Someone Explain Surge Slots To Me.

[Rolls]

Now I understand the principle fairly well of surge slots, I am also aware that they do in fact work so it is my understanding that is lacking.

From what I understand they basically leak air back out the compressor into the intake, so under high pressure (surge) instead of the blades skipping on the air as the engine cannot ingest anymore air, it just flows back into the intake much like a recirc bov. Now at low rpm when surge happens this makes perfect sense and I can see how it works, however my question is at higher rpm when the engine is well out of the surge zone and it can in fact ingest all the air, what stops the air from being leaked out the holes back into the intake again?

The intake (pre turbo) is ALWAYS under vacuum so the path of least resistance is always going to be to go back out the surge slots, what stops it happening at higher rpm and basically just wasting half the compressors flow? Is it because at high rpm the surge holes can only flow so much air so a static amount of air is wasted (recirculated) at all rpms or is there something more fancy going on? Also how much flow do they waste at high rpm, is it a tiny amount, or does it visually change the flow map of the turbocharger?

Look forward to see if anyone can explain it.

[JonnoHR31]

disco explained it very well not that long ago, but since this new forum version i can never find anything relevant with the search function, its useless.

edit: found it without search

No its more of a pressure balance thing . What happens is the leading edges of the lower or splitter blades are just inside or below where the radial slot is located . When the pressure is higher in front of the splitter blades the full height blades bypass some air out through the slot purely because its the lower pressure or least path of resistance . At higher revs the engine can swallow more air so the pressure in front of the splitter blades decreases so the air previously being bypassed goes through the housing to the engine .

The whole idea of port shrouded comp housings is to allow a larger compressor wheel or wheel trim to be used while avoiding compressor surge .

The other thing is that it allows smaller A/R turbine housings so be used on biggish turbos and avoid compressor surge issues .

A good example of a turbo most know of is the Garrett/HKS GT3076R/GT3037 56 trim . Turbine housings from HKS alone start a 0.61 A/R and go up to from memory 1.12 and Garrett's range from 0.63 to 1.06 A/R . I reckon you'd have a hard time avoiding surge with the 0.61 or the 0.63 housings given that the compressor is good for 500+ horsepower , people who've use both those housing never mention comp surge so the PS'd housings do work .

A .

http://www.skylinesaustralia.com/forums/topic/342457-hks2835-tune/page__st__100

Edited December 30, 2010 by JonnoHR31

[Rolls]

The funny thing is that post was actually a reply to my question! however I still don't quite follow the exact details of what is going on, I don't get why it leaks air at low rpm stopping surge but doesn't leak more air at high rpm essentially wasting the compressors flow.

[JonnoHR31]

i think he basically means it only leaks from 1 part of the blades to another, evening out the pressure difference that is only there during surge

[Rolls]

I dont get why there is only a pressure difference there during surge, there is always going to be a pressure difference, eg the intake is always under vacuum and the blades are always under pressure, so there will always be a large pressure difference, it will be even larger during surge so I guess more will flow out of them at this time, but why don't they continue to leak flow at higher rpm?

[JonnoHR31]

yer but during surge some of the air starts flowing backwards. from this pic it looks like the surge slots bleed air from halfway along the blade, so under normal conditions that area woulnt be under boost, it'd still be in vacuum. but in surge the backwards flowing air can go through these slots.

[Rolls]

ah ok, that pic makes it more obvious, I can see how it normally would be under vacuum at that point, but when surge occurs it be earlier in the blade, I assumed the surge slots went directly to the blade and there wasn't a right angle like in that pic.

ty