I am trying to get my '99 with a BTR stage 3 cam and TSP 225 heads to the next level. When I had it built, Covid was making parts hard to find. I tried for weeks to score an LS6 intake I could send off for porting, and searched for a Fast 92 in good condition, but wasn't having any luck there either. We ended up putting my stock 78mm LS1 intake back on.

Here is my question. When the tuner was working with it, he kept complaining about the intake temps and the drivers side cat temp. To get my peak HP run, they had to hold two carpet dryer fans over the intake. It was only 80 degrees that day and the shop doors were open. I kept hearing them also mention a velocity problem. Obviously with high flowing heads, I need to do something with this intake. Is it possible the 78mm throttle body is compressing the air causing the temps to rise? Or is the air not flowing fast enough to have that problem? I do notice the car is definitely more powerful in the early morning hours when ambient temps are coolest.
I'm no mechanic or engineer, but I am curious if a larger intake/TB will lower the intake temps?

Also, if any of you have any recommendations for an intake with minimal mod and a stock hood, feel free to mention it here.

Thanks,
D

 

May have limited your opportunity to find an intake if you were only looking for a used LS6 intake. Any 2001-2004 intake would suffice, as they are all the same for LS1 and LS6.

I would like to know more about the left side cat temp. What was the disparity between the left and right banks? I assume you have headers and hi-flow cats with that setup?

 

Left cat was 910 degrees, right cat was 800. The exhaust is stock. Because I live in I/M territory, I have to pass a visual and an OBDII check. In 2029, I am eligible for a vintage tag and can make some mods. However, I have an exhaust shop that worked on my truck last week that claims a Magnaflow kit with high flow cats and an x-pipe paired with some BBE shorties would keep me CARB legal, drop the temps, and maybe even pick up 10-12 HP. With a bigger intake, I would hope for another 15-20 on top of that. I just wonder if the intake and cat temps now are pulling timing?

 

Could be one of the Cats (Left one) is starting to clog. And as far intake temps what were they? Sitting on a dyno there is a lot of heat soak due to restricted air flow. I know you had two fans on it but just the heat coming off the radiator is enough to heat up the IAT's.

 

IAT was at 125 on the last pull. I've seen a $20 harness that moves it down closer to the CAI. Mine is a Donaldson Blackwing. Does moving it further from the TB really make a difference, or is it snake oil?

On the cat, when I get on it, it does smell like burnt matches. I don't know if it's any indicator, but the two sides don't sound any different, nor does one side blow harder than the other. I know new cats are part of the equation at some point, I just don't know when to tell that time has come.

Edit: It passed I/M two weeks ago with everything at Ready. I imagine a bad cat will throw a code and it fails, correct?

 

Are you in CA and therefore subject to typical CA smog test requirements?

These are Magnaflow's CARB-compliant offerings for your 1999: https://www.magnaflow.com/collection...alifornia=true
I highly doubt their CARB-legal cats are "high flow", perhaps higher flow than stock, but not on the order of a typical 200-cell high-flow cat.
Also, going this route would apparently cost you $$thousands, hardly seems worth it for maybe 10hp.

Sounds like you'd be better off "California swapping" the car, i.e. running a different cat setup except at smog time. Note: this is not a recommendation to run a CA-illegal exhaust setup...

 

Moving the IAT sensor closer to the CAI does not cool the IAT. It just reads it at a cooler point upstream. Essentially fooling the computer, but it should prevent the computer from pulling timing out. The harness moves the sensor away from the radiant heat source. The problem is the radiant heat it's exposed to. I would try some reflective( the foil faced stuff) insulation around the tubing between the TB and the air cleaner. Only the foil stuff will reflect the radiant heat. And a clogged Cat may not show a code. The burnt match smell is Sulfur Dioxide, the cat is starting to fail. 125°F IAT is not that uncommon and the computer will pull timing as IAT's increase.

 

If the driver side cat is starting to become restricted the hot exhaust gases have nowhere to go...what do your fuel trims look like ??...may be negative on bank 1...best thing is hook up a back pressure gauge...max backpressure is 2 psi at 2000 RPM…being drivers side instead of attaching a back pressure gauge to the O2 sensor bung you can do it right at the left AIR air tube…maybe even disconnect it and see if your IAT temp decreases…I have a fitting I can adapt to my back pressure gauge for the AIR check valve.

 

 

This makes a whole lot of sense. When I approach a stop sign or light, as soon as I let off the throttle and push the clutch, the car dips down to around 300 RPM, surges and hunts for idle a couple of times, and finally settles in at 750. It's worse when cold and I brake as well. (Almost like I am producing vacuum here.) I didn't connect the dots until you posted this, I chalked it up to an issue with the tune. I've got to go buy a couple of parts to make the jumper hose you pictured below. I'll make this my weekend project.

 

So, some heat wrap similar to what guys put around their headers?

One more question. If heatsoak is an issue, do headers make it worse from more radiant heat? Or do they make it better with more flow? I assume moving the IAT isn't going to gain me more than a couple HP then. I was looking to see where I left about 50-75 on the table. The cam is supposed to run 450 WHP. On a good cold day with plenty of headwind, I might crack 400. She's making 390 on the dyno.

 

No, I'm in Utah, but have a CA car. It came out of San Diego.

So here is where I'm at. If the cats are clogged, I have to replace them anyway. The Magnaflow is bolt on and could hypothetically be done in my garage on jack stands. I realize without LT, I'm not going to pick up a ton of power, but a "higher flow" than stock is definitely better than a plugged stock cat. I may as well throw on some BBK or BBE shorties while I am under there. Used they run around $300 - $500 a set. The emissions station down the street says as long as there is a CARB number on the cat and the header, it passes the visual.

It's funny how price becomes an issue.. Some guys blow $1500 on a CAI and cat-back to pick up 10 HP.. lol I imagine spending the same amount to pick it up on the back side is just as good.

 

Well with your scan tool if you have one is to look at your IAT then disconnect the AIR fitting and see if the IAT is now lower…if it is lower then that is most likely your issue and no need to do a back pressure reading…curious what your fuel trims look like and your driveability issues like your idle surging…you may have negative trims on the left back and positive on the right side.

 

I have a Foxwell NT301. It says it logs live data, but I have never used it for that. Only checking and clearing codes. I'll read up on it and try this method later today. Thanks for the tip!

 

Well, why would you put a stage 3 cam in there along with 225cc heads, no headers and no fast intake, that’s why you’re making 390 wheel horsepower. These cars run all day long with high intake temps, that’s the way it was designed. I don’t have a manual car, but from my understanding, that thing is going to surge at low RPM with that cam in there. Need a good tune, 3.90 gear, shorty headers if it’s legal in CA, maybe new cats, leave your intake setup alone until you find a LS6 intake you can port to match the 225cc heads.

my .02

 

Engine at temp, ambient temp outside 90 degrees. IAT 111, Short trim 7 % at idle.

At 2000 RPM, IAT drops, FT unchanged.

IAT pulled out and tucked into hood liner as high away from engine heat as I could get it. It went up anyway to 123. FT went to 10.2 at idle. I swear I could hear it pull timing.

IAT plugged back in, revved a few times, temp drops and FT drops momentarily. MAP PSI doubled for a split second.


What do you think? Sounds like there is no vacuum being built. Cat is still functional?

 

Is these numbers with the AIR connected ??…if you removed the IAT sensor which is after the MAF sensor this is unmetered air that the MAF sensor doesn’t read so your fuel trims will go positive as you saw !!…you have a big vacuum leak…you would have to block that off…are you getting these values on the enhanced side or global OBD2…in global you don’t enter make, model etc…in global OBD2 the numbers you see can’t be substituted…what the engine computer see you see…in enhanced you may see a different temp than global.

 

I must be misunderstanding. I'm not sure what the AIR is? Do you mean the fitting on the exhaust manifold from the air pump? The Foxwell is in global mode.

 

Yes !!

 

I ask myself that question a lot. Rick at AFR recommended a stage 2 cam with AFR 205 heads. The guy at the performance shop said no, you want maximum power.. go stage 3 with big heads. Then the heavy lifting is done and the rest are just bolt-ons. It's a convertible that will never see the track or a strip. Some days I wish I had listened to AFR and had better low-end torque and better street manners. But, what's done is done. If I can add a better intake and new I/M legal exhaust to get to 420+ I will be happy with it. Don't get me wrong.. from 3000 RPM all the way to 6700, it rips and is very fun to drive. It shuts up WRX and 5.0 folks, but with the money I dropped on it, I should see a bit more.