Exhaust Not Worth It

[georgeleedom]

I have a '99 GLT. I recently upgraded to a 16T turbo, blue injectors, and a SRI. I have ordered a 3" down pipe a sport cat, & 2.5" exhaust. The various parts sites, such as IPD, recommend their 3" dp. So does about everyone on these forums. However, I spoke to two different people who build race engines for a living. They both said adding a 3" pipe without modifying the rest of the exhaust adds nothing. They also told me that a 3" dp and 2.5" back is too much exhaust, making the 16T spool up too slowly, as it requires more resistance than that to fire.

I would appreciate any and all opinions on this, especially anyone who has made the mods and can provide actual comparisons.

Thanks,

George

[TorqueSteer]

How much experience do they have specifically with this engine?

The 16t has a tiny exhaust housing, even with an open 3" downpipe, it will never spool "slowly."

That being said, from my personal experience, a 3" downpipe will slightly decrease off the line spool time, but the loss will be made up for in the top end of the powerband. The question is, what are you looking for in the car? Low end grunt, or more top end power? ECU tuning and cam timing will also play a large role in spool time and the overall powerband.

[Che'_Moderator]

I have a '99 GLT. I recently upgraded to a 16T turbo, blue injectors, and a SRI. I have ordered a 3" down pipe a sport cat, & 2.5" exhaust. The various parts sites, such as IPD, recommend their 3" dp. So does about everyone on these forums. However, I spoke to two different people who build race engines for a living. They both said adding a 3" pipe without modifying the rest of the exhaust adds nothing. They also told me that a 3" dp and 2.5" back is too much exhaust, making the 16T spool up too slowly, as it requires more resistance than that to fire.

I would appreciate any and all opinions on this, especially anyone who has made the mods and can provide actual comparisons.

Thanks,

George

Been saying this for 10 years now and no one listens. If you want to go fast run 2.5 all the way back. If you want to be a paper racer 3" or gtfo......

[TorqueSteer]

Been saying this for 10 years now and no one listens. If you want to go fast run 2.5 all the way back. If you want to be a paper racer 3" or gtfo......

I always thought it was you who changed my thread title and poked fun when I suggested that my exhaust was too big, I guess it was one of the other mods..

That being said, if you want to retain close to stock driveability and gain power at the same time, I agree that a 2.5" turbo back would be the best option for you.

[ozzimark]

They also told me that a 3" dp and 2.5" back is too much exhaust, making the 16T spool up too slowly, as it requires more resistance than that to fire.

Walk away and never talk to them again. Turbines work off a pressure difference, the less pressure it sees at the exducer, the more potential there is for the turbine wheel to extract work out of the moving exhaust flow, given a constant inducer pressure. That means better spool and typically higher efficiency.

Plus, it'll bring down exhaust backpressure and EGTs

[Fudge_Brownie]

Been saying this for 10 years now and no one listens. If you want to go fast run 2.5 all the way back. If you want to be a paper racer 3" or gtfo......

Is this to obtain the highest exhaust velocity?

Does this have any negative impact on EGT's, assuming a TD04 exhaust housing?

[Che'_Moderator]

Is this to obtain the highest exhaust velocity?

Does this have any negative impact on EGT's, assuming a TD04 exhaust housing?

Biggest issue you will notice first is screwed up A/F ratios

[Fudge_Brownie]

I thought we were assuming the tune took the 3in dp in to account?

[scandalo]

I will say that a 2.5 DP is not as plentiful as say... 3 inch OBX pipe.

And for the record the 3 inch DP with an angled exhaust housing on my 16T the boost was almost instant. These turbos are too small and not pushing 1.5bar (like some other tuned turbo cars) to worry about lag.

[Che'_Moderator]

I thought we were assuming the tune took the 3in dp in to account?

I like assuming! Lets assume this topic has not been beat more times than justins meat.

GAME ON!

[Dammit]

My MTE tune specifies a 3" downpipe and 100 cel cat as requirements to run the tune (Stage II).

[Fudge_Brownie]

I like assuming! Lets assume this topic has not been beat more times than justins meat.

Yet the 3in -> 2.5in combination is still the standard recommendation around here. So, make like Justin and beat it again.

2.5in DP given no ECU adjustments.

3in to 2.5in catback, make sense given ECU tuned to accomodate 3in DP? Or do you still maintain 2.5in makes the most sense?

[georgeleedom]

Walk away and never talk to them again. Turbines work off a pressure difference, the less pressure it sees at the exducer, the more potential there is for the turbine wheel to extract work out of the moving exhaust flow, given a constant inducer pressure. That means better spool and typically higher efficiency.

Plus, it'll bring down exhaust backpressure and EGTs

I don't know that I'm going to walk away from good friends who build cars in the 7's, but that doesn't mean I'm going to take their advice without considering input from you guys either. I suppose what I'm not understanding is the 16T exducer/inducer size ratio is constant. So if the trim stays the same, wouldn't the exhaust restriction affect whether the turbo thinks it needs to get to work faster? Like if your washing your car and how much of your thumb is covering the hose always stays the same, then you can only change the speed of the water hitting the car by being closer or farther away (or turning up the water, as you suggest with changing the cam timing)

OK, I just re-read that and I'm beginning to think maybe the medication isn't such a bad idea.

[Che'_Moderator]

I don't know that I'm going to walk away from good friends who build cars in the 7's, but that doesn't mean I'm going to take their advice without considering input from you guys either. I suppose what I'm not understanding is the 16T exducer/inducer size ratio is constant. So if the trim stays the same, wouldn't the exhaust restriction affect whether the turbo thinks it needs to get to work faster? Like if your washing your car and how much of your thumb is covering the hose always stays the same, then you can only change the speed of the water hitting the car by being closer or farther away (or turning up the water, as you suggest with changing the cam timing)

OK, I just re-read that and I'm beginning to think maybe the medication isn't such a bad idea.

What you said is spot on for manifold and head dynamics.

[RZT5]

Biggest issue you will notice first is screwed up A/F ratios

How does the 3" dp cause AFRs to be off?