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

 

lurk around zilvia.net to grab some sr20 t25