Reducing Back Pressure In Exhaust.

[NYTSKY]

Hi all,

After i had my car tuned on E85 we could see that boost was dropping higher in the rev range.

After trying various things to keep the boost steady we determined it was too much back pressure in the exhaust system (even with a screamer)

I have since plumbed the screamer in and now boost has dropped again (21psi to 18psi) which i think confirms the fact that the back pressure is reducing the boost in higher RPMs, now that the added gas from the wastegate is joining the exhaust system.

What can i do to reduce the back pressure in my exhaust system??

I was thinking the turbo is too small for my setup and was considering an upgrade.

[Cjmartz2k]

Can't help you if the turbo is to small, but as for exhaust, do you guys use "cutouts" down under? They are pretty popular in the states, like this combined with this.

[joeyjoejoejuniorshabadoo]

Can't help you if the turbo is to small, but as for exhaust, do you guys use "cutouts" down under? They are pretty popular in the states, like this combined with this.

i dont think that's gonna help him... running a 2.5" cutout on a 3" system...

bigger gate maybe? and keep the screamer

[allthewaytotheskyline]

what turbo are you running exactly and what size is your exhaust system at the moment?

[TiTAN]

or upgrade the system to 3.5/4"?

[Cjmartz2k]

i dont think that's gonna help him... running a 2.5" cutout on a 3" system...

I didn't look and see what size dump it was, I just chose the ones closest to the top of the list. They have them in 3" and larger.

Bigger wastegate does seem like a good option to me, but I think somebody with GT3037 specific experience could say better. I had a single 50mm WG and I would loose boost on the top end. I ported out the manifold WG runners and and WG, then it would hold boost all the way to red line. As the weather got cooler however, I then started having boost creep problems, so we chopped up the manifold at the WG runners, re-angled them, and added a second 50mm wastegate. Now it holds boost rock steady at 1.7 bar and we were able to put 12 degrees of timing back into the MAP at peak torque due to a LOT lower backpressure in the manifold.

[SiR_RB]

a larger bore exhaust no smaller then 3" all the way....what cat u got on there? make sure u run a genuine highflow cat, hell i'ld even run a 4" highflow cat in there

[juggernaut1]

Upsize to mandrel bent 3.25 or 3.5 inch exhaust using in 5 inch body metal 100 cpi cat with 3.25/3.5 inch in/out of the cat with X number of straight through mufflers to suite your noise requirements.

Edited April 11, 2009 by juggernaut1

[NYTSKY]

GT3076

50mm Synchronic gate

3inch custom dump

4inch body MetalCat

3.5 to 3.75 HKS exhaust.

Would a larger body cat help? Or a 3.5inch dump pipe?

[juggernaut1]

Some data I came accross recently regarding the Ballistic Cats http://www.surefloexhaust.com.au/frame1600x1200.htm:

Summary info from the above URL - as you'll see the 5 inch body seems to be the way to go

r. Measurements: -

5” cat with 3” ends - 706cfm

5” cat with 3.5” ends - 885cfm

5” cat with 4” ends - 1015cfm [/i]We tested the “Magic Cat” and “Metal Cat” using their own 3” fittings welded to each end and compared the flow with our Ballistic Cat with 3” pipework welded to the ends and here are the results:

Magic Cat 4” body with 3” ends - 546.6cfm

<LI>Metal Cat 4” body with 3” ends - 629.6cfm

<LI>Ballistic Cat 5” body with 3” ends - 706.2cfm

There are some rubbery figures going around. Magic advertise the flow of their 4” cat as 750cfm and their new 5” cat as 890cfm. These tests are obviously done with no ends on the catalytic converter. So if you compare the 5” cat from Magic @ 890cfm against our 5” Cat with 4” ends @ 1015cfm it shows ours flows 125cfm or 14% extra. This is extra performance. This cat is also available as a “Bolt-On” replacement unit for Nissan R32, R33, GTR, GTST, GTiR Pulsar, 180SX and S13 Silvia.

So assuming your running 3" in/out on your metal cat - it is flowing 629.6 cfm..........................compared to a 5" body Balistic cat with 3.5" in/out would be 885 cfm (or 1015 cfm if you went 4" in/out)

Edited April 11, 2009 by juggernaut1

[R31Nismoid]

Where is the restriction?

Drop the exhaust before the cat

Then from after the cat

Then from the next flange near the rear (if there is one)

3.5" should be flowing enough, no question there. I reckon its also the cat

[Guilt-Toy]

how much back pressure do you have in your exhaust? mine has about 4psi on 330rwkw and nearly 8psi!!! at 360rwkw.

need more information, if you have back pressure how much do you have ? id say the issue is your turbo not your exhaust, my 3" can make 370ish with no issues.

[paulr33]

agree with the others, id say its in the turbocharger housing

how is the manifold setup, it could be the backlog also

[joeyjoejoejuniorshabadoo]

i'd say the turbo is stretch to its max..... BUT

as ash said, drop the zorst and see...

i'd go 3.5" all the way with a decat

[joeyjoejoejuniorshabadoo]

where is your 3" plumbed in btw?

3"? 3.5"? 3.75"?

you could try plumbing it in a little later where it opens up a bit...

[NYTSKY]

Where is the restriction?

Drop the exhaust before the cat

Then from after the cat

Then from the next flange near the rear (if there is one)

3.5" should be flowing enough, no question there. I reckon its also the cat

I had the guys at the shop look over everything for me, they said all looked fine. Im thinking ill try a 5" body cat.

how much back pressure do you have in your exhaust? mine has about 4psi on 330rwkw and nearly 8psi!!! at 360rwkw.

need more information, if you have back pressure how much do you have ? id say the issue is your turbo not your exhaust, my 3" can make 370ish with no issues.

Not sure how to measure it Anth, but i agree the system should support well into the 300s easy.

agree with the others, id say its in the turbocharger housing

how is the manifold setup, it could be the backlog also

Manifold is a 6BOOST item. Im leaning towards the turbo itself. With the extra timing and boost added for the E85 tune id say the turbo is almost at its MAX.

Some will say you can get more out of it but my tuners dyno reads lower than most.

Maybe its GT35R time

[R31Nismoid]

I had the guys at the shop look over everything for me, they said all looked fine. Im thinking ill try a 5" body cat.

no i mean do a run, drop, do a run, drop another place

you'll soon see whats causing it

[SLAPS]

Manifold is a 6BOOST item. Im leaning towards the turbo itself. With the extra timing and boost added for the E85 tune id say the turbo is almost at its MAX.

Some will say you can get more out of it but my tuners dyno reads lower than most.

Maybe its GT35R time

Get the boost drop sorted and it will make some decent power.

Plumbing the wastegate in to the dump kills boost a fair whack. :S

E85 produces more exhaust gas than regular pump fuel so no doubt not helping your problems.

As others have mentioned drop the exhaust after the front pipe to see where the restriction is.

All though its likely to be a bandaid you may wish to look at fitting the gate with a spring to suit the boost your after, then use the EBC to tweak spool.

Another option is to fit a smaller gate that will reduce the surface area and prevent the gate being blown open once the pressure backs up.

If you have to redo the exhaust consider a 3.5-4" dump and then if need be taper it down to 3-3.5" or twin 2.5" after the cat. Works for the XR6T's to make decent power and keep noise under wraps. Twins do an excellent job of keeping noise down while still flowing damn well but $$.

I had the guys at the shop look over everything for me, they said all looked fine. Im thinking ill try a 5" body cat.

Some times if a workshop is busy they will say it looks fine but they really don't know until they drop the exhaust and find out the good ol' fashioned way.

Edited April 12, 2009 by TheRogue

[jeffworld2]

dave run it on the dyno with the cat off, its two bolts; takes 10min max to remove and Adam even has a hoist down there. If its your exhaust you will know straight away

[The Mafia]

just letting you know -

I have a GT3037 with a 0.63 rear and a 0.6 compressor cover, 3 inch off the dump, 3 inch pipe through the cat, and 3.5 inch all the way with a muffler and cannon. Standard exhaust manifold, and intake. Standard cams, standard head.

Car is making 310rwkw and holds 20psi from 3200rpm in third right to the limiter and doesn't spike or drop off. I have a gizmo boost controller and a fairly tight 9psi wastegate actuator.

What size rear housing do you have? I don't recommend the 0.63 unless you have water meth. 0.82 is the best.

Edited April 15, 2009 by The Mafia