Boost loss after gutted cats?

[brainpan]

I had my cats gutted last week and since then I can't peak anything over 15psi on my stage 2 giac. 2 weeks before the gut job I had my ecu reflashed for troubleshooting reasons. At that time I would go into limp mode with anything over 5 psi.

Anyways, i cant get back to the shop till late next week, they are 100% certain they reflashed stage 2 software, my flash loader isn't working either. I checked my boost gauge for kinks/lose fittings and it seems fine with good vac at idle 20-22hg.

Best psi I pulled on the torque app was 17 and change when before I was up to 20s.

So can the lower psi I'm seeing be related to the gutted cats or did they flash stage 1 software by accident?

Pre cat issues
https://youtu.be/gFkCNzaRdGE
Current boost
https://youtu.be/01Ghgn8epZM

Thx.

[theswoleguy]

Torque reads requested boost, sounds like you're on stage 1

Sent from my Nexus 6P using Google's ProjectFI

[brainpan]

Torque reads requested boost, sounds like you're on stage 1

Sent from my Nexus 6P using Google's ProjectFI

cool thx, annoying but easy fix

[BlownOne]

Less backpressure with gutted cats, creates less boost (boost pressure is directly related to backpressure), but it will make the same power.

[theswoleguy]

On turbos maybe but I don't think that applies on supercharged cars.

Sent from my Nexus 6P using Google's ProjectFI

[DBL R]

Less backpressure with gutted cats, creates less boost (boost pressure is directly related to backpressure), but it will make the same power.

+1

[timzcat]

Not even on turbos. less restriction post-turbo will not reduce pressure and may improve it.
The whole reason for large exhaust is to let the engine breathe since a turbo is a big restriction in the exhaust.
Less restriction improves the pressure differential and may not make more boost but can often allow it to come on sooner.

[bhvrdr]

Less backpressure with gutted cats, creates less boost (boost pressure is directly related to backpressure), but it will make the same power.

I've been curious about this. Its commonly stated that "boost is a measure of restriction" and doing things like adding headers or cams will reduce boost.


I added longtubes to my other car and I actually didnt see any change in boost at all. It got me thinking...uh oh right.


For all naturally aspirated cars the manifold pressure is always 14.7psi or just over 1 bar at WOT. Why does this pressure not change no matter what cams you use or even if you ran no exhaust at all? Its because thats the maximum pressure available to put into the engine (natural pressure or ambient pressure). Isnt boost pressure actually how much the turbo or supercharger is compressing the air over ambient? If so, is this really impacted drastically by cams or exhaust? I've been trying to wrap my head around it better.

Thanks,
Mike

[skiptowncat]

Yep, less backpressure reduces the boost making the turbo run more efficiently, when you turn the boost back up to previous levels you get more power. Running too little backpressure can cause the turbo to weep oil past the seals or cause boost creep.

[-leman-]

The difference between NA and forced induction in this respect is that NA has no compressor efficiency issues. So basically whatever the engine may consume at any given intake manifold pressure and any given engine speed will be there. When you reduce backpressure you theoretically increase airflow through the engine on certain engine speeds (probably even everywhere through the rev range), and it should be seen as higher MAF reading despite the intake manifold pressure stays the same. On MAFless setups however you won't even see the air mass reading increase as air mass would be a modelled value, and the model would be now slightly off (as it would still be based on old backpressure assumptions).

For forced induction cars you may face an issue where the engine may theoretically consume a lot at any given intake manifold pressure and any given engine speed, but the compressor / turbo is not capable to deliver such air flow at this pressure rate. This would mean that at such engine speed the intake manifold pressure will have to be lower. When you would further reduce backpressure on such engine you could further increase its potential air consuption, thereby reducing the boost even further as you have already maxxed out compressor / turbo. On the other hand, if your compressor / turbo is not maxxed out from its airflow viewpoint (i.e. it is capable to deliver whatever boost ECU requests from it throughout the rev range) you wont' see boost drop when reducing backpressure, you would just have ECU forcing the compressor / turbo to work harder to keep up with requested boost and then you get same boost, higher airflow but also higher IATs (that is unless some safety routines ECU has would kick in and reduce requested boost).

[brainpan]

Some good info thanks! I see a lot of info referring to turbos which is great but do the same principles apply to a blower?

I guess the ultimate way of knowing is to do some 3rd gear logs. It's thanksgiving this weekend so I may be busy with family gatherings but may be able to squeeze a few in.

My only hang up with this the peak levels I'm getting in the torque app. The boost tap is right off the neck of the blower so I figured the gauge is measuring the levels produced by the blower?

Thx

[bhvrdr]

Some good info thanks! I see a lot of info referring to turbos which is great but do the same principles apply to a blower?

I guess the ultimate way of knowing is to do some 3rd gear logs. It's thanksgiving this weekend so I may be busy with family gatherings but may be able to squeeze a few in.

My only hang up with this the peak levels I'm getting in the torque app. The boost tap is right off the neck of the blower so I figured the gauge is measuring the levels produced by the blower?

Thx

turbochargers are slightly different mostly in that nowadays they are many times coming with electronically controlled waste gate or at the very least electronically controlled boost control. a Turbocharger therefore can be spun up to still meet boost targets after restrictions are reduced or flow is increased. The supercharger on the other hand is spinning at a set RPM based upon the cranks RPM.

[doug97gxe]

On turbos maybe but I don't think that applies on supercharged cars.

Sent from my Nexus 6P using Google's ProjectFI

exactly which is why exhausts do nothing on these cars but can see a huge increase on turbo cars..

[BlownOne]

I've been curious about this. Its commonly stated that "boost is a measure of restriction" and doing things like adding headers or cams will reduce boost.


I added longtubes to my other car and I actually didnt see any change in boost at all. It got me thinking...uh oh right.


For all naturally aspirated cars the manifold pressure is always 14.7psi or just over 1 bar at WOT. Why does this pressure not change no matter what cams you use or even if you ran no exhaust at all? Its because thats the maximum pressure available to put into the engine (natural pressure or ambient pressure). Isnt boost pressure actually how much the turbo or supercharger is compressing the air over ambient? If so, is this really impacted drastically by cams or exhaust? I've been trying to wrap my head around it better.

Thanks,
Mike

Hey Mike, yes you are correct boost is a measure of restriction on any air pump (engine) and the more efficient you make it from the cylinder heads to the exhaust tips, the less boost you will make at the same or greater power level while seeing lower iats. Imo the reason you didn't see a drop in boost pressure from adding headers on your other car is because the exhaust wasn't a restriction at that boost level. Same thing with cat back exhausts on the 3.0T with just a stage1 or 2, stock system isn't restrictive so there's no change until you remove the cats, which are the restriction. Also the size of the air pump (engine) directly effects the gains when improving airflow...the larger the pump, the better the gains.

When you change cams the vacuum of the engine will be different and depending on lift/duration you should see a drop in boost pressure as you are improving the air pumps efficiency. Boost is always just a measure of restriction...if you're spinning the blower at 24k rpms and making 20psi, but then add headers with full catless exhaust, now you might only be at 18psi at 24k rpms, but it's making more power, more efficiently, with less restriction. Grab your wife's blow drier and turn it on high. Now cover the air outlet with a bag that has one little hole in it, listen to the sound of the drier and feel the flow blowing out of it...then remove the bag, listen and feel how much more blows out of it. I wouldn't say cams or exhaust drastically change efficiency, but they help move more air.

Hope that helps brotha!
Will

[jprice]

Lot of bad info in here.

Turbo:
If using a wastegate frequency valve at the same duty cycle, boost increases with reduced backpressure. If using boost feedback (PID), less wastegate duty cycle required to hit same boost level.
If using an MBC or just direct pressure line to wastegate actuator (basically using the actuator as the MBC), then boost stays the same with less backpressure, but you get to it quicker/earlier.

TVS Supercharger:
Less backpressure = less boost, but likely higher airflow and more power. I observed this on a 3.0T with the armytrix exhaust. With the valves closed, boost was higher, but car was slower. With valves open, boost was less, but car was faster. (This all of course assumes your boost bypass valve is staying closed, we're talking mechanical system here)

Less backpressure on a turbo car allows exhaust to exit the turbine housing faster, which in turn spins the turbine wheel faster, which of course spins the compressor wheel faster, and that's what is pushing air into the motor. On the TVS supercharger, the speed of the charger is mechanically linked to the speed of the motor through the pulleys. Air is being forced into the motor at the same rate at a given RPM. If the exhaust is able to more fully and efficiently exit, there's more room for air coming in to the cylinders, so there's less build up between charger and cylinders. Less pressure is good in this case, since the more you compress the air it heats it up.

[bhvrdr]

Good conversation. ThankS for all the input fellas.

Mike

[withENEMYin]

So your saying i should remove my Cats?

[soltez]

Some good info thanks! I see a lot of info referring to turbos which is great but do the same principles apply to a blower?

I guess the ultimate way of knowing is to do some 3rd gear logs. It's thanksgiving this weekend so I may be busy with family gatherings but may be able to squeeze a few in.

My only hang up with this the peak levels I'm getting in the torque app. The boost tap is right off the neck of the blower so I figured the gauge is measuring the levels produced by the blower?

Thx

Hey man you ended up going catless eh? What exhaust are you running? My car is way to loud catless with my awe non res track. I'm debating converting the track to a touring if it bring the dbs down around 3500-4500RPM I know AWE says they are the same volume in the upper RPM band just not sure where.

[brainpan]

Hey man you ended up going catless eh? What exhaust are you running? My car is way to loud catless with my awe non res track. I'm debating converting the track to a touring if it bring the dbs down around 3500-4500RPM I know AWE says they are the same volume in the upper RPM band just not sure where.

Haha, oh man track without cats must be mental. I'm running borla and it's not bad at all. Cold idle is pretty loud but I'm sure it's nothing next to your track system. Not much drone either considering dsg and all.

[-leman-]

Turbo:
If using a wastegate frequency valve at the same duty cycle, boost increases with reduced backpressure. If using boost feedback (PID), less wastegate duty cycle required to hit same boost level.
If using an MBC or just direct pressure line to wastegate actuator (basically using the actuator as the MBC), then boost stays the same with less backpressure, but you get to it quicker/earlier.

TVS Supercharger:
Less backpressure = less boost, but likely higher airflow and more power. I observed this on a 3.0T with the armytrix exhaust. With the valves closed, boost was higher, but car was slower. With valves open, boost was less, but car was faster. (This all of course assumes your boost bypass valve is staying closed, we're talking mechanical system here).

No trying to be smart but MBC is actually a fixed duty wastegate valve (direct pressure line to wastegate actuator is 0 duty). So it's not clear how you get different results for these scenarios. In both cases you would have more energy driving the turbo (hotside) and more airflow, but whether it would result in more boost depends on the particular config (compressor wheel/housing and the engine).

Speaking of TVS supercharger on 3.0T I would concur under assumption of closed bypass valve at WOT, which would basically mean that the compressor is kept "maxxed out". On the other hand, as correctly methioned above, the 3.0T OE exhaust proves to be sufficiently effective to handle even tune+single pulley, so I wonder if armytrix with closed valves is actually more restrictive than OE exhaust.

[jprice]

No trying to be smart but MBC is actually a fixed duty wastegate valve (direct pressure line to wastegate actuator is 0 duty). So it's not clear how you get different results for these scenarios. In both cases you would have more energy driving the turbo (hotside) and more airflow, but whether it would result in more boost depends on the particular config (compressor wheel/housing and the engine).

That's a good point, I was specifically calling out a setup that has a frequency wastegate duty valve that's operative above 0% duty... Like an n75 on a 2.7T with fixed WGDC operating at say 65% duty cycle. You end up with more boost after removing exhaust restriction even though the DC is the same.

Speaking of TVS supercharger on 3.0T I would concur under assumption of closed bypass valve at WOT, which would basically mean that the compressor is kept "maxxed out". On the other hand, as correctly methioned above, the 3.0T OE exhaust proves to be sufficiently effective to handle even tune+single pulley, so I wonder if armytrix with closed valves is actually more restrictive than OE exhaust.

I believe you're right on.. I think the armytrix with valves closed is more restrictive than stock exhaust. With people pushing more air now with dual pulley setups now, there may be more gains in power to be had on the exhaust side vs just stage 1 or stage 2.

[LittleDozer]

Lot of bad info in here.

Turbo:
If using a wastegate frequency valve at the same duty cycle, boost increases with reduced backpressure. If using boost feedback (PID), less wastegate duty cycle required to hit same boost level.
If using an MBC or just direct pressure line to wastegate actuator (basically using the actuator as the MBC), then boost stays the same with less backpressure, but you get to it quicker/earlier.

TVS Supercharger:
Less backpressure = less boost, but likely higher airflow and more power. I observed this on a 3.0T with the armytrix exhaust. With the valves closed, boost was higher, but car was slower. With valves open, boost was less, but car was faster. (This all of course assumes your boost bypass valve is staying closed, we're talking mechanical system here)

Less backpressure on a turbo car allows exhaust to exit the turbine housing faster, which in turn spins the turbine wheel faster, which of course spins the compressor wheel faster, and that's what is pushing air into the motor. On the TVS supercharger, the speed of the charger is mechanically linked to the speed of the motor through the pulleys. Air is being forced into the motor at the same rate at a given RPM. If the exhaust is able to more fully and efficiently exit, there's more room for air coming in to the cylinders, so there's less build up between charger and cylinders. Less pressure is good in this case, since the more you compress the air it heats it up.

Finally someone comes in with the correct explanation for both lol

[jprice]

OP: Log RFP_AV and make sure you're not getting any boost bypass at WOT.

[doughboy17]

Meow!