Turbos

In reply to jere:

S15 is dual ball bearing with .64 exhaust housing. My friends was a gtir with journal bearings and .86 exhaust (finally googled it those ARE correct numbers) and he had full chooch as i said in a 1.8 before 4k. Somethings wrong for op to have boost not until 5k.

OP can you find the turbo tag and post some numbers or google yourself to see what its off of? Seriously the only reason im advocating for the current turbo as OK is because i thought the same turbo on a SMALLER motor was too big (exh wise) and i was proven wrong my first drive out.

Pretty sure if op can get his mits on another silvia turbo he can swap housing but as i said i think thats chasing the wrong goose here. I belive its got other problems, clogged cat ahem.

It sounds like a ridiculously big turbine housing for that size compressor. You are likely giving up a lot of response for marginal extra top end power. That said, all kinds of mismatched turbos will work. If other similar engines are reaching the boost threshold at much lower RPM, I agree there is something else wrong. Is it an internal or external wastegate? How much boost is full boost? I'd check for a wastegate problem first.

Check for leaky everything as well. Exhaust manifold, turbo to manifold, every junction you can think of from whatever air measuring device you have pre-turbo inlet all the way to the O2 sensor.

I think the tag is missing.

This says ar .86 Divorced wastegate. Don't worry about the little splattery things. It was drizzling when I opened the hood. It's not leaking oil.

This is what I'm seriously beginning to think might be the problem. That lever goes to a valve in the exhaust after the cat. Turning the valve does pretty much nothing to the sound of the engine. I'm thinking it might be broken and stuck in some sort of sound reducing quiety mode.

Also, I should clarify the lag. If I'm in third gear full throttle it's about 3 psi at 2500 rpm, 5 psi at 3000 rpm, 7 psi by 3500 rpm, maybe 9 psi by 4000 and then it shoots to 14 between 4500 and 5k.

I guess im not getting you but the .8x was factory for that 2.0l i mentioned but you said earlier its way too large and needs something smaller. Excuse my simple mind.

Alright, i thought you were just mentioning that the t25/t28 family in general was factory on 2.0l's, so i misinterpreted your post.

If I'm in third gear full throttle it's about 3 psi at 2500 rpm, 5 psi at 3000 rpm, 7 psi by 3500 rpm, maybe 9 psi by 4000 and then it shoots to 14 between 4500 and 5k.

Im starting to think your valve theory is right and i'm really curious about that setup since i may want to replicate it.. Another possibility is improper (retarded) cam timing.

In reply to DaewooOfDeath:

Yeah look for exhaust restriction if there is some kind of valve in the exhaust Judging from that you are going to be in for all kinds of surprises. Is that brass nipple on the wastegate supposed to vent to atmosphere?

What does the rest of the system look like as far as plumbing? Is there up and down side to side play in the compressor wheels shaft?

You can pressure test the intake for boost leaks with some pvc plumbing parts as a cap, a schrader valve from a tire, and a bicycle pump or tire inflator of some sort. It helps to turn the crankshaft so all the intake valves are closed but will give you an idea either way. Boost leaks are very common in homebrew turbo cars

As for the too big too small discussion what rpm does everyone think would be ideal for full boost to come about? I am thinking right around 3200 or so if the motor only revs to 6500. What about just raising the rpm limits, many cars seem to do well with this?

In reply to jere:

I thought the same about wastegate hose routing so looked it up and that is correct per tials specs. I second the exhaust clog and boost leak idea but id check clogged cat first cause it does make 14 or whatever psi just late. And cam timing, i mentioned it earlier and someone else did as well, those are the three things id check first

Oh yeah-look for manifold cracks as well.

Alright. I checked the bearings and they're in good condition. They are also ball bearings, so cool.

The manifold isn't cracked and it's not leaking.

I tested the intake piping by cleaning everything off and the introducing some colored oil behind the filter but in front of the turbo. Much hooning later, no leaks.

It's looking like cats, the valve in the exhaust or cam timing.

My man, be honest here, how poser mobile is this thing? Reason I ask, there could've been some real work done to this car, but then it may have been sold to some poser that slapped a turbo on and didn't do things right.

Here's a scenario: that rush you feel at 5k is the cams and head work that "may have been done". And the turbo is just along for the ride. (Yes I know you said the boost gauge does boost gaugey things)

Although, who does a cam install for a cam that lives in the upper rpm, but doesn't do the supporting mods to raise redline? I bet that engine screams at 7500rom.

Trackmouse wrote: Although, who does a cam install for a cam that lives in the upper rpm, but doesn't do the supporting mods to raise redline?

Almost everybody...

Well maybe the problem is you need a new tach.

Trackmouse wrote: My man, be honest here, how poser mobile is this thing? Reason I ask, there could've been some real work done to this car, but then it may have been sold to some poser that slapped a turbo on and didn't do things right. Here's a scenario: that rush you feel at 5k is the cams and head work that "may have been done". And the turbo is just along for the ride. (Yes I know you said the boost gauge does boost gaugey things) Although, who does a cam install for a cam that lives in the upper rpm, but doesn't do the supporting mods to raise redline? I bet that engine screams at 7500rom.

Doesn't seem poser mobile to me. The parts are pricey and the car looks stock. It's also quiet. It's also effing fast. Like, it'll burn the tires from a roll in 2nd once it gets up around 9 or 10 psi at 4k.

I don't know what's done on the top end, if anything. It honestly doesn't feel that different from a stock beta off boost. I haven't gone hunting for fuel cut, so it could be 8k for all I know.

I've been restricting myself to a 5200 rpm "redline" until I can cut open an oil filter and check the magnetic plug. Break in stuff. (I've only revved it to 5.2k once or twice.)

After that, well, we'll find out how it revs. Even if the top end is stock it should be safe to 7500 ~ish.

Mystery solved and I feel like kind of an idiot.

There was a tear in the wastegate signal hose. I think it was holding the wastegate partly open, which is why it wasn't building boost at low rpm and also why it was overboosting at high rpm.

Fixed the tear and now it's got good power from 3500 rpm and screaming power from 4k all the way to redline. Also, screamer pipes sound cool.

What is going on with the downpipe? Looks like it's been welded onto the turbine housing - as well as the bolts! If so, tres' ghetto.

In reply to kb58:

Eh, it works. ¯_(ツ)_/¯

This is a GT2860? And it htis boost at 5000 RPM? That's not right. This was a stock turbo on a SR20DET which is the same displacement and hits boost like a road car should.

People have put these on RB20DET, a low tech engine with notable lag on any turbo, and still are fine.

I'm actually going to put it's more modern cousin, GTX2860R, on my RB20.

edit: lol I guess i'm super late to this.