3s down on power

[MR2SIK]

Im starting to get really confused by this, as its not really showing any symptoms.

A while back I noticed the car didnt feel as quick as it used to be, but I put it down to just getting used to the increased power. Last season I took it to the drags, and I was struggling to lay down any decent times, as the season went on, my trap speeds slowly dropped.
This season Its just the same.
Ive had it back on the dyno to see if anything showed up, air fuel ratios are the same, tunes the same, boost was holding at 17psi instead of dropping back to 15 as it used to, but ive sorted that now.

Dyno graph of when it was tuned compared to the other day show the line following the same trend, just at a lower hp reading. ending up about 20hp down at the peak.

Just thinking now, could a worn out turbo still pump the same pressure (15-17psi in my case) and not show any difference on the dyno except for overall power output?. Cos the graph started dropping around when boost builds?

[Lith]

I'd say that a drop across the rev range would be more likely to be engine related. Have you done a compression test?

[MR2SIK]

I'd say that a drop across the rev range would be more likely to be engine related. Have you done a compression test?

No, thats what I was meant to ask my mate at the drags yesterday, forgot all about it. will try and get that done today.
Any idea on what results I should get?

[rollaholic]

i'd be more interested in a leak down test personally, if thats the way you are leaning (good place to start if nothing else)

with regards to compression tests, you are looking for even compression across all cylinders - the actual pressure is not AS important.

[Akane]

I'd say compression test first, and then the same old 3S "Spark plugs and dizzy" business.

Time to get a sweet Nissan S14 Facelift IMO

[fivebob]

boost was holding at 17psi instead of dropping back to 15 as it used to, but ive sorted that now.
......
Cos the graph started dropping around when boost builds?

Just remember that boost is an indicator of restriction in an engine, not a measure of potential performance.

What was the issue that caused the boost not to drop like it was, and how did you fix it?

Given that power levels drop as boost builds it's likely that here is a restriction in the intake/exhaust that wasn't there before.

[Lith]

Given that power levels drop as boost builds it's likely that here is a restriction in the intake/exhaust that wasn't there before.

Could be a leak....

[fivebob]

Given that power levels drop as boost builds it's likely that here is a restriction in the intake/exhaust that wasn't there before.

Could be a leak....

How so given the other symptoms?

Boost would drop if there was a leak wouldn't it?

[MR2SIK]

In saying I fixed it, I just altered the boost control settings so it controlled it earlier to stop it from spiking.
I know boost pressure isnt all about potential performance, just noting that when it was built and tuned, it used to hit 17psi before dropping back to 15, like it couldnt hold more than 15. Now it can hold 17.

[rollaholic]

Given that power levels drop as boost builds it's likely that here is a restriction in the intake/exhaust that wasn't there before.

Could be a leak....

How so given the other symptoms?

Boost would drop if there was a leak wouldn't it?

not if the leak was inside the combustion chamber

[fivebob]

In saying I fixed it, I just altered the boost control settings so it controlled it earlier to stop it from spiking.
I know boost pressure isnt all about potential performance, just noting that when it was built and tuned, it used to hit 17psi before dropping back to 15, like it couldnt hold more than 15. Now it can hold 17.

Which probably means, when you couple it with a power loss, you are likely have a restriction in the system somewhere.

Have you changed anything in the exhaust system?

[fivebob]

Given that power levels drop as boost builds it's likely that here is a restriction in the intake/exhaust that wasn't there before.

Could be a leak....

How so given the other symptoms?

Boost would drop if there was a leak wouldn't it?

not if the leak was inside the combustion chamber

Nice try, but with less energy in the exhaust the boost would drop

[MR2SIK]

hope its nothing that bad im over spending money on it

[Lith]

Given that power levels drop as boost builds it's likely that here is a restriction in the intake/exhaust that wasn't there before.

Could be a leak....

How so given the other symptoms?

Boost would drop if there was a leak wouldn't it?

Not necessarily if he has modified boost controller settings to fight boost drop off unless its really bad. It is possible to make up for lost air so instead of the main cause of the measured pressure being from heat resistance, its from heat - so you have the same psi value but with less air density.

Its just an idea, yours just as easily could be wrong- but it may not be. A lot of Nissans have a similar kind of issue when using rubber intake pipes, with heat/higher boost they eventually lose a bit of their strength and collapse providing people with lots of fun and games trying to explain power loss.

[Lith]

Given that power levels drop as boost builds it's likely that here is a restriction in the intake/exhaust that wasn't there before.

Could be a leak....

How so given the other symptoms?

Boost would drop if there was a leak wouldn't it?

Not necessarily if he has modified boost controller settings to fight boost drop off unless its really bad. It is possible to make up for lost air so instead of the main cause of the measured pressure being from heat resistance, its from heat - so you have the same psi value but with less air density.

If the boost controller setup got its pressure measurement at the compressor outlet then boost might be a bit more likely to drop off up high due to a leak, but if it reads from (and they usually are plumbed to) air pressure from a in the plenum area then it will keep spinning the turbo harder until it reaches the target boost. This is one of the reasons boost leaks can blow turbos, as if the leak is big enough then the turbo is basically allowed to free boost and spins its poor little spinniness off.

Its just an idea, yours just as easily could be wrong- but it may not be. A lot of Nissans have a similar kind of issue when using rubber intake pipes, with heat/higher boost they eventually lose a bit of their strength and collapse providing people with lots of fun and games trying to explain power loss.

[fivebob]

Given that power levels drop as boost builds it's likely that here is a restriction in the intake/exhaust that wasn't there before.

Could be a leak....

How so given the other symptoms?

Boost would drop if there was a leak wouldn't it?

Not necessarily if he has modified boost controller settings to fight boost drop off unless its really bad. It is possible to make up for lost air so instead of the main cause of the measured pressure being from heat resistance, its from heat - so you have the same psi value but with less air density.

That's possible, but IME on a low restriction engine a CT20B won't hold 17PSI above 5500rpm, best I can get is around 16psi with the wastegate firmly shut. It simply does not have the flow capacity to supply that much air. If you add in a boost leak I can't see how it could maintain 17PSI, no matter how you adjust the boost controller, without there being a restriction.

FWIW at 17psi, and the flow levels required to produce 200+kw, the CT20B is pretty inefficient already so I don't know how much extra heat would be generated if the turbo had to supply more air to ovecome a leak.

Its just an idea, yours just as easily could be wrong- but it may not be.

Indeed it could be wrong, I would never discount anything till I identify the problem. I was pointing out what, IMO, was the most likely reason for the problem based on my experience of the 3S & CT20B and the symptoms described.

[MR2SIK]

the exhaust has had a flange fitted so I can quickly swap between the stock muffler, and my normal one for wofs etc.

and the downpipe has also been rewelded near where it connects to the turbo after it split last summer.

Going to get a compression test done after work tomorrow, at least it will rule it out, then go from there.
Keep the ideas coming in though, great way to learn about all this kind of stuff.

[fivebob]

When you changed the boost controller settings did it change the spool up characteristics of the turbo?

Have you tried going back to the old settings to see if that was the cause of the problem?

[MR2SIK]

When you changed the boost controller settings did it change the spool up characteristics of the turbo?

Have you tried going back to the old settings to see if that was the cause of the problem?

yeah, its slowed the rate that it spools and stops it shooting over 15psi, it felt down on power before i changed it too.

[johndoe1025]

tried doin simple things, like changing your air filter etc? im religious to doing shit like that aye, imo i rekon its quite effective to your cars performance, just general maintanence things