I'll PM Blkmgk and see if he can chime in on here. IIRC (and don't quote me!), he picked up around ~100 RWHP in the midrange when going to TTC. Peak power I believe was unchanged. This was with back to back dynos pulls.98mkiv said:
There is no power to be gained from this at all, but the people who do this mod, love the feel when the boost finally becomes positive.
Oh, and for all the people who will say, bu there is power to be gained, I would like to know from where. You are still using the same turbos. So if the ONLY thing you do is go from seq to TTC, and all other mods are the same, the power will be the same.
answer to above:
ttc only gains power during the transition from 1st to both turbos. i think it does actually have an appreciable gain here, and it actually goes for quite a large range of engine speed since the transition doesn't happen that quickly. not completely sure, it would be good to look at dynos, but this is what people mean by ttc gaining power i think.
>I'd like to see a dyno sheet, or some explanation of how more power is gained from the same set of mechanicals, just sequenced (or lack of sequenced) differently.....
why make a sequential system identical to toyota's? there is a point to using two differently sized turbos, especially when people want like 800hp with the lag of the stockers (or even less). no one is saying to use an identical setup to toyota's because clearly you are right that it would not work very well as-is.
> I mean that I would keep the same control scheme, and the same system schematically. I would make the system with much more freely flowing tubular manifold and I would use larger identical turbos. In this scheme you would still have minimal lag and huge boost with a nearly seamless transition. Some output signals of the stock ECU might need to modified in order to keep the transition smooth, but you get my point ?
but it's possible, by either completely switching from small to big once the engine's flowing enough for the big to spin by itself w/o surge, or maybe even running both.
> I take this to mean that you want to run a small turbo till it hits peak boost, then shut it off and transition to a large turbo? This might work. You may be able to pipe the turbos so that the output of each turbo is parallel valved and when the small opens, the large closes proportionally. You will likely run into problems on the output side, you would need a reed valve since it naturally will balance pressure across itself to protect the turbo from backspinning overspinning. I don't see a point to trying to do it this way, I haven't ever seen it done. Like I said, to me it makes no sense to do it this way.
if you did run both at the same time, which would probably buy you a bit more surge avoidability... why would one spin the other backwards?
> follow me here: If you have a pair of matched turbos, and you run them in TTC or parallel (a la 300zx, callaway TT, etc) then each turbo puts out the same pressure. Neither will overpower the other because they are balanced.
If you have sequential turbos, matched set, and run them, think of this scenario: At peak boost, the EGBV is open, both turbos are spinning 100% of capacity. What is the output of each? The same. No pressure differential means that they are both safe. If you understand that, then you understand how they act from the moment the EGBV opens. The pressure in the exh. manifold is balanced, since both turbos see the same pressures on the inlet and outled side of the exducer. The pressure on the compressor sides is also the same. One side sees atmospheric and the outlet sides are balanced. The turbos are safe this way.
Now, if you take a set of mismatched turbos, in TTC: The smaller turbo (less pressure) will make less pressure than the larger turbo. Knowing that turbos work entirely off of pressure differentials, what happens? Think of it this way. The compressor wheel takes atmospheric pressure air and compresses it centrifugally to a higher pressure. It does this by the differential across the exducer, produced by the exh. gasses. For some pressure differential across the exducer, you will get some turbine rpm and some pressure dependant on compressor resistance. If you operated the turbo and capped the intake of the compressor, the turbo over spins and roasts the bearings or becomes inbalanced and breaks a shaft,etc. Also, when the large turbo spins in TTC, the output pressure of that turbo is working directly against the pressure differential across the exducer of the smaller turbo. Let me know if you don't understand, I may not be making this clear enough.
if the turbos both came on at the same boost, that is, if the check valve opened when both outlet pressures were the same, then you would have niether a pressure differential nor any backwards flow to spin one of them backwards. you would just have to have some way of ensuring that the check valve opened under this condition. basically, you would need to ensure that the big turbo spooled itself up all the way to whatever boost level you wanted the small turbo to run alone. that would be the problem-- allowing the big guy to spool up at 12 or 15psi or whatever you wanted the small turbo alone to spin up to.
> that is why the stock turbos have a reed valve. Because without it, #1 would spin #2 backward (remember #2 is essentially off because of the EGBV) until #2 could match the pressure of #1. #2 could NEVER get to spin in the right direction if there was no reed valve, because from the moment you made boost with #1, #2 would spin backwards and never be able to reverse direction. One other point with the reed valve and mismatched turbos: A pressure differential in the wrong direction (recall it is a one way valve) closes the valve. So you will dead-head the smaller turbo and over spin it.
garrett powerstroke diesels use an "exhaust backpressure valve", which is a thick butterfly like an inch from the turbine outlet. i think they are 3 or 3.5" in diameter and are clearly made to withstand reasonably high temperatures. i saw one on ebay the other day go for next to nothing... might be a good place to source the exhaust control and bypass valves
> Could be a good source, but one problem for us: Gasoline engines have a much higher exhaust gas stream temp than a diesel, so it would probably need to be looked at to see if it would work. The EGBV bearings (and I think part of the housing, but I am not sure) in the supra are ceramic for this reason. I also think that they are using large, thick butterfly valve as an engine brake in the diesels, but I would need to see it to know. We don't need anything thick, but proper sealing is critical (again, that is why I think it is ceramic in a supra - thermal stability) but a diesel needs a thick butterfly because that's the "jake brake". This how diesels use that super high compression motor to brake on the downhills.
reply to above:
i see your point on why you just want to create a higher flow version of the stock system, htat's not a bad idea at all, i think.
not just flow, pressure.
but you are missing the point that in a mismatched system, both turbos are not just free-spinning, they will of course be wastegated to control how fast they spin so that they produce a steady outlet pressure. again, if both turbos are producing the same outlet pressure, there will not be any backwards spinning. if they were spinning all-out with no wastegate, then of course, you are right, the small turbo would make less pressure, because, as you said, they would have similar (but not the same because of the different restrictions of the ex. housings), pressure and energy differentials pre and post turbine. but this is not the case in like 99% of gasoline turbo applications... we use wastegates. so the difficulty now becomes making sure that the big turbo spools all the way up to whatever boost marks your cutoff for the small one. if it is unable to spool this much, then you will have problems 'cause the outlet pressures won't be the same. otherwise you'll be good to go.
no, you are missing the point. If you want to balance the pressure so there is no pressure differential betwen the two, then they have to be the same. Otherwise, you can only use the large turbo to make the same pressure as the smaller turbo, in which case you have identical turbos (at least pressure wise) and you are underutilizing the second turbo. Also, in balancing the pressure, you would need to be able to balance the pressures thru the waste gate of each turbo as they spool. The large turbo and the small turbo will certainly have different curves and because of this, you cannot just balance the pressure so easily. If the turbos were linear in spool characteristics, this would not be easy to do. They are certainly not linear, and this is an almost impossible task. I ask you to show me an example where this system has worked **well**.
diesel exhaust temps are not *that* much lower than gasoline exhaust temps. like 1350-1450 deg F compared to 1450-1500ish. maybe this difference is quite relevant depending on those bearing materials, but i just kind of assumed it was not that important. *shrug*
we are talking a valve that was designed for a stock diesel engine. Temps are lower. I don't recall exactly, so I wont state numbers, but lower. You and I are talking about a high horsepower gas engine, that will be pushing limits, and it will have siginificantly higher temps. The butterfly in the exhaust tract of a supra is thinner than a jakebrake butterfly. It has no significant pressure to deal with, it is a flow interupptor, but the flow isn't cut off by it, it is simply redirected thru turbo #1. You see? The jake brake needs to be able to pressureize that short tract of manifold between iteself and the piston no near cylinder pressures. It stops the flow and the flow has no where to go. Different animals.
i think the blade is thick to deal with heat and pressure, because when it is fully open (ie, when the gas is mashed and the engine's supposed to make a lot of power) they still want to maximize flow (use a thinner butterfly). flow is actually really important on a diesel because EGTs rise rapidly with exhaust backpressure and kill performance, so i'm assuming they were in some way forced to use that thick a butterfly. the toyota one is actually pretty thick too.
The blade is thick to deal with heat and pressure but you want a thin blade to increase flow.... YES, but which do YOU want? Flow thru a 3.5" diameter tube is pretty signficant, so I am guessing they factored in the cross sectinal area of the butterfly into that equation. In any case, I don't think fabricating a valve is a show stopper for someone intent on developing a sequential TT system with matched LARGE turbos. I'd undertake this - it would be a great project and I have thought about it alot. To do it RIGHT would take some $$ and there really is no return for it, few would buy a system, regardless of how great it was...... it's costly and the overall pool of supra owners is too small to justify me spending my money. I would design the system and fabricate it, but I need someone to chub up some funding assistance! Anyone who wants to have one of these, developed by a senior Mech E. can get one..... contact me (OK shameless self promotion ends here)
you are missing it again my man. you want to balance the COMPRESOR OUTLET pressures. if you do not, that is how you will get reverse spinning. you can easily do this with wastegates, that is what they are meant to do.
I'm not going to go too much further with this.....
A wastegate doesn't give you enough sensitivity to control pressure as well as you would need to.
if you are worried about reverse spinning due to mismatched turbine pressures, that would mean the turbine outlet pressure would have to be higher than turbine inlet pressure. there is no reason to worry about this happening with a half-decent exhaust system. for both turbos, turbine inlet pressure will be relatively close to manifold pressure, maybe a bit higher on the smaller turbo. the turbine outlet pressures will be relatively close to atmospheric. the differences here do not matter, because you will never have a turbine outlet pressure of the small turbo exceding its inlet pressure with an exhaust system that isn't completely and utterly choking. so you do not need to balance the pressure difference across the turbo. in fact, you will want to have unique pressure and heat energy differentials, controlled by the wastegates, in order to maintain the same compressor outlet pressure, because, as you and i both agree, the small turbo and the big turbo will require different differentials to make the same compressor outlet pressure.
you need to explain, are you referring to the compressor or the exducer wheel. I'm trying to explain this to you as well as I can. All I can tell you is, your apporach won't work. Theoretically it could work if you had a good enough controller, but technically, you would be trying to control one turbo to be the identical of the other (map). Why do this when you could simply use two indetical turbos? Your limiting factor for the large turbo would be the performance of the smaller turbo, so you would never see any benefit from the larger turbo.
hey buddy, not to be an ass, but you were wrong and did not think it through all the way. you ought to come to grips with it instead of coming across as stubborn and foolish. sure, when you first think about it, diffferent sized twins seems like it won't work. if you think about it there's really nothing wrong on the exhaust side of things! properly sized wastegates control compressor outlet pressure very well. again, to have a good twin setup with no backspinning you need:I'm not going to go too much further with this.....
A wastegate doesn't give you enough sensitivity to control pressure as well as you would need to.
it's specified rather clearly in my explanation. the turbine wheel is the exhaust wheel. the compressor wheel is, well, the compressor wheel.98mkiv said:you need to explain, are you referring to the compressor or the exducer wheel.
what are you talking about? like really, what are you talking about? you are not by any means trying to control one turbo to be the identical of the other in terms of their compressor map. i mean, i don't even know how that's possible, i barely even understand what it means, because it doesn't make any sense! they are two different turbos and they can work fine independently until one of them runs out of gas (when this happens, switch to the large turbo only, or lower your hp expectations).98mkiv said:I'm trying to explain this to you as well as I can. All I can tell you is, your apporach won't work. Theoretically it could work if you had a good enough controller, but technically, you would be trying to control one turbo to be the identical of the other (map).
each turbo in a twin setup looking for an efficient 800 or whatever hp will still be a pretty big turbo. you can get less lag by offsetting the sizes. the limiting factor need not be the small turbo. this setup would easily have the means to completely shut off the small turbo at higher RPMs and higher boost levels where the small turbo is at its limits, but still be able to run both the small and the large at the same time to avoid such a harsh transition.98mkiv said:Why do this when you could simply use two indetical turbos? Your limiting factor for the large turbo would be the performance of the smaller turbo, so you would never see any benefit from the larger turbo.