Rotrex boosting 13+ psi on a 9 psi pulley... what gives?

I've been tinkering with a Kraftwerks kit on my FRS for too long. After sorting some belt alignment issues I started capturing logs with the ECUTEK software to review and send on to my tuner. From those logs I noticed that I'm getting .6 bar at 5800 rpm, .7 bar around 6500, .8 bar at 6900, and .9 bar at 7100. See datalog file below to review.

https://datazap.me/u/dh318xgmailcom/log-1581308900?log=0&data=30-56

First thing first. I now know the car is running lean at 7100 rpm up. That I plan to resolve with the tuner. I really just want to focus on the boost issue.

I have the proper diameter pulleys on this kit to provide the standard ~9 psi. Why am I getting well over the amount? Is there something I can check other than pulley size? What can I do to restrict the boost to no more than 10 psi? I am running 91 octane, and to run above the 10 psi level safely requires e85. As of right now I don't want to mess with e85.

So I'm pleading to those much more experienced than myself to help me diagnose or at least band aid the issue.

Also, am I reading the logs right under MAP Gauge Pressure or even just MAP Pressure? If you subtract Atomospheric Pressure from the MAP it reads basically the same as the MAP Gauge Pressure.

Do you not have an adjustable blow off valve.

I thought most pullies were to to deliver on superchargers. IE if you get a 9lb pully it will make it at a specific RPM and boost beyond that above that RPM. Does not happen very much in the small block chevy world where I have most of my experiance because they never rev high enough to overdrive the superchargers. 

My Rotrex kit on my car (Miata) is only supposed to be 12psi, but near 7000rpm I was seeing almost 15psi on my boost gauge. I'm not sure if that rating is at a certain RPM range and we're operating above it. Also, worth noting that a restriction in the system, like a poor flowing cylinder head or intake manifold or will cause pressure to read higher.

Because it wants you to make more power

wearymicrobe said:

Do you not have an adjustable blow off valve.

I thought most pullies were to to deliver on superchargers. IE if you get a 9lb pully it will make it at a specific RPM and boost beyond that above that RPM. Does not happen very much in the small block chevy world where I have most of my experiance because they never rev high enough to overdrive the superchargers. 

I bought the kit used, but it came with a nice Tial diverter valve. My rudimentary understanding was that this is mainly used to divert unused pressure when the throttle valve closes either to atmosphere or back into the intake system.

NickD said:

My Rotrex kit on my car (Miata) is only supposed to be 12psi, but near 7000rpm I was seeing almost 15psi on my boost gauge. I'm not sure if that rating is at a certain RPM range and we're operating above it. Also, worth noting that a restriction in the system, like a poor flowing cylinder head or intake manifold or will cause pressure to read higher.

I've read that some read higher in certain areas than others. At this point I may install a boost gauge, but depending on location the accuracy is still suspect.

captainawesome said:

wearymicrobe said:

Do you not have an adjustable blow off valve.

I thought most pullies were to to deliver on superchargers. IE if you get a 9lb pully it will make it at a specific RPM and boost beyond that above that RPM. Does not happen very much in the small block chevy world where I have most of my experiance because they never rev high enough to overdrive the superchargers. 

I bought the kit used, but it came with a nice Tial diverter valve. My rudimentary understanding was that this is mainly used to divert unused pressure when the throttle valve closes either to atmosphere or back into the intake system.

I think most of them are to keep pressure/vacuum from building up very quickly when you close the throttle very quickly. 

So personally as long as 15psi is safe and you don't think you have a restirtion at the head I would tune for what you have. I don;t know your injector or pump setup but but a couple extra psi is not the end of the work when it comes to tuning. If you know that your shortblock will fail at this PSI eventually then get a boost regulator and install it. 

General consensus for the twins is that 9 psi is safe on pump gas. Any higher than that needs more octane. As much as I want to switch to e85 right now, I just don't have the funds to go down that path right now. I've got larger injectors and an upgraded clutch ready to install for the power increase, but the cost for a flex fuel sensor and a new tune is not a priority.

captainawesome said:

General consensus for the twins is that 9 psi is safe on pump gas. Any higher than that needs more octane. As much as I want to switch to e85 right now, I just don't have the funds to go down that path right now. I've got larger injectors and an upgraded clutch ready to install for the power increase, but the cost for a flex fuel sensor and a new tune is not a priority.

Well cheap and easy way to fix this seems like a 6psi pully. 

wearymicrobe said:

captainawesome said:

General consensus for the twins is that 9 psi is safe on pump gas. Any higher than that needs more octane. As much as I want to switch to e85 right now, I just don't have the funds to go down that path right now. I've got larger injectors and an upgraded clutch ready to install for the power increase, but the cost for a flex fuel sensor and a new tune is not a priority.

Well cheap and easy way to fix this seems like a 6psi pully. 

If that were an option I would go that direction, but I'm running the smallest boost pulley set made for the system.

One thing that I just thought of that could be completely related, or not but it seems worth mentioning to just rule it out. The car was running a bit off the other day on my way in to work. Like boost was intermittant and power seemed down considerably. I noticed the PVC hose was collapsed at the time, but it's been a few sleep cycles since then. In fact, I started the car up to drive home from work, and it was back to normal, so I never looked into that at all. I did install a catch can on the PVC side along with a check valve when installing the supercharger. Maybe I fudged up something there? Also I've never noticed the common whooosh noise of the diverter valve. I'm grasping at straws here.

It is making boost by backpressure, from trying to cram mass flow into the engine. 

If there is a restriction in the exhaust, that will increase "boost" pressure.

Knurled. said:

It is making boost by backpressure, from trying to cram mass flow into the engine. 

 

If there is a restriction in the exhaust, that will increase "boost" pressure.

No cats in the header, no cat in the front pipe, stock resonator, and stock muffler. I believe the muffler is flowing like intended and hasn't been considered a bottleneck until the 350+ range. Can't hurt to check though.

Also the kit at the 9 psi rating should be running that amount of pressure on a fully stock exhaust.

Call/email the manufacturer. 

CAinCA said:

Call/email the manufacturer. 

 

They have been absolutely unhelpful when trying to get a pulley alignment issue sorted. I can't completely blame them as I bought the full kit used, then bought an upgraded wider pulley set for better track reliability through Amazon.

So I checked my bypass valve flow when I got home. I disconnected it from the air intake return, and it is allowing all air to escape back into the system at idle. It's not sealing, so maybe the wrong spring is installed? Color appears to be a pinkish red, so I've got to dig into this a bit more. Could this be spinning the intake impeller faster than it's supposed to be?

Edit: Tial Sport QRJ BOV pink spring is 1.5 psi and recommended for supercharger applications.

A supercharger mounted upstream of the throttle body MUST allow all air to escape.

Knurled. said:

A supercharger mounted upstream of the throttle body MUST allow all air to escape.

Okay. So that's not an issue by the sound of it. The vacuum source to the diverter valve shows 16.5 inches of vacuum. Don't know how that is supposed to correlate to what spring I should need. Tial has charts, but for the version of valve I have, it just says the pink one is needed for supercharger applications. Sounds like I've got that correct.

A Rotrex is more like a turbocharger that makes boost in direct relation to engine speed than it is like a "supercharger". The latter term is usually used to refer to a positive displacement supercharger. And let's not get pedantic about "turbosuperchargers", I'm referring to general usage of the term. Things you do for superchargers don't always apply to centrifugals.

It's a compressor, not a blower. The blowoff/diverter/bypass valve is there to dump excess pressure when you cut the throttle, just like in a turbo. It is closed under boost and opens when there's vacuum in the intake manifold. It's not usually used as a pop-off valve to control peak boost, although I've seen that done to increase boost down low and bleed it off up top.

Manifold Absolute Pressure is relative to absolute vacuum, so it should read roughly 1 bar when the engine is not running. MAP "gauge" is probably calculating what a boost gauge would show, which is pressure relative to ambient. These are very similar numbers with a fairly consistent 1 bar offset until you get to altitude.

If you want to limit the boost of a Rotrex, you either limit engine speed or you play with gearing. Is your Amazon pulley set the same size as the "stock" setup?

In reply to Keith Tanner :

So the bypass valve should be closed under low vacuum conditions and only open under "x" amount of vacuum? Am I understanding that correctly?

The pulley set I have matches diameter to the previous pulleys which are the 9 psi set. The only thing that has changed is belt width from 20mm to 30mm. I never ran the SC with the 20mm set, so have no numbers or previous baseline to compare to unfortunately.

I guess it depends what we mean by "low vacuum" :) Lower vacuum = higher pressure? 

The bypass valve will be open at idle and cruise and when you cut the throttle under boost (ie, shift). It's a simple thing, simply a piston with a spring behind it. When pressure in the intake tract is less or equal to the pressure in the intake manifold, the piston stays closed due to the spring. When pressure in the intake tract is higher than the pressure in the intake manifold (the throttle is mostly closed), that pressure will overcome the spring, force the piston open and dump air out. 

High boost at high rpm isn't as likely to cause detonation as high boost at torque peak is, so you have to be careful when comparing max centrifugal boost to max turbo boost. What you're most likely to do is run out of injector because high engine speed means a short window of opportunity for the injectors to open, and weird things can happen to DI engines when they get past 25% injector duty cycle. As long as you're not running lean or exhibiting driveability problems near redline, you should be okay. If not okay, the solution is simply lower the redline until you are.