Over boosting/Boost creep
 

I am having some issues with over boosting and boost creep.

This is a large street port 13B RE with BW S366 .91 AR twin scroll, dual tial 38mm WGs, straight pipe 3.5" exhaust, water to air IC, bellmouth intake, E85.

In tuning the car we initially faced boost spikes up to 23 psi with a 14 spi WG spring setup.

The springs were changed to 8 psi to help the problem.

The results showed that with the electronic boost controller closing the WG any amount of duty caused serious boost spikes up over ~25psi by 4000rpms before backing off. It was unknown if the boost would ever level off as the rpms rose. With no boost controller the 8 psi springs are able to hold boost around 12 psi @ 4000rpms before creeping to 19psi by 7000rpms and holding to redline.

Now I realized that my problem is most likely poor WG runner position but due to space constraints and lack of motivation to modify the runners further I am in search of alternative solutions to this issue.

I would like to try increasing my turbine AR to 1.00 or 1.10 so that i can run 20-25 psi.

I doubt there will be any boost lag with the larger housing since it is currently spiking but I cant figure out what effects this will have on boost creep in the higher rpms.

My theory is that a larger AR housing will reduce boost spike at the low end. looking at the dyno graph it seems like the turbine is currently choking at 7000rpms and the boost tapers off at 19 psi in this range to redline. Reduced back pressure could cause even higher boost pressures in this range and pulsing the WG closed could cause the same boost spike issues experienced down low. I am honestly unsure and not interested in wasting money on fruitless fixes.

I have read up on this topic and found a mixed lot of answers. Some saying this solution will help and others saying it will have no effect.

I am hoping someone can shed light on how I can tackle this problem short of modifying my manifold. Or make it clear why modifying the manifold is my only option.

Thanks

I hope this question is clear but I can also give more info if needed.

Bigger ar will not fix it, just shift it up a little bit. I have tried it personally on 2 cars.

i'd like to see your manifold...

Now I realized that my problem is most likely poor WG runner position but due to space constraints and lack of motivation to modify the runners further I am in search of alternative solutions to this issue.

Luckily for you since you lack motivation to fix the problem there is a really easy solution!

You just need an exhaust restriction to limit boost creep. I found that placing it as far from the turbo as possible will provide the best spool while still limiting exhaust flow enough to impact Engine VE.

Place a gasket with a 1" hole in it at the furthest back flange location in the exhaust and work your way up in size to where it *just* starts to creep again (do it at night when the air is cool and dense).

I have had exhausts that just needed a 2 1/8" hole restrictor and I have exhausts that needed a 1" hole restrictor. Good luck.

damn, that must be a really terrible manifold.

1 synapse 50mm wastegate that i have will hold 6Psi on my S366 (.91 ar)
I do only have 3" exhaust, but i dont see that being a restriction at the low power i was putting out.

pictures of the manifold are here:
https://www.rx7club.com/single-turbo...-991863/page3/

Thanks for the info!


https://cimg6.ibsrv.net/gimg/www.rx7...3a22451d55.jpg


https://cimg0.ibsrv.net/gimg/www.rx7...d4e34bfbda.jpg


https://cimg1.ibsrv.net/gimg/www.rx7...df51d51545.jpg

In my opinion the WG position is suboptimal.

I will try this thanks for the info.

Yeah, when the wastegates are close to the exhaust port like that it really helps to angle the wastegate runner or make it out of a radius piece.

It is because the exhaust velocity is so high it would rather keep going down the runner than making the hard turn out the wastegate(s).

Also, looks like you used 1.5" pipe instead of 2". Great for spool, but also means higher exhaust velocity so wastegate placement is even more critical.


Gilgamesh damn, that must be a really terrible manifold.
1 synapse 50mm wastegate that i have will hold 6Psi on my S366 (.91 ar)
I do only have 3" exhaust, but i dont see that being a restriction at the low power i was putting out.

I found a huge difference between 3" and 3.5" exhaust and boost creep on my car. Even the 3.5" downpipe and midpipe were fine with 3" catback, but when it was all 3.5" turbo back it was boost creep city.

Yea, your manifold design is a lot better since your WGs are coming off the runner bends. Mines are on the straight sections requiring a lot of change in flow direction.

Thanks, I will be sure to consider this if I make another manifold. Ideally I would have angled the WGs but the watercooling lines for the WGs are in an awkward position for me and made it hard to orient them any other way.

I really appreciate the info guys.

Whats the inside of the WG mergers look like? Can they be cleaned up or they already pretty good?

I tried my best with a die grinder but they could be ported more cleanly i just dont have appropriate tools. I was thinking of using a flexible honing tool like this:

Flex-Hone Tool BC3424 Flexible Honing Brush - 3/4" at SkyGeek.com

That would be a fairly easy and inexpensive fix on the manifold. Pull it apart and fix it correctly. Stuffing a restriction in the exhaust would fix the symptom but is a terrible idea for proper cure.

Yes, you can keep the wastegates exactly where they are and how they are oriented if you like and just swap the short straight piece you used to attach the wastegate flange to the main runner out.

You can use an off center reducer union and cut it right where it transitions in size placing the extended section toward the engine ports.


https://cimg2.ibsrv.net/gimg/www.rx7...e3ec08e17b.jpg

Or you can use a short radius section and face the inside radius toward the engine port.


https://cimg9.ibsrv.net/gimg/www.rx7...bcb7bfe896.jpg

^ This x 10⁴

I cut the collectors and made new ones. Boost holds solid at 1 bar on a 15 psi spring.


https://cimg1.ibsrv.net/gimg/www.rx7...268ebdd3b.jpeg


https://cimg2.ibsrv.net/gimg/www.rx7...27ca6233c.jpeg


https://cimg4.ibsrv.net/gimg/www.rx7...58466b917.jpeg


https://cimg6.ibsrv.net/gimg/www.rx7...1c972e25a.jpeg

:icon_tup::icon_tup::icon_tup::icon_tup::icon_tup:

Top effort buddy. Mine must have been the same angle as yours. I couldn't help kinda being impressed how the holes ended up being a perfect egg shape.

Thanks. A hole saw and angle grinder did the trick.

Good work mate!

well done! can mods change the thread title to resolved. or fixed..?