Slight compressor surge with gt3071

[archangel]

Hi peeps. Would like some advice. The tubby is a forged rev1 turbo build with a bmic, gt3071r (0.63AR) turbo, sard fuel rail and 880cc injectors. It's running nicely on the current map but accelerating from mid rpms in 5th gear results in some compressor surge. All other gears are fine and it's fine in 5th gear higher up the rpm range so isn't a big deal but I'm considering options for springtime to look at what I can do to improve the VE slightly to improve the flow.
I'm considering getting a better flowing rev3 turbo head, and maybe going with some 264 cams with appropriate springs, buckets, etc. but I'm not going for crazy power (would like to keep it around 350-400bhp) so I don't want to move too much of the power band too high either. Would it be worth upgrading cams on the current head or is it really worth doing the whole head conversion if it's coming out anyway? Thoughts appreciated.

[MisterToo]

Not sure on the compressor surge mate, but I can certainly offer a little advice on the cylinder head.

My old car had a seriously modified head. Larger valves, ported and modded valve train. I had Piper 264's and the original turbo was a GT3071r too, so a very similar setup to the one you're planning. The thing is, the modded head lead to a lot more lag than the stock one, which is more than capable of flowing enough for over 500bhp, so do yourself a favour and save some serious money by sticking with the stock one. Spool will be quicker and for a motor that will certainly make well over 400bhp (my first outing made 450+ on the dyno) it will be perfect.

Glen.

[archangel]

Thanks Glen that's good to know. Just out of interest which rev did you have? I'm curious to know how much difference there is between the rev1 and rev3+ turbo head.

[MisterToo]

Mine was a Rev 1. There's not a lot of difference in the port sizes between revisions, but I believe the later head has slightly larger valves. Maybe one of the other guys can confirm.

[ashley]

My guess would be that it's not the head that's restricting flow, but rather it's struggling to push air out of the head...on a GT3071 you're doing really well to flow enough gas through the turbine on a 0.63 housing to make the power you are seeing.

It may be that the turbine housing is creating enough back pressure on the exhaust ports to cause surge at higher flow levels.

What exhaust set up have you got?

[MisterToo]

My guess would be that it's not the head that's restricting flow, but rather it's struggling to push air out of the head...on a GT3071 you're doing really well to flow enough gas through the turbine on a 0.63 housing to make the power you are seeing.

It may be that the turbine housing is creating enough back pressure on the exhaust ports to cause surge at higher flow levels.

What exhaust set up have you got?

Good call!

[archangel]

Interesting. Thanks guys. I think the exhaust is a mongoose. Not a particular fat exhaust but it isn't too offensive.

[ashley]

Stock exhaust manifold? Internal waste gate? What diameter downpipe? Still got a cat? Downpipe wrapped? Exhaust bore?

[Willenium]

Stock exhaust manifold? Internal waste gate? What diameter downpipe? Still got a cat? Downpipe wrapped? Exhaust bore?

Stock exhaust manifold, Tial external wastegate and think it's 3" diameter on the downpipe. Decatted but not wrapped.

[ashley]

Yeah- so it's nothing downstream of the turbine wheel then.

How much boost are you running and how much power are you currently making?

[archangel]

Running about 22-23 psi. Haven't had it dyno'd yet but I'm expecting it to be around 350-400bhp. It doesn't surge if I setup the boost to come in more slowly so the wastegate starts opening at around 14psi, but I like a more positive delivery so the high setting on the boost controller starts opening the wastegate at around 17psi. Like I said it only surges a little in 5th gear accelerating from say 3000 rpm so it's just on the edge, but I'd like to have things flowing well enough to maximise torque without too much sacrifice on lag.