Cutout Valve Power Gains / Turbo Spool Increase

[stormgreysupra]

Hello everyone!

I currently have a top mount turbo kit with a Garrett G35-1050, 0.83 AR with the factory exhaust.

Does anyone have any experience going from the factory exhaust to a turndown dowpipe? Is there a noticable power gain and more importantly increase in turbo spool?

I’m considering adding a cutout valve right behind the downpipe. Thanks!

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[cbrummell]

Hello everyone!

I currently have a top mount turbo kit with a Garrett G35-1050, 0.83 AR with the factory exhaust.

Does anyone have any experience going from the factory exhaust to a turndown dowpipe? Is there a noticable power gain and more importantly increase in turbo spool?

I’m considering adding a cutout valve right behind the downpipe. Thanks!

i'll try and see if i can find the article i read but it was complete test done on a mk4 supra making around 700whp testing the differences between 3 inch - 3.5 inch - 4 inch exhaust and lastly an open dump all on a dyno.

by the time they fitted the full 4 inch setup there was enough difference from the flow gains they had to make revisions to the tune!

bigger exhaust not only allowed for faster spool but also made more power on the same boost!

i think you will definitely pick up power by going to a cutout but it should spool faster at minimum!

 

[Thraxbert]

It'll probably pick up ~15-20HP on a stock turbo, and maybe upwards of 30-50 on an aftermarket turbo.

 

[cbrummell]

i'll try and see if i can find the article i read but it was complete test done on a mk4 supra making around 700whp testing the differences between 3 inch - 3.5 inch - 4 inch exhaust and lastly an open dump all on a dyno.

by the time they fitted the full 4 inch setup there was enough difference from the flow gains they had to make revisions to the tune!

bigger exhaust not only allowed for faster spool but also made more power on the same boost!

i think you will definitely pick up power by going to a cutout but it should spool faster at minimum!

https://www.motortrend.com/how-to/modp-1102-exhaust-test-tech/

 

[stormgreysupra]

i'll try and see if i can find the article i read but it was complete test done on a mk4 supra making around 700whp testing the differences between 3 inch - 3.5 inch - 4 inch exhaust and lastly an open dump all on a dyno.

by the time they fitted the full 4 inch setup there was enough difference from the flow gains they had to make revisions to the tune!

bigger exhaust not only allowed for faster spool but also made more power on the same boost!

i think you will definitely pick up power by going to a cutout but it should spool faster at minimum!

yes I’m familiar with that article. Thing here is that my downpipe diameter will remain the same with the 3.15 inch connection.

 

[stormgreysupra]

It'll probably pick up ~15-20HP on a stock turbo, and maybe upwards of 30-50 on an aftermarket turbo.

that sounds very much like it’s worth it!

 

[Thraxbert]

You could probably pick up even more with a true 4" downpipe that just open dumps. There's going to be some loss because the turndown still has to squeeze exhaust through the 3.15" section.

Also: it's going to be phenomenally loud.

 

[stormgreysupra]

You could probably pick up even more with a true 4" downpipe that just open dumps. There's going to be some loss because the turndown still has to squeeze exhaust through the 3.15" section.

Also: it's going to be phenomenally loud.

i would have to get a whole new downpipe custom made and then the connection would still have to be 3.15 inch where it meets the factory exhaust.

i’m not worried about how loud it is because I could always close and open the cutout depending on what i’m doing with the car.

 

[Thraxbert]

Wait: are you doing an active cutout or exhaust removal + turndown tip?

 

[stormgreysupra]

Wait: are you doing an active cutout or exhaust removal + turndown tip?

active cutout behind the downpipe with factory exhaust, that’s my plan.

would I still get power and spool gains?

 

[NiceBMW]

There are a few full 3.5"+ exhausts out there if you end up going that route. I bought DAWs 3.5" exit catless DP and injen's 3.5" exhaust. Just cut the 3.15 connection end off the exhaust and used a long 3.5" clamp to connect them. Don't have a ton a data to share but was the best option for me.

 

[Thraxbert]

active cutout behind the downpipe with factory exhaust, that’s my plan.

would I still get power and spool gains?

I'm not sure the gains would be that great. The exit orifice of an active cutout is very small. When you said "turndown" I assumed you meant removing the exhaust and installing a turndown tip (e.g. ETS turndown tip).

 

[stormgreysupra]

I'm not sure the gains would be that great. The exit orifice of an active cutout is very small. When you said "turndown" I assumed you meant removing the exhaust and installing a turndown tip (e.g. ETS turndown tip).

There would be a turn down tip welded into the midpipe with a valve in it. I didn’t want to confuse anyone so just asked about a turn down exhaust. It should be very similar.

 

[NiceBMW]

There would be a turn down tip welded into the midpipe with a valve in it. I didn’t want to confuse anyone so just asked about a turn down exhaust. It should be very similar.

If you have a full 3.5-4" DP that goes to a cutout, I can't see why spool and power wouldn't be improved over the restrictive exhaust although we will be guessing what the actual gains are without before/after testing. Remnant shows some great gains with thiers which gives us an idea.

 

[stormgreysupra]

If you have a full 3.5-4" DP that goes to a cutout, I can't see why spool and power wouldn't be improved over the restrictive exhaust although we will be guessing what the actual gains are without before/after testing. Remnant shows some great gains with thiers which gives us an idea.

i have a 3.15 inch downpipe, I want to add a cutout valve with a turndown tip there. Not add a new bigger downpipe.

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