TwinCharging!

[Max965]

New hear... I came accross this forum because of Stocky's impressive Twintubo setup.
I am interested in doing one myself so and I see that its becoming popular- more so because for performance wise theres little downside.

Anyway I just wanted some discussion about the compressor flows and how it can work. I will use Stocky setup as that is im my view an excellent route to take.

So setup is.. Air> Turbo > SC > IC > Manifold

Please comment/correct on my points on these situations.

1) On idle/low throttle/vacum there is a 2-5spi spring wastegate that routes the air from SC outlet back to Turbo Inlet. Under load the wastegate is shut.

2) As turbo builds pressure the SC should be increasing this pressure at any time. Therefore SC outlet will always be equal or greater than the Turbo outlet/SC inlet, so then the manifold pressure will reach a max setting (e.g 1Bar) and there will be another wastegate set at 1 Bar which will progressively divert the turbo outlet direct to manifold inlet bypassing the SC. When fully open the SC is not underload and so small HP is used. (No clutch used due to pulley size issue/slipping/complications.)

3) Something I have not read about is looking at the comressor maps of turbos at low rpm when inlet pressure is high due to the SC, the SC might need more CFM than the turbo can suck in itself- also known as Choke, i.e the turbo is spinning at this moment in time at low rpm so only a small can be CFM be sucked in, just like sticking a small turbo on a large engine. Is this an issue for the SC?, to me you could say the SC is sucking air through the turbo and it will be ok but this means theres a vacum and its then the same as the choke on a big engine. So if this is the case (for a small section on the map) could you then have a vacum BOV or wastegate to open up under this choke condition and the SC will bring in its extra CFM from the turbo inlet? (So in theory youll get part of its CFM boosted from turbo and the other at 0bar). Then the turbo begins to spin up and give the SC the CFM it needs, comes back on its compressor map and so the valve closes. I guess this 3rd point (if valid) is an added extra for higher boost at low rpm...

What do you guys think? This is a simple but effective setup...

The great point is that you can get the biggest turbos you can find so max efficency is at max rpm.

Having a completely flat torque is what you can achieve with a twincharge, I added a dyno from a Mini thats twincharged, two runs, one before and after...

Thanks
Miles

[Max965]

Pic..

[stockymcstock]

you're a bit mixed up on a few points there, i'll clarify.

at WOT, ALL the air from the turbo is going through the supercharger. there is no switching point where the flow paths are diverted. the bypass gate is completely closed. the turbo just pressurises the blower inlet. the supercharger then multiplies the pressure change (NOT adds!) by a fixed amount.

at partial throttle and idle conditions, the bypass gate progressively opens to release post-blower pressure in the pipework. it is plumbed back into the turbo-charger connecting pipe as per pics of my car on here somewhere. it is possible to simple air-vent it, or plumb it back into the turbo inlet if you wish. it is very loud unfortunately.

sizing the turbo is important, you can run a very large rear housing on it but the compressor side must be reasonably well matched to what you want it to do. for example, my compressor side is being asked to deliver ~370hp worth of air at just 2-3psi of boost. generally, a larger number on the compressor A/R helps in a twincharge by increasing the available flow at low pressure levels.

cheers

[Max965]

Now I am bit confused about point 2, at WOT you always have the SC blowing. On the Delta's there was a bypass pipe so the turbo can go direct to the manifold when the turbo overcame the SC but you dont have this?
So what do you use to regulate the manifolds pressure? if the turbo wastegate then this must be set at a much lower level than what you need as the SC multiplies the boost? I take it you dont mind the SC running all the time at WOT.

Point 3 - I see that a large A/R will have a wide CFM rate and so the issue wouldnt be needed.

Cheers

[GTSBoy]

Now I am bit confused about point 2, at WOT you always have the SC blowing. On the Delta's there was a bypass pipe so the turbo can go direct to the manifold when the turbo overcame the SC but you dont have this?
So what do you use to regulate the manifolds pressure? if the turbo wastegate then this must be set at a much lower level than what you need as the SC multiplies the boost? I take it you dont mind the SC running all the time at WOT.

Point 3 - I see that a large A/R will have a wide CFM rate and so the issue wouldnt be needed.

Cheers

Point 2. The answer is "So what?". A wastegate setting is an arbitrary thing anyway. You wind the adjustment (either spring or bleed) up or down until you get your target boost pressure in the manifold. Job done. No need to even care about what is going on at the wastegate itself (unless it isn't working right somehow).

[The Pupat]

Stop worrying about getting a flat torque curve and just start using twincharging is good for, getting lots of torque everywhere in the rev range. Basically you get a supercharger that will blow say 7psi into the motor from idle, this helps spool the big turbo quick. Turbo then blows air into the supercharger which is then compressed with a multiplication factor. Turbo keeps blowing more air into the motor increasing the SC intake pressure until until the wastegate says that's enough and starts throttle back the turbo. You won't get a perfectly flat torque curve per se, what you will get is a motor that will "spool" a big turbo very quick and as such will make a lot of power across a lot of RPM, which is what you want.

[stockymcstock]

the turbo controls final boost pressure via the conventional turbine wastegate. it is referenced to total boost pressure and hence bypasses the correct amount of exhaust gas to maintain 9psi in the plenum. what the supercharger is doing to the pressure changes is irrelevant to the turbine's wastegate.

my method involves no complex valving to switch the flow paths, which introduces the power adding effect of having inlet pressure above exhaust back pressure. since the turbo is only making 2-3psi, it can *hopefully!* be assumed that exhaust back pressure is less than the 9psi being seen in the plenum. this means less hot exhaust gas residuals in the chamber and hence more power at the top end. it is counteracted by the parasitic loss of driving the blower via a belt, but i would hypothesise that it latter does not outweigh the gains to be had, judging from my dyno sheet.

cheers

[Max965]

Ok I see your reasoning but if I am right I was going to say the SC losses and also that the SCs are 50% vs the 70% turbo efficency, which is roughly 70% more air density on the same pressure.

I just would of thought that would outweigh the gains on the backpressure.

There could be an easy way for it to be tested, just dyno the MAX HP with the bypass permantly on and then off.

Still if its better to bypass it you could use the same wastegate and so no more plumbing would be needed.

[stockymcstock]

the SCs are 50% vs the 70% turbo efficency, which is roughly 70% more air density on the same pressure.

i'm not sure what you mean there, but those sorts of figures are totally useless when you consider the system that both compressors are operating in.

i'd be all in for trying it out with some extremely complex valving and piping, but it's a logistical nightmare to switch the flow paths smoothly, and i can't see it being any better that what i currently have.

if you were switching flow paths you would want to turn the blower off at high RPM by using a clutch system to eliminate the belt drag on the crank. so in effect, once the blower was off and the turbo doing everything, it would produce exactly the same peak power as if the turbo was the only thing on there.

so the way to do it would be to tune it with the twincharge, then take the blower off and fit up some replacement piping into the intercooler from the turbo and re-tune it on the same boost, same dyno, same day. that would be a very interesting comparison, and may be something that i would consider doing actually. it would certainly go a long way to showing the area-under-the-curve gains with the twincharge anyway. it would also be a useful method of generating customers for twincharge kits......

[Max965]

Perhaps I got my maths wrong...

I was thinking that at high rpm/boost the lower efficency of the SC would produce more heat and so less dense air than the turbo being on its own (or bypassed)

Looking at your pictures... the set up you have is an outlet of the turbo going straight into the inlet SC and the idle/vacum bypass pipe as well coming from that turbo outlet into the outlet of the SC. I just thought if you dyno'd it just activate that wastegate (pull the vac line) to open at high rpm on WOT and see if that torque number changes.

You know what Im just going to build this thing and try all this stuff out, im sure this is the tip of the iceberg and there could be some interesting things to try with different turbos/SCs...

Cheers

[GTSBoy]

Max,

Efficiency of a compressor is only a measure of how much input energy to the compressor gets turned into heat over and above the amount of temperature rise that you get from just compressing the gas. So if you have a pos disp blower that runs (say) 50% efficiency when working at (say) 2:1 pressure ratio, then it will heat the air up a fair bit when boosting from atmospheric to 1 bar of boost. Same as a turbo compressor would if it were operating on it's map at a point that had the same efficiency. But here is the thing with Stockie's twin charger. Say his turbo is operating at 3psi. If he has it on the efficiency island, then it will not be adding much temperature to the gas. If he then passes that through the blower, instead of starting with 14.7 psi he is starting with 17.7 psi. To reach a total boost target of 10 psi, he only needs to add another 7spi of total boost to get to 24.7 right? But the way this is done is by the blower operating at a pressure ratio. It is operating at the ratio of 24.7 / 17.7, which is 1.4 ..... So he gets to operate his blower at a lowish pressure ratio, which keeps it towards the more efficient end of it's operating range.

If he were to try to achieve 10 psi with only the blower, then the pressure ratio is 24.7 / 14.7 = 1.7 .....that's a lot higher ratio and more likely to get out of teh efficiency band that is happy.

cheers

[Rz]

i would love to see some before and after dyno runs of any car thats gone from a turbo , to a twincharged setup.
theres obviously merit in the idea because its been done enough times over the years , but some overlayed charts to actually see the gains in response would rock.

[Max965]

thanks GTSBoy, ok makes sense the turbo offsets the SC.

If you wanted to get really sceintific on this stuff- a combined compressor map would be good- i guess you could build one and adjust the crosover points if any.

[GTSBoy]

If you wanted to get really sceintific on this stuff- a combined compressor map would be good- i guess you could build one and adjust the crosover points if any.

You could. You would probably be institutionalised afterwards, but you could.

[Max965]

Ok I found something interesting...

A wastegate called a Part Throttle Wastegate by some guy called 'Stanely UpDike'. It was a 3 ported wastegate with two springs, one to turbo outlet, one to atmosphere and the other to manifold, it basically allowed the WG to be open on part throttle, closed on boost and then reopened on max boost. One spring compressed under vac, then the other comrpessed under boost. It helped keep backpressures low when not on boost.

This could be used for the bypass

I cant find if it exists anymore or who sells it. I read it one of my turbo books.