Technical Much more boost, less power at high rpm

[nyt]

Posting this in a dedicated thread so it's not buried in my build thread...

So now that I've gotten past most of the fun issues with my build and started turning it up, I've run into yet another problem. More boost is resulting in less power up top. Everything is smooth and logs are good, but 15psi of additional boost is making the same power at 6200rpm and less above. Similar happens at 6400rpm with an 8psi difference. The 24-22psi pull kept building power until I stopped the run at 7600rpm.

I'm wondering if this could be valve float as I'm running the factory valve train or if the factory exhaust with cats swapped to resonators is too restrictive. I need to disconnect the flap actuator and ensure that stays open to test during a pull, but I'm pretty sure I have it programmed correctly. Update: tested with flap static open, same results.

Any thoughts? There are no boost leaks. The turbo is a 6466g2 on a speedtech manifold and operating at under 35% WGDC.

Here are three 3rd gear pulls at various boost levels. You can see the higher boost definitely makes more power down low, but as revs increase, it falls down below the lower boost pulls.

Interestingly, the issue is less pronounced in 4th gear, but still very apparent.

And comparing a higher boost 4th gear pull to lower boost 3rd gear pull...

This is the exhaust

Log from a 32psi 4th gear pull.. pretty much looks the same as all of them.

datazap.me | nyt | Log 1656042884

datazap.me

 

[STE92]

Exhaust and back pressure definitely. What A/R is you turbo?

 

[nyt]

Exhaust and back pressure definitely. What A/R is you turbo?

1.0

I'm wondering if the back pressure is causing some valve float. I have an idea to test a decreasing boost curve earlier. If at the high rpm, with low boost tapered down from high boost, it doesn't make the same power as the low boost pull, that to me would indicate float and the lifters expanding.

I may swap the Ferrea valve springs in to see if it helps at some point. I don't want to get rid of the quiet exhaust.

 

[fmorelli]

I may swap the Ferrea valve springs in to see if it helps at some point. I don't want to get rid of the quiet exhaust.

An exhaust doesn't have to be loud or droning to flow. I rebuilt my exhaust system and it is quieter than my son's stock 335is. Maybe the same at WOT. Mid-pipe high flow cats help with that. Flex pipe connection helps with drone issues. But you need a good exhaust builder ...

 

[STE92]

My vrsf street exhaust so quite . I bet is quieter than what you have right now. Why would exhaust cause valve float?

 

[Brule]

I compared your logs with my multiple turbo and exhaust setups.


This is for a 6466 1.0AR 3.5 inch out bonnet, 4 inch straight pipe and 4 inch with resonator and muffler. They are all that close in logs I cant tell which is best.

Depends on wastegate spring to compare properly (mine 17psi spring) but im in high teens mid 20s wgdc at 30psi.

34psi 7200rpm its in low . 33 average.


Other setup
6266 .84AR through 3 inch dump pipe and stock exhause with no cats and straight piped muffler

28psi high 70s wgdc. (14psi wastegate)

I believe your stock exhaust is restricting power.

 

[nyt]

I compared your logs with my multiple turbo and exhaust setups.


This is for a 6466 1.0AR 3.5 inch out bonnet, 4 inch straight pipe and 4 inch with resonator and muffler. They are all that close in logs I cant tell which is best.

Depends on wastegate spring to compare properly (mine 17psi spring) but im in high teens mid 20s wgdc at 30psi.

34psi 7200rpm its in low . 33 average.


Other setup
6266 .84AR through 3 inch dump pipe and stock exhause with no cats and straight piped muffler

28psi high 70s wgdc. (14psi wastegate)

I believe your stock exhaust is restricting power.

If there were images here, I'm not seeing them.

I just tested with the flap static open, same results. Next step is to try new springs or find a quiet exhaust that can flow. Any recs?

For now I'll target 32 psi at peak and taper off to 22psi up top.

 

[nyt]

My vrsf street exhaust so quite . I bet is quieter than what you have right now. Why would exhaust cause valve float?

uh, no, it's not. it's not even close to as quiet.

 

[Brule]

No images were uploaded.

I watched a dyno of a 1JZ with a 3584rs and 3 inch dump pipe struggle to make over 28 psi at high rpm.
They cut the exhaust after the dump pipe and boost shot up over 30psi in lower rpm.
Then they put a 4 inch in it snd it picked up 70hp in midrange 40 odd hp in the top end same boost.

What wastegate spring?

 

[Brule]

My point, the exhaust will be holding off some power.
I recently dynoed mine and at a flat 28psi boost curve power climbs all the way to 7000rpm.
No taper off in power just keeps climbing.

 

[RNM]

uh, no, it's not. it's not even close to as quiet.

It is very quiet if you add a vibrant resonator.

 

[STE92]

VRSF Street version, single top 3” dp to exhaust.
Cold start little louder that stock and when you get on it sound decent nothing crazy. My stock exhaust was with bmw performance muffler was louder. Cruising their is zero drone and u can barely hear it windows up AC on, been stopped cops had zero issues with exhaust.

 

[Tempergarage]

Hey there first post

 

[mwpa71]

Old post but did you get a resolution?

 

[nyt]

Old post but did you get a resolution?

It was exhaust backpressure. My shop build is nearing completion, so will be fabbing up a new exhaust as one of first projects, as well as engine teardown.

 

[mwpa71]

It was exhaust backpressure. My shop build is nearing completion, so will be fabbing up a new exhaust as one of first projects, as well as engine teardown.

Thanks. I have a similar issue on my 135i n55. I upgraded rear section to 3.5 but still have standard midpipes same as yours. I'm swapping them out to single 3.5. My logs look perfect but draggy times are not against similar builds. Good luck with it.

 

[wheela]

Thanks. I have a similar issue on my 135i n55. I upgraded rear section to 3.5 but still have standard midpipes same as yours. I'm swapping them out to single 3.5. My logs look perfect but draggy times are not against similar builds. Good luck with it.

I'm curious if part of the issue could be the wastegate not flowing enough. The turbine can only flow so much.

Here's some screenshots from a Garret g35-900. If you look at the turbine chart, you see that even with the largest 1.21 a/r turbine housing, flow through the turbine chokes out around 30 lb/min. More back pressure doesn't push more flow through it beyond an asymptote. Compare that to the compressor map, which can flow up to 80 lb/min. You can't flow 80 lb/min into the engine if you can only flow 30 lb/min back out. The other 50 lb (actually slighlty more because of the added fuel mass) has to flow through the wastegate to balance mass flow through the engine.

So what I suspect may happen if the turbine and wastegate can't keep up with the flow demands, then back pressure against turbine spikes up increasing turbine speeds but with no significant increase in mass flow across the turbine. Faster spinning turbine builds more boost from the compressor, but the higher boost doesn't yield more load/airflow into the engine because mass flow across the engine is being choked on the exhaust side.

I believe this is why some people run dual external wastegates.

 

[nyt]

except if you add more flow via wastegate, you cannot maintain pr. This is a simple issue. @mwpa71 if you want to test, just disconnect downpipe from exhaust and make a pull. You'll need to make adjustments to keep boost in check if you're maintaining properly now.

 

[mwpa71]

except if you add more flow via wastegate, you cannot maintain pr. This is a simple issue. @mwpa71 if you want to test, just disconnect downpipe from exhaust and make a pull. You'll need to make adjustments to keep boost in check if you're maintaining properly now.

I have open dump tested. Mids definitely restrictive just don't know by how much as the car was so loud I was cop bait and had to cut test time short. My trims jumped high into the positive which demonstrates more fuel and therefore more flow and power. 3.5 midpipe being fitted this week so we shall see.

 

[Jern54]

Engine teardown?

It was exhaust backpressure. My shop build is nearing completion, so will be fabbing up a new exhaust as one of first projects, as well as engine teardown.

Why?