Why Low Boost

[NomadTW]

a thought occured to me about how people have trouble running mroe than around 6 lbs of boost on the stock engine... the engine obviously should be able to handle a little more than that.
it hit me a couple minutes ago, that the IRIS system is probably a problem, since it causes positive pressure in the chamber to begin with, someone estimated appx 2psi positive pressure. is it possible that the reason people can't go higher than ~6 reliably is because 6 is really 8 or 9 and 8 is really 10-12psi?

anyone else have a take on this?

I always thought it was because the SVX is already running at 10 to 1 compression.

What I've heard is that the SVX needs a turbo capable of spitting out high volume, not high pressure.



Dave

[Mr. Pockets]

Please post modification-related posts in the 'Mod Mania' forum. Thanks.

[NomadTW]

people in 2.5 RS's can usually run around 8psi and they run like 11:1 stock comp is there any way to completely disable the iris system and test it on someone's car with some sort of FI? perhaps this is something easy street will address in they're obscene power output program

[UberRoo]

The IRIS butterfly is a tuned system. It's tuned for both flow rate, and air density. By putting a compressor on the engine, all those figures get knocked out a whack. You're quite right that the resonance valve may go a little too far and increase pressure. I think it's more likely that it would instead not function correctly and decrease pressure, or at least do nothing at all. It could do either - I don't know which.

The high compression is certainly a major factor. Yes, 10:1 is fairly high compression these days, but it's not that high. The advent of unleaded fuel has really killed performance and brought the death of high compression engines.

The high volume or high pressure problem is, I think, largely the same thing in this case. If you don't produce enough volume, you won't get any pressure. The EG33 is a little larger than most turbocharged engines, so turbos of the right size are not as common, or at least more expensive.

I'm having a little trouble with the "reliability" issue. This term keeps getting tossed around as though we have some way of quantifying it. The fact is that nobody seems to have any idea when reliability becomes an issue. As far as I know, the EG33 will happily take twenty pounds of boost without any problem. I really doubt it, but has anybody tried it? I've heard of NOS gone wrong, but people screw up NOS all the time, so that's nothing new or unique to the SVX. I'd wager that a proper boost setup would run without problems at 10psi on a stock motor - premium fuel, functioning knock sensors, ignition retard, integrated O2 sensors, adequate fuel supply, etc. Maybe not the ideal setup, but I would think that it'd be reliable.

...but back to the IRIS butterfly. Yes it might overdo the boost, but probably not.

[NomadTW]

well the nos point is a good one... most that run it seem to be able to run a 75hp shot with no problem, but nobody can seem to get that out of a FI system... not without further modification

[oab_au]

The Inerta Resonate Inlet System won't cause any problems in a blown system.

The system is two different systems. The Inerta section runs from about 2000 rpm, developes maximum pressure about 3500 rpm, and is turned off at 4000 rpm. This then allows the Resonate section to run the engine, using tuned length inlet tracts to peak, at maximum torque about 4800 rpms.

The Inerta system utilises the pressure developed by a column of fast flowing air, to continue forcing the inlet charge into the cylinder, even though the piston has stopped moving down the bore, and is starting to move back up again. The inlet valve won't close for another 54 degs, with the piston moving up on the compression stroke but the flow still forces the air in.

This is a normal action on any engine. The difference with the Subaru system is the higher pressure that the large tapering inlet pipes provide. The other unique feature of the Subaru system is the way they have harnessed this energy at the other end of the inlet cycle.

With the inlet divided into two seperate systems, by the closed butterfly valve, we have three cylinders feeding on each side. The inlet valve is open for 236 degs. The full engine cycle is 720 degs, dvided by 3 cylinders gives 240 degs between inlet cycles. To put it simply, as one inlet valve closes, the next valve opens, so the flow continues, uninterupted.

Subaru has set the inlet valve duration to 236 degs, so that there is 4 degs, where all the valves are closed, this causes the air to flow into a closed chamber, raising the pressure by about 4/5 psi. 4 degs later the next inlet valve opens to a positive pressure, that flows through the chamber blowing the last of the exhaust gas out the still open exhaust valve, then into the cylinder, even though the piston is stopped at the top of the stroke, to increase the induction duration of the engine.

This system can only operate on three cylinders, due to the valve duration. Thats why they don't use it on the four cylinder engines. To my knowlage no other engine uses this system to this extent. It is an integeral part of the SVX design, as they could not have spread the torque across the rev range that this car needs, to be able to pull top gear from 1500 rpm to 7000 rpm, without it. Many have tried, but to have an engine that will cruise at 140 mph at maximum torque, shows that this car was designed as a high speed touring car, not a stopwatch sprint car.

Harvey.

[b3lha]

Nice explanation Harvey.

[NomadTW]

so is the iris system mechanically activated by the rotation of the cam? i was always under the impression it was electrically activated

so basically no matter how fast the air flows the system is dependant on rpms as to how fast it will go

in that case could the iris system reduce the maximum rpms that a tuner could make the engine run to?

[oab_au]

Originally posted by b3lha
Nice explanation Harvey.

Thanks Phil, not an easy concept to get across. You do need a background of engine operation, to understand it, I think.

I don't really expect everyone to get it, but as their knowlage of the engine expands, I think it will make sense.

Harvey.

[cdigerlando]

I was under the impression that the iris opened at a certain RPM to effectively vary the intake length, which gives better performance down low with a short length, and up high when open with a longer length.

It is an electrically actuated mechanical system. Vacuum is stored in a small holding tank with a check valve and is opened to actuate the iris "throttle plate" when the ECU tells it. I thought this was done at higher RPMs.

With my turbo I do not notice any turbo spiking through the lower rev range, although there is a certain point when boom...I get a lot of boost on. I don't know if this is a poorly designed wastegate or the effects of the iris valve, or both.

[oab_au]

Originally posted by NomadTW
so is the iris system mechanically activated by the rotation of the cam? i was always under the impression it was electrically activated

so basically no matter how fast the air flows the system is dependant on rpms as to how fast it will go

in that case could the iris system reduce the maximum rpms that a tuner could make the engine run to?

Hi Nomad,
Yes the IRIS butterfly valve is operated by manifold vacuum, and activated by the ECU electrically.

I am not too sure what you are saying. The pressure developed by the Inertia system increases as the engine speed increases. Starting at 2500 rpm building pressure, peaking to maximum at 3500. As the rpm rises from there the pressure starts to fall, till at 4300 rpm, when it is turned off. This is because the inlet duration time, is becoming too short for the slow moving mass, to build enough pressure.

Yes, this would limit the maximum rpm that the engine could attain, if it was the only system that the engine had. This is when the butterfly valve opens to allow the Resonate section, of the IRIS.to run the engine, from 4300 rpm, on up to red line.

When the butterfly valve opens, the two separate inlet tracts are joined, to form a large chamber, that each of the 6 cylinders induct there air from, fed by two 60mm throttle bodies (no lack of air here). This is like having 6 tuned length ram tubes, breathing from the atmosphere.

Due to the high speed of the engine now, the piston produces a sudden depression as it starts down the inlet stroke. This causes a negative pressure wave, to travel up the inlet tract, till it spills into the large chamber, to be replaced by a positive pressure wave, that travels back down to the cylinder, to increase the induction pressure.

The resonate speed is set for the maximum torque rpm of 4800, where it produces its maximum pressure. This is set by the length of the inlet pipe, to allow for the wave to travel up and down, in the time it takes for the engine to rotate about 180 degs, at 4800 rpm. A shorter pipe would resonate at a higher rpm.

The length of the 6 inlet pipes are different, spreading the resonate peak over about 750 rpm. This reduces the peak torque, but gives a more useable torque range. Subaru do the same with the exhaust tuned length, using the different lengths to resonate over the same range as the inlet system.

Putting the two systems together has the Inertia system working from 2500 rpm, providing about 90% of the total torque at 3500 rpm, then the Resonate section continues up, to peak the torque at 4800 rpm. To my knowledge no other engine uses this system to this effect. It’s no wonder that this engine has a torque curve that would put a V8 to shame.

Harvey.

[oab_au]

Originally posted by cdigerlando
I was under the impression that the iris opened at a certain RPM to effectively vary the intake length, which gives better performance down low with a short length, and up high when open with a longer length.

It is an electrically actuated mechanical system. Vacuum is stored in a small holding tank with a check valve and is opened to actuate the iris "throttle plate" when the ECU tells it. I thought this was done at higher RPMs.

With my turbo I do not notice any turbo spiking through the lower rev range, although there is a certain point when boom...I get a lot of boost on. I don't know if this is a poorly designed wastegate or the effects of the iris valve, or both.

Hi Chuck, I am a bit slow in the responces, some times the hand goes on strike and won't type

Yes that is the way most engine inlet systems work. They usually have two tuned length inlet tracts, a long one to resonate at low speed and a short one to resonate at a higher speed. Most just have a long tract, with a valve that shorts half of it out to make the short one.

The disadvantage these have, compaired to our Inertia low speed system, is the pressure that they can generate. The low speed resonate systems don't generate anywhere near the pressure that our low speed system does.

The pressure that the resonate sound wave has, depends on the speed of the mechanical action, that creates it. If the piston is moving slowly and the valve is opening slowly, then the intensity of the pressure wave will be low. This type of system works better as the engine speed increases, and the intensity of the sound wave increases.

Our Inertia system uses the momentum of the air, to create the pressure. The results can be felt, by the large amount of torque that hits around 3000rpm. Drive any other 3lt with a resonate low speed inlet system, and you will hardly notice it is doing anything, except maybe stopping the engine from bogging down.


You are right in noticing that the IRIS valve uses a reserviour to hold vacuum, as when it is to operate the valve, there is no manifold vacuum. or in your case, a positive pressure of about 7psi.

I don't know for sure, but I think the IRIS valve is opened at idle to prevent inlet tract pulsing, that would cause noise and uneven filling. it would then close at about say 1500 rpm, to divide the two tracts, opening again at 4300 rpm.

No I can't see it affecting the turbo, it would help, especially in the valve overlap peroid, to clear and cool the combustion chamber. As long as there is not a positive pressure coming up from the exhaust system, to cause the exhaust gas to flow back into the cylinder on overlap and into the inlet manifold , to kill the fresh charge and push the boost pressure up.

All the best mate I know you are going to succeed.

Harvey.

[cdigerlando]

One thing I really wanted to try some day, was to enable the iris system, tune to that and dyno map the rpm range. Then disable the iris sytem, tune to that setup and dyno the rpm range. That way I could see when under boost, what (if any) new cutoff point could be identified. The Tec II has some outputs so that I could control this solenoid with the tec II if I needed to.

[oab_au]

Originally posted by cdigerlando
One thing I really wanted to try some day, was to enable the iris system, tune to that and dyno map the rpm range. Then disable the iris sytem, tune to that setup and dyno the rpm range. That way I could see when under boost, what (if any) new cutoff point could be identified. The Tec II has some outputs so that I could control this solenoid with the tec II if I needed to.

Chuck there is a very good diagram in the FSM of the volumetric efficiency that this system has with the IRIS valve open and closed. Unfortunatly I can't scan and post it, at the present. If some body could scan and post it here, it would explain the way the system operates. It is in book 2, section 7, page 8, fig 8.

It shows a graph of volumetric efficiency, or cylinder filling, that takes place through the rpm range with the valve open and closed. You can see the effect that would be if the Inertia system was used only, or if the Resonate system was used only.

It also shows the pressure curve that takes place in the inlet system, and at the inlet valve, through the 3 phases of the rpm range, 3500, when the Inertia pressure is at its maximum, 4300, when the valve opens to change the systems over, and 5300 when the Resonate system is developing its maximum pressure.

I think you would find it interesting.

Harvey.