Bypass Valve Opening Late?

Isn't the pump in the back for just for secondary air? On my 5.2, vacuum comes from the intake section on both sides, through two check valves, and directly to the accumulator. The dashpot control to open the secondary air path gets its vacuum from the accumulator.

 

Yes. I meant tank not pump. My bad. That's what I get for doing this without my notes handy.

Solenoid valve (8) closest to the center of the car manages the bypass valve.
The other solenoid valve operates the air injection valve which lets the air from the air pump through to the header rail.

Hose 35 hooks to a small metal tube that is tack welded to the air injection tube and pops back up by the bypass valve. You can where it pops up as it is right under the #37.


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So much wrong here.
Vacuum comes from manifold, not emissions pump.

The thermocoupler is only there to sense heat, it does not look for cold. I believe the workshop manual states this as well.
Have you tried this? I have and have spoke to others that have.
My cell light is working and I do not get a 1448.

Many 355s, specifically the challenge cars never had a bypass from the factory.
The notions that reducing the exhaust pressure by allowing a 2nd stream of exhaust somehow damages the headers is absurd.
Remember, the bypass is open most of the time when the engine is really working. It's only closed at low RPM when there's really not a lot of heat anyway.

Remember, the purpose of the bypass valve is to make the car quiet sometimes and loud other times.

 

Lack of heat...better?

I have tried exactly this and chased the hell out if it three years ago.
In fact I had the damn vacuum line teed and a gauge plumbed into the cabin so I could check the initiation of the vacuum for the valve at different RPM / Gear / Throttle position.
If you leave the vacuum off the valve and the ECU tells it to open and does not, it throws a CEL. Been there done that. Lack of heat...
If you leave the valve open and drive around that way it throws a CEL because it is sensing heat when the ECU says it should not..... Mine does anyway.

Challenge cars are a different matter.

 

I believe the vacuum/header theory was about the stock muffler which has a shared exhaust path and was supposed to occur even with a functioning bypass valve. When the valve closes from lifting off the accelerator it creates a vacuum which sucks hot gas back into the headers. Can’t speak time the validity of it but have heard a few times. Aftermarket exhausts supposedly solve this with separate channels.

 

I assume that leaving the vacuum line off the capsule won't degrade the vacuum for the Secondary Air Solenoid Valve for long enough to cause the secondary air system to throw a CEL.

Maybe someone can come up with a theory on how all the experts can be right

 

Thanks, understood. The text does say "though only at full power", so I assumed that the throttle was involved.

 

"I'm not an expert, but I play one on Ferrari Chat."

 

based on this description, this would be an advantageous case to remove the bypass completely in order to protect the headers.

There can be some scavenging in good header designs. However there is a y pipe at the end of the headers, nothing should go backwards at this point regardless on any theory of a bypass valve.

If anything, with a working bypass valve, the quick closing would cause pressure, (and thus heat). The only place there be a vacuum is on the muffler side of the valve,

 

Ahh, 3rd gear it was. Ignore my post #2. It is not 2nd gear that opens later.

 

This. Mine has always been a bit sluggish to open. I replaced the solenoid valve last night, and that baby opens up WICKED FAST now.

 

Just an FYI to an old post of mine from several years ago (not sure where to find it, but here is the general info from memory): My 1997 F355 spider I owned at the time, had QV London rebuilt oem exhaust manifolds, factory catalysts, Capristo bypass valve and Capristo exhaust system. My local dealer experimented with wiring a bypass switch in my car to see how that might work. They were able to figure out the wiring, and even spoof the Motronic not to throw the P1448 code. The concerning thing they saw after driving under various conditions, was the fuel trims were getting out of wack, as so much exhaust was going through the bypass, and not enough over the pre catalyst O2 sensors. They figured the solution would be to move the pre catalyst O2 sensors to the merge collectors on the exhaust manifolds. I wasn't interested in modifying the car like that, so we ended the experiment. Interesting though.

Anyone running with the valve open all the time, should probably check their short and long term fuel trims.

 

From the Capristo website

“Under certain driving conditions with the original bypass valve in the open position, a reverse flow through the silencer causes overheating and damage to the headers on Ferrari 355 vehicles. Under these conditions, once the exhaust gas valve is open, the exhaust gases do not all flow through the main catalytic converter but mainly through the bypass. The exhaust gases flow off the exit which leads to the main catalytic converter and generate a vacuum. Thus the hot exhaust gases in the muffler are aspirated backwards through the main catalyst which additionally heats up the gases.​

Then the gases are guided though the bypass and recirculated by the exhaust gas valve into the muffler again. When the exhaust gas valve is open, this gas flow is circulating ceaselessly generating more and more heat. We are of the opinion that this problem is known to Ferrari. For this reason the F355 was changed since 1996 from engine 2.7 to 5.2 and the headers were modified exactly in the place where the vacuum is generated. But the problem was not eliminated by this measure.

Now you can imagine that extreme back pressure is created in a case where all of the exhaust gases coming from the engine and those which are aspirated backwards have to flow through 1 exhaust gas valve. That’s the reason why the F355 does not provide the power as indicated.After some testing it was obvious for us that our theory was right and that the bypass causes this problem. One exhaust gas valve is not sufficient and thus leading to extensive heat due to the back pressure.Our exhaust systems, irrespective of the racing or the usual model, are so designed that the exhaust tubes from the main catalytic converter lead to the outer end pipes and those from the bypass to the inner end pipes. These tubes do not lead to each other. Consequently it is not possible, when the vacuum in the main cat is generated, that hot exhaust gases are recirculated.​

Instead cold air through the outer end pipe is drawn backwards through the catalytic converter and the bypass and the valve. Once cold air is mixed up with hot exhaust gases, the temperature of the exhaust gases decreases and also the temperature of the complete system.The original single bypass valve also did not flow enough gasses at full throttle. After recognizing this problem, CAPRISTO exhausts set out to eliminate both problems with a new design.”​

Anyone have a sense if this is real? Sounds like some pretty heavy marketing.

 

Antonio Capristo seems to have well engineered products, so it may very well be true, but using their exhaust will not prevent failure of the oem manifolds. (While I had QV London remanufactured manifolds, they still failed using the Capristo. I discovered my manifolds were repaired replacing the pipes with thicker 304 SS, but only up to the first bend. So they subsequently failed just past. I sent them back and they properly repaired/replaced the pipes all the way to the collector).

Whatever cooling improvements, it is not having a significant enough impact on that. More power? Never did before and after dynos, but I doubt it makes much more than any other muffler. Capristo DOES make a muffler system that makes significant power (I forget the specs, 22hp?) and it eliminates the bypass Y pipe and installs the bypass valves (one on each side) similar to the 360/430. Only problem, is the sound is not the high pitch scream of the other Capristo 355 exhausts that utilize the Y pipe bypass.

 

That muffler is called the twin sound and the associated claims on the Capristo website relate only to that exhaust. Not the F1 scream sounding one that is what most get. Not sure why they include that description on the muffler when it doesn’t even apply.

 

Going back to this. Does “though only at full power” imply WOT? As in the valve doesn’t actually open unless you’re at WOT regardless of RPM or gear?

 

For 2.7…


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This is also interesting.

2.7’s are probably operating differently as challenge cars do not run bypass, therefore there should be no fuel trim issues running open bypass all the time.

 

Perhaps full power has some margin of error. I've been trying to make sense of this chart for the TPS...

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Perhaps the grey area defined by 3 indicates WOT?

 

Maybe. Only way to know is the watch/read the short and long fuel trims.

 

Rather difficult to understand by reading only the 355 manual. Also, the terms used in it seem to be related to the "parameters shown by the Diagnosis System" (the Ferrari diagnostic computer, I believe).

Since many parts of the 355 manual evolved from the 348 manual in which the 2.7 related descriptions were built following (on top of) the 2.5 descriptions, reading the manual for the 2.5 348, the TPS of which is not a pot but has two micro switches, makes it is easier to understand what the Ferrari terms "fully open" (i.e. WOT) and "full power" mean. The first TPS switch marks the "idle position" and the second switch marks the start of the "fully open" position.

In the description of operation of the TPS on 2.5 348, the "fully open" throttle position is described to be from when the second TPS microswitch closes which takes place when "the throttle opening exceeds 72 deg." Then, in another paragraph, the 2.5 348 manual states that the "Full power enrichment" takes place from when the "fully open contact of the microswitch is closed" (meaning the second TPS microswitch closed at 72 deg.). So, based on the quoted descriptions, it appears that "Full power" zone of the throttle valve is from 72 deg. and up.

Based on the above, it appears that the areas on the TPS graph shown in the 355 manual basically mean: shaded area at the bottom and to the right - "idling"; white area - "intermediate"; shaded area at the left and up - "fully open" (from 72 deg. up). The 355 graph with a protractor superimposed:
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I was thinking the same thing, but wasn't sure of the relationship between the throttle butterfly and the TPS, specifically if it was calibrated right on the limits.

Does the protractor represent the butterfly? If so, and you could only get the TPS to "zero" at 2? or is the calibration limit at 1? Would the butterfly be fully open at only 72 degrees on the TPS? In which case, you would need max pedal input to open up the bypass valve.

 

It is not clearly explained in the manuals but I can only interpret the angles as follows:

1. The "zero" deg. (the horizontal graph line) is only a theoretical butterfly position; the 10 deg. line means that the butterfly is actually at 10 deg. when fully closed (sitting against its stop); 72 deg. line represents the butterfly opened at 72 deg. and the graph line near the 90 deg. graph vertical line means the butterfly limit of travel (appears to be at ~89 deg.).

2. With regard to the TPS - its voltage at 10 deg. (butterfly at stop) and up to 17 deg. is understood by the ECU to mean still "idling"; TPS voltages from 17 deg. to 72 deg. are understood by the ECU to mean "intermediate" (butterfly positions); TPS voltages from the line at 72 deg. and up are understood by the ECU to mean "fully open" (butterfly positions); this zone appears to also be called "full power".

The line at 30 deg. is just an example of an intermediate butterfly position.

If the above interpretation is correct, then it may mean that the 355 bypass valve will open at the RPMs as shown in the manual, provided that the throttle position as, at the time, 72 deg. or higher.

With regard to whether the bypass valve could just be kept open all the time, I believe that this would cause the engine to produce somewhat less power at lower end since the bypass valve alters to exhaust back-pressure. More back pressure is beneficial (more power) at lower/medium RPMs and moderate loads whereas less back-pressure enables more power at higher loads and towards the high end. More back pressure (bypass closed) should also contribute to lower fuel consumption.

 

I still don't understand this. "1" is the "offset variability field" which I understood to be the random idle TPS starting point with an 18% (or 20%?) ohmage variation

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Wouldn't that produce an 18% (20%) variation at the top end? Is 1 the same angle as 3?

Makes sense. I think I was confused by the "4" sitting on the top of the 1 line. Redrawn...

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EDIT: Perhaps 18 or 20% ohmage variation is not as much as I think it is. If the angle is 10 degrees, 20% variation is only 2 degrees (if there is a fairly linear relationship between ohms and degrees).

 

The graph represents geometric positions of the butterfly and it does not relate to any of the POT resistance tolerance percentages. As stated in the manual (page C16), there are three zones of the butterfly angular positions, recognised by the ECU based on the voltages received from the Pin 3 of the POT:

- idling zone
- intermediate zone
- fully open zone

It appears that the ECU, for "Normal operation", does not work with the continuously varying voltages received from the POT but only "checks" whether the voltage coming from the Pin 3 is any voltage from the 10-17 deg. zone (use the "idling" table), or any voltage from the 17-72 deg. zone (use the "intermediate" table), or any voltage from the 72-UP deg. zone (use the "fully open" table + enrichment + open bypass at rpm etc.). Basically, in "Normal operation", the ECU does not want to know the exact voltage value at each butterfly position but only whether the voltage received at any given time is from the "idling" or from "intermediate" or from "fully open" zone of butterfly angular position.

It is only in the "Emergency operation" (failed MAF) that the ECU will use the actual varying voltage received from the Pin 3 of the TPS in lieu of the absent varying voltage from the MAF, i.e. the ECU will make a "crude" calculation of the amount of fuel to be injected.

The graph description in the manual is not totally clear but I understand that it means the following:

- the bottom shaded area is "idling" - any Pin 3 voltage representing the butterfly positions between 10 and 17 deg.
- intermediate clear area is "intermediate" - any Pin 3 voltage representing butterfly positions between 17 and 72 deg.
- the left shaded area is "fully open" - any Pin 3 voltage representing butterfly positions between 72 and MAX (~89 deg.)

Take note that the bottom shaded area below 10 deg. does not actually exist as the butterfly, by the TB's construction/geometry, cannot travel to that zone.