Technical Much more boost, less power at high rpm

nyt

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Sep 15, 2019
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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.

3-png.png


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


And comparing a higher boost 4th gear pull to lower boost 3rd gear pull...
34low-png.png


This is the exhaust
touypzs-jpg.jpg


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

 
Last edited:

nyt

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Sep 15, 2019
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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

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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

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

Brule

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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.
 
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nyt

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Sep 15, 2019
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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

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Sep 15, 2019
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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

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Feb 20, 2017
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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

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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.
 

STE92

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Mar 3, 2017
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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.