Twincharging Cut Off?

[ynhrgt]

I'm doing a fair bit of research, and I ask a question yesterday regarding twincharging but I guess nobody knew the answer to.

I'll ask a more general question however, can anybody suggest how the supercharger would disconnect after the engine hits a certain rpm so the larger turbo(s) can take over? I've heard things like using a by-pass valve, but wouldn't the supercharger suckup horsepower since its still connected to the drivebelt? I've also heard somebody suggesting an advanced pulley system lol.. I don't think it needs to resort to that but if it must.

I know all this sounds complex, but if anybody could offer some suggestions.

[RANDY]

That Nissan Micra (March) had a bypass for its super/turbo setup. I'm not sure of any easier way?

[StockyMcStock]

I am. don't use a switchover point. it defeats one of the great advantages of a twincharge anyway, in that your inlet pressure gets back up to close that of your exhaust pressure. using a simple blow-through (turbo outlet into blower INLET) your inlet pressure will always be substantially higher than exhaust back pressure.

where did you ask the question before, i can't find a thread?

[Abo Bob]

This question was asked about a month ago and I provided an answer.

Check the second most recent zoom magazine for a full diagram showing it.

[Cubes]

StockyMcStock,

Have you any links on other motors using your setup?

I'm keen to look in to this more, not that I'll be doing it, more so out of curiosity.

Hurry up and get it running.

[ynhrgt]

I am. don't use a switchover point. it defeats one of the great advantages of a twincharge anyway, in that your inlet pressure gets back up to close that of your exhaust pressure. using a simple blow-through (turbo outlet into blower INLET) your inlet pressure will always be substantially higher than exhaust back pressure.

where did you ask the question before, i can't find a thread?

Can you elaborate on this? I'm not quite what you mean by turbo outlet into a blower inlet.

[RANDY]

I think he means:

turbo -> super -> intake.

That way, the super pulls air through the turbo for low-end, but when there's enough exhaust to spool the turbo, it will force air through the super.

[Warpspeed]

Hi ynhrgt, have only just found this thread. I e-mailed you yesterday, and pm'd you today. It may be best if we just talk here, so others can join in.

Stocky (Neill) is the guy in Queensland I mentioned building a twincharge system right now. As he is already here on this thread, no need to go over to the Commodore Forum to reach him.

A twincharge system is really just a supercharged engine with a turbo fitted to it as well. The turbo compressor feeds straight into the supercharger intake, there is no need to turn off the supercharger.

A bypass system around the supercharger is essential if you plan to leave the throttle body in the original location. As you have an RB26, leave the six throttle bodies right where they are. That is definitely the best way to go about it.

It is fairly obvious that you cannot just fit a positive displacement supercharger up stream of the throttle body. When you close the throttle at high engine Rpm (when changing gear), either something will burst, or the drive belt must slip or break. A bypass arranged around the supercharger will eliminate that problem.

A bypass will also effectively unload the supercharger at small throttle openings. Although the rotors still spin, the supercharger effectively draws no drive power from the crank. It will improve fuel economy by about 10% and reduce noise and heat buildup in the supercharger. Floor it, the bypass slams shut, and away you go. Back off, and the bypass opens. It works wonderfully well.

[Bl4cK32]

Or just run it like Mad Max

[Warpspeed]

Not a good idea.

If you just de-clutch the blower, air pressure will spin the rotors o/k but the pressure drop is fairly high. The engine will lose a LOT of power that way and it will be really gutless. Toyota use a blower bypass as well as the clutch. With the bypass open it does not really matter if the rotors are spinning or not.

A clutch is a nuisance, and it wears out. Better not to have a clutch at all. A clutch also rather limits you when selecting pulley sizes.

[ynhrgt]

Hi ynhrgt, have only just found this thread. I e-mailed you yesterday, and pm'd you today. It may be best if we just talk here, so others can join in.

Stocky (Neill) is the guy in Queensland I mentioned building a twincharge system right now. As he is already here on this thread, no need to go over to the Commodore Forum to reach him.

A twincharge system is really just a supercharged engine with a turbo fitted to it as well. The turbo compressor feeds straight into the supercharger intake, there is no need to turn off the supercharger.

A bypass system around the supercharger is essential if you plan to leave the throttle body in the original location. As you have an RB26, leave the six throttle bodies right where they are. That is definitely the best way to go about it.

It is fairly obvious that you cannot just fit a positive displacement supercharger up stream of the throttle body. When you close the throttle at high engine Rpm (when changing gear), either something will burst, or the drive belt must slip or break. A bypass arranged around the supercharger will eliminate that problem.

A bypass will also effectively unload the supercharger at small throttle openings. Although the rotors still spin, the supercharger effectively draws no drive power from the crank. It will improve fuel economy by about 10% and reduce noise and heat buildup in the supercharger. Floor it, the bypass slams shut, and away you go. Back off, and the bypass opens. It works wonderfully well.

Warpspeed, you truly are the man to talk to regarding this. Again, excuse my ignorance.. but by bypass system, you mean something like a blow-off valve or re-circulating bov? So if I'm understanding this correctly, and I'm seriously trying to.. it would look something like this

Two Pod Air Filters >> Turbo + Turbo >> Supercharger >> Blow Off Valve >> Intake ??

Basically, air will go into two air filters for the twin turbos, then the twin turbos will spin and push the air into the supercharger (which spools very quick) then it will blow all of that into the intake. And if the throttle gets cut off, the air would get pushed back through a blow off valve or re-circulating blow off valve. My other question when the turbos are spinning really fast and producing a lot of pressure, exactly wouldn't that affect the supercharger? I mean since the supercharger is blocking the flow sort of?

One thing I have in doubt is.. exactly HOW would the supercharger NOT take away power? Its still connected to the belt. And what do you mean by it would unload the supercharger at partial throttle? I need a bit of clarifications on this heh.

Besides Opcon Autorotor, are there any other good twin-screw superchargers?

Edited February 26, 2006 by ynhrgt

[Warpspeed]

The first thing is that if you add a supercharger, the engine is going to flow a lot more air. The original twin turbos will be far too small on both the compressor side and the exhaust side. They could be replaced with larger twin turbos, but a big single turbo would probably be a much better alternative.

The purpose of the bypass system around the supercharger is to open up a flow path direct between supercharger intake, and supercharger outlet. That does two things. First it removes all supercharger contributed boost when open (obviously) by allowing the supercharger to recirculate air around through the open bypass valve back to the supercharger inlet.

When you go to change up a gear at redline Rpm, you quickly close the throttle. Without a bypass of some sort, the supercharger would continue to pump air up against the completely closed throttle, causing a MASSIVE boost spike. It would blow a hose, or burst your intercooler core for sure. So you want the bypass to open fully whenever you suddenly back off on the throttle. But it must also close fully when you want some acceleration, to allow the supercharger to work normally.

When driving around at fairly light throttle, (or at constant highway speeds), the bypass can be held open. This has the effect of removing all back pressure from the supercharger, and it removes almost all the drive torque required to turn the supercharger rotors. It is like having the rotors spin around in free air without any outer casing. So there is essentially no power loss, no air heating, no noise, and it will improve fuel economy significantly.

This bypass must operate SMOOTHLY and PROGRESSIVELY. The usual (noisy) turbo blowoff valve is very sudden and violent, and totally unsuitable for this application. A much better way is to adapt a turbo external wastegate to perform the bypass function. All it needs is a suitably light spring fitted, and it will work fine.

In order to make any boost pressure at all, the supercharger must pump more air volume than the engine would normally flow by itself. It can never therefore be a flow restriction to the turbo. If there is a higher boost pressure after the supercharger than before, it is certainly not restricting flow. So don't ever think that the supercharger will hold back the turbo at the top end by being a restriction.

What DOES restrict top end horsepower is the exhaust turbine back pressure. Now nobody ever talks about how total exhaust back pressure costs horsepower. You never hear turbo people say my turbo engine makes 400Kw but if it had zero total exhaust back pressure it would be making 500Kw. Unfortunately that 100 lost Kw is the power needed to drive the exhaust turbine to make the boost in the first place.

People just look at the cherry red exhaust pipes and the glowing full moon exhaust housing on the dyno and marvel at how powerful the engine is.

But the same people point at a supercharger and look at the big pulleys and say how awful, that supercharger is sucking 100 Kw out of my engine. My 500 Kw engine only makes 400 Kw because of that damned INEFFICIENT supercharger.

The truth is, that 500 Kw engine needs 100Kw to compress the air. You can do it with a turbo, or you can do it with a supercharger, but that 100 Kw still has to come from somewhere.

A further truth is that a supercharged engine runs with high boost and very low exhaust back pressure, the heat can get out. You never see red hot exhaust pipes even on an 8,000 Hp top fuel dragster. They run cool. Supercharged engines are relatively detonation free because the heat can get out.

A turbo engine is the exact opposite. The heat is trapped by the high total exhaust back pressure, and as a result turbo engines are horribly susceptible to detonation problems. Everyone knows that.

With twincharging it is the ideal compromise. Boost pressure can be kept just above exhaust back pressure, even with a full street exhaust system, and detonation is much less of a problem. It will also be much more responsive than a turbo engine down low. And it will have better top end airflow than a straight supercharged engine would have.

The Opcon Autorotor screw supercharger is the best and most efficient supercharger available. A Whipple screw supercharger (made by exactly the same company) are a cheaper mass produced version of the same thing. A new Eaton roots supercharger costs about the same as a Whipple but is nowhere near as efficient. A secondhand Eaton off e-bay is a budget way to get started. Toyota superchargers are just complete crap, and are hardly worth the very low cost they can be bought for.

Edited February 27, 2006 by Warpspeed

[ynhrgt]

I'm beginning to understand what your saying here, and I have a "very rough" idea of what your talking about here. Basically your saying, the supercharger will not take any power if it re-circulates the air it creates back into itself, thus it would spool itself up. This way it doesn't suckup power from the drivebelt.

Could you possibly draw a diagram to better explain this? I mean understand the use of a wastegate as a bypass valve, but exactly how would the supercharger outlet be re-circulated back into the inlet? I'm a bit confused on this one.

In order to make any boost pressure at all, the supercharger must pump more air volume than the engine would normally flow by itself. It can never therefore be a flow restriction to the turbo. If there is a higher boost pressure after the supercharger than before, it is certainly not restricting flow. So don't ever think that the supercharger will hold back the turbo at the top end by being a restriction.

So if this bypass valve needs to OPEN when the throttle closes so you don't blow a hole in your engine, and it needs to be OPEN to re-circulate the air the supercharger makes back into the supercharger itself.. then why do you need to use a bypass valve period? I mean couldn't you just make a pipe that does both? Sorry if that doesn't make sense.

[discopotato03]

Supercharging/turbocharging . Supercharging is really just another term for forced induction . What everyone calls a "supercharger" is a mechanical engine driven air pump of which some are constant displacement pumps ie lobe or screw and others are rotary or like the compressor side of a turbocharger .

The disadvantage of the mechanically driven pump is that its out put does not match the rising air demands of the engine . This is why the mechanical supercharger (MSC) is overdriven by step up gearing so that a sane sized compressor can develop a head of pressure over the engine .

The typical Roots or lobe type SC is a very inefficient device because it absorbs considerable crankshaft Hp to drive and its compressing efficiency is quite low . Its low because the temperature rise between its inlet and outlet is quite high meaning charge density is low and therefore some weight of oxygen is lost . High charge temperatures are bad because it puts the compressed air fuel charge (on the compression stroke) that much closer to the detonation and or pre ignition point .

The rotary or "half turbo" style SC's suffer from low boost at low revs and probably too much at higher revs which has to be wasted .

To quantify

1) If you add a SC an engine will flow a lot more air .

I would say that if you put positive pressure across the inlet tract it should be capable of flowing a greater volume of a compressible medium . With respect to internal combustion engines we really should look at inlet pressure as absolute pressure meaning ANYTHING above a total vacum is positive pressure . Na engines only have sea level air pressure to work with at best so you could say they are supercharged at 1 Bar or one atmosphere . So if you can raise this pressure to above 1 Bar the tract should flow more untill the restrictions limit and maximise the total flow .

2) SC bypass open at idle ? It doubtful its turning fast enough to generate any real extra airflow .

3) Bypass open = no energy loss . Not really , you will get drive losses through step up gearing or belt

drive frictional losses plus the innertia of the lobes or screw gears .

4) What does hold back top end Hp is turbine backpressure ? This is only the case if the turbine side of the

turbocharger is not properly sized in relation to the expected power output and necessary exhaust

throughput . If this is done its the system designers fault not the turbos .

5) No one ever talks about how total exhaust pressure costs horsepower . Many of my turbo related posts

mention turbine inlet pressure which is between the exhaust valve and turbine housing but any

measurable pressure south of the housing affects it . I think it would be more accurate to say that the

100Kw may have been lost to pumping losses against an exhaust restriction , sure when the turbine inlet

pressure goes up the exhaust gas temp rises with it but its not diffucult to get around .

6) If the total exhaust pressure were zero . Well if it was I'd say we would have no scavenging going on .

Just on this point SC engines do have dramas with their exhaust systems because they have to pass the

exhaust gasses the engine produces in a supercharged state so the manifold runners and tail pipe +

mufflers need to be large enough to handle it . Its a bit sad at low engine speeds with large pipes trying

to get the noise legal let alone much extraction going on . The turbine side of our turbo (provided its

dump pipe is properly made) tends to act like a partial muffler to a degree .

7) Truth is 500Kw engine takes 100Kw to compress its air ? This could be true with a mechanical SC though

doubtful with a properly designed competition turbo engine . It is possible with modified production four

valve engines (porting/cams/manifolds/turbos) to have greater inlet manifold pressure than exhaust

manifold pressure so volumetric efficiency would have to be good .

8) Further truth is that SC engines run high boost with very low exhaust back pressure . Fact - could run

high boost/high charge temp/high crank power loss and low exhaust manifold pressure if everything

starting at the exhaust valve was sized large enough not to pose any restriction .

9) The heat can get out ? Maybe , but its too late if the damage was done on the way in and in the early part

of the cycle .

10)Top fuelers would not use pump fuels and alcohol burns with less heat output . Large diametre stacks

should not form any restriction so heat buildup should not be an issue in under 10 seconds .

11)SC engines detonation free because heat can get out , see 9) .

12)Turbo engines horribly succeptible to detonation because of heat trapped by high total back pressure ?

Yep if you set it up to do that sure . If you set it up properly with the appropriate turbine / housing /

manifold / wastegate / dump / exhaust - no chance . Please do not try to tell me that the turbo F1

engines could not get the heat out and would have been better off with mechanical superchargers .

To round off , I believe the last of Lancias Grp B super cars used a mechanical SC/turbo combination . I think they were run in parallel with a priority valve that could feed the inlet manifold from either . So the SC got the ball rolling and the turbo took over when the SC tapered off . I guess the priority valve stopped one from backfeeding through the other when not loaded up . The idea was to have the best of both worlds and a broad power range from limited engine capacity .

There is a fellow at the American site High Altitude Imports who is running a turbo and SC on a Honda engine and the development story is very interesting if you like that sort of thing .

Cheers A .

[StockyMcStock]

you're reading way too much into what tony has said there, he was merely using examples to illustrate his point. you could waffle on all day about how turbos are better, the same as some guys could waffle on all day about how their GM 6/71 is better.

at the end of the day, remember everything in an engine package is a compromise of some sort. tony and i just have different views of what is important in a real street engine than most people do.

i'll see if i can dig up my explanation post over on calaisturbo of the bypass system, i've written it all out too many times already to do it again.

[Warpspeed]

i'll see if i can dig up my explanation post over on calaisturbo of the bypass system, i've written it all out too many times already to do it again.

Thanks Stocky, if you can dig up your explanation it will save me a lot of typing and answer ynhrgt's supercharger bypass question.

Disco, I take issue with you on a few points.

You say that boost pressure is higher than turbine inlet pressure on a four valve street engine. That might be true if a maximum sized turbine is used, so full boost pressure is not reached until almost at redline Rpm. If you want a reasonably low boost threshold and a flat boost curve the exhaust turbine will need to be made smaller than that. On my own turbo engine, full boost is reached at about half of maximum RPM, and turbine inlet pressure is exactly TWICE boost pressure. Oh, and it is a four valve DOHC engine too. That is fairly typical of a reasonably sized modern ball bearing turbo.

The other point is that turbos only actually reach their maximum efficiency over a very limited range of flows and pressure. Any compressor flow map will quickly tell you that it will be operating well away from the peak island on the flow map for much of the time. A supercharger has a MUCH wider operating range, and although the peak efficiency may be less, the average efficiency can be a lot higher. Screw supercargoes operate at 70+ percent over perhaps a 10:1 flow range. Turbo may operate at 74 percent over a 1.5 : 1 flow range if you are lucky and well below that everywhere else. I will not mention compressor surge, or lag. There is not the slightest doubt which is going to be more efficient.

If you have a reasonable intercooler, the actual discharge temperature of the supercharger or turbo becomes much less important.

What causes detonation is excessive charge temperature at the end of the compression stroke causing unstable combustion. The temperature of the charge at the beginning of the compression stroke is vitally important, (as is the compression ratio) in determining the final temperature reached.

Low induction temperatures coming out of a good intercooler are not going to be worth a damn, if the combustion chamber is filled with very high pressure exhaust residuals at up to 1,000 degrees celsius.

A turbo engine will have a lot of trapped very hot, high pressure exhaust gas, that simply cannot get out past the exhaust turbine. That is going to mix with the cooler incoming induction air, raising it's temperature VERY significantly. If your exhaust manifold is glowing red hot, the exhaust gas going through that manifold is red hot too. Your low compression turbo engine, with the dished pistons is going to hold a lot of trapped exhaust gas that cannot escape. THAT is why turbo engines detonate so readily.

Now with a supercharged engine, there is essentially minimum exhaust back pressure after the engine. If the engine has tuned pipes, the exhaust port pressure may even be negative during valve overlap. All that hot gas gets sucked right out. And if not, a higher boost pressure will drive it out during valve overlap.

A turbo engine that has a higher exhaust manifold pressure than boost pressure cannot use valve overlap in the same way to scavenge the engine. Only a turbo race engine can. Your turbo race engine with the big turbine and high overlap cam can make massive top end power, but it will be a very peaky laggy engine.

A supercharged engine can run milder cams, and have a very broad flat torque curve. I know which I would prefer in a road car. It is a fact that given an identical engine, a turbo version will be far more prone to detonation than a supercharged version.

There is also the myth that turbo engines are more powerful. If that is true, why are all the fastest drag race cars supercharged ? Where are all the four second turbo cars ? Don't get me wrong, I like turbos, my own road car has a turbo, but they are not as perfect as some people like to believe.

[Abo Bob]

Well doesn't my first post look foolish now...

There are some awesome ideas in here. Real intelligent thinking which I like.

[StockyMcStock]

here we go, i found it. it relates back to the SC-14 blower that i was using at the time but describes the bypass system pretty well:

"the bypass system is used to vent air around the blower (thus unloading it completely) at idle and light throttle openings. you will need an external turbo wastegate (i use a 32mm garrett item, but the bigger the better!) and whoever does your piping will make a flange/pipe attachment to the piping somewhere before the throttle body.

the premise behind it is that a wastegate has 2 fittings, on either side of a diaphragm which "see" different pressures and thus act on the diaphragm. the diaphragm (and hence poppet valve which normally opens at a certain boost level and vents exhaust gasses) has a fitting above and below it, on the "cold" and "hot" sides of the wastegate respectively. the cold side is connected to your plenum (i use the brake booster line) and "sees" the pressure changes AFTER the throttle body. the hot side is connected to the intake piping after the blower, but before the throttle body. thus, it only ever sees atmospheric or above (boost) pressure.

the idea is that the gate sees the pressure drop across the throttle body.

now at idle, plenum vacuum sucks hard against the diaphragm and pulls the poppet valve open. this allows air to freely circulate out the open gate, the blower runs cooler and draws almost no power from the engine.

when you close the throttle fully, plenum pressure = intake pressure, and the only thing acting at the wastegate is the internal spring, which holds the poppet valve closed hard. thus, you get full upstream boost.

at any throttle openings between these two, the gate progressively closes and is fully proportional to your throttle opening. if you get your spring rate (stiffness) just right then you can cruise around all day without ever seeing upstream boost, but as soon as you put that foot down it's all there. mine is set up so that positive pressure is seen at about 3/4 throttle opening.

i would suggest (if you have an intercooler) to position the wastegate upstream of the intercooler, this will keep air circulating through it at idle (may keep it cooler a bit) and will eliminate the SC-14's growly note. the closer the gate is to the blower, the more blower noise you will hear out through it. another option is to plumb the gate's outlet back into the blower inlet (or turbo inlet if it's twin charged) to reduce the noise.

hope that helps,"

[ynhrgt]

I understand that much. But this is a bit different from what warpspeed mentioned as in the bypass valve would remain open during full throttle, partial throttle and even at idle (fully closed). With yours, why would the bypass valve be closed during hard accelleration? Wouldn't that defeat the whole purpose of the supercharger using its own charge of air to power itself?

I mean basically I was told the bypass should stay open pretty much all the time, the only change would be how much it would open by. Your saying it should be closed at full idle (fully closed)?

I plan on using as large a wastegate as I can get my hands on. If you can include some sort of diagram into this or draw a crude one that would be great. I know I'm making progress here but lol its a bit difficult to understand.

here we go, i found it. it relates back to the SC-14 blower that i was using at the time but describes the bypass system pretty well:

"the bypass system is used to vent air around the blower (thus unloading it completely) at idle and light throttle openings. you will need an external turbo wastegate (i use a 32mm garrett item, but the bigger the better!) and whoever does your piping will make a flange/pipe attachment to the piping somewhere before the throttle body.

the premise behind it is that a wastegate has 2 fittings, on either side of a diaphragm which "see" different pressures and thus act on the diaphragm. the diaphragm (and hence poppet valve which normally opens at a certain boost level and vents exhaust gasses) has a fitting above and below it, on the "cold" and "hot" sides of the wastegate respectively. the cold side is connected to your plenum (i use the brake booster line) and "sees" the pressure changes AFTER the throttle body. the hot side is connected to the intake piping after the blower, but before the throttle body. thus, it only ever sees atmospheric or above (boost) pressure.

the idea is that the gate sees the pressure drop across the throttle body.

now at idle, plenum vacuum sucks hard against the diaphragm and pulls the poppet valve open. this allows air to freely circulate out the open gate, the blower runs cooler and draws almost no power from the engine.

when you close the throttle fully, plenum pressure = intake pressure, and the only thing acting at the wastegate is the internal spring, which holds the poppet valve closed hard. thus, you get full upstream boost.

at any throttle openings between these two, the gate progressively closes and is fully proportional to your throttle opening. if you get your spring rate (stiffness) just right then you can cruise around all day without ever seeing upstream boost, but as soon as you put that foot down it's all there. mine is set up so that positive pressure is seen at about 3/4 throttle opening.

i would suggest (if you have an intercooler) to position the wastegate upstream of the intercooler, this will keep air circulating through it at idle (may keep it cooler a bit) and will eliminate the SC-14's growly note. the closer the gate is to the blower, the more blower noise you will hear out through it. another option is to plumb the gate's outlet back into the blower inlet (or turbo inlet if it's twin charged) to reduce the noise.

hope that helps,"

[Warpspeed]

I understand that much. But this is a bit different from what warpspeed mentioned as in the bypass valve would remain open during full throttle, partial throttle and even at idle (fully closed). With yours, why would the bypass valve be closed during hard acceleration? Wouldn't that defeat the whole purpose of the supercharger using its own charge of air to power itself?

I mean basically I was told the bypass should stay open pretty much all the time, the only change would be how much it would open by. Your saying it should be closed at full idle (fully closed)?

Excellent description Neil,

Ynhrgt, I think you misunderstood me. All this is a fairly simple idea, but horribly difficult to explain, and put into few words.

The wastegate is shut tight only during near full throttle, go back and read exactly what I said. It must be shut (obviously) or there could not be any boost pressure at all.

Yes, the bypass will remain open pretty much all of the time while driving, because you will be driving around at light throttle most of the time, at least on the street you will be if you want to keep your licence. The bypass only closes full above perhaps 3/4 throttle, or whenever you see any boost, and that will only be for a few seconds in each gear during acceleration.

Don't go too big on the bypass. Try to imagine how much boost pressure you would lose if you drilled a 32mm hole in your supercharger pipework ! Believe me, even a 25mm hole would be a MASSIVE boost leak. A very effective bypass need be no larger. The bypass only needs to dump most of the pressure, which even a small wastegate will do. Look how big the holes are in internal wastegates, some are surprisingly small.

Another very important aspect of this, is that the control diaphragm and spring must be proportioned to open and close the wastegate at the desired throttle opening. Somewhere about five to ten inches of mercury vacuum upstream of the throttle is where you want the wastegate to open/close. That is around 2.5psi to 5psi pressure differential across the throttle. So the spring will need to be fairly soft to work in that range.

Now the spring still needs to be stiff enough to hold the poppet valve shut against full boost pressure without leaking, and the spring HAS to be suitably soft to get the correct control operating pressure range. As a result the poppet valve must be made a lot smaller than the diaphragm. If the diaphragm is twice the diameter of the poppet valve, the area ratios will be 4:1 . That means the maximum boost pressure that would force the wastegate open uncontrollably would be four times the control pressure. Perhaps 10psi to 20psi boost if the control pressure is 2.5psi to 5psi.

So make absolutely sure that the wastegate diaphragm is at least twice the diameter of the poppet valve, the bigger the difference the better. Good quality wastegates will be like that, Garret, Turbonetics, and some of the better Japanese gates. Avoid the cheap and nasty Chinese gates. Some of those have like a 60mm poppet valve, and a 60mm diaphragm, with a 42mm outlet. That is exactly what not to use.

Something like a 32mm gate with a 65mm diaphragm, or a 42mm gate with an 80mm diaphragm would be pretty good. Even better would be a homemade hybrid wastegate, 80mm diaphragm with a 32mm poppet valve.