Boost cut after exhaust mod
 

Guys,

Ok got a local exhaust shop to check the main cat under the car and it was blocked up big time so i made the decision to just take the cat out all together and replace it with exhaust. The car all of a sudden seems to be going very hard and sometimes hitting the boost cut? :wallbash: Surely the removal of one cat wouldnt be enough to make the car hit the boost cut would it???? Any ideas would be appreciated.:scratch:

In colder weather, my 94 consistently hit the boost cut after I replaced the pre-cat with a downpipe and the stock muffler with a Racing Beat dual tip catback. The car is stock otherwise. Removing your main cat might be enough to cause it.

I changed out the stock .64" restrictor pill in the wastegate actuator hose and put in a .72" Holly jet. That lowered the boost peak a bit, but I still need to lower it further.

do you still have the precat on the car? if not then both cats are off and you are running a straight exhaust. you need a boost controller and a upgraded ecu like the pfc. you are going to blow your motor if you can't control the boost and 10 psi is the max you can run on the stock ecu. stop boosting the car until you can resolve this problem. read up on the 10 psi rule.

Jeff

Nah doesnt have the pre-cat its a 97 aus spec and they only had the one cat which i got removed today.....so essentially its just a straight exhaust now....I noticed there was some sensor connected to the cat, that wouldnt have anything to do with it would it?

https://www.rx7club.com/3rd-generation-specific-1993-2002-16/what-i-need-w-full-exhaust-543979/#post5699718. This is a good thread to read about running full exhausts. Unless you feel like spending some decent money on upgrades and tuning I would just get a new main cat. If you don't want to do that you should port your wastegate, get a boost controller, get a tune, and an aftermarket ecu (like a PFC as Jeff said).

you need to put the cat back on or control your boost to 10 psi max. I would get a highflow cat and install it. next time you hit fuel cut the motor could blow. no sensor on the cat its an air control valve for the air pump which you will need when you install the new cat.

Jeff

nubyrx797, it sounds like you have boost creep. You can't fix this with a boost controller.

Do you have a boost gauge? What do you see it read?

You are either getting two things, boost creep or a boost spike. A boost spike happens when the wastegate doesn't open fast enough (to divert exhaust gas pressure around the turbo, keeping it from spinning too fast). You end up building too much boost for a second, then the wastegate catches back up and is able to control it. You can fix boost spikes by adjusting your boost control. Either by modifying the ecu, replacing the restrictor pill, adding a manual boost controller, or getting an electronic boost controller. This is very dangerous and could easily destroy your engine.

Boost creep happens when the wastegate isn't able to divert enough exhaust around the turbo and causes the turbos to spin faster and faster uncontrollably. This is very common for FDs with straight open exhaust. This is very dangerous and could easily destroy your engine. Stop driving the car until you get this resolved. I think this is probably what you are experiencing. There are only a couple things you can do to control boost creep. You can add restriction back to the exhaust by adding a cat or putting an exhaust restrictor plate in. A restrictor plate is just like a little gasket with a small hole in it (<2.5in probably). That adds enough restriction to allow the wastegate to do its job. The other option is to upgrade the wastegate to give it the ability to divert more gas. You can do this by porting the stock piece (cutting a bigger port around the flapper) or by upgrading the turbos all together and getting a completely new wastegate (would need thousands of dollars more in other parts as well).

Yes. It is enough.



What he said. I'm sure you are getting both creep and spike with a full open exhaust. :)

usually a boost controller alone isn't enough to keep boost at stock levels as the system is flowing more air than the wastegate can manage. Go to the FAQ thread and look for links on the boost creep, boost spike and also search on posts from rynberg on midpipes. Lots of good info out there.

Thanks for the advice everyone, had a read up on the boost creep. hooked a boost guage up to my car and noticed the boost is creeping up around 10 but i never saw anything over that??......Should i still be hitting the cutout if the boost guage is not registering over 10??

Oh yes it would be enough.

Get a boost controller to start. Set it for 11psi, no higher. If that doesn't keep you at a rock solid 11psi, you may also need to port your wastegate.

If you're cutting out at 10psi something isn't right. Double-check the gauge setup for leaks and accuracy.

First, some boost gauges may not react quickly enough to display a steep spike, especially when rpm is increasing rapidly.

Second, I hit boost cut at 5500 rpm with only about 11 psi. Boost was 10 psi at 5000 rpm but without sufficient exhaust backpressure, it quickly jumped to 11 psi at 5500 rpm under hard acceleration. By 6000 rpm, boost would drop back to 9.2 psi. Meanwhile, the stock boost cut curve drops steeply from 13.25 psi at 4500 rpm to about 11.3 psi at 5500 and then decreases further at a slower rate to 11 psi at 6000 rpm and finally to 10.75 psi at 6500 rpm and up. So, with cold weather and a little bad luck, your unrestricted exhaust and stock turbo setup can easily take you into boost cut territory. Don't do that. :slap:

Stock-ECU Over-boost Fuel-Cut Pressure v RPM
 

Check out this chart...

Here's a graph showing the stock boost curve, the overboost fuel cut curve and my measured boost running with just a downpipe and stock pills.

Guys,

Update I went back to the exhaust shop and they had another cat from a similiar model rx7 that was very clean so i just had that one put back on and problem solved! no over boosting.

Thanks for all the information tho, I find the rx7club and its members very helpful! :icon_tup:

TRUST WHAT EVERYONE IS SAYING.:sadwavey: i just said good bye to my roto
i was messin around today, car kept hittin boost cut and then the car died on deceleration as soon as i came to the stop sign. after about an hour the car finally started and i got it home at 1500 rpm or else it would die...... ran like s#$@.