Maximize spool - should I ditch the anti-surge compressor housing?
 

Quick back story - I just picked up 91 coupe with a moderate street port and a GT3582r with a T4 1.06 a/r hot side. It has a cast anti-surge comp housing.

It seems like it is spooling later than it should from what I have read - it currently has a 7 pound spring in it, but I don't hear the wastegate opening until close to 4k RPM (I still haven't figured out how to log with the Power FC...) It honestly doesn't seem much better than the S366 on old car, and that thing was a lag monster. I want as much spool as possible, and from what I have read the anti-surge housings can delay spool by as much as 500 RPM. Is it worth swapping out, and will I have any downsides? I'm still not savvy with compressor maps, so not sure if its in the surge range at all. Also, are there multiple different compressor housings to choose from, or they all pretty much the same?

On a side note, it has a 3" downpipe, but the RB 2.5" catback, so as soon as I get the cash I will go with a full 3" catback which I know will also help. It also doesn't have a boost controller, which I know will also help, but even with those things it still seems sluggish.

On a further side note, it made 270 HP with only 7-8 PSI, which seems really promising!

I know you're not going to want to hear this, but a modern Link, AEM, Haltech, Fueltech, etc ECU + 3.5" full exhaust (downpipe and full exhaust) will do WONDERS. There are VERY few PowerFC tuners left out there, and owners with them aren't going to pay the required time to tune them properly (older ECUs take a lot longer to calibrate in Open Loop).

In the meantime, you should hit up Nelson Siverio who's right in your neck of the woods. He does PowerFC + is one of the world's best rotary tuners. Pay the guy a grand, let him go through the whole car, and dial everything in... Boom, done.

Sorry, forgot to mention that - I'm getting rid of the power fc ASAP. Going with a megasquirt (had one on my old car and loved it).

Edit: the cart actually runs beautifully - it was built and tuned by Rotary Performance in Garland. Still, I hate the ecu and want something I can actually work on myself.

New ecu, lower temp thermostat/fan switching if the mech fan is gone and decent airbox/intercooler will improve response as well as the exhaust being mandatory. Is it a divided manifold? Divided manifold and 1.01 divided turbine housing should lower boost threshold too.

Divided manifold, but not sure if housing is divided (going to be pulling it soon to add EGT probes in the manifold.) Why would a lower temp thermostat help? Thats something I've never heard of. Anyway, I think I'm going to wait on swapping out the compressor housings for now until I get the rest of that stuff sorted out. I've got a Miata I picked up for $400 that I'm going to fix and sell, so I won't be able to afford anything past the ECU until I get that done. Knowing me that could be between a couple weeks and 6 months...

Also, I forgot about the charge temperature. It has literally the worst intercooler design I can possibly think of (so bad I'm starting a whole thread about its awefulness). Its placed like a v-mount, but the radiator is in the stock location and it is mounted over the shroud so it gets air from AFTER the radiator (and the clutch fan pulls air through it when stationary, so it gets hot when stationary). Air temps on cruise, no acceleration, were 140 when it was 90 outside. WTF? I'm pretty sure it was done like that because he ran out of money to have them make a v-mount for him, but I'm honestly really disappointed with RP for installing it like that. They would have been better off keeping the stock intercooler. That is 100% the first major thing I am fixing on the car because I'm kinda scared even driving it at 7 PSI.

High intake air temps definitely affect spool.
I noticed that when my car would get high heat soaked under hood temps in traffic.

So, its worth addressing the intercooler and intake design not just for peak performance, but for the spool issue you are working on as well.

Like others have said, big exhaust will help the low rpm spool and power immensely. Full 3" is better than what you have, but if you could swing 3.5" or 4" especially in the downpipe area it will help quite a bit in my experience.
When I went from RB 3" downpipe/midpipe to 3.5" downpipe/midpipe it reduced full boost rpm (14psi) from 3,800rpm to 3,400rpm on my T04B w/ P trim exhaust. The amount of boost achievable in the 2,000-2,500rpm range was several psi higher as well and so was off idle driveability. That was still with the RB 3" Y-pipe and mufflers. When I later changed out to 3.5" single rear section, I didn't notice any better spool or power on the dyno (but it weighed 30lbs less).

Good to know! The thing that sucks is the RB 3" catback is almost as much as the full exhaust. I might just get something custom made up whenever I get some money. Still debating on whether to keep the cat; I'm sure it hurts spool and peak power, but I just hate smelling the exhaust.

Lower heat flux throught intake system and lower/block manifold temps make the biggest difference at low rpm when there is more time for the charge air to absorb heat. even goinf from JDM 82 to aftermarket 72 degree thermostat improved pickup on mine, if you still have the hot USDM thermostat you will definitely notice more from off idle into the midrange. Thats why the fuel models in current ecus have ait/coolant temp weighting settings that use residence time. Remeber if you can drop low rpm intake temp 15C that is 5% more torque and energy onto the turbine too.

Got it, that makes sense. It definitely has the USDM thermostat (lowest I see is 85, can climb into the 90's in our current 40+ degree heat. Sounds like between that, an intercooler that doesn't heat up the intake charge, maybe some heat shielding, and a modern ECU I should see a good improvement, then even more once I can enlarge my exhaust.

Thanks for the tips!

No ill effects to doing this? I could swear my rx goes heaps better when its not fully warmed up but I try to avoid putting my foot in until it's warm. Why would OE thermo be 80 something c if the 72c thermo running temps are so much more efficient?
I'd like to do this since its so easy, but a bit dubious of ill effects.

I ran a 160 deg thermostat for years and finally switched back to a 180 deg thermostat on my FC because, yes the engine just runs so much smoother and better mpg when it is up to temperature.

I didn't notice any difference in peak temperatures when racing, but when I had the 160 t-stat I had the fans kick on at 176 anyways and on the 180 t-stat I had it kick on 185. The fan cycles coming off the freeway and into traffic were definitely shorter, but not much more cycling at idle because the engine thermal input is so low it really is more the thermostat position controlling the temperature through the big radiator I had rather than the fan.

Emissions. US were significantly hotter than JDM Australian for that reason, not about power or engine longevity.

mine runs smoothly at 72C there was a post by a guy who did some race engine development on an engine dyno (marine so heaps higher duty than auto) and he got best power and reliability around 70C water temp and 120C oil temp. For an endurance race car it probably doesn't matter as 99%of people won't have the heat exchanger capcity to keep temps below mid 90s but in a street/strip/hillclimb car it makes a difference.

Did you had to adjust the fuel table when switching the thermostat? Only asking because you said your MPG improved when switching.

The only mapping I changed when I changed t-stat was the fan on/off temps and the idle air control valve.

I already had the mapping optimized because I had run the 160 t-stat for over 10 years and learning about tuning was my hobby for about the first 5 years.
That means the coolant temp fuel correction tables were dialed - when the engine was on the 160 t-stat and running at 176F right as the fans kicked on it was ofc running lower inj duty cycle than running at 160F because it COULD be run leaner hotter.

If you looked at that fuel coolant correction map it was a fast decrease from ambient to about 120F and then a very gradual decrease to ~186F and then I had it slowly increase again with a safety ramping up to 100% correction from 230-250F.

When I switched to the 180F t-stat I spent more time driving in those hotter, leaner areas of the fuel coolant correction map.

Interesting. Do you have more details (like a screen shot) of that fuel correction map? I've gotten pretty dang good at the regular fuel tables, and OK at ignition, plus I've got the drivability stuff down, but I am still learning on the correction maps. I've basically tuned for when it is at operating temperature, and the only correction I've played with is for IAT's, and even then not too much.

No, I have since sold the car off.

Dang, worth a try! I'll play around with it once I have everything dialed in.

Thanks for the explanation! I soon will be going through the same thing you did 10 years ago (?), as I move to an e-water pump, no thermostat, and controlling everything through ecu maps.

None, just spring pressure. I doubt an electronic controller would help to much, but every little bit counts.

I'm wiring up my megasquirt right now, so hopefully I'll be able to get the controller going in a couple weeks and see for sure. After that I've just got to save for the exhaust...

Running a stronger wastegate spring also helps spooling.
Running a good EBC also helps with spooling. I run a PFC and have never used it to control boost as it sucks at that.
The spring pressure should be about 1-2 psi lower than your target. My FP HTA GT3582R 1.06 T3 runs about 1.5 psi higher than the spring.
It takes longer to spool to your desired boost level in the lower gears when compared to the higher gears due to the rate of intake volume increase.

2.5" RB catback is definitely restrictive. But how about the obvious - check for boost and exhaust leaks. Pressurize the engine with an air compressor and make sure now air is leaking out?

I have checked for leaks and it doesn't have any, unless there's a very small boost leak I missed (definitely no exhaust leaks). I think the biggest thing is the 2.5" catback, which I knew going in, plus the god-awful intercooler/air heater. At this point I just have to get that Miata fixed up so I can sell it and pay for everything!