Hi guys,

I got another FD and have some questions about it.

First it's a stock car besides from a shitty exhaust. Its still got the stock down-pipe but from there it's a straight pipe until a silencing box at the end, now I thought if you don't have a resonator or something more restrictive you could cause boost spike?

Which brings me to my next question. If I pull away WOT, 1st and 2nd gear is good until red line then I change to 3rd and as-soon as I hit about 6500rpm its as if the car hits fuel cut, makes like a rapid backfire noise. I'm thinking this is caused by the boost spike from no back pressure?

When I got the car the second turbo wasn't coming online in 1st and 2nd or sometimes it did but delayed at about 5500-6000rmp. So I tried a different turbo control solenoid that I had lying around and now it's boosting in 1st and 2nd all the time but still feels a bit delayed?

Thanks in advance

 

You need to reduce your boost to stop the 3rd gear spiking.
If you have an older model car you need to either get rid of the pil in the wastegate actuator hose (this will bring you boost down to 7-8 psi, and should stop the fuel cut in 3rd + gear), or get rid of the pil and run a manual boost controller so you can adjust it as close to fuel cut as you care to.
If you have a later model car the restriction is built into the nipple on the turbos, in that case you need to drill it out and run a MBC.

My vote is get an MBC, and a set of Dale Clark's check valves and i think you will be boostin pretty

 

Do you have a GOOD boost gauge? What does it show?

 

But the cars bone stock so why would I need a MBC, Surely the pill will restrict the boost to 10psi unless I'm getting boost spike from the loss of back pressure from the nonrestrictive exhaust that I've got causing the waste-gate to become overwhelmed? Cars on stock ECU and it's a 94 model.

 

Not yet, Fitting a Greddy Boost gauge tomorrow

 

Well from your description your car is not bone stock, and it appears in your situation the exhaust is enough to make you fuel cut from boost spike..

By all means run no MBC if you want, you don't NEED one...you can remove the pill and drive with 7psi only if you like!

The MBC is used to bring your boost BACK UP TO STOCK LEVELS/JUST UNDER FUEL CUT, after you have removed the wastegate pill.

EDIT; I think you misunderstand the fact that the pill in the wastegate line ADDS restriction, so makes it HARDER for the waste gate to open. If you get rid of the pill / REDUCE RESTRICTION, then the wastegate will open EASIER. If you get rid of the restriction completely then you will run off wastegate spring pressure which is 7-8 psi.
Ideally you should remove the pill completely which will bring you down to about 7psi, then you tune it up from there with the MBC to the highest safe level.

 

96fd3s is correct in the matter, but lets see what the boost gauge is reading? Start with the fact/hard evidence and work from there.

 

You really do have a shitty exhaust. Get rid of the stock precat (likely clogged), get a downpipe, and a good catalytic converter. Your car will thank you for it.

Good advice above regarding your boost levels, boost control, and getting a gauge

 

I understand what you are saying about the pill in the wastegate line, but the way I see it is that if the pill allows the the wastegate to open at 10psi then why would you replace it with a boost controller to do the same thing the pill does, unless the pill is faulty and is causing the gate to open to early or late then yes I'd say use a MBC or get a new pill?

Sorry if I come across as arrogant I'm just trying to understand what you are saying. I will see what the car is boosting tomorrow with the gauge maybe it isn't even over boost that's causing the problem.

 

My car is from Japan, I believe they didn't come with a precat? The air pump goes to nothing, literally the pipe is just hanging under the car.

My question is can a less restrictive exhaust cause boost spike on a stock ecu?

 

^yes a less restrictive exhaust can cause boost spike.
And the problem is due to the increased flow.
Due to the increased flow of your exhaust. You build boost too quickly (spikes) before the wastegate can open enough to bleed the boost.

If you reduce the restriction to the wastegate it can open/react quicker therefore getting rid of your boost spike.

As said above, when you remove restriction to the wastegate your boost will reduce. The only way to do that is to

A. Remove the pill completely, replace with nothing (you will boost 7psi).

B. Remove the pill completely (you will boost 7psi). Then use MBC to tune it back up to a level you can safely run without fuel cut. (best option)

C. drill out the pill in your wastegate line (not ideal because whatever size you drill it out to, is what you will be stuck with. Which may be too much or too little boost)

D. restrict your exhaust

 

Okay thanks for that, well explained

So What I would see as the two best options is to either restrict my exhaust and carry on with the pill at 10psi (more power)

or keep exhaust and get a MBC but might only be able to boost say 9psi to prevent fuel cut (less power but quicker spooling)

 

listen... Your car is clearly not operating safe. It should not fuelcut at 11 psi, so either you are boosting way more, or if it actually cuts at 10 psi, you have a problem with fuel delivery...

As we have said, do not beat the car around. Get a GOOD boost gauge and measure your boost level. Start from there.
I recommend you to run safe. FDs does not handle fuel cuts well. All it takes is a ping, and it can shoot apexes across the atlantic and hit somewhere in australia....
Cheapest thing would be to just get the boost level down to acceptable standards with a boostcontroller, and handle it from there.
I mean, do you really care about boosting abit lower, and probably saving your engine until you have figured out the main problem? Id say the small down in power is DEFINATELY worth it, south africa does not have many rotary specialists... Rebuilds can and will be costly then...

Your car is a prime example of a rotary car that has been tuned as a piston engine, and will die from it too if you dont fix it asap...

 

Okay so I installed my boost gauge and it says that my car is boosting 0.8bar (11.6psi) in third until it cuts out at 6.5krpm (makes noise as if hitting the rev limiter) In first and second it's boosting 0.7bar (10psi) till red line.

 

South Africa actually has quite a number of rotary specialists, it's actually a very common engine here.

If you read my previous posts you would see that I said the car has its stock ECU therefore how would it be tuned as a piston engine?

The only modifications to this car is a straight mid-pipe and a cat-back. everything else is as if it came from the factory in 1994.

 

The problems are-

1) Mazda created a complex sequential turbo system and it does NOT reference boost pressure to operate. It was made to only work correctly 100% stock.

When you change the exhaust/intake you change how and when the two turbos make boost and you get boost creep on the primary turbo, spikes on the secondary transition and boost creep on the secondary turbo depending on how free flowing your intake/exhaust is.

2) Second problem you are having is Mazda wisely put an overboost fuel cut on the car so people didn't blow it up.

Its a little confusing, because it allows more boost in the lower rpms and especially around the 2ndary turbo switch over point and then decreases the maximum allowed boost in steps in the higher rpms.

3) Boost creep, there is just a small wastegate on the primary turbo.

With an aftermarket ecu or manual boost controllers on the wastegate and precontrol actuators you can regulate boost on the primary turbo well as it has the wastegate and the prespool door to regulate boost.

Once you transition to the 2ndary turbo you begin to have boost creep if you add an intake or exhaust as now you only have the small wastegate regulating boost.

I suggest keeping the stock airbox, adding a highflow cat to the exhaust and putting manual boost controllers on the wastegate and precontrol actuators.

You can then safely run the car at 10psi boost on the stock ecu.

 

Okay so I've taken the exhaust (that has a cat in the midpipe) and the downpipe that was on my other car off and put it onto this one and the problem is still there whereas on my other car there was no problem at all. I am starting to think this isn't a boost problem but something else causing the problem..

Now while I do see the point in installing a MBC for prespool and wastegate to control boost spike I found this somewhere on this site..

In my original post I referred to"spike" instead of "creep" I somehow mixed the two around.. My bad

 

So, you symptoms are car drives/boosts well but cuts out like fuel cut still after 6,500rpm?

You have stock intake box on it?

Any chance you can get a boost gauge on it?

There is a small chance the new FD has fresher turbos or less boost leaks than your other FD, so even the small increase in flow from non stock exhaust is causing problems.

When I raced my FD in SCCA stock class it had boost cut from overboost in 3rd,4th and 5th gear right around 6,500rpm as you describe with just a more free flowing 2.5" crush bent straight pipe cat back exhaust.

When I wasn't racing in that class I put the manual boost controllers on it and no problems with the overboost fuel cut. Drove very smooth with no hesitations.

Putting boost controllers on the FD can actually decrease boost creep as well as spiking in my experience.

However, it is easy to increase exhaust/intake flow so you will still have boost creep with boost controllers- the wastegate is just too small.

 

Yeah, that's exactly what's happening, but only in 3rd. I haven't tested 4th and 5th but I would suspect the same would happen. 1st and 2nd no cut.
Stock air box.

I did fit one and in 1st and 2nd the car is boosting 0.7 (10psi) but in 3rd it's boosting at about 0.8 but I did see it go to 1bar (14.5psi)

There were absolutely no leaks in my other FD as I replaced all the couplings and pipes. Although the turbos are different where my other FDs turbos are aluminium around the compressor turbine and these look like they have a plastic sleeve in them of some sort?

Thanks for the help

 

Although the turbos are different where my other FDs turbos are aluminium around the compressor turbine and these look like they have a plastic sleeve in them of some sort?

Those turbos on the new to you FD are the '99 spec "high power" (280hp) turbos.

They have the plastic lined "abradable compressor housings" as you noted, smaller exhaust wheels and housing I believe and the actuator restrictors built into the boost reference nipples on the compressor housings instead of using "pills" as earlier FDs used.

It makes sense the '99 spec turbos would boost creep more easily, though I have no direct experience with them.

I do know when tuners in Japan do all out stock sequential builds they actually use the older "low power" spec turbos for the larger exhaust sides higher flow potential as this is the choke point on the stock turbos.

Its clear you found the problem- that is you are overboosting and the overboost fuel cut is doing its job.

Easy fix.

I have found that the best way to restrict the exhaust to decrease boost creep is to put the restriction as far away from the turbo as possible.

This way it has the good flow and volume of the remainder of the exhaust system to help with spool and then the restriction to limit overall system flow once you have reached your peak desired boost.

You can use an exhaust tip "silencer" or make a "gasket" out of stout material with a smaller hole in it to place at the furthest back exhaust joint.

It will take some experimentation to get the orfice size right- remember to account for increased boost creep with higher air density (lower altitudes and cooler air).

 

I've got a thought here....

My car is a 94 model with a 99spec motor.. the harness I suspect has been cut and joined with a newer engine harness to support the new solenoids and what not on the engine otherwise the whole cars electronics would have to be changed? which is not the case because everything electronically is still 94 spec..

Now if the newer engine was tuned for 280hp like increased boost that would mean that the engine is trying to make 280hp but the ECU is only giving fuel for 255hp possibly causing the fuel cut?

Maybe the exhaust isn't even the problem and there's actually no creep....

 

Boost creep is when there is so much flow that the boost just keeps creeping higher and higher. You dont have that. You have overboosting. It can be controlled with a manual boost controller, but you will have to drill the wastegate nipple on the turbo. This will likely fix your problem.

 

What you're saying is actually incorrect, or maybe just improperly worded. Boost creep happens solely because a wastegate cannot divert enough exhaust energy around the turbine blade. Period.

 

What I wrote is exactly correct. Boost (and the increased airflow thru the turbos) will continue to increase. It wont just rise a little higher and then stop.

You should find something else to do.

 

It seems to me that boost creep is exactly what the OP is having trouble with.

On the primary turbo you have the wastegate and the pre-spool door to divert exhaust gas around the primary turbine, so boost creep is unusual on the primary turbo.

At transition into the 2ndary you have the whole 2ndary turbo area opened up for flow so once your boost pattern stabilizes after the slight drop in transition it takes a while for the wastegate to be overrun.

In addition to this, you have the FD ECU which allows more boost on the primary turbo and 2ndary switchover and then trims down the allowed boost in steps to 10psi.

So, you start to get boost creep right where it isn't allowed at 6,500rpm and fuel cut happens.