Turbo Surge Stress

[Veno]

Hello! Okay so, surge stress. I'm not sure where to draw the line. i used to aim around 2% stress maximum but i thought to myself while building my ost recent engine: "How far can it go before it explodes?" and found i can get a 3.4l inline 6 with twin 55mm compressor + 42mm turbine turbos to hit 40% surge stress and still not have a reliability warning. But 40% sounds really bad right? I noticed the mechanical stress tends to cause catastrophic failure at about 15% so i could just do with someone explaining where i should draw the line with it while tryoing to make a realistic engine. Thanks :)

[barracuda-rostigraben]

Personally I like keeping it under ~10% for my "not send it and make it kinda reasonable" builds. Others may vary.

[Medu Salem]

To add to this topic...

... with playtesting around a lot I found that there is an optimal point for the surge stress.
That is if you aim to get into the "notch" at the left at 1 bar boost (which mostly I do, as I design my turbos on the lower limit around the surge stress and upper limit around the mechanical stress, simply to cut material costs).
Then there is usually a "sweet spot" where you are almost perfectly in that notch with about 5-15% surge stress.

Going 15-20% or beyond usually starts to drop the reliability also more significantly than it does while still about 5-15%. That is if you care about your engine's reliability score.

So usually I tweak my turbo so that I get it to spool the earliest possible (getting into the notch), but trying to avoid the significant drop in reliability that happens "suddenly" if you would make the turbo another "tick" smaller.

You can totally see that "jump" of increased surge stress & corresponding reliability drop, if you play around a lot with different engines. You will notice that "sweetspot" where you say "not more than this".

At least I don't find it worth sacrifciing a lot of reliability to make it spool even earlier. And I think it will matter even more in one of the future updates because of some planned changes to how the relaibility score will be viewed by the markets. Then I might lower the stress even further not to annoy the target audience.


Overall it really depends on the year you are designing the engine in as well. In the early years I rarely ever get it to spool soon enough that it would even hit the surge stress. xD
But in later years with variable geometry and smartboost stuff you hit the surge stress more easily.

[Veno]

I'll keep this in mind thanks guys :)

[Johno]

remember you can change surge stress by moving idle rpm by using balancing weight or cam