Higher Spooling Turbo Setups?
 

I'm doing some research on some turbo options for my wife's miata, planning on doing the install next summer (this summer is paint, cause according to the wife, paint before turbo....before you ask the obvious...divorce is not an option). I've been doing research on this forum for almost 6 mo. now and thought I knew what I wanted to do. I had my choices down to a td04, t25 or the 13g. MS for management and was thinking of ebay IC, diy manifold and downpipe. BUT! That was when it looked like she was going to have a short commute on back roads. Now it looks like she will be driving highway, or at 3500 rpm most of the time. So now I want to try to find setups, or turbos, that wouldn't start spooling until around that point or a little later so that she's not dumping gas.

So my goals have changed from 6-10psi, 180hp(ish), lower spool to a more commuter friendly car on the highway with umph when you need it. Someone recommended a 16g with a 20g housing to me, but I wanted to check in with the experts here to see what their thoughts were. Including those of you who do commute on the highway often. Any thoughts? Thanks to all those that have posted, I've learned a whole lot in the past 6 months.

The turbo won't spool up until it is under load... Just so ya know. Just take my puny K03 turbo on my Audi. No boost at all unless I give it half throttle or more. And trust me, that thing is SMALL.

Yeah, the fueling is based more on MAP load than RPM, so light cruising even with a turbo in vacuum won't realistically use any more fuel than a nonturbo car. Actually, it can be better as you will have MS and can tune cruise AFR's to be higher than the stock stoich 14.7. So, I'd still consider doing the T25.

go t25 for cheapness, availability, easily upgraded, and available hot parts

If turbos spin due to exhaust gas flow, and your at higher rpm (more exhaust flow), then why won't the turbo create boost? The waste gate must be full open to not build boost right? So, does that mean that when the engine is under low load (cruising), there isn't enough vacuum to actuate the waste gate?

T25 is hot on the list. Just found one for $75 from a 98 eclipse.

Make sure the t25 from an eclipse is one of the ones that works with Begi or FM Manifolds and downpipes if your going that route.

As for cruising as long as your not full throttle you can stay out of boost. I know I did between 90-100mph following a vet one day on I95 in NC and would only get into boost when we would both floor it.

at cruise the throttle is mostly closed. no air in, no exhaust out.

Its about pressure differentials and load, not just flow. With the throttle plates partially closed, there isn't much pressure before the turbine, so you stay out of boost. When you floor it (or for smaller turbos, give it a reasonable amount of throttle) Your engine starts to work more efficiently because those pesky throttle plates arent hindering its function. More efficient = more flow = more pressure pre turbine. As the RPMs build, the pressure before the turbine starts to overtake the pressure in the exhaust system, and the turbo starts to spin faster and faster as that air tries to force its way out. Eventually (this all happens very quickly BTW) the compressor is turning fast enough that the amount of air that it is taking in and blowing into the engine is more than the engine itself can digest in 1 cycle, so the air starts to become pressurized (boost). And that's how babies are made.

It wont fit anything but an ebay manifold. Skip it.

You want a T25 from an SR20DET... They are PLENTIFUL!

Either that, or a t25/t28 cxracing/ebay turbo would work just fine.

Ah. Hadn't thought of it as a delta p across the turbine. Makes more sense from that standpoint, Pressure*Volume=Work. Without the delta p, you get no work. Thanks!

No luck with the sr20 T25 on CL, guess I'll have to search some of their forums. Thanks for the heads up on that and the eclipse style t25.

No problem. Just remember to name your children after me.

I had significantly better MPG when I had the begi kit that made target boost at 4400rpm, rather than 3200rpm with my AF stuff. Tiny little freeway throttle inputs make 7psi pretty easily.

TPS based boost control can prevent you from making too much boost at part throttle.

only to mechanical limits though. you cant make it lower than your wastegate unless you have some kind of 2 step wastegate that i dont know of :P

ps, find me one of those.

i run an external gate with a 3.8 psi spring....

Yes...you still do :(

My 300zx t25 turbo setup made boost in reverse. If I hit an incline on the highway, the only way to make it not create pressure was to have the throttle completely off. So there is a problem with going too small. The SR20 T25 should be just fine though.

You can also run a quieter (more restrictive exhaust) to delay spool. My gf's favorite thing about adding the turbo was that my car is quiet again...for now :D

lurk around zilvia.net to grab some sr20 t25

Couple of things:

First, turbos spin due to heat energy. Diesel trucks will spool turbos at high RPM going downhill, but we're talking like 5psi, not the 30+ they actually run under throttle.

Second, boost is an arbitrary measurement of intake manifold restriction. Never forget that. It just so happens that it conveniently correlates with the flow rates from a specific turbo, but that's about as far as the usefulness goes. Case in point, you can alter the boost that a supercharger runs by changing the intercooler, intake manifold, or even the camshafts. Turbos are much less prone to this since their method of boost control is pressure based.

The wastegate can be totally closed, but if the throttle plate is closed, the intake manifold will be under vacuum. Only when you open the throttle enough to develop some heat in the exhaust will the turbo spool up and pump enough air into the intake manifold to register positive pressure.

Constant boost of 1-2psi on the highway correlates to a cruise speed of approximately 110mph. An 80mph cruise requires something like 10in.hg to maintain, a little LESS than an equivalent naturally aspirated car since the turbo essentially negates any intake restriction there might have been. Only when you tip into the throttle will you enter boost - it is not entirely RPM dependent.

also you can tune the car to cruising afrs at 1-2 psi of boost without much danger

My turbo spools very quickly, and I still only see boost if I tip in on steep grades during cruise.

Off idle boost, and still 29mpg combined. Some of it is turbo sizing, the rest is tuning. If you still need a responsive turbo gas mileage is no reason to stay away. My 75mile commute consists of lots of hills, sweepers and roads through the forest from the Lake Conroe area to the Brazos valley. Having a small turbo is great around town.

Now I can see having a bigger turbo for better highway passing power. If you're already at an engine speed that would spool a big turbo then fine. Mine peters off at high rpms, simply runs out of airflow, but it's much smaller than an SR20 T25 even.

Thanks for all the help guys! I'm sure I'll post more questions once I start getting parts in hand.

or SR20forum.com, that's where I found mine. The T25 from the W10 Avenir SR20 engine fits FM/Bell manifolds, has the .64 A/R, and doesn't have the bolt-on style compressor inlet. There always seem to be a few on SR20forum, got mine for $85 shipped (east coast to west coast) and stuck in a GPopShop rebuild kit for $88.