Boost control on Rx7 Turbo II

I don't think I can do much of anything with the exhaust pre-turbo. The header is literally one cast piece that is maybe 3 inches from the engine to the turbo. I can't even see the manifold with the turbo in place.

The stock exhaust has 3 catalytic convertors in it. It also costs about 2,ooo$ to get a new stock exhaust (if you can find one).

The boost was right at 7 psi when I had a catalytic convertor on it. I could put one back on, but I would rather not (due to the way the exhaust is now, i would need to cut the flanges off and weld on 2 sets of new ones...) Pain in the ass!

Rob R.

wvumtnbkr wrote: In reply to Leafy: Because the manifold is not accessable. The manifold is about 3" in legth. I am not sure how it could fit. If anybody has any ideas, let me know! P.S. Curmudgeons picture shows an S5 turbo. Maybe that is what I need in order to solve this issue. My turbo had one stock port (the size of the smaller port in that pic). Now, it has 1 port that is slightly larger than the larger port in that pic.

I have an S5 turbo sitting around...

Some previous posts addressed the intake side. Does this car still have the stock airbox?

No, it doesn't have teh stock airbox. The reason being that the Turbo inlet ducts are unobtainium. Therefore, it has a steel piece of curved tubing that hooks to the AFM and a cone filter.

I suppose I could try to get the stock airbox back on. It may take some additional welding and cutting.

In reply to Swank Force One:

Do you have the manifold too?

Rob R.

wvumtnbkr wrote: In reply to Swank Force One: Do you have the manifold too? Rob R.

I don't. I acquired the turbo in a trade and it's just been sitting around.

Just a couple random thoughts (that don't cost anything):

Have you pressure-tested the wastegate actuator to make sure it's actually trying to open at the expected PSI and that the arm fully extends?

Where is the vacuum/boost source for the actuator coming from? Right off the compressor housing or all the way back at the intake manifold? That can make a huge difference. Also worth considering, where is your boost gauge source?

Have you pressure-tested the vacuum system? Any leaks in the control system can show up as extra boost (yea, that sounds counter-intuitive I know).

What vacuum system do you mean?

I can pressure test the actuator to see what pressure it is reacting at. The source is the compressor housing.

The boost guage source is the intake manifold. It seems to correspond fairly well with my AFRs and seat of pants. This thing is stinking quick right now. It is definetly making more than 6 or 7 PSI.

What if I just wire open the wastegate all the time? That would make it ramp up slower for sure. It might not allow it to ramp up quick enough to over boost.

This is just a thought. What are the negatives (besides less HP)?

Rob R.

Wired open it will be super laggy. And if it only hits 6-7psi wiered open that means your waste gate actuator isnt opening far enough and you just need one with a longer stroke.

I see what you mean. However, it is the stock actuator and arm.

Poop. I wish I had more time to drive this thing before the race....

Well have you used the air compressor to determine if it opens all the way?

I did. However, that was when I had it apart.

Me thinks I should do this again while under the car and watching it move.

Make sure you dial down the compressor regulator before testing. You probably don't want to expose it to more than 10 or 15 psi or it could be damaged.

If you leave the wastegate wired all the way open you may not be able to get ANY positive boost. Even if you do it'll lag like the bejeezus and be a real turd to drive.

I don't know anything about the FC boost control system (as stated earlier) but in order to test normally I cap off the turbo inlet with a rubber plumbing fitting and then introduce boost pressure into the system to fully pressurize the intake/intercooler/vacuum lines/etc. You'll need to rotate the e-shaft till the pressure stops passing through the motor. Any leaks in the system will become apparent as the pressure level rises. If the system can't hold 10 or 15 PSI you probably have leak(s).

It sounds like you've got things set up correctly though with the wg actuator drawing off the compressor housing the boost gauge from the manifold.

The S4 FC wastegate is easily overwhelmed (S5 is significantly better) once you free up the exhaust and remove the stock airbox. I ported the snot out of mine, but it would still overboost. Way back when I was changing parts on my car, the downpipe and 3" exhaust made a relatively small difference. What really woke the thing up was pulling the stock airbox and replacing it with a cone filter. From personal, and admittedly not overly scientific observation, I would say that the intake restriction would be more effective at muzzling your car than an exhaust restriction.

I will look into that immediately.

Thanks!

Rob R.

Maybe an external wastegate will be the way to go?

http://www.rx7club.com/2nd-generation-specific-1986-1992-17/pics-my-external-wastegate-stock-s4-turbo-tial-38mm-d-567832/

http://www.rx7club.com/2nd-generation-specific-1986-1992-17/really-need-some-help-boost-creep-external-gated-1049424/

i have to wonder if the 1.5 to 2.0 lbs of boost pressure your trying to shed really validates the effort. i understand its LEMONS and reliability is more important than speed, but could you raise the fuel pressure a little or swap to a few cc larger injectors and call it good?

if not, might also consider porting the WG hole beyond what the flapper can cover. say you added 20% to the opening, towards the main exit. you'd have a 20% bleed off all the time, sure, and that would result in a bit more boost onset lag in the system, BUT, the slower boost build will be easier on the clutch and the motor, (and the turbo), and will likely net you the control your looking for.

-J0N

jmthunderbirdturbo wrote: i have to wonder if the 1.5 to 2.0 lbs of boost pressure your trying to shed really validates the effort. i understand its LEMONS and reliability is more important than speed, but could you raise the fuel pressure a little or swap to a few cc larger injectors and call it good? if not, might also consider porting the WG hole beyond what the flapper can cover. say you added 20% to the opening, towards the main exit. you'd have a 20% bleed off all the time, sure, and that would result in a bit more boost onset lag in the system, BUT, the slower boost build will be easier on the clutch and the motor, (and the turbo), and will likely net you the control your looking for. -J0N

It would also be mega hard to drive. Its hard enough driving a 1980's tech turbo system because of the lag, dont make the lag worse.

I confirmed the actuator is working properly at the correct pressure. I also installed a slight restriction in the intake.

I will report back after this weekend.

It is running a steady 6 psi with a 1.25" exhaust restriction.

Make a cheap dump pipe. Had one on my Galant vr4 and it solved all boost creep problems on a b16g. I had one made with the stock cast O2 housing.

The boost control was excellent at 6 this entire weekend. However, we had some fuel issues with our gas tank.