Part throttle compressor surge - VF35/FMIC
 

Just posting to chew over some ideas. I just fitted a FMIC on my MY98 WRX and I've got a serious problem with compressor surge now, which I never had with the standard TMIC. It'll probably help to know the spec which in brief is FMIC with wrapped pipework and obligatory induction kit, Walbro 255lph fuel pump & 440cc injectors, ported Harvey headers/up-pipe + VF35 turbo, sports catted downpipe, exhaust all heat-wrapped from head to cat, Forge recirculating DV on weakest (green) spring, Z32 MAF sensor, silicone turbo inlet pipe, all mapped for this spec on a Power FC for 1.4 bar.

Anyway, I am getting the classic chuff-chuff-chuff effect of the charge bouncing back and forth round the boost circuit on PART throttle, due to the way that the VF35 is spooling up so fast. At full throttle it still does it a little around 4000rpm, but you can drive through it. From memory, it'll hit full boost of slightly over 1.4 bar on half throttle at 3500 rpm.

Although I do a fair bit of full-on driving, a lot of the time I'm cruising or commuting and the 3000-4000rpm part throttle zone is crucial to driveability. Weakening the DV spring (by stretching it to twice its length then cutting it in half and refitting into the DV) has made a significant difference, but I can't see how I can weaken it much further and still have it actually do its job at all. The downside of the weakened spring smoothing out the part throttle performance a little is that it's doing a lot of hissing instead of chuffing, but at least it's not backing up through the MAF so much.

What other factors should I take into consideration? A bigger DV? Twin DVs, one close to the compressor outlet as well as the one near the TB? An atmospheric DV, even though I don't really want the noise but tbh it's bloody noisy already anyway? Reducing the boost solenoid duty? Fitting an AVCR or something to give me more tailored boost profiles? A bigger turbo that's more capable of shoving boost through a partly open throttle? Porting out the TB and fitting a bigger plate?

All comments welcome. BTW it does absolutely fly along now and the FMIC is totally in charge of intake temps without a doubt. Just this compressor surge is a downside of the FMIC install that I hadn't considered beforehand and I need to fix it!

Try a standard dump valve, if you can fit a metal one that doesn't leak.

Yes I'm already considering that. I've just realised that I can back off the spring tension even further by unscrewing the spring seat on the top of my Forge, and yet again it's made a slight improvement all round, but still the surge is there and I don't want it at all.

I'm just wondering whether there's some fundamental mismatch between a VF35 and a FMIC and I'd be better off just changing to a bigger turbo.

Dump valve takes 5 mins to change, and I could never live with a Forge DV recirv or VTA on MAF based ECUs and FMIC although not because of part throttle surge but lift off.

VF35 is a surgey thing.

I'll definitely give it a go. A full throttle lift off on the Forge is very clean and positive; always was with the TMIC and still is with the FMIC. Clearly it's more a problem with the FMIC than anything else, but that's the one thing that's got to stay - it does its job so well.

I've got the same problem on a UK MY99, I've still got the standard TD04 on though, but have the HKS SSQV. Problem started after fitting the front mount. What effect does comp surge have on the MAF if any??

What power are you running,curious because my car is almost finished with exact same mods,bit worried now though about the vf35+fmic??

Well after it was mapped I could see the trace on the laptop showing the MAF signal bouncing up and down all over the place every time the surge came in. I can't see how it can be good. Obviously with the MAF getting these crazy backflows of air coming through, the ECU's not going to be able to get the fuel and ignition right while it's surging.

It's running over 300bhp, but I couldn't put an exact figure on it; it's not something I worry about a great deal! I fitted the FMIC mainly because the standard TMIC was not up to the job at 1.4 bar. When it was remapped with the FMIC it was able to take another 4 degrees of ignition advance and I also now have the confidence that I can keep my foot down for pretty much as long as I want and not worry about charge temperatures. But this surge is a pain in the arse for sure.

I think you've hit the nail on the head - I run this combo and find it very 'surgey' although Bob Rawle has worked wonders and improved things significantly with his mapping :notworthy

I'm about to strap on a bigger turbo next month so will be interested to see what happens re surge.

hi mate,
that's a lovely spec car you've got there...nice one:thumb: my guess is that you've got your intake and exhaust losses so low that you're flowing more than the compressor can deliver. i'd go down the avc-r route and profile the boost response to suit your hardware.

The only DV that cured my part throttle surge problem (and I tried loads) is the APS Twin Vent DV, with the screw wound out to its max.

No more part thottle issues. Very smooth.

There is a new boost controller that may be the nuts for times like this. the very latest HKS EVC 6 has 3d mappable boost control that you can map for TPS vs RPM, or Speed vs RPM and a few more options besides. Part throttle surge is easier to control on an Ecutek car because you can deliberately reduce boost at part throttle. not so with even an AVCR or PowerFC, even though with the AVCR you can reduce the boost at low RPM.

There is a lot of info on surge, and certainly the silicone inlet pipe makes it worse, but I don't know if anyone has tried putting a restrictor/venturi before the turbo to help it. WRC cars can only ru the high boost at low RPM that they do, because of the restrictor causing a very high turbo inlet speed. But as always, you want to increase the speed without really restricting the flow.

i am experiencing same problems with surge but mainly on full throttle between 3-4k the spec of my car is

my99 rb5
ecutek 3
avc-r
hybrid fmic
forge re circ
vf 35 ported wastegate
ported and wrapped headers
wrapped down pipe
still on 440 injectors
walbro 255 fuel pump
k&n induction kit
fully decatted 3" exhaust

the biggest problem i have is gear changes i get a loss of power for a split second if at full throttle i can cure this by running a forge vta with red spring but that causes the turbo to stall so went back to forge re circ so i have 2 problems i am trying to solve

Well if you have time, post up on here what happens!

Marklemac - I'm gonna carry on down the route of fettling the dump valve, whether by trying to loosen off the Forge even more, or refit an OEM DV which has been known to work, or even go to a bigger DV (e.g. APS/Turbosmart etc) that will flow more. Whatever way you look at it, the OEM DV has a much bigger inlet and outlet than the Forge, which is 25mm in and out. When I first fitted the Forge it definitely seemed to give a much more positive dump at lower boost plus a rather pleasing kerching sound when it went off. But I'll happily regain the 'woolier' response of the OE valve if it'll smooth out this surge problem.

Paul - I'm going to have a go today at restricting the inlet pipe close to the turbo end, with a jubilee clip, this was suggested by Harvey and a bit of trawling around elsewhere seems to blame big silicone inlet pipes for these kind of problems. Anyway, it's an easy enough mod to do and worth a go. You know my car Paul - you think this HKS controller could do the job? At least that way I've got a final solution if all else fails, although without knowing how much they are I can't help thinking it might be cheaper just to chop in my VF35 and get TD05 or something?

Benny - sounds like we're in the same boat!

Anyone had any experience of running twin dump valves? Surely on a long FMIC inlet tract there must be a benefit of dumping charge at both ends, one near the compressor and one near the TB?

EDIT: Now fitted an OEM v4 DV and it has definitely had a good effect, moving the surge up to 1 bar and above and lessening it at the same time, meaning I can drive on part throttle without really encountering the surge at all. I can't do anything else to get rid of the full throttle surge but on full throttle it pushes through it so quickly anyway that it's not so much of a problem. I just hope it's not damaging my turbo!

UPDATE:
Swapped VF35 for a TD05H-16G and it still does it. So it's not the DV, doesn't seem to be the turbo, what else is left? WHere it surges is nowhere near the surge line of its compressor map, it's at 4000 revs full throttle. The only things I can think of is to try an even bigger turbo, a bigger engine or rip out all the FMIC install and stick a TMIC back on. Help!

I take it you've had the map optimised for the TD05..?

Was there absolutely no problem while running the TMIC..?

Keep an update on this as it's an intriguing problem..;)

What spring & shim combinations are you running in the Forge Re-circ? I'm using a red spring (Hardest) in mine but have the same problem whilst running a similar level spec and a VF34. How does a softer/harder spring affect the DV in it's operation?

No it's not remapped for the TD05 yet - I only just had it mapped for the VF35 last month! Even at 0.8 bar 'safe' setting it's surging at 4k revs full throttle.

P1 - I got rid of the Forge in the end. I put the OE valve back on and its way better than the Forge even on the lightest spring (green).

THinking I should maybe just have the inlet manifold off and replace my big silicone intake pipe with an OE plastic one... but I can't bear the thought of taking the whole lot off again.

I had the same surge problem with my TD05/06.20G so a bigger turbo isn't necessarily the answer. I also have a PowerFC and AVCR. Surge was a real pain when the boost needle was hovering around the 0 point (ie just starting to boost) when feathering the throttle. I fitted the softest spring to my Forge DV but that didn't help much either. I have an APS fmic where the dv is just behind the throttle body and also a Samco inlet. I removed the rubber connecting hose from the metal dump pipe and the stub on the inlet that it goes on to replacing the stub with a 30mm (I think it was) copper bend that I got from a plumber's merchant securing it with a clip and marking it with a black marker so that I could check that it wasn't slipping out - which it did first time because I hadn't pushed it far enough in. The pipe points back towards the bulkhead and is connected to the dv by a short length of black flexible hose meant for a water feature that I got from a Garden Centre!! The end result is that the dump pipework is now almost a quarter the length it was (approx 10" rather than nearly 3' - you can metricate it yourself if you like :) ) and surge has practically disappeared due, I suspect, to the dumped boost still having enough velocity to keep the compressor spinning because of the much shorter pipe length. This mod was straightforward to carry out with the inlet in situ. Hope this helps :)

I have the the same set up.

i have solve the Part throttle surge by fitting a Forge Recur. using red spring.

but i still cant solve the surge @ 3800-4300Rpm when on full throttle.

called Powerstation who mapped the car, they said its normal as i am running an induction kit and it will make it worst with FMIC and upgrade inlet hose.

Rich saids as long as it doesnt dets its fine. :)

their is a way round it if you adjust your driving style. as it only surge on 4th and 5th.

raymond, interesting. do you have a picture of the dv now in postition ?

what about the hose that goes from the dv to the manifold, do you have to make a new pipe up for that ?

Great stuff so far, keep it coming. I think at first I had two separate but related problems, the 3000 rpm part throttle surge which was hideous but has been totally solved by reverting to the OE dump valve; and the full throttle surge which is still there and has survived a turbo swap. And can be nothing to do with the dump valve.

I think I can finally get my head round why the full throttle surge happens - if the compressor's outlet is relatively restricted compared to the inlet, boost will naturally want to go back through the housing if given the chance. The long, twisted and fairly narrow FMIC pipework presents more of a restriction than the short, wide, straight inlet with a bloody great induction cone on the end. It's like a dustbin compared to a reel of garden hose.

So looking at it from that point of view, if I am indeed right in what I'm thinking, to cure full throttle surge I either need to work on improving flow through the FMIC plumbing or on restricting flow through the inlet side. The former option isn't realistically possible - the best I can hope for is for the FMIC pipes to all join up with no leaks or fouling. And regardless of the other problems it's given me, there's no doubting the FMIC has charge temps totally nailed down.

So it looks like inlet restriction may do it. I've already jubilee clipped the middle of the silicone pipe, but obviously that's not enough. Maybe it won't be so bad getting the inlet manifold off again. Who am I kidding LOL...

Anyone got an OE inlet pipe for a v4?

BTW I'm gonna do some testing of boost pressure at the compressor outlet vs. inlet manifold. I wonder how big the difference will be? RESULTS: inconclusive because at the compressor outlet it's always in positive boost. What an idiot I am, having thought about it. There's no way there could ever be a vacuum showing, because if there's vacuum in the manifold, the throttle's closed. Derrrr.

Any more updates on this silent running?curious cus mine is exacly the same spec as yours and i have the same problems?

Well I've not had the chance to work on it for ages; I really want to take the car off the road for a week or so to do various job on it. Should be able to crack on in the next fortnight or so. Of course, while the manifold's off it would be silly not to fit some insulating spacers in there as well, and I also need to fit an adjustable fuel pressure regulator, so while the fuel system is in pieces I'll do that. Plus I'm going to change the coolant while I'm there, so a cooler thermostat and a high pressure header tank cap are going on as well...it never ends LOL

The upshot is that I've got an OE narrow plastic inlet pipe ready to go on. I'm sure it will fix the problem. At 0.8 bar on my TD05 I'm still getting mild full throttle surge from around 3500-4000rpm, and I know from a bit of experimentation that it definitely gets more pronounced, but at the same place in the revs, when you up the full throttle boost to 1.0, 1.2 and 1.4 bar. At 0.8 bar it's fairly quiet, but I can hear it and don't believe in 'driving around' problems like this if I can help it.

Looking around different threads and forums, full throttle surge problems have a common factor of an aftermarket inlet pipe. Bigger turbos which require bigger inlet pipes even come with 'surge slots' machined into the compressor inlet side. Feel free to PM me about this as I want to share any knowledge about this issue.

Yes will do cheers!!

Oh dear. I've just found this thread. I had mild surge with the VF35 and Samco inlet pipe. IIRC there a 3 level of surge with mine being chuffing but no significant effect on Maf/driving feel to me (although Paul said he felt it). The worst is upset Maf and big problems with jerky driving etc.

I swapped for a TD05 and remapped which cured it. I hated the TD05 but that may be down to another problem that I'm too embarressed to talk about ;)

I now have a twin scroll, which gives more power and torque and similar spool to the VF35 but I get mild chuffing. I'll put up with it. In hindsight I should have dismissed the mild surge with the VF35 as it performed so well. I should have also swapped the Samco for a OE MY03 STI inlet pipe - I discovered it is almost as big as the Samco but should reduce the surge and costs only £80!

F

. I should have also swapped the Samco for a OE MY03 STI inlet pipe - I discovered it is almost as big as the Samco but should reduce the surge and costs only £80!

Hmm interesting will this 03 sti pipe fit the ver 5/6 ?and is that price from the dealres?
:)

That should read the inlet pipe from a blobeye Spec C STi. The std STi is no bigger than any other model of its age.
The inlet manifold on a blobeye Spec C is a single cast item, so it allows the room for this bigger pipe due to no TGV bodies getting in the way.

It'll probably fit the ver 5/6 - just check that the breather, boost signal and boost solenoid dump conmnecitons don't foul anything on the earlier car.

nick

I can't see if this has been checked or not, but compressor surge at part throttle is usually due to the BCS position not being mapped correctly at part throttle, if the BCS is in the position to give 1.4bar at wide open throttle and the throttle is restricting flow, you could well have 2bar+ between the compressor and throttle. the BCS position before boost is seen in the manifold needs to be set carefully otherwise surge can be very common, although this usually afflicts smaller turbo's at low speed. Its also could be more of an issue on FMIC mapped cars as the open loop position of the BCS is usually set more aggresively to combat the extra lag with the FMIC.

It can be helped or eliminated by softening the recirc valve so it stays open longer is rather than stiffeneing it, the stock Forge spring is far to stiff (a cynic would say thats it's so you have to buy the tuning kit at extra cost to fix it!) and is far stiffer than the stock item (despite what it says on the website about it being matched - my arse!)

Simon

Yes, from the dealers.

"That should read the inlet pipe from a blobeye Spec C STi. The std STi is no bigger than any other model of its age."

Well I haven't checked but it is bigger than a V5/6 and the late classics. I would have tried it if I had time.

F