300rwkw - What wastegate

[Sydneykid]

I'm currently getting 300rwkw from a GT30 and a 45mm wastgate at 17psi, so when my forgies are in and I want to push for 400rwkw you think the wastegate is oversized?

Hi GTS-TVSPEC, how are you going to achieve the extra 100 rwkw?

More boost from the same turbo?

Or larger turbo at the same boost?

[Sydneykid]

391rwhp/11 = 35.5mm

35.5/1.3*1.4=38mm

or 35.5/1.3*1.2=32mm

Hi boosted32, I use BHP in the formula, not RWHP.

So 470BHP / 11 = 43 mm

Since 1.3 bar is neither high nor low boost, no boost correction factor is needed.

You have a 50 mm wastegate, which the formula would indicate is a little to big. According to the published theory, this means you may be loosing some response. It would also indicate that the wastegate diaphram may wear prematurely due to it having to open and close rapidly and often to control the boost. A smaller wastegate would not have to do this as its flow would more closely match what your turbo needs to hold the shaft rpm (and therefore boost).

Hope that helps

[DoughBoy]

 The only issue occurs if there is varying efficiency amongst the engines.

What about turbo efficiency? Exhaust manifold design/efficiency? What if the engine is highly tuned to operate at a specific rpm (powerband)? I'm not sure if you are formulating a generic calculation to determine wastegate sizing or just providing a guide to wastegates based on current data.

[boosted32]

Hi boosted32, I use BHP in the formula, not RWHP.

even though I wrote RWHP, i think the calc i did was for BHP anyway, cos:

230rwkw should = 230*1.35 = 310rwhp

230rwkw = 230*1.7 = 391 bhp

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so 391/11=35mm

[email protected]

250*1.7=425bhp

425/11=38.6mm

38.6/1.6*1.4=34mm

anyway... as you said, I could probably get better response and a longer lasting wastegate with a smaller one, thx for the info... I think the guy that owner by car in japan didn't quite know what he was doing... there's quite a few odd things on the car!

[Sydneykid]

Hi Doughboy, I need a bit more information on your "hypothetical" to really give a decent fact based answer.

I would need to know usual stuff, the BHP, boost and wastegate sizes.

In the interim I'll try and answer this one....

Also, about this "response" of a wastegate. Just say you are "coming on boost" very rapidly (as most larger, aftermarket turbos tend to), wouldn't you think that a wastegate that only needs to travel ~5mm to maintain the desired boost level (i.e. a large gate) as opposed to a smaller wastegate that would need to travel ~10mm to vent the same volume of gas would be able to do this faster, hence be more "responsive"? I concede the smaller one would be able to be more accurate in it's flow limiting, but then again wouldn't an "intelligent" boost controller be able to manipulate these regardless?

I am not sure that the travel "distance" of a wastegate is really an issue. Remembering that a 50mm wastegate valve is going to weigh twice as much as a 35 mm one. So there is an inertia issue to be considered. I think that is really the crux of the problem. The diaphram has to move twice as much weight, in and out very rapidly. This has 2 undersirable effects incomparison to a smaller (more correctly) sized wastegate.

Firstly the too large a wastegate has to open and close more often, as it has too little exhaust flow when closed and too much exhaust flow when open. This wears the diaphram, due to the requirement for more movements and more weight.

Secondly all this opening and closing of the wastegate affects the amount of exhaust flow though the turbine, this leads to fluctuations in the boost control. eg; I have seen a relatively low powered engine, with a very large wastegate, move up and down 0.3 bar in its boost as the boost control circuit struggles to keep up with this open, closed, open, closed requirement.

Hope that helps

[Sydneykid]

Hi DoughBoy, I’ll try and look at each one separately…

“What about turbo efficiency?”

Efficiency, is that heat, low boost, high shaft rpm, poor response low airflow etc there are so many ways that a turbo can be inefficient. I don’t know which one to pick for an answer.

“Exhaust manifold design/efficiency?”

If the exhaust manifold is so restrictive it limits the power output, you would need a smaller wastegate to match that power. This is logical as the manifold is doing some restricting, so a larger (than ideal) wastegate would achieve nothing.

“What if the engine is highly tuned to operate at a specific rpm (powerband)?”

Thus it would produce more power at that rpm and therefore need a correctly sized wastegate for that power. You have to size the wastegate for the maximum power (airflow) of the engine. Since you have gone to considerable trouble and expense to have a tuned engine, I would assume that you would want to use it in that rpm range all the time.

“I'm not sure if you are formulating a generic calculation to determine wastegate sizing or just providing a guide to wastegates based on current data”

Both actually. Currently there is no simple formula for determining what size wastegate you use. There seems to be 2 schools of thought, the first being to buy the “biggest” one you can afford. This presumes that bigger is always better, but we know that’s simply not the case with wastegates. Secondly the car owner has no idea, so he has to ask his wastegate supplier. If he is genuine, he will give an answer based on his experience. But what if he isn’t genuine? What if he wants to make the most money he can? Then he will sell the one that makes the most profit for his business.

In either case you can get the wrong answer (and the wrong wastegate), the first because his experiences may not match your requirements and in the second because he doesn’t care whether it is right or wrong for you.

So what I thought I would do is use my experience, then supplement it with real world data from 30 or so volunteers. This would then enable me to come up with a simple formula that anyone can use to check and see if the advice they are getting from their local supplier is good or bad. Stuff like…..

My local supplier tells me I need a 60 mm wastegate for my 200 rwkw Skyline. Why is he telling me something that is obviously wrong? Because it’s on special this month? He has 1 in stock that he hasn’t been able to sell? He only makes $100 profit on a 35 mm wastegate and $250 on a 60 mm one?

It’s all about empowerment, so you (the Skyline owner) can sift out the BS.

Hope that helps

[Ustasa]

You can also throw a spanner in the works by saying that higher boost equates to a higher back pressure in a street engine which means for a given waste gate opening, a greater amount of exhaust gas will bypass the turbine.

[Sydneykid]

Thanks Ustasa, that is one I hadn't thought of. What effect do you think it would have on the calcs?

[Dr_Drift]

Secondly all this opening and closing of the wastegate affects the amount of exhaust flow though the turbine, this leads to fluctuations in the boost control.  eg; I have seen a relatively low powered engine, with a very large wastegate, move up and down 0.3 bar in its boost as the boost control circuit struggles to keep up with this open, closed, open, closed requirement.

Hope that helps

Surely it's not just a matter of open / closed???

Wouldn't the amount the gate has to open be a cruital factor as it will need to flow varying volumes to maintain a set boost level across a certain RPM range??

As for the varying 0,3 bar... could that not be caused due to a poorly setup or inappropriate boost controller? I have seen boost fluctations, allthough not quite as severe, on small internal gates... On nearly every occasion this has been due to the boost controller, but a few times I have seen mismatched turbos which generated compressor surge...

Doc.

[GTS-t VSPEC]

Sydneykid, I was planning on gaining the extra 100kw with increased boost and flow on the same GT30, by porting the head and using cams.

[Sydneykid]

Good questions Dr....

Surely it's not just a matter of open / closed???

Wouldn't the amount the gate has to open be a cruital factor as it will need to flow varying volumes to maintain a set boost level across a certain RPM range??

The wastegate responds to boost pressure. The more boost pressure on the diaphragm and it's spring, the further the valve opens. Ignoring early opening for a minute. If your target boost is 1.5 bar, when it the actual boost gets to 1.51 bar, the valve should open a little tiny bit. Now we all know that doesn't happen, the boost goes up so fast it flies past 1.5 and the valves opens vary fast. If the valve is too big, it lets too much exhaust gas bypass the turbine, boost drops fast and the valve closes. Then boost climbs rapidly again because the valve is closed. Valve opens and the cycle starts again.

With the correctly sized wastegate with the exactly same spring rate, the valve will open just as far but it is smaller and so will bypass less exhaust. Thus it may not completely shut again. If it is correctly sized it should not have bypassed way too much exhaust gas, maybe only a little too much. So the valve just moves a little more open and closed to control the boost.

Secondly it's a lighter valve (remember a 35 mm valve head is 50% of the weight of a 50 mm valve head) so it has less inertia and can open and close faster.

As for the varying 0,3 bar... could that not be caused due to a poorly setup or inappropriate boost controller?

Well that's what I thought, so I replaced the wastegate spring with one of the right rate for the target boost ie; no AVCR. Same result, fluctuating boost. So I used one of the Autospeed boost control systems (they always work for me), still with the right spring rate in the wastegate. Same result. So we changed the turbo to another with a slightly less aggressive compressor. Better, but not much.

It was at this stage I suspected the 60 mm wastegate was too big for 230 rwkw. But I couldn't convince the owner, he believed and was continually told by everyone he spoke to, that bigger was always better. So I recommended that the guys put on a smaller wastegate (35 mm) with the original turbo and not tell him. Bingo no boost fluctuations.

Oh, and it never lost 1 rwkw from the smaller wastegate, but it picked up a noticeable amount of response. This happened with another car (it had a heap more problems as well) and that's when I started thinking about a formula to pick up ridiculously (small or large) sized wastegates. Save a lot of time and money chasing other stuff.

Hope that adds some more discussion to this thread.