poppet valve. An even bigger difference in diameter would be much better. Anything less than twice is not going to work unless the desired final boost pressure is unusually low.

3/ There will be two hoses coming from the control diaphragm on the wastegate, one from either side. These go to either side of the throttle body so the diaphragm sees the pressure drop across the throttle body. Idle vacuum should fully open the wastegate against the internal spring force. That tells you which way around the hoses connect.

4/ Spring pressure should be such that about 2.5 psi just lifts the wastegate valve off it's seat. 5psi should fully open the wastegate against spring pressure. These are only rough guide pressures to initially selecting a spring. Those pressures will definitely work, but testing may decide you to change things slightly. But those spring figures will definitely get you a working system.