Yeah, that is a problem.
It turns out HKS did a twin charger setup for the Toyota 4 pots. I'm going to try and find some info on them and see if it would be possible to do mine the same.
Twin Charging
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This is just a thought and may not be a correct assumption, but isn't the idea of a twin charge system mainly to plug the hole created in the lower rev range by a large laggy turbo? i.e. a supercharger provides boost at revs below that at which the turbo is able to provide, and the turbo spools up to take over when the supercharger is running out of gas, thus keeping the positive pressure more constant over the entire rev range.kiwicar wrote:Hi
I am sure it can be got to work, bt just thinking about the control side
I would have thought the tricky one to sort out would be the situation where you are driving along a at fairly rapid rate, with the super charger clutched out and bypassed, your slightly bigger than nomally matched turbo is spinning round giving say 1or 2 psi boost and say 240 bhp some annoying pleb in a Bently beatles up behind you . . You obviously have to put the little tick in his place so you boot the throttle. . . Then what does your ECU do? Presumably the first thing it does is slam the waste gate shut to spool up the turbo, but that will not give you instant enough throttle response as you matched it a bit bigger for efficiancy. . . so do you drop the clutch in on the super charger? seems like a yes, however if you do then the load of accelerating the blower and the asociated air column is going to pull out the 30 or so brake horsepower you wanted to save by having the clutch and bypass set up there in the first place and it feels like you lifted off not floored the throttle. After a second or so the clutch is in and the supercharger is producing boost and off you go, now the extra exhaust gass has hit the turbo and it is spinning up nicely and we are really beginning to make boost, what happens next, open the waste gate again. . ? ok but with the supercharger now shoving in all that boost you run the risk of surge, do you drop the clutch out on the supercharger again and add 30bhp to the mix but instantly dropping out the blower's contribution just as the turbo is opening it's waste gate anyway. . .yes I can see slowing down all the valves will help but that somewhat further negates the point of adding the blower, the instant throttle response. To me it just seems a control theory nightmare, you can probably get it to work, but can you get it to work better than a better matched turbo or blower?
I can see the 2 differnt sized turbos working well as their response is in the same control loop dependent on the same energy input, the exhaust gas, and they do not add in the +- 30 bhp penalty load on start up.
I know the setup of turbo charger and supercharger worked very well on post war radial aero engines where silly levels of break mean efficiency were avaliable over a very wide range of operating altitudes but . . .
Best regards
Mike
If so, when you are cruising the engine would be at low RPM anyway and the tubby would be unable to provide boost as the rpm is too low, so wouldn't it already be running on the supercharger? Therefore dumping the right foot at low RPM wouldn't suddenly engage the blower, as the switch over would have already happened when the revs dropped to cruise levels earlier.
I would have thought the control would have been rpm dependent. Say the tubby comes in at 2500rpm, then the blower provides boost to, say 3k before being shut off, possibly with some kind of bleed or slip to smooth out the step between blower and tubby boost. There probably wants to be some delay in re-instating the blower as rpm drops, or it may click in and out as you change gear or whatever, but then once rpm falls below a set rate for a set time, the clutch engages (or disengages, depending on your perspective) and the blower starts providing boost again as the revs slow to under 2,500 or whatever.
Although I'm probably talking crap
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teamidris
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Sounds a fair description to me.
"I'm sure you've calculated your blower speed with the crank pulley dia and the aircon pulley dia but I'm just a bit surprised an aircon pulley is small enough to give you enough supercharger speed?"
Kind of customer led. A work mate worked it out that the original constant-drive pulley was too fast. I was more worried about belt contact. The new crank pulley was made with 3 rows of pulley drives. (avoid supercharger space issues) and is quite a diameter.
I suspect that the ideal is to take the supercharger to max speed just as the turbo is getting going and then drop the clutch? So a handfull of 'test' pulleys would be needed for the best match maybe?
But I'm still for electric. You get full air flow right off and a chance for any air flow at any engine speed with gradual drop off. It smells the right technology, in the same way electric water pumps work well.
"I'm sure you've calculated your blower speed with the crank pulley dia and the aircon pulley dia but I'm just a bit surprised an aircon pulley is small enough to give you enough supercharger speed?"
Kind of customer led. A work mate worked it out that the original constant-drive pulley was too fast. I was more worried about belt contact. The new crank pulley was made with 3 rows of pulley drives. (avoid supercharger space issues) and is quite a diameter.
I suspect that the ideal is to take the supercharger to max speed just as the turbo is getting going and then drop the clutch? So a handfull of 'test' pulleys would be needed for the best match maybe?
But I'm still for electric. You get full air flow right off and a chance for any air flow at any engine speed with gradual drop off. It smells the right technology, in the same way electric water pumps work well.
I think (in theory) the best/easiest way to run this would be to have a switch on the dash for the chargers. Either off to save fuel (and air goes around the charger) or on and the bypass is used to feed additional air at high rpm.
Lets say I go charger/intercooler/turbo/aftercooler/throttle body/engine
The chargers are controlled from factory to boost up to 8psi, once it goes above this a bypass valve is gradually opened to allow air to re-circulate back around and limit boost. Once the pressure goes over 8psi (either from the charger alone or pressure multiplication between the charger/turbo) and the bypass opens, rather than recirculating air back around the charger, the turbo would start drawing extra air above and beyond what the charger is delivering. As there will no longer be much of a pressure difference between the inlet and outlet of the charger, it shouldn't require much power to spin and it shouldn't be adding significant amounts of heat to the air. My only question with this is; if the charger delivers 1400cc per revolution, when the bypass opens and the turbo has spooled; will it still deliver 1400cc per rpm (or there-abouts), allowing the bypass to be sized only large enough to provide air when the chargers are switched off at cruise and supplementary air to the turbo. Or does it need to be sized large enough to deliver all the air the engine will need?
If this is possible, it means that I don't need to control switch over points etc, which seems to be the trickiest bit of twincharging. The charger is always on and feeding air. Once the turbo has spooled the draw from the charger should drop significantly and the turbo able to feed air above and beyond the capability of the blower
Make sense?
Lets say I go charger/intercooler/turbo/aftercooler/throttle body/engine
The chargers are controlled from factory to boost up to 8psi, once it goes above this a bypass valve is gradually opened to allow air to re-circulate back around and limit boost. Once the pressure goes over 8psi (either from the charger alone or pressure multiplication between the charger/turbo) and the bypass opens, rather than recirculating air back around the charger, the turbo would start drawing extra air above and beyond what the charger is delivering. As there will no longer be much of a pressure difference between the inlet and outlet of the charger, it shouldn't require much power to spin and it shouldn't be adding significant amounts of heat to the air. My only question with this is; if the charger delivers 1400cc per revolution, when the bypass opens and the turbo has spooled; will it still deliver 1400cc per rpm (or there-abouts), allowing the bypass to be sized only large enough to provide air when the chargers are switched off at cruise and supplementary air to the turbo. Or does it need to be sized large enough to deliver all the air the engine will need?
If this is possible, it means that I don't need to control switch over points etc, which seems to be the trickiest bit of twincharging. The charger is always on and feeding air. Once the turbo has spooled the draw from the charger should drop significantly and the turbo able to feed air above and beyond the capability of the blower
Make sense?
Been looking at some threads on another forum with people who have built their own twin charger systems (with great success). Seems the best way to do is have no high rpm bypass at all. Just feed the turbo into the charger using a turbo that flows what you want in total at half the boost you intend to run. Bypass is only for idle/part throttle/charger off.
Apparently leaving the charger on means you get the full amount of air you require but at a pressure that greatly increases cylinder scavenging with little overlap, meaning more air/mixture in the chamber and less chance of detonation.
Not a very thorough explanation but apparently doing it this way gives spectacular results.
Apparently leaving the charger on means you get the full amount of air you require but at a pressure that greatly increases cylinder scavenging with little overlap, meaning more air/mixture in the chamber and less chance of detonation.
Not a very thorough explanation but apparently doing it this way gives spectacular results.
