Compressor surge on acceleration?
Turbo Mini Forums

Hey, i am running a 1300 engine with a 9:1 compression and a gt1752 turbo at currently 10psi boost. i Run the SC Delta ECU and the SC engine management system.

I am using a forge recirculation blow off valve wich goes back right after the airfilter - so i still hear the hiss but dont use a open one wich is not allowed here.

The Engine runs really well and most of the time boosts perfectly smooth at all rev ranges. One problem is when i reduce a bit from full throttle to a part where it still runs in boost but with less pressure - the compressor goes into compressor surge. I guess this means the spring of the blow off is too strong right? I will change that soon.

The other problemthat sometimes occures is that when i accelerate from a low boost level it also feels like the compressor is in surge. it still boosts up and accelerates good but with a "vibration" in the boost level. Any idea what that can cause? Fueling seems fine in this situation.
I thought maye its the dump valve getting in some kind of eigenfrequency. Do you think changing the frequency of the dump valve would help?

Try blocking off the DV and see if it still does it.

Or if you have one, exchanging it for an atmospheric one for the purposes of test.

Surge and stall is massive issue if it occurs in axial flow compressors. There Shouldn't be any detrimental effects to the redial compressor in a turbo but the bearing life may be adversely affected

Own the day

This isn't something daft like lack of a decent vacuum connection for the DV is it?
Shane

I find it hard to believe a GT1752 will surge on a 1275. I suspect this is DV flutter?

I checked the spring in the DV and it is the softest one available (for 5-15psi) so it should fit my 10psi quite well.

I also checked the vacuum connection for the DV and everything seems fine.

The flutter on acceleration might be DV flutter as Dave mentioned. I will try installing the DV in the other direction. Then it shouldnt be able to flutter. But i will also loose the hiss sound.

Ok the change in direction of the DV did make it way better. Its overall more smooth now. No compressor surge on decelleration any more.

But it still sometimes has this flutterin on acceleration

I’ve always found it interesting how DV flutter characteristics; and the sound of it, changes with engine changes. When I went to the K1100 with DBW throttle, the (same) DV was massively different; at first it sounded like a grp B rally car. Changing some settings on the DBW made it sound different again, closer to what it used to sound like (but still not the same).

I imagine it’ll be different again once I change to a smaller TB (the current VW one is way too large).

OP - if you aren’t happy with the characteristics, just try experimenting; pull the vac port from a different location on the manifold; try a smaller(or maybe larger) bore signal hose to the DV (ideally the hose and fittings should be larger at the manifold end than at the DV end).

Edited by TurboDave16V on 16th Jun, 2020.

Ok, i turned around the DumpValve and the flutter after taking away throttle is completly gone. The vibration on full throttle didnt change much.

Now i have been doing some fine tuning on the cruise area of the fuel map since it has been running super rich most of the cruise area. Especially at low rpm it was too rich and started ratteling.

While testing i often had a too lean mixture at the transit to boost and i felt lean rattle on the transit. On boost it ran fine with 12-11AFR. First thought it would acceleration enrichment, but it turned out i had way too much enrichment. I turned it back and mad the area on transit to boost a bit richer on the map. At this point it started running really good.
Always between 14 and 15 afr on cruise and 12-11 on boost.

Just at this point at a stop a coolant pipe from the turbo blew and i was leaking coolant so i couldnt go on.
But another thing i noticed made me worry more:

When the engine is ideling (smoothly) at 13-14AFR i can see almost no exhaust gases. It revs up perfectly while standing and also when reving i can see no exhaust gases. But when i let it fall back to idle, while the revs are falling i get a huge white cloud out of the exhaust. When its at idle it is all gone again.
Usually i would say white cloud is water (its bright white so no oil) from a blown head gasket. But wouldnt it smoke all the time then?
Gonna make a leak test soon to check the gasket.
What do you think - is it posiible that it is a blown gasket and if so - what did it blow? the short lean times on transition to boost?

Ok, i did a leak down test and i have almost no leakage ( 3-4%) and exactly the same at every zylinder. Guess the head gasket is not gone then.

Anyone a idea where this huge white cloud can come from then?

check the servo hose for brake fluid?

I dont have a servo.

I will check the turbo next since the water was leaking from there - maybe it is cracked and sucking water into the turbo?

sounds feasible, if theres that much water going in Im sure youd notice having to keep topping it up?

Firstly I'm sure that the compressor will stall first then surge. So what your experiencing is compressor stall thats the juddering/vibration your feeling the compressor shouldn't surge as there is only one stage of compressor. If your experiencing this over pressure in the compressor housing without the dump valve releasing the over pressure, the over pressure may have found a path past the bearings into the turbine housing dragging fluids with it when on part throttle.

It would be interesting to know what the differential is in boost pressure at the compressor exit compared to that at the inlet manifold at part throttle. I assume your running an inter cooler?

Own the day

I never had to top it up and i really only notice that huge white cloud when i rev it up without load and then let it go. there is no smoke while reving.

I do run a huge intercooler from a BMW 120d. I never measured the pressure drop across it, but i assume it shouldnt produce much back pressure at a small engine like that.
I run the map sensor from the inlet manifold and the wastegate from the compressor exit - could that be a problem? The Wastegate has a bleed valve, controlled by the ECU over the map sensor.