15psi on Boost Gauge, stock turbo?
 

Hi there,

When test driving a 1994 FD with a boost gauge installed, but otherwise stock, the gauge peaks at 14-15 psi under full throttle. I was under the impression it should peak at around 10psi for both turbos. Is this normal? A gauge problem perhaps? What else could cause this?

Some more details about the car:

Year: 1994
Make: Mazda
Model: RX-7
Mileage: 47598
Color: White

Specifications:
5 Spd Manual
Leather
CD
Sun Roof
Just under 48000 mileage
Touring Package including BOSE speaker system in trunk

Modifications (all these were done for reliability / maintainability reasons):
M2 Downpipe
Autometer Sport Comp boost gauge
NGK Platinum spark plugs

With the downpipes it is very possible to hit 15 psi on stock setup. The stock wastegate can only flow so much and when you get rid of the pre-cat or main cat you can get boost creep. When I got my FD it would hit 18psi before I ported the waste gate.

If you have eliminated the possibility of an aftermarket boost controller I would say that is why. it should climb to 10psi, drop down to 8 psi during the transition to the second turbo and jump right back to 10 psi. Does it slowly creep up to 15 psi?

If you end up getting it you should think about getting a standalone since the stock ecu cannot handle that much boost and probably porting the wastegate some time.

Thanks Sideo, I appreciate the feedback. I'm actually the one selling the car. The first turbo spiked to 15psi then briefly dropped down to around 12-13psi, then the 2nd turbo kicked in and spiked to 14-15psi before tapering to 11-12psi.

From what I understand about your reply, there's nothing actually wrong with the car, is that right? Could it be indicative of something else wrong or is it probably just a consequence of having the downpipe without the necessary porting / ECU? Would something as simple as lubricating the wastegate help?

The car is actually in Reno, NV.

Thanks for your help, I just don't want to sell it for less than it's worth if there's nothing actually wrong with it.

Yup sounds like boost creep from the downpipe.

Can't this type of high psi creep potentially lead to detonation and engine damage without being addressed?

It is not boost creep causing the issue with just a downpipe, but boost spikes caused by boost regulation.

The FD3s does not reference boost pressure in its factory boost regulation (I know crazy right?).

So when you change the turbo system dynamics (with intake or exhaust) the ECU is still pulsing the Prespool and Wastegate solenoids with the preset factory maps and instead of 10psi you get 15psi (or whatever).

Boost settles down in the high rpm since the Wastegate Solenoid is operating full open at that point.

Real boost creep will be when the boost keeps rising in the high rpm even though the wastegate is full open.

Can't this type of high psi creep potentially lead to detonation and engine damage without being addressed?

Yes, 15psi (or whatever over boost) will put more strain on the motor- But if you have the stock ECU it has the fail safes of over boost fuel cut and knock control incorporated.. so it is pretty safe.

The factory ECU allows higher over boost levels in the lower rpm (primary turbo) and around 4,500-5,000rpm (2ndary turbo switchover) because the factory boost control is far from perfect and there will be variations in these areas.

If you have over boost over what the factory ECU deems safe it cuts all fuel and you get a hesitation.

since you're running this one, look at how much boost you're running with the engine off, it can be a couple psi, so 14psi on the gauge could be more like 12psi in real life...

yes, but the stock ECU actually seems to be able to handle quite a lot of boost on the primary turbo without knock, my friends car was malfunctioning, and would easily do 20psi on the first turbo, no knock, and it was like this for years with no trouble. i don't recommend trying it, but the stock ecu does a good job here

I feel the best solution is to put Hallman Pro RX manual boost controllers on the Prespool and wastegate actuators- bypassing the stock solenoids.

Then you can dial in a perfect 10-9-10 boost pattern (yes, you can get a perfect seamless 2ndary transition).

With this manual boost controller mod you can then run a downpipe, catback and intercooler on the stock ECU and have no boost control issues or boost creep. You can even crank the boost up to 12-11-12 with no issues.

This is a safe, reliable set up for and FD with ~250rwhp.

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If you add intake and or a highflow cat/midpipe to the above set-up you will start to get boost creep and factory ECU fuel cut in the high rpms and it starts to run pretty lean on the wideband.

With some trickery adding restriction at the back of the exhaust you can run intake, intercooler and full exhaust with the factory ECU and manual boost controller mod and keep under factory over boost fuel cut, but you are on the edge for Air Fuel Ratios (into 13s) at that point so its more of a race gas set-up.

Thanks for the feedback everyone. I recently test drove the car and it's a very nice example, I just have some concerns about this issue before pulling the trigger since the car would be seeing some daily driving duty (weather permitting ;)) and there isn't anyone in our area who can work on rotaries.