so, to the title, i am running 7psi by choice - no boost control pills and solenoids disconnected. it looks as though my CAT my be on its way out and i was wondering what it will do to my boost levels if i put on a midpipe instead.

everything else is stock - just wanna make sure it isn't going to pop the engine.
my search has bought up sound and smell - so i am ONLY worried about boost.

has anyone done this? surely someone has!

 

Well, if you're running 7-8psi of boost, that means that even more air must pass through the wastegate than when you're running 10psi. So that means you're more likely to get boost creep, although it will have to climb over 10psi before it presents any risk. But this sorta only applies to cars with basic bolt-on flow mods (muffler, downpipe, intake)

But you say *everything* else is stock. If that means stock intake, air filter, muffler, and downpipe are all stock, boost creep becomes pretty unlikely. It's not as much restriction as a north american FD that has a catalytic element in the downpipe, but probably still enough. I think I remember hearing that the stock muffler alone is enough to prevent creep.

I will theorize that if you get no creep at 8psi boost, you can safely re-enable your wastegate and precontrol solenoids and run at 10psi.

Dave

 

but theoretically i thought opening up the exhaust would raise my boost pressure, is that not right?

 

No it isnt right. If you have restriction elsewhere, then it can't open up as much. However, if you have more air comming in than stock, then you'll let more go out and thus flow more air and pop. Get some fuel management and then worry about a MP. For not, get a stock main cat and put it on.

 

More air is not necessarily more "boost". For instance, 10 PSI on the stock twins is not the same as 10 PSI on a large single turbo.

Is is possible to control boost with a midpipe? Absolutely.

https://www.rx7club.com/3rd-generation-specific-1993-2002-16/266-rwhp-stock-ecu-yee-hah-213367/

 

Why is it that its not the same? please explain

 

The difference is pressure compared to flow. Sorry this is long, but hopefully it makes things clear. (warning: I'm using imaginary numbers for psi and CFM. This is NOT an exact measure of the pressure or volume of air flowing through the 13B-RE engine!!!)


Think of a garden hose that is just barely turned on. This is low pressure (5psi), low volume (50CFM). You can't fill a bucket quickly, and the water won't really hurt your hand if you sprayed yourself. You couldn't knock the petals off a rose-bush, even if you sprayed it up close.

Now, without turning the faucet up, just put your finger over the top, or add a jet-spray nozzle. This is high pressure (20psi), but low volume (still 50CFM). It will take the same amount of time to fill a bucket, but you can feel that the water is moving faster through the air. If you spray your hand, it might hurt; you could spray a rosebush and knock some petals off the flowers due to the high pressure.

If you have the faucet on full blast, but there's no jet-spray nozzle, you've got low pressure (5psi) but high volume (100CFM). You can fill a bucket quickly, but the spray won't hurt your hand or knock the petals off a rosebush.

Full-blast through a jet-spray nozzle would be high pressure high volume; bucket gets full quickly, rose-bushes fall apart when you spray them.





Here's the root of the problem: the RX-7's ECU is not able to compensate for increased flow (CFM), because it's only measuring the pressure (10psi) and ASSUMING the flow volume.


Here's what happens: the stock twin-turbo setup, when boosting at 10psi, will force a certain volume of air into the engine (maybe 50CFM, for instance). The ECU adds the proper amount of fuel, and a little extra for safety.

Minor modifications (such as intake, exhaust, downpipe) can increase the volume of air that the factory turbos are forcing through the engine (maybe up to 60-70CFM), even when the pressure is the same (still 10psi). The flow hasn't changed too much, so the ECU's 'extra fuel for safety' will be enough, and your car will be safe.

Major flow modifications, mainly midpipes, will increase the pressure as well (maybe 15psi). Flow will increase a LOT (possibly 90-100CFM), and now you're beyond the safety margin built into the ECU. You don't have enough fuel, and you experience detonation and destroy your motor.


Hope that helps,
-s-

 

To the original poster: I know a person with a '94 RX-7 running intake, IC, downpipe, midpipe and exhaust on a stock ECU with the wastegate solenoid disconnected. The wastegate spring limits the car to 7psi, and he told me that he doesn't experience spikes. He's dyno'd the car, and his AFR charts looked pretty safe from what I remember. He made a lot of power on the dyno (over 300whp), but the car didn't feel much faster than my car on the street, especially at low RPM's and partial throttle.

 

done it with all but mid-pipe. It's not "No boost contol" it's dircet boost control based on the wga spring preload. Wg cracks open at 8 psi, full open at 10-12, you you run about 9-10 psi, no spikes, ( but not much low end also ).

 

im also running about the same setup intake v-intercooler hks downpipe midpipe and exhaust and i ported the wastegate and have my boost controller set at 7 psi and on the coldest days boost spikes to 13 psi.

 

The spring will st the pressure the wastegate starts dumping pressure at but if the waste gate can't handle the amount it's dummping then you will get creep. Obviously your friend is not exceeding the amount his waste gate can handle if it's staying at 7 psi.