Turbo Performance Question

[jhayes63]

I'm relatively new here having bought a red 1990 turbo Grand Prix this past fall. I'm not sure of it's history but one modification is that it has an aftermarket K&N air filter instead of the original air intake system to the turbo inlet. I used to have a 1985 Buick Grand National turbo which is before they had intercoolers. Of course the later Grand Nationals had intercoolers to boost horsepower. My 1985 GN had an aftermarket filter to improve air flow, similar to my 1990 GP. The other mod that we would do is to run ducts to get the air from the front of the car behind the grill.

So my question is since the 1990 GP stock air intake was an intercooled system, why replace the air box and duct work taking air from the front of the car with the aftermarket open filter taking air near the turbo? Since intercoolers provide more horsepower via cooler intake air, it would seem the stock system would provide cooler air to the turbo than the open filter under the hot engine bay next to the hot turbo, all things being equal. This would seem to result in lower temperature after the intercooler hence more power. Was the original set-up so much more restrictive, robbing horsepower, or was there another reason?

I would like to return the intake back to original but want to make sure that I wouldn't cause performance issues. By the way, I'm looking for the original system parts - any ideas?

Edited March 27, 2023 by jhayes63
Clarification

[Jaywags]

There is no benefit for an aftermarket cone filter pulling air from the engine bay.. besides it already being intercooled.. it’s designed perfectly fine from factory(besides maybe a bigger turbo). I had a cone filter on my 3rd gen RX7 because I swapped the twin turbos to big single… never got around to adding an air duct to get the air from the front. Anyway, the TGP also has a oil cooler underneath the passenger side that is connected to air intake duct … it needs that air flow that’s being pulled through by the turbo suction …please anyone correct me if I’m wrong.. but that’s my thoughts. 

[jhayes63]

Thanks for the input! I hadn't realized that the oil cooler was connected to the air intake.

[Jaywags]

On 4/3/2023 at 4:30 PM, jhayes63 said:

Thanks for the input! I hadn't realized that the oil cooler was connected to the air intake.

I’ll try to get a picture and post it on here… I noticed my tube to the cooler was crushed a bit… but I’ll get it fixed eventually.

[Jaywags]

Here are the pics, in order from air inlet on the bottom, tube to oil cooler, under the headlights you can see the hose clamp to the plastic tube and then through the fender cover to the filter housing 

[Jaywags]

9 hours ago, Jaywags said:

Here are the pics, in order from air inlet on the bottom, tube to oil cooler, under the headlights you can see the hose clamp to the plastic tube and then through the fender cover to the filter housing 

Apparently the pictures loaded backwards…

[White93z34]

Wait, what.

On these cars the oil cooler is a liquid to liquid design, its basically a water cooled stack cooler that sits on the engine block.

What you're showing is pictures of the auxiliary transmission cooler that is crammed over in I believe the passenger side wheel well, I forget where exactly it pulls air from, I think from down low by the valance. The Air intake tubing and Trans cooler air intake are not connected in any way.

As far as the actual factory air intake parts, I want to say someone figured out at some point that the factory setup was less then good, I want to say the factory air filter element was taken from a Sunbird or something along those lines.

Granted I have zero data to back that up with, perhaps in the past people just wanted to hear the turbo sing better and that obviously meant racecar. I guess it would be interesting to see if it actually is restrictive or not. Honestly I can't say I've thought about the factory air intake this much before . Given that this application of the T25 was technically undersized for the displacement of the engine you'd think that whatever ASC put on was fine for a factory application. Maybe all that changes with the different tunes/chips that people put on these cars. I've been driving a chipped one way way way longer then in factory form. I guess TLDR all I know is what's been reported on the forums for the last 20 years and have never really stopped to consider the factory intake ducting in such detail. 

[Jaywags]

5 hours ago, White93z34 said:

Wait, what.

On these cars the oil cooler is a liquid to liquid design, its basically a water cooled stack cooler that sits on the engine block.

What you're showing is pictures of the auxiliary transmission cooler that is crammed over in I believe the passenger side wheel well, I forget where exactly it pulls air from, I think from down low by the valance. The Air intake tubing and Trans cooler air intake are not connected in any way.

As far as the actual factory air intake parts, I want to say someone figured out at some point that the factory setup was less then good, I want to say the factory air filter element was taken from a Sunbird or something along those lines.

Granted I have zero data to back that up with, perhaps in the past people just wanted to hear the turbo sing better and that obviously meant racecar. I guess it would be interesting to see if it actually is restrictive or not. Honestly I can't say I've thought about the factory air intake this much before . Given that this application of the T25 was technically undersized for the displacement of the engine you'd think that whatever ASC put on was fine for a factory application. Maybe all that changes with the different tunes/chips that people put on these cars. I've been driving a chipped one way way way longer then in factory form. I guess TLDR all I know is what's been reported on the forums for the last 20 years and have never really stopped to consider the factory intake ducting in such detail. 

I'll check my shop manual tonight to verify... very possible its a trans cooler... 

[Jaywags]

5 hours ago, White93z34 said:

Wait, what.

On these cars the oil cooler is a liquid to liquid design, its basically a water cooled stack cooler that sits on the engine block.

What you're showing is pictures of the auxiliary transmission cooler that is crammed over in I believe the passenger side wheel well, I forget where exactly it pulls air from, I think from down low by the valance. The Air intake tubing and Trans cooler air intake are not connected in any way.

As far as the actual factory air intake parts, I want to say someone figured out at some point that the factory setup was less then good, I want to say the factory air filter element was taken from a Sunbird or something along those lines.

Granted I have zero data to back that up with, perhaps in the past people just wanted to hear the turbo sing better and that obviously meant racecar. I guess it would be interesting to see if it actually is restrictive or not. Honestly I can't say I've thought about the factory air intake this much before . Given that this application of the T25 was technically undersized for the displacement of the engine you'd think that whatever ASC put on was fine for a factory application. Maybe all that changes with the different tunes/chips that people put on these cars. I've been driving a chipped one way way way longer then in factory form. I guess TLDR all I know is what's been reported on the forums for the last 20 years and have never really stopped to consider the factory intake ducting in such detail. 

The cooler is definitely attached to the air intake duct… it’s bolted directly to the intermittent housing… 

[White93z34]

its connected to a intake duct but not the same one that the engine draws from, look down by the valance there is another opening there, that is where the transmission cooler draws air. 

Actually, you circled both entry points in green even.

The one for the engines air intake draws from a scoop behind the passenger side headlamp.

And yes, that is the auxiliary transmission cooler 100%, trust me here I've had enough of these cars fully apart and back together again.

If you look above the oil filter you'll see an aluminum oil cooler with coolant lines coming and going from it.

[White93z34]

Just to prove I’m not full of it. Here from the parts manual is the entirety of the LG5 engine air intake system 

[Jaywags]

17 minutes ago, White93z34 said:

Just to prove I’m not full of it. Here from the parts manual is the entirety of the LG5 engine air intake system 

Yes sir, I stand corrected… I just saw it tonight when I got home.. I appreciate you clarifying that.

[Amanita]

Obviously the NA engines that I have experience with are different from the turbo but I always thought the cylinder heads were the choke point for air intake, not the air box.

Edited April 6, 2023 by Amanita

[jhayes63]

Thanks! Those are great pictures. Your car looks very original. Nice to have parts numbers on the hoses!