[New Owner]

I've been told by a few people that if you pull the cats off the car and put in the Downpipe then your boost will increase.
Now I was wondering how this is possible since boost is actually a measure of backpressure in the engine. Plus, the wastegate should always limit the boost to 11.5 psi. If all things being held constant, ie. the cfm from the turbos, then boost levels should actually decrease from removing the cats since there is less backpressure in the engine. Since the supra turbos are capable of handling increased cfm, the boost levels return to 11.5psi where it is capped by the stock wastegate. So, what is really happening when the cats are removed is, you are decreasing backpressure in the engine and increasing cfm from the turbos with the boost levels still being regulated at 11.5 psi by the stock wastegate settings. More flow at the same boost levels.

 

[2 RIP]

Oh man, ok, the stock wastegate isn't regulated at 11.5 psi, back pressure is for all motor. When a turbo can flow better, its going to make more booost. stock is 11.5 to 11psi, w/ intake, downpipe, and exhuast it can jump all the way up to 22psi.

2 RIP

 

[Mike Hughes]

Backpressure?!

Come on now, let's be real: backpressure after the turbos has almost zero meaning on Supras. In fact, the less, the better.

Removing the cats will increase exhaust flow. Wastegates regulate exhaust flow like a trap door. Sometimes so much exhaust flow is created that the wastegates can't "keep up" with the exhaust being pushed through the turbos. This will increase compressor performance, more air will flow, and more boost will be produced.

 

[dzeiler]

Rip is correct turbos motors hate back pressure non turbos need to scavange some of the exhasut gases. turbos work best when there is a big pressure difference on each side of the turbo. The stock cats are relativly restrictive. Dp and an aftermarket exhasut will allow higher boost. The waste gate can be disconnected or modified by various measures you will be able to boost tillthe car hits fuel cut( a back upf rom the ecu which turns off fuel for a microsec) at 13.7 psi or ust undr 1 bar. By adding device to the ecu ( ususal a greddy boost cut conroler or bcc 100 bucks) you can boost your little heart out. the higher the boost the less durable the turbos are supposedly over the years a figure of 18 psi has been deemed reasonably durable( don't ask me how long they will last at that level your milage will vary) The stocker crap out of producing boost at something higer than 20+ psi they get inefficent. another neat trick is TTC or true twin turbo conversion changes the turbos to twins rather than sequential at higher levals of boost you get a smoother boost curve. Don't do all this stuff at once learn a little about your car. I presume it is older and has some miles on it things break and are puzzeling and can be expensive to fix. go to www.mkiv.com ask questions enjoy your car.

 

[BPUed]

Wanna clear some things up first.

Backpressure does exist in a turbo car, but it exists between the turbo and the engine, and is determined my manifold size and turbo size. That's why (to some extent) bigger exhaust is better on a turbo car. A debate going on right now is whether or not you can go to big. Some believe a slightly smaller exhaust will improve midrange power, but with a decrease in top end. With the turbo able to spool faster the system is capable of producing more boost.

 

[swamibob]

Yes it will get more boost, I got 12.7 when I put on a downpipe and exhaust.

 

[New Owner]

Today I had the cats pulled off the car and had a downpipe put on. Now the car still has the stock intake. So here is what I found out. The car definitely feels much quicker after pulling the cats. But, the boost is still reading 11-11.5psi. So there was no increase in boost pressure. I have noticed that the turbos are reaching 11.5psi earlier in the rpm band, but the boost pressure is stopping at 11.5psi. Tomorrow I'm going to pull the aircleaner and see what happens to boost and then I'll do the bleeder T mod along with the FFCD, EGR mod, and 12v mod to see what happens to boost then. I would expect the boost to push to 17-18 psi. I really don't expect the boost to increase until I do the bleeder T mod. I'll see if I can get a few other supra owners in the area to witness the boost test to make sure I'm doing this right.
I'm a former mustang owner and I don't have much experience with turbos, but I do have experience with superchargers. With the superchargers any obstruction in the engine, including the exhaust, will cause an increase in boost levels. When you port heads, intake manifolds, port match and increase the exhaust size, the boost put out by the supercharger will decrease, but there will still be the same amount of airflow through the engine.
From what I know of turbos, they are driven by the heat of the exhaust. I am relating this to my current job which is building powerplants. When a turbine is built, it's performance is measured by it's heat output. Now there are also emission requirements on these turbines and we use a catalyst similar to the catalytics found on cars to keep emissions down. These catalysts are placed in the exhaust manifolds and they decrease the performance of the turbine due to back pressure, but they do clean up the emissions. It's kind of a trade off. I would think the same principles apply to turbos since they are essentially small turbines.

 

[Hua]

Don't do the 12v fuel mod.

 

[swamibob]

I don't believe it's due to heat but do to the flow of the exhaust leaving the engine and harnessing the power similar to a hydro-electric plant built into a dam harnessing the water flow. It's the airflow that spins the tubos.

 

[dzeiler]

airflow is a measure of heat and pressure same thing different measure. BTW don't do the FFCD see the tech section of mkiv.com spring for the BCC only a 100.00