Power drops off when more boost added ???

[M.D.]

I've not been on here for a while but thought id ask you guys a question regarding my strange power issue I've had.

Basically I've recently fitted a turbo technics hybrid TD05 style turbo. I've got keen and made a decent exhaust manifold for this set up to help things flow as nicely as possible.



And at a bar boost it's making a good amount of power and torque

Blue line is what it's at now with the new exhaust manifold. My previous one was restrictive. (Red line)



But the issue is when we try adding more boost. (Above 1.2 bar) the power rises with the boost but as soon as it gets above 1.2 bar the power just dives off to below what it was at the lower boost setting and you can hear the engine holding back.

It doesn't sound like a misfire and it's giving the symptoms of a restriction somewhere. But we know the bolt on parts such as exhaust. Filter and intercooler are all proven to over 400bhp when I had the RB in it.

So the only thing that is standard is the engine and cams. (SR20det)

But I know standard SRs with more than my power which is what is confusing us. The engine is a little smokey but makes 8 bar compression equally accords the board. With a 1.6mm steel head gasket. And doesn't breathe any oil.



The suggestions I've had and looked at were. Exhaust manifold being restrictive. Hence the new version
Inlet collapsing. (Removed filter to test and no change)
Exhaust restrictive. (Same as before with no issues)
Possibly valve float. (Head is from my old engine that got very hot before it died)
Plugs or coilpacks (bcr8es and replacement coilpacks with no change. )

The spec of the set up is.

SR20det
Steel HG.
Rocker stoppers
Standard internals.
Greddy style plenum
Tubular top mount manifold.
Hybrid billet TD05 turbo.
Apex type 1 intercooler.
3" exhaust
k+n filter (ran without filter to test and no change)
Vipec (link) Ecu
740cc injectors.

If anyone can shed any light on this mystery it would be brilliant .

[Actual_Ben_Taylor]

What do the AFRs look like?

My first guess would be valve bounce...

[M.D.]

AFR is good. 11.5- 12 even when the power drops.
The only changing factor is the boost that causes it.


Is it possible for valves to bounce at such low revs with boost?

[Actual_Ben_Taylor]

The standard SR valve springs are pretty shit, so possibly

[-ghost-]

I know very little about this kinda of thing but I always thing that style of manifold flange is very restrictive Not sure if it is.

[bahis]

That bend in the inlet might collapse with enough boost. Mine cut out completely when I had a silicone bend in the inlet and boost was over 1.5Bar...

[sideways14a]

Where is it dropping grunt?
Is it holding up torque through the rev range only to drop off high in the revs?

Stock SR valve setup sucks big style though.

[R3K1355]

That bend in the inlet might collapse with enough boost. Mine cut out completely when I had a silicone bend in the inlet and boost was over 1.5Bar...

That'd be dead easy to see happen on a Dyno tho, which I'm assuming is where this occured.

[ANDY black s13]

I'd go with the 'Bic pen' valve springs the SR is blessed with don't like equal length longish runner manifolds,
someone on here had issues with a Apex type 2 long runner/equal length mani at over 1.2 bar something like the valves were being blown open
swapped mani and could go over 1.2 I think was the result, maybe consider some harder poundage springs?
as manifold looks good imo,looks a bit full race stylee at collector area

[raddy.]

is there any change in afr when it loose power? weak ignition/spark blowout? Ignition timing issue?

[skyshack]

Suspect its just making hot air.

Turbine speed pushed it out the top of comp map? Or worse too near the stall line.

Comp Effy was 80ish% now down to less than 70%?

Get a temp sensor in the comp outlet. See what the temp does with 1bar and 1.2bar.

[M.D.]

The turbo made 430bhp on a car at 1.6 bar the week before so we know the turbo will happily do the power so it's 100% not that.
The AFR doesn't really change much when it happens either. So it's not like it's running mega rich or lean at that point.

And yes we have checked for any collapsing pipework which is all ok.

I can't remember what the torque did up the rev range as the power literally drops like a stone and it would barely accelerate anymore so we lifted off. The boost rises from 3500 and the power rises with it. when it reaches 1.2bar and above it just curves off on the graph and heads downward very quickly. I'll get a pic of the graph tomorrow if I can.

The valve springs being weak does seem to be coming top of the list so far. I'm assuming the boost pushes the closed inlet valves open? Would this be correct?

[M.D.]

Thankyou for the compliment on the manifold by the way. Spent a lot of hours making and researching what works and doesn't. This seemed to be the most effective design for good spool and power. It's just a shame the engine isn't quite as good at making power lol.

[baker_001]

How are you measuring air flow? Intake valves blowing open would increase air flow and reduce boost so you can diagnose it if you have a MAF (but I can see you're not running a cold side one at least).

Does it do it at that boost level no matter what the engine speed?

[big_mac]

Id rule out a valve problem personally, you already have a good run through the revs with no problems, valve float is rev related so id find it hard to believe its happening if you arent revving it harder.

To me it points at a restriction of some kind limiting the engine or perhaps an ignition problem

[skyshack]

Was it a 2L 4 pot that it made 430bhp @1.6bar on?

1.2bar, 430 x (2.2/2.6) = 364 bhp.
1bar, 430 x (2/2.6) = 330 bhp.

If so at 1 bar you are about 35bhp short to begin with.

Id rule out a valve problem personally, you already have a good run through the revs with no problems, valve float is rev related so id find it hard to believe its happening if you arent revving it harder.

He thinks his valve springs are weak or lack preload and have a seating force of around 40lbs instead of 60-70ish. Are the spring seats / washers installed?

To me it points at a restriction of some kind limiting the engine or perhaps an ignition problem

Close the plugs gap down in 0.1mm steps.

[ANDY black s13]

Id rule out a valve problem personally, you already have a good run through the revs with no problems, valve float is rev related so id find it hard to believe its happening if you arent revving it harder.

To me it points at a restriction of some kind limiting the engine or perhaps an ignition problem

The problem someone else had that I mentioned was not valve float or bounce from the revs the engine was making
It was never fully explained what was going on as manifold was swapped for another design and it made more boost and power
It was pointing towards the equal length primaries and the OE valve springs didn't like anymore than 1.2 bar
it could be the inlets are being blown open off their seats,it could be the exhaust pulses hitting the collector all at the same time
as boost and exhaust flow increased it was reverse pulsing and blowing the exhaust valves open
Or a combination of both is happening ?, it only has to move them a tiny amount and power will drop off rapidly
the turbo has made good power already as in 150 more at 1.6,
an ignition fault is another possible as in boost related is spark blow out, but I think md had that covered mentioning swapped coil packs etc
Sr valve springs are very soft, I was amazed they spin to 7.2>7.5 k revs without valve bouncing on springs so soft
CA's RB's use a noticably harder OE spring, (resisting strong urge to type what I really think about SR20's)

[M.D.]

Sorry for the late reply. I thought I'd try and reply to the suggestions as I'm at a total loss and just going by what my mapper believes.

The issue is only boost related. It doesn't matter what RPM it's at. When we dial in more boost (it's Ecu controlled) the power drops immediately.

The engine the turbo has made good power on is an impreza.

The plugs are at 0.7mm and I've swapped my coil packs and checked the volts at the feed for them too. Just to make sure there is decent voltage.

I've been told that the SR springs are poor and the boost can push them off there seats under heavy loads.

It makes the symptoms of having a restriction somewhere but nothing fitted could cause one which is very confusing.

Unless the engine has bad blow by under the boost but I'd expect breather misting if that was the case. Which there isn't. Although we are still not sure where the smoke comes from under lift off and heavy boost :-/

[skyshack]

Remove the throttle, make and fit a blanking plate.
Remove the cam or rockers from open valves.
Blank and plug all other holes in plenum.
Remove plugs.
Put a compressed air supply on the plenum regulated at a bit less than 1bar and then try to wind it up to 1.2bar.
If the valves are being blown off the seat the pressure will start to pulse as they blow off pressure and reset allowing pressure to build up again.

[M.D.]

I like your way of thinking mate. That's a fairly simple yet affective way of testing. . Thankyou.