I just introduced myself over in the n00bie section, and I have a question for you guys. My friends S4 turbo is on its way out (in and out play, pretty significant, and he changes his oil every 1500 miles) so I was looking into finding another S4 turbo. Digging up some information on google, I realized that the S4 turbo is smaller and inferior to the S5 turbos, except for the twin scroll setup. That and the wastegate is stupidly restrictive. Does anyone have pictures of an S5 turbo? how is the wastegate flow compared to the S4? We put an HKS intake on the car for a day, and the boost climbed like crazy (16+ psi). The car is an 87 TII, with a full 3 inch RB exhaust, turbo back. Right now, the stock intake is what is keeping the boost steady..restricting airflow to it.

I want him to get rid of the stock intake, but we have to cure the boost creep first, which will involve some wastegate work and a possible dump.



I was playing around with some ideas over the past few days. One is to take the downpipe, cut out the flat flange that sits behind the wastegate actuator. WRXs have this type of downpipe, and it causes the wastegate stream to slam into the plate, turn 90 degrees, then turn 90 degrees to get into the downpipe. Im not sure if he is cool with a wastegate dump (might be too loud in his case). After cutting out the flange, I was going to take a piece of pipe, cut it long ways down the middle, and shape it to the side of the downpipe, allowing for more room for the wastegate stream to merge into the downpipe. WRXs have quite a few downpipes made like this, anywhere from an enlarged pipe that covers both exits, to a separate pipe for the wastegate, which returns into the exhaust down the line.



My friend doesnt want to go really crazy (external wastegate and all), but a dump pipe or some kind of modified downpipe should cure the boost creep situation. My other question is does the S5 turbo and manifold bolt up to the motor? Should I expect any different response or turbo lag compared to the S4 unit? Does it need as much port work as an S4 unit? Hopefully Im not forgetting any information, and I dont sound like a total n00b to you guys lol....hes only looking to make 250 to the ground. We dynoed the car last year on the old motor, and 4th gear with just the exhaust on it, it made 210 to the wheels with an 11.3:1 a/f ratio. With an intake and the supporting fuel mods (injectors are already in, regulator and a fuel pump and a wideband 02 will be coming next), im sure we will break that without a problem.

Generally speaking the s4-5 compressor is only good for round 220 rwhp. The turbine wheel is different s4 to s5. The wastegate is bigger on s5 but not much, you can port them out a bit but not much. The back plate modification you speak of is quite common and helps abit. Running an external is no problem but requires a lot of work for small dividends. You can bolt the s5 manifold to s4 engine but not s5 turbo to s4 manifold.

The s5 turbo spools up better than s4 been divided from engine to turbine. s4 manifold is not divided.

Other points worth mentioning, the bov and intercooler plumbing are bigger on S5 , the engine is also higher compression . But the main restiction is the compressor which is the same s4-5.

gotcha. So I would assume fitting a new compressor wheel to the stock turbo would be his best bet. The air pump and emissions stuff have all been removed from the motor as well. I would rather take as much heat out of the turbo as I can to take the load off the intercooler, even though it is a larger top mount. Trying to push the stock turbo to 14psi or more sounds like a fruitless effort lol

I was looking around online, stumbled upon BNRs staged upgrades for the S4 and S5 turbos...going to try giving them a call tomorrow to see what it involves (might be better off sending out a good unit for upgrading rather than his beat up one)

Just get a t28 from a s15 silvia https://www.nopistons.com/forums/pub...1047683329.gif

why? its smaller than the stocker

Difference between s4 and s5 turbos is w/g and turbine housing. They also use a different exhaust manifold. S4 and s5 turbos only work with their resprective manifolds. They will bolt to any engine, however oil feed lines are different( oil return lines will work if you swap front covers). Anyhow s5 turbos spool faster in my experience, usually by about 500 rpms or so. Wg valve diameter is larger, can't remember off hand, from s4 to s5. S4 w/g runner only contacts one runner, while s5 has two w/g runners. Ok blah blah..

You can make more than 220rwhp, I've personally done it a few times, however you have to concentrate on AID and other things.



Main restriction is the turbine runners, and after that the turbine wheel itself.