Quick question, my boost gauge is reading 5-6psi at full pressure but when let off it jumps to 9-10psi , is this normal?

Also if I change stock cat to a 200cell what would the psi read roughly or too hard to tell.

Cheers guys.

Edited 05/01/16 12:21 PM05/01/16 12:21 PM by BlakxrT

Changing to a high flow cat on a xr6t without altering tune will cause overboost/boost spike (so I have read).

No idea about the boost spiking when letting off?

Assume with the lower boost you have a BA or BF? I think stock they are supposed to be 4psi max? And the FG has 10psi. But not 100%....

Im sure someone will chime in if I've given the wrong info [emoji14]

Its normal for pressure to spike momentarily when the throttle is shut suddenly under boost.

5-6 psi is correct for a b series.

People have changed cat with no retune and been fine but it wil be running on the leaner side. Possibly only a sh*tty batch of fuel away from a detonation event. Run at your own risk.

5-6 psi is correct for a b series.

People have changed cat with no retune and been fine but it wil be running on the leaner side. Possibly only a sh*tty batch of fuel away from a detonation event. Run at your own risk.

that's not good. Must tune. lol

I'm probably forgetting something stupid, but I'm very interested to learn how this works,this being my first turbo car. I can understand that reduced back flow = easier/faster turbo spool up = increased boost pressure = increased air quantity which has the potential to impact on AFR. I'm assuming that within the factory boost range, the car detects the incoming air and can adjust fuel appropriately to give correct AFR.

So on part throttle, boost should come on easier at a given engine load and the car will adjust accordingly on factory tune.

Under harder throttle I guess is where the potential is to make higher than factory boost. What I don't understand is wouldn't the waste gate just bleed off the excess?

This doesn't cover a boost spike on sudden throttle close of course. Is that the only scenario where the risk exists?

The wastegate is not large enough to by pass enough gas. The moment you try to push more than factory boost the wategate port is no longer sufficient. This is why pretty much everyone suggests the "flapper" mod to be done almost immediately.

The wastegate is not large enough to by pass enough gas.

Thanks for that - makes sense. I can understand from that how it is possible to overboost.

I'm assuming then that this is in a sudden, uncontrolled boost spike scenario (e.g. sudden throttle close), where as under a steadier load, if the wastegate can't bleed boost quickly enough and boost builds beyond what the ECU,s factory tune is happy with, the ECU shuts everything down.

I guess the question is whether or not doing the cat alone would cause enough of a boost increase to push the car into this zone. My experience with a 100 cell on an otherwise stock BA XR6T is no. The advice I got before fitting was that this wouldn't happen with a standard dump. Notwithstanding that my boost gauge is probably not super accurate, before the cat change I would max at 7psi (I doubt it is actually that high). After replacing the cat, I get to 7psi more easily but still peaks at that pressure. I haven't ever triggered the overboost cut-out in the ECU.

Every car and cat is different though and everyone has to be comfortable with what they are doing to their own car. A tune would definitely be best to make the most of the cat and I''m planning to do exactly that, with some mild mods, in the future once a couple of other priorities are out of the way!