Boost control on Rx7 Turbo II

I have a 1987 RX7 Turbo II lemons / chumpcar. We have a free flowing exhaust on the car and now the car is boosting to 12 psi. I took the turbo off and ported the internal wastegate. Now, the car will boost to 9.5 psi (when it is 50 degrees out).

Stock boost is 7 psi. I would like to get down to 7 or 8 psi for reliability reasons.

What is the best way to do this? I do not think I can port any more of the wastegate.

When the car had a cat on it, the boost was 6 to 7 psi. I was thinking I could make an "exhaust block". What I mean is that I would make a 1.5" steel plate that I would put between the downpipe flange and the tail pipe.

The downpipe is 3" that goes into a 2.25" dual exhaust. It apparently flows VERY well.

Any help is appreciated!

Rob R.

Sorry I don't have any experience with FCs in particular but I'm going to make an assumption based on my FD and other turbo cars I've had. I assume there's a solenoid attached to the wastegate actuator that allows the ECU to bleed off a bit of pressure to increase boost. If you remove the solenoid from the equation, you should get the true WG actuator spring boost level, the lowest possible given your porting. If that's still not low enough, you may have to put an intake or exhaust restriction in, change the actuator spring, port the WG more, bolt on an eBay-gate, etc.

How much did you port your wastegate? Did you weld on a washer as described here: http://www.aaroncake.net/RX-7/s4wastegate.htm My reference, that's my old turbine housing in the pictures. :) I ran stock boost with a 2.5" DP, no cat, 2.5" higher flowing single exhaust. Even the washer wastegate wasn't able to completely handle a big intake and 3" RB turbo back exhaust.

yep, I used Aaron Cakes website as my example. I made the port look almost exactly like the one in the pictures (yours? Cool!) I also have welded a washer on to it in order to cover the now larger port.

I have a borla stainless steel cat back exaust, no cat, and a 3" downpipe. Even with the porting, it still boost creeps (3rd gear pull uphill in sub 50 deg weather = 9.5 psi).

I am thinking of just putting a restrictor plate in teh exhaust between the flanges. I am thinking 1.75" should do it.

BTW, the system has no electronic controls. It is a diapraghm that is controlled directly by intake pressure. The diapraghm directly controls the wastegate.

There is no easy way to install an external wastegate (all cast parts and no room anyway).

Rob R.

Have you made sure the iwga is stroking fully, or considered swapping to an iwga with a longer stroke? The turbo on my racecar is limited to a minimum of 10psi on a 7psi iwga can because that can doesnt make the full stroke.

Yea, it opens fully (I put grease on the rod).

Thanks for the idea!

On the FC, running the stock, unmodified airbox with a paper filter helps a lot. Even so, It will probably also need some exhaust restriction to help control boost. Your idea with an exhaust restriction will help. Make a few plates of varying sizes. 1.5" may be too small.

You may also try a smaller diameter downpipe.

Boost control on a turbo rotary is always a challenge. I had 4 modded Turbo II and 2 FD. still have FD. :)

^ Reducing the intake flow or the DP were going to be my suggestions as well. The restrictors could easily work, but it just seems a bit crude rather than properly reducing the pipe size.

Reducing flow could increase temps...

Wouldnt it just bring temps back to stock?

In theory, but it's also quite normal for factory turbo racers of that time period to run bigger downpipes and o2 housings for increased reliability, despite the boost increase.

Mazdas, even.

The restrictors are crude but effective.

If this wasn't a Lemons/Chump car, I'd suggest an electronic boost controller and more fuel. :)

You could also consider clipping the turbine wheel. That should help with this issue and lend an increase in top end power.

The weird thing is that I have enough fuel. I have a walbro fuel pump with stock injectors. Even when I was pushing 12 psi, my AFR was only 11.2:1.

wvumtnbkr wrote: The weird thing is that I have enough fuel. I have a walbro fuel pump with stock injectors. Even when I was pushing 12 psi, my AFR was only 11.2:1.

Don't want to go much leaner than that.

I saw the aaroncake porting info on snrub's S4 setup. The engine I swapped into my car is an S5, I need to rebuild the turbo anyway and was wondering if the porting thing is a good idea on the S5's as well for the same reason (the article says 'to a lesser extent S5's'). I'm running 2.5 inch tubing with a Dynomax at the very end and have adapted the S4 ECU etc so I can run a mechanical OMP and not go into limp.

In reply to amg_rx7:

I agree. However, it is about 11.0:1 at 9.5 psi. It is in the tens everywhere else. (except idle and off throttle).

In reply to Curmudgeon:

If you are rebuilding, I would do it. There is no reason not to. It is simply better boost control.

wvumtnbkr wrote: In reply to amg_rx7: I agree. However, it is about 11.0:1 at 9.5 psi. It is in the tens everywhere else. (except idle and off throttle).

Yep. That's about standard for OE Mazda tuning on a turbo rotary :) Safety in rich AFR.

Agreed with above, all FC wastegates need to be ported. For your setup I'm not sure if you need to be as extreme, you may simply want to make the hole and passages wider. It's pretty easy to remove the turbine housing and a little grinding is pretty easy to do. :)

Okay, update time.

I tried to use an exhaust restrictor that was 1.75" in diamater. It didn't help that much.

I am now trying a 1.5" restrictor. My issue is that if this doesn't solve it, I don't know what to do.

The car will not be able to be driven until the green flag drops. Therefore, i am not going to know if this solved it. If it did not solve it, I need a quick way to drop the boost even more.

I was thinking that I could take the stock BOV off the car and just run a .5" I.D. hose from the exit of the compressor back to in front of the compressor. I figured this would limit the boost a bit. This would be like having the BOV open all the time (while recycling the air back to the intake - the car has a MAF sensor).

Whatcha think? Worth a try? Am I not making sense?

Thanks!

Rob R.

wvumtnbkr wrote: I was thinking that I could take the stock BOV off the car and just run a .5" I.D. hose from the exit of the compressor back to in front of the compressor. I figured this would limit the boost a bit. This would be like having the BOV open all the time (while recycling the air back to the intake - the car has a MAF sensor).

Thats a quick way to over spin the turbo and blow it up. And if it doesnt kill it it will put you into the hair dryer portion of the compressor map. Why not just weld an ebay external wastegate to the stock manifold? Put it under a heat shield, no one will ever know.

Update from me too: I replaced the seals and bushings in my turbo, the manifold has TWO wastegate openings and a large oval flap door. I'm going to leave it alone for right now.

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.

Can you create a pre turbo exhaust exit and the route that back into the main exhaust post turbo and waste gate? This should lower your exhaust pressure to the turbo. It's a jack leg way of lowering exhaust pressure but may work. I can't promise it will as I've never tried it.

If money was no object I'd just say to permanently close the turbo waste gate and to create an external waste gate that dumped to atmosphere.

A cheaper way to go would be to go back to the stock small diameter exhaust. If your exhaust is less efficient you'll make less boost. Also, this exhaust solution as oodles of hours for reliability from the factory. The true way to win chump races, reliability...