Having a little issue with a car. The car is a 87 RX-7 (S4) which has been streetported and a powerfc installed with 1600cc secondaries. Full 3" exhaust with a ported wastegate. Otherwise everything else is pretty much stock.

Basically its overboosting to 16 psi before my preset fuel cut kicks in. I have tested the line going to the wastegate control actuator and it moves if pressure is applied to it and holds the pressure as well so as far as I can tell its functioning as it should. I'm not so familiar with the 2nd gen's so does anyone have any ideas. I attached a log of the boost curve...



thewird

 

Needs to see vacuum....not pressure

 

How would it ever see vacuum if its hooked up to the turbo. This is wastegates 101 >_>. Pressure is what opens the gate.



thewird

 

maybe the wastegate spring is too hard ??

 

It's all stock stuff which I believe is 6 psi spring.

thewird

 

Wastegate 201....control the pressure then, take a look at boost-control 101

 

You can't give it anymore pressure then what its getting at from the compressor cover... You can only give it less which raises boost, not lowers it.

Does anyone know if a faulty twin scroll or related setup could cause an overboost situation? I would have assumed no but I'm not sure.

thewird

 

The wastegate May not be ported enough...

 

wait is this a stock turbo?


The S4 wastegate SUCKS. it's like the size of a penny.

Port the snot out of the wastegate or change turbochargers. An exhaust valve from a SBC makes a good new wastegate door once you grind the stem off. Grind some material off of the backing plate too to allow the WG to open up more so it can flow a little better, it needs all the help it can get.

 

^ Yah were' coming to that conclusion now. More things I learned as I talked to the owner...

1) twin scroll vacuum line wasn't on before which is why it only boosted to ~10 psi before (im assuming with the line off it doesn't open the other side so its a restriction)
2) Wastegate was ported as much as it could go without causing a leak

So I take it the wastegate could be ported further if the door is changed as well as grind the backplate.

Some options we were discussing was upgrading to an s5 manfiold and getting a BNR stage 3. Also talking about just changing the whole thing and go gt3574 or presicion billet 6265. Thanks for the help guys.

thewird

 

Get an S5 Turbo .
OR buy that BNR Stage 3 on CDN section for 4 bills Wird..its a good deal!

 

Think he has decided on a billet 6265 kit from a-spec. Thanks though.

thewird

 

mm the above kit sounds sexy ,

And to control overboost on a S4 turbo , go with a smaller exhaust or weld an external wastegate setup to the stock S4 manifold.

It doesnt matter how much you port that wastegate ( even changing the flange door ), it will boost creep, specially in fall with a 3" exhaust.

 

Well, at least I learned something with this for future S4's, S4 turbo + full 3" exhaust = no

thewird

 

I don't see the problem. You can run full exhaust on a stock turbo. Did it with my S4. It sounds like you have a boost line incorrect and running no boost control.

 

Well, considering there is only one line that controls the wastegate and its fairly straight forward, don't see how it could be wrong. The engine has a streetport as well and there should be no restrictions anywhere else either as he has a nice FMIC. RX7(613), may have a point in that the cold weather may have something to do with it as well since 4,000 RPM isn't exactly what I'd expect to get full boost by on a stock turbo. Really, my boost graph looks exactly like boost creep from a 6 psi spring (its just a steady climb).



thewird

 

turbo just needs more porting.


with the twin scroll vac line off the car will have soggy bottom end compared to with it on and being properly controlled


with the twin scroll wired shut the car will have lots of low end and no top end but will make a ton of boost.

Just port the WG more. alot more. You can port them enough to make a 3" exhaust work, I've done it on two different cars, and know of a 3rd local car that did it too. Maybe not to make it hold 5.5PSI but certainly enough to keep the boost below 10.

 

Would the original flapper still do? You guys made it sound like with the S4 it wasn't possible at all with the original flapper.

thewird

 

People usually weld on a larger washer to the existing S4 flapper to enlarge it but you need to machine out a bit of the side casting. Personally you're wasting your time on the S4 housing. The internal wastegate only dumps from the inner turbine scroll leaving it essentially crippled at high exh. flow,whereas the S5 has two i.e. one for the inner and one for the outer portion of the turbine scroll.

 

Thanks, I'm only asking for future knowledge sake. The owner already decided on what he's going to do (the 6265 kit).

thewird

 

Hey marco,

I have the same boost issue with my S4 and i got the 2.5" dp, when we meet next u should take alook and tell me.

hybrid

 

I ported mine to the max with stock flapper, and never got it to hold boost correctly .

( had healthy motor, stock ports ) and 3" full exhaust.

the best I ever got was to not creep over 10 psi on a very hot/humid august day.

it mightttt hold with a new flapper door, and stock DP then 3" exhaust.

I personally ran 3 different S4 turbos, with twin scroll working and deleted. working wastegate actuators ( tested ). on a 3" DP to muffler, and never any luck controlling boost.

If you have a Racing beat 3" exhaust, then you can control the boost to around 10 psi with a good port job ( the mufflers work as a good restriction ).

but that being said, once its fall-cold air temps. I still would trust it.

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I also had a S5 turbo, better success , but still not "satisfactory" /.