I bought the car about a month ago and just trying to get the regular maintenance done. Did some searching no this subject but came up empty.

Hooked up my boost gauge to the car and it is doing the regular 10-8-10 transition but in the higher rpm's around 6k or so it seems to drop back down to 8 or 8.5psi.

Wondering if this is normal for stock boost on an FD. The car is almost stock. Mods are K&N air filter in stock box, HKS downpipe and 3" muffler on stock 2.5" catback i beleive.

Wondering if this is normal or if i might have an intake/vaccum line leak somewhere?

Thanks

 

While 10-8-10 is the "perfect" boost pattern, my own FD has a similar boost fade at the higher rpms. My rotary mechanic has been through the system and never been able to find any specific problem. He did say that it is pretty normal to have the boost fade to 7 psi on these cars. Not that there isn't a problem with your car, but it may be a tough problem to find and resolve.

Here is some information on boost troubleshooting that may help.

 

you have nothing to worry about...its more than normal to see a 2-3 psi drop after 6k rpms.

 

Great thank you. From what they are saying this would be normal.

7) Road test with boost gauge

RPM PSI
3,000 10
4,500 8 as secondary turbo comes on
4,500 + 10 recovers almost immediately from short dip
6,000 + from 10 to about 8 gradually, and holds to redline

 

That looks really good to me. I wish mine held 8 to redline. I start to drop at 5500 to 6000rpm and will go down to about 7psi. Of course, at that point I'm not watching my boost gauge very closely but am watching the road.

 

With standard bolt-ons and an SMB high flow cat I hold maximum boost solidly until redline. However, with a stock cat this is less likely.

 

Are you running the stock pre-cat and main cat?

When I had boost fall off at high RPMs in my close to stock FD, it wound up being the main cat clogging/melting. I was able to verify this by unbolting the two front bolts and shining a small flash light down the cat. The front of it looked like a clogged shower drain from the honeycomb catalyst internals melting.

 

i have this exact same problem and it is not how mr mazda intended it to be.

boost after 5.5krpm drops off to 7psi and when i dynoed the car the power and torque dropped off at the same point, hence the car only recorded just over 200rwhp (229 @ the fly)

i have the graph if anyone may be able to help as have searched to solve this problem for over 9months now.


The dyno shows 180 fwhp on primary and increases rapidly on second until 5.5k then just dies off.

only mods are filters/hard pipes/rear box.

All check valves have been changed to vitton and all hoses/connectors changed for new. Rats nest has been fully zip tied and all ic connections checked.


Gonna do the cat next although it looks near new.