dburt86

iTrader: (2)

I wanted to post this here for a couple reasons. 1. I think ill get a bit more feed back in regards to the higher power stuff and 2. the owner of the car wants to keep it out of the spotlight

I tuned a turbo bmw the other day ( ive done many in the past) and we were expeirecning some weird things.

Car was making great power. Made like 700 on 28ish psi. It was responding well to boost, made about 760 at like 32 or so (ill check the logs)

But then it started falling off hard. After about 34lbs, the gains were very little per pound. The more we threw at it, the harder it would fall off.

So we are wondering whats going on. Its a 6870, .96 open t4, bigger cams then stock (stocks have made 800+ no problem), dual valve springs and retainers, over sized valves, ect....

Plugs looked ***** on the money for timing mark and coloring at about 9 degrees up top.

So heres the graph, just give me what your first impressions are when looking at this, and why its falling off so hard.

Car doesnt break up at all. We did 3 runs at this level (36psi) and they all yeilded very similar results.

Another weird thing to note, is it only took 75% boosty duty cycle to get 36psi, but even when at 100% duty, it wouldnt raise the boost.

Open turbo inlet, and a 1 foot up pipe.

So back pressure? valve float? I dont know. Exhaust manifold flow?

Ill add more info when i get a chance, but im at work so im trying to fire this off quick.

a05c

Possibly pushing the compressor out of its efficiency range?

What were IAT's?

Do you log backpressure?

dburt86

iTrader: (2)

Not logging back pressure yet but probably will when we go back. Don't really know where it's at effiency wise because precision doesn't offer compressor maps. ******* joke

dburt86

iTrader: (2)

It's were around 100 on a 80ish degree day. Treadstone cooler. Temps didn't rise during the pull at all, most the temp rise was heat soak from sitting

svslow

That turbo should make more, they do 1000+ on 2J's. If the IAT's were solid I don't think the turbo is the issue. Back pressure could be a factor if the increase from 75%-100% made no change.

dburt86

iTrader: (2)

Kind of what i was thinking also. Car has a custom top mount manifold with bigger primaries and a open .96 housing with the smallest up pipe youve ever seen (can literally see the exhaust wheel even with a 90degree bend.) So its hard to believe that it is back pressure, but at the same time, i guess it could be.

asubennett

iTrader: (6)

its back pressure. too many cubes for that turbo to be pushed as hard as you are pushing it.

dburt86

iTrader: (2)

just to confirm, this is a 3.2l inline 6

svslow

Solid 250 hp @ 4000 RPM

dburt86

iTrader: (2)

the car weighs 2600 with everything, and is on a r888 tire. Its already too fast.

O yea, and its a convertible, and a 1987.

Incredibly safe.

ls3fox

wow that graph looks like **** up top and according to the dyno its showing 38.8psi so im wondering if maybe between the cams and 39psi the valve springs just aren't strong enough

stevieturbo

What sort of cams are in it ?

I would highly doubt back pressure is an issue. Effectively I've a 6468 on each side of mine which would be 3.15 litre per side and even at 30psi boost I'm around a 1:1 pressure ratio at all times.

Yes you would be moving more air...but you've also a larger turbine housing and turbine wheel.

Oddly using your same turbo on another car at around 45psi boost....performance whilst very good...was a little disappointing given how much boost it was taking. Much smaller motor than yours though

Strangely on a 4cyl car ages ago....we have fairly big spec cams and it struggled to make over 30psi boost. It performed ok, and it did rev ahrd. But was also very laggy.
Changed to much milder cams, car spooled an easy 6-700rpm sooner, made far more power, went faster everywhere and made boost far easier.

So cams can make a huge difference both in good and bad ways.

dburt86

iTrader: (2)

Kind of our thought too. But the dyno was reading pressure much higher then the MAP was. We were seeing about 36.5.

dburt86

iTrader: (2)

Funny you mention cams, because thats what started this. Back story time.

Car was put together with some not good advice. Owner was sold a solid lifter cam, with no cam advance (vanos) and the cam specs were 12mm lift, 260in/252ex duration with a 104/104 centerline.

With those cams, it made 720 at 30psi (basically same as now) but took about 6 more degrees of timing to do it. Car ran like **** down low, idled at 1400rpm with like 70kps manifold pressure. At this time, we thought the power was low. So we went to a smaller cam.

Fast forward to now, with the smaller cam, it makes almost the same power, but takes less timing, but we got choked up at 36ish psi.

Im working on getting the specs on these new cams.

dburt86

iTrader: (2)

But i agree, i thought this turbo was gonna make much more. The thing only made like 500 or so on 15ish pounds.

It didnt start seeing big gains until upper 20's, so i was a bit suprised it choked at 36

93camaro_zzz

iTrader: (13)

Is it a manual or auto? What size stall if auto?

My dyno graph looked kinda similar when I dynoed my car. It turned out to be of how my converter acts on a dyno. But the car felt fine, pulled fine, ran fine at the track, etc. I researched a little and found many other turbo auto cars experiencing the same issue.

dburt86

iTrader: (2)

This is a manual car

93camaro_zzz

iTrader: (13)

You have graph smoothing on 0 from the looks of it. Turn it on 5 and see what it looks like. It actually looks like the HP curve kinda follows the boost curve. Looks like the boost falls off a little bit up top between 6 and 7?

a05c

Any details on the head that's on it? If the head isn't capable of moving that much air and its just piling up in the intake. Just a thought.

dburt86

iTrader: (2)

The weird thing, that i probably should have mentioned before, is that peak HP happens at about 6200 in all graphs before this.

This run here is with 100% boost duty. Previous runs would be at about 75% and the boost would stay pretty flat and end up close to the same boost.

Im just wondering if its all related.