Jack

iTrader: (2)

I am looking for the sharpest boost rise with just spring pressure, using the 50 mm HKS racing wastegate with the adjustment screw/plate. If the target boost is 13psi, what spring would have a sharper curve the 8.6 or 11 psi spring. Remember I can get the target boost of 13psi due to the preload adjustment on the wastegate with both springs. Your feedback is welcome.

pistonsuk

The lower psi spring will react faster, but may yield more unstable boost levels.

The difference between the two springs is the "k" of the spring. This variable defines the force of a spring under a displacment in the equation F=kx. the "F" in this case is the boost pressure seen at the wg diaphram.

Lets say that the wg has to be open 1/8" (x) to bypass the required amount to hold 13psi at xxxxrpm. This means that the 11psi spring will require a larger boost change to get the required amount of displacment. i.e. the spring will have to be preloaded to 11psi to fully open at 13psi. Where as the 8psi spring could be preloaded to 12psi to get the required displacement at 13psi. As mentioned above this means the 8psi spring will be more sensitive to boost flucuations and may yield unstable boost.

Justin

Jack

iTrader: (2)

Justin

Thank you, so for my clarity, the 8.6 spring preloaded to 12psi will net a sharper boost rise, or I will get to my target boost (13pi) sooner or lower rpm.

Zero R

iTrader: (1)

Some more info on the same thing I think your getting at.

https://www.rx7club.com/single-turbo-rx-7s-23/spec-fd-gt35r-wg-spring-tests-412649/

pistonsuk

Yes. The fastest boost rise rate possible is with no wg or a wg that doesn't open. Anytime your wg is open you are sacrificing boost rise rate by diverting exhaust gas aroung the turbine.

Idealy you want the wg to be completly shut until exactly 13psi and then open just enough to maintain 13psi but not restrict to the point that boost goes to 14psi. In reality wg open a bit sooner to get a jump on the flow (opening displacment) needed to not over boost.

As far as "unstable boost". What I was referring to is the operation of the closed loop that the turbo system runs on once the desired boost level is reached.

The closed loop system is this: The wg opens more if it is fed a larger manifold pressure. If the wg opens, the turbine is fed less exhaust gas and therefore lowers manifold pressure. This is what regulates your desired boost pressure.

Spring rates affect the sensitivity of this system. If a lower psi spring is used (smaller k) the spring will displace more given the same change in manifold pressure (say 13-14psi). If this displacement is large enough to divert more exhaust gas than needed the manifold pressure will drop(say from 14 to 1-12psi). This in turn will make the wg close more and then the boost will go back up (maybe back to 14 or 15psi). In short, if too low of a wg spring is used there is a greater chance of the manifold pressure oscillating.

For your situation try the 8.6psi spring and if you find this oscillation to be a problem go with the other.

Justin

twokrx7

iTrader: (1)

The higher spring setting will build boost faster if that's what you are looking for.

pistonsuk

This ^ is not true assuming that he preloads both springs to run 13psi. The reason for this is explained here:

Justin

Jack

iTrader: (2)

Thanks Justin...

twokrx7

iTrader: (1)

I'm afraid it's still true, even if you preload a lower k spring the k does not change. It will still deflect more than a higher k spring below it's so called setting. If you preload a lower k spring to increase it's setting to 13 psi it will still deflect more at 12 psi than a higher k spring with a non-preload setting of 13 psi, therefore the lower k spring will be more open below the target psi and therefore not spool up as fast. I guess I'll add IMO since everything is debatable on this forum, even mechanics are subject to interpretation but not often on such simple applications.

MARTIN

^thats right...

Montrealer

F=kx

in most basic terms, to get 13 psi with the softer spring, you are just lowering k and increasing X.
The resutling force F acting against the manifold pressure will be the same no matter wich spring you use.
So technically they should start to open at the same time.

I do understand what twokrx7 is explaining, but this will only occur once the springs starts moving. The spring with a greater K will see a greater value of F with the same increase in X. The greater value of F required to move the bigger spring means it will take more time for the larger spring to get to the fully opened position (more resistance). So the smaller spring should be more responsive. But the travel delta X of the spring and the delta K of the two springs is so small that I doubt there will be much difference.

Anyhow thats the way I see it...

pistonsuk

We are splitting hairs on this one. In actuality there is probably not much difference as Montrealer said. However, I do want to better explain my reasoning.

First of all the preloading of the spring makes it possible for either spring, regardless of spring rate (k), to open at the same time (or one later than the other i.e. the lower k after the larger k). My point was the lower could be preloaded more, and to open later.

Lets take the 13psi target senario with both an 10 and 8psi spring to be available to us. The wg diaphram has a surface area of 1 square inch. The 10psi spring has a k of 24lb/in and the 8psi spring has 16lb/in (these are made up for purpose of explanation). To maintain 13psi at xxxxrpm the wg must be open say 1/8". We are looking for the sharpest boost rate (keeping the wg shut as long as possible).

10psi Spring:

To fully open the wg the required amount (1/8") at 13psi the 10psi spring must begin to open at 10psi.

As seen in the equation F=kx: (13lb/in^2-10lb/in^2)x1in^2=24lb/inx1/8"

8psi Spring:

To fully open the wg the required amount (1/8") at 13psi the 8psi spring must begin to open at 11psi.

As seen in the equation F=kx: (13lb/in^2-11lb/in^2)x1in^2=16lb/inx1/8"

I hope this helps.

Justin

MARTIN

I dont understand why you guys go through this much trouble to explain something that is not too difficult... You guys dont have to give a theory... ITs been done and proven... There is a reason when you use a boost controller that after a certain psi you need to put in a bigger spring... boost controllers preload the spring for accuracy. So for example, if you want to boost 25psi and you have an 8lb spring, you might have problems reaching that boost level because the ammount of pressure that the BC is preloading the spring is greater than the spring.

twokrx7

iTrader: (1)

In a nutshell, my explaination was not technically correct cause I thought I could wing it without putting pencil to paper. The lower k spring with more preload will crack open a hair later but as Martin stated there is little difference. Perhaps that gives the higher k spring a better "response" using Montrealer's terminology although he associated this term with the lower k spring. As long as the two spring settings are close, there should be little difference. Biggest issue occurs when two springs are far apart and it takes alot of preload on the lower k spring to equal the higher k spring setting; excessive pre-load can limit the available travel of the spring and ultimately how much the wg can open, effectively making the wg perform like a smaller wg.

MARTIN

plain and simple, if your max boost is 20 and low boost is 15, then get a 15lb spring... you will find that the highest spring rate will give you the best boost responce...

Jack

iTrader: (2)

personally I like the engineering answers, due to the fact I learn why things work...

MARTIN

and thats good.... but simple answers are the best answers. Unless, like you, want to know how **** works...

pistonsuk

This is wrong.

Boost controllers are a leak controller. They simply reduce/eliminate the pressure seen by the wg diaphram.

For the most rapid boost response you should use the smaller spring with the exception of:

1) As mentioned above, you run out of available needed travel by over-preloading.

2) The response is unstable due to the F=kx explanation above.

The most effective setup, in theory and on paper, would be to use a spring rate very near the desired boost level and then electronicly boost control it. This setup benefits from the very stable high spring rate to precisly control wg opening and minimizing the chance of running out of wg travel but does not suffer from the early opening of the stiffer spring b/c the solonid will not show it any pressure until the desired boost level is reached.

Justin

MARTIN

They still let pressure get to the diaphram. Im my book, its called preloading. Boost controllers preload the spring depending on the ammount of boost you are pushing. But it is constant percentage. There is a reason why in a profec b when you turn the **** all the way up, it doesnt mean unlimited boost. There is a limitation, and its not the BC its the spring.

Jack

iTrader: (2)

With respect to running an MBC and HKS 50mm 8.6 spring and targets boost 13psi. Should I leave the spring at 8.6 and control boost with the MBC or should I create some preload on the wategate spring and fine tune with MBC. Again the purpose to create the sharpest boost rise to 13psi.

Asleep

I don't know the theory enough to split hairs.

What I will tell you is that testing experience on my car shows the HIGHER the spring rate (closer to target boost) in my wastegates yielded the fastest, sharpest, quickest, necking snappingist, response.

I logged the 12psi spring, and the 17psi springs for my car. Target boost was 18psi (the lowest boost setting I wanted to use.) Test scenario was executed over two different wastegates and exhaust manifolds on the same car and turbo system.

An Engineer tests their understanding of a theory. Test it out for yourself and share back your results.

rx713bt

iTrader: (4)

Just design a PID controller.. Damn, you guys remind me of controls class.