2.5 vs 3 inch downpipe
 

Currently I have the Racing beat 2.5 inch downpipe, NA presilencer and NA catback. I just installed an external wastegate on my S4 exhaust manifold using a Tial MV-R wastegate and 11 psi spring. Boost goes up to 15 psi, and drops back down to 10-11 psi. What can I do to minimize that drop? If I install a stronger spring, is it going to hit 18 psi and drop down to 15?


What kind of gains would I expect to see from a 2.5 inch downpipe going up to 3 inch while retaining the rest at 2.5 inch? What I want is boost to stay solid.

not much of a gain with the 3 inch downpipe since the exhaust flow would be restricted by the 2.5 inch catback almost like a funnel going larger to smaller. a 2.5 inch downpipe to 3 inch exhaust would flow better than the previous. with the wastegate 15 psi is just the spike before the wastgate opens. once it is open boost is going to drop off to the pressure the spring is set to which in your case would be 11 psi. if you get a stiffer spring say for example a 15psi spring it would spike to whatever pressure then drop off to 15 psi once the wastegate opens

is your wastegate going back to the 2.5" exhaust?

wastegate vents to atmosphere.

I would like a pic how you installed the external wastegate on the stock exhaust manifold.
Is your stock wastegate flapper door still opening? I would think that has something to do because there is still spring pressure on it and will fluctuate boost unaccordingly with the 2nd wastegate.

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in my experience the opposite has been true.

header off the turbo needs to flow optimally and on turbo engines 3" is fairly standard for a wide range of use.

tapering to 2.5" midpipe and stock catback will give decent flow for upwarards of 350rwhp.

more info is needed on the engine and turbo though, in most cases upgrading the spring will not allow the transition to move up. in short putting a spring to spike to 18psi it will probably still drop to 11psi. most of the time when you have a drop in boost it is due to the turbo losing efficiency either due to bottlenecking in the intake or backpressure in the exhaust. running an EBC sometimes you can correct this by upping the duty cycle to hold the wastegate closed more in the upper range.

You are getting boost creep above wastegate spring pressure and then it settles back down to wastegate spring pressure on the high rpm?

Add a manual boost controller and set it to run your desired boost.

Or, go ahead with your plan for a more free flowing exhaust so it boost creeps in the high rpm as well.

Either will work, but in my experience enlarging the downpipe and midpipe will help boost response, but if there is a restriction further down stream boost will still drop.

To get boost creep in the high rpm you may have to make your intake and exhaust much more free flowing- which will result in much more power than just increasing the boost as you are improving the volumetric efficiency of the system.

But if you tune your desired boost with boost creep you will find that lower altitude denser air (cold night on coast) will really give you more boost than you intended.

I am not saying either way is wrong or right, just letting you know what to expect.

Blue TII - I dont think it is boost creep. As we all know, the stock turbo does not like to hold higher boost and will drop down at high RPMs. Lots of back pressure opens up the wastegate causing the boost to drop from what I understood. I picked up the external wastegate hoping to flow that extra pressure away keeping the intended boost level (since I can adjust the spring pressure).

I dont want it to creep, or drop. I just want it to hold at a solid amount, doesnt everyone? Yes the 11 psi spring holds 11 psi up top, but around 4500-5500 its up at 15 and then tapers down to 11 by 7000. I dont mind putting a stiffer spring in to get it to hold 15 psi - but currently as it sits I'd expect it to hit 18 and then drop. I dont want that and with me tuning personally I dont feel comfortable going over 15 psi until I get a local tuner to fine tune it for me.

RotaryEvolution - Its a 13BT, FD rotors, S5 irons(thick) S4 rotor housings. intake casting cleaned up, and a medium/large exhaust port, ALS apex seals.
BNR stage 1 hybrid,
full RB exhaust,
vmount intercooler
megasquirt 2 ECU
4 x ID 1000cc

The BNR was a local custom made turbo. I had him model it, I wanted him to use a T04B compressor housing but he didnt, he said he machined the stock housing a good bit. I'm sure there is more in this turbo - It's just a matter of getting there the right way.

i would probably run a lower spring and try a cheap manual boost controller to see how it reacts.

Blue TII - I dont think it is boost creep.

Well, its easy to know or not with an open vented wastegate.

If you hear your wastegate open at 15lbs boost but then close on the top end when it is down to 11psi boost then you are correct that you have tightened the spring to raise the wastegate boost to 15psi and restriction or lack of compressor flow is causing the boost drop.

If the wastegate stays open in the high rpm when the boost drops to 11psi then you are running at 11psi wastegate pressure and boost creeping to 15psi in the midrange.

I would hook up a solenoid to your megasquirt and use that to control boost. it's RPM dependent so it can change the boost the wastegate sees at higher rpms to keep the boost up.

Blue, I will confirm later. I havent paid attention if the wastegate opens at 11 and goes up to 15 or not.

Eage, I have one in the engine bay but never hooked it up. I have a feeling I screwed up something in my MS cause I could never get it to control my fan relay either.

I forgot how the s4t2 manifold was made. is that wastegate only effecting one runner or is it opening both rotor 1 and 2 exhaust pulse?

i also wouldn't trust that weld on the stock flapper, a single spot weld isn't going to be enough to hold it closed and it is probably leaking. not to mention it is barely welded on the flapper, and the weld will degrade under the heat over time.

I'm okay with that. This mod was done with the intent of fixing the boost drop at high RPM. I can always add more weld, Its harder to undo if it were welded more.

In the next year or so I will have a whole new turbo setup - In the mean time I wanted to push the hybrid and see if it can be made a more viable and worth while option for my low HP goal of 350 WHP.

GrossPolluter I'm not sure if both or only a single runner hit the factory wastegate. Why would it matter if it is blocked off?

If the manifold is divided, and if your external wastegate is only relieving one rotor from spooling the turbo, the other rotor will be continuing to spool the turbo. I don't remember how the s4 manifold was. That size wastegate should be enough.

I would also look into the spring that is inside the wastegate and also the adjustable tension. Maybe a pressure test on the wastegate to see when it opens.
Maybe the spring is too stiff, or the adjustment is very tight.
Diaphram having issues?

oh sorry, looked at the picture and the external wastegate is relieving both rotors.

After reading, it looks like I get boost SPIKE. it rises above the chose level and then quickly goes back down, then the boost DROPS below the chosen level. i replaced the 11 psi spring with a 13 psi spring and now i can get it to hold a solid 11 psi. I think I will need to try a 15 or 17 psi to get it up where I would like it.

strange that a higher spring rate would eliminate the spike..

My apologies. It still spikes. I guess I should re-read my posts before hitting submit. The second point was that it does not DROP as much as before with the stiffer spring.

just be weary of spiking above 18psi on pump gas, i won't even tune beyond 17psi on premium due to the risk of detonation involved beyond that without a knock suppressor(AI).

it sounds like the lower spring and a boost controller should give you a good constant boost level of 13-15 with the 11psi spring.

I've been weary of even going over 15 psi at peak torque. My timing table is very conservative and I'm probably leaving a lot of power on the table until I get meth or water injection and a proper tune.

I will get back to you next weekend when I have another opportunity to take the car out :) I need to go for a cruise on the highway one night and play with the MBC until the boost is set right.