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300rwkw - What wastegate

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Hey everyone.

Rather than abduct SydneyKid's thread about wastegates i thought better to start my own.

I'll hopefully have 300-320rwkw after a number of things.

So i may as-well but the wastegate now to suit this.

Basically, what should i get, and why.

please provide reasons for bigger/smaller or whatever.

I know roughly how one works. But not TOO technically.

Any advice here would be good as it'll be going on this week :)

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basically you go for a bigger wastegate to vent larger amounts of air, the problem is, the bigger the wastegate, the bigger loss in response when it opens because it dumps a heap of air rather than a smaller one which will dump a smaller amoutn of air. If you get what im trying to say lol

basically for [email protected] you wouldn't need a very large wastegate as you wont need to dump a relatively large amount of air to stop the boost from creeping up.

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Hi guys, this is not such a bad place to start some wastegate debate. So let's get into the theoretical stuff.......

1. The wastegate size is relevant to how much horsepower you are targeting

2. If you want to run high boost, then a smaller wastegate is better than a larger wastegate. The idea being that in order to generate high boost you need most of the exhaust gas going through the turbine, not through the wastegate.

3. If you want to run low boost then a larger wastegate is better than a smaller wastegate. The idea being that in order to keep the boost low you need a lot of the exhaust gas going through the wastegate not through the turbine.

But what is high boost? Well for the sake of this discussion I have settled on 1.2 to 1.4 bar (18 to 21 psi) as being the divider, thus 1. 2 bar and under is low and 1.4 bar and above is high. How did I arrive at this number? Well based on the results of the surveys, this seems to be the most common point where the wastegate sizes change from theory 2 to theory 3 (above).

The next bit of theory is that it takes 1 lb per minute of airflow to make 11 bhp in a current generation 4 valve engine. This is a pretty well established piece of turbo sizing philosophy. But how do we relate this to wastegate sizing? Well referring to the results of the surveys, it seems a straight 1 to 1 relationship is not too far from the average, so 1 lb of airflow = 1 mm of wastegate diameter.

So R31Nismoid, if we want to use your car as an example we need the boost target since we have the power target.

Hope that stimulates some thoughts in the mean time.

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Oh... debate.

This is what i want. And i know with the knowledge around this thread could become VERY useful.

I'm installing a GT30... might not get me to 300rwkw, but it'll come close.

possibly 270-280rwkw.

I would envisage running approx 20psi and not much more than that.

So a larger wastegate would be for me going on the above. (theory #3)

This is also what my mate said, get a bigger one 45mm eg. He is doing the upgrade for me and knows a lot about this sorta stuff.

But he was happy to have me post and see what responses we get.

keep the info coming. As i might one day go a GT35/40... and run similar boost levels but with more power being chased.


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OK R31 Nismoid, let's use your car as a discussion starter.....

300 rwkw = 480 bhp

480 bhp = 44 lbs of airflow = 44 mm wastegate

Now if you want to run 20 psi, then that's statistically the right size wastegate.

My experience indicates that to get 300 rwkw out of a GT30 you would need to run around 1.5 bar (22psi). So using the draft formula...

44 / 22 X 19 = 38 mm.

So based on the 75% success rate of the formula in the samples so far, I would say a 38 mm wastegate would be the go.

Hopefully that should get some discussion going....

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Interesting, only thing that I can see is missing is the size of the turbine housing - a larger housing would equate to needing a smaller gate, a smaller housing a larger gate - thoughts?

anyway - may throw a spanner into the works, but I have no probs running 1 bar with my 40mm gate - this was with a small (0.61AR) turbine housing, but as it was on a 3037 - the 'R' of the AR wouldnt be that small. It also worked, as may be imagined, with a larger 0.87 AR turbine housing.

Bottom line though, it would appear 40mm is ample to control boost on a GT30 size turbo -as Sydneykid has already pointed out:)

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Hi Steve and Dr Drift, I try Steve's question first............

What I am trying to do with the formula is give a baseline, something to think about. A 75% fit type of thing. At the moment there seems to be a majority of guys wanting external wastegates who don't even know where to start. They have been told for big horsepower you need a big wastegate, which is not necessarily true. They have been told you can't have too big a wastegate, which is definitely not true. They have been told for high boost you need a big wastegate, which again is not necessarily true.

In order to do this there is one main assumption, that the turbo is pretty much the optimium size for the target power and boost. If it is totally wrong then whether the wastegate is sized correctly or not is really irrelevant. The turbo / engine mismatch is gunna kill horsepower / response anyway.

The other issue is exhaust flow rates through the wastegate, some manufacturers claim very different flow rates for the same size inlet / outlet / valve. Now I haven't tested every wastegate out there, but I have yet to see any difference in popoff valve type wastegates that I have tested. So for this discussion and the formula I have ignored the manufacturers claims. The Turbonetics flap style wastegate is an exception, they flow much better than the same size popoff valve on our flow bench.

Ok moving on to the good Dr's question, "So if he's chasing more in the future and had to bear that in mind something like the turbosmart 45mm racegate or similar would be a good option?"

Sorry, but more what? Boost or horsepower? It is possible to make more horsepower with less boost ie; with a turbo capable of more airflow. In that case the formula says you would need a larger wastegate. If you wanted more boost (and therefore more horsepower) the same size wastegate may well be OK. You can get to the stage with "more boost" when a smaller wastegate is needed.

Interesting discussion???

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ok.. he want's more power, understandable that can be achieved with more or less boost depending on turbo sizing, but more power would generally indicate more exhaust flow...

Can you please explain a situation where chasing more power would require a smaller gate? Surely a larger gate would prevent boost creep from not enough flow through gate?

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More power (at less boost) - bigger turbo - more turbine gas flow - smaller wastegate?

I dont fully understand the bigger gate = lower boost.

With my 40mm gate, I ran 1 bar all day, it still holds fine at 1.7bar except for a small (0.1bar) boost drop at above 7000rpm, I believe this is due to my wastegate spring being rated at 10lbs. I also did a run at 10lbs on a dyno, and it held boost perfectly. The different runs resulted in power difference of nearly 100rwkw (10lbs vs 25). I fully expect now that I have a freer flowing exhaust my top end has shifted right too, so the difference will be greater now (seat of the pants only so far:))

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OK, the good Dr first this time...........

Try this;

More power = more exhaust flow

More boost = more exhaust flow through the turbine

More boost = maybe the same exhaust though the wastegate, thus same size wastegate

Lots more boost = less exhaust flow through the wastegate, thus smaller wastegate

Moving onto Steve's question.........

More power = more exhaust flow

Less boost = less exhaust flow through the turbine

More power = more exhaust flow the wastegate, thus larger wastegate required

Steve, try the formula on your car "40mm, 1.7bar, 309rwkw"

309 rwkw = 520 bhp = 47 mm

47 mm / 1.7 bar X 1.4 bar = 39 mm

So the 40 mm wastegate looks OK.

I suggest you have play with the numbers you are thinking about for your car, post them up and we can discus.

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my tuner is running a 35mm turbosmart wastegate on his t04 powered sr20 - power is approx 300rwkw on 28psi with no problems

my car should have the same

any idea sydneykid if a rb25 would need a 45mm due to the extra 2cyl or wouldnt this make much difference?

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Im really glad i asked the question now!

some VERY helpful responses, and damned good questions too! Very interesting discussion. Pity i have little to offer but be the guninea pig :D

Whilst some responses are nice and technical for me... if i read them for about 4 hours i understand them all :rofl: so it's all good

If anyone wants to abduct my thread a little then you can.

It's all for the greater good of the wastegate ;)

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What about a scenario where more boost = more power, but the amount of exhaust gas used to bring about more boost is by far less than the volume of exhaust gas created by the additional boost?

Just to throw in some (hypothetical) numbers, say 5lbs more boost (on a turbo just entering it's center efficiency island) creates 200cfm more exhaust gas, but only requires 30cfm to put out this additional boost. I know these numbers are way off, but just wanting to illustrate the situation I am trying to describe.

Edit: 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?

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am i doing this correctly

i have:

[email protected], 50mm ext wastegate

391rwhp/11 = 35.5mm


or 35.5/1.3*1.2=32mm

(i'm not sure about those last two calcs... is that what I'm meant to do??)

so the wastegate is waaay too big??

And i would need an even smaller one if I wanted to get to 250rwkw (at say 1.6bar?)...



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Sorry guys, I am a bit tied up at the moment, racing season is fast approaching.

KamikazeR33, it doesn't matter how many cylinders you have. The logic is that it takes X amount of air and fuel to get X amount of horsepower. So whether the engine has 4, 5, 6, 8, 10 or 12 cylinders to make 500 bhp is irrelevant. The only issue occurs if there is varying efficiency amongst the engines. Since I am limiting this formula to production, DOHC, 4 valve engines, I seen no real big problems in using the same calculations. BTW this is also why RPM is not considered.

More answers in the next post.

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