[tazandjan]

General

Both the 550 and 575 cars were fitted by the factory with pneumatic bypass valves on the two inside tailpipes. Look at most Maranellos/SAs with OEM exhausts and you will see much more discoloration on the outside tailpipes than on the inside tailpipes as a result of the inside valves being closed during normal driving. The purpose of the valves, which prevent exhaust gasses from passing through the lower restriction inner tailpipes, is two-fold. First it provides a little more back-pressure and increases torque at low revs, and second, it helps Ferraris pass European noise regulations. On some Ferraris, like the F355, noise is the primary reason for the valves, but on the Maranello, the valves do actually provide a boost in low rpm torque. As if our torque-monsters needed it, but free torque is free torque, so sign me up.

There are different schemes for pneumatic valve actuation on different Ferraris. Some, like the F430, are fail open valves. On the Maranello/SA, however, the valves are fail closed (spring loaded to close with no vacuum, like when the engine is shut off). So if your car is strangled at high rpm, check and make sure the bypass valves are operating correctly. The 550 has a relatively crude opening schedule and opens at wide open throttle (WOT) at an undocumented rpm, probably somewhere around 3000-4000 rpm, like the contemporarty F355. The 550 WSM has a rather cryptic instruction to check and ensure the valves are open at 5000 rpm and WOT.

The 575s have a more sophisticated schedule, probably a result of the 575's more advanced 7.1.1 DME. Idling at ~ 750 rpm vs the 550's ~1000 rpm, the 575 valves are open at idle for a nice stoplight burble, close at 1000 rpm, and reopen at 2720 rpm and 64% throttle. Opening varies with rpm and throttle setting for the 575s, so lower throttle settings will result in the valves opening at higher rpm and WOT will result in them opening at 2720 rpm (as if anything was that precise).

Bypass Valve Actuation

So how do these valves actually work? A vacuum accumulator near the engine provides vacuum for both the brake booster and the pneumatic bypass valves. Look at the parts drawing below and you will see a long vacuum line that goes from the vacuum accumulator to the bypass solenoid. The bypass solenoid receives a signal from the Motronic DME and opens the solenoid valve to allow vacuum to pass through a T connector to the two individual pneumatic bypass valves, opening them. Hysteresis built into the solenoid valve has them close at a lower rpm than the opening RPM, preventing the valves from fluttering at opening rpm/throttle combinations.

Defeating the Bypass Valves

For the 575s, the bypass valve schedule is the most generous of any Ferrari, and mine are still fully functional on my HGTC exhaust. I like the OEM bypass schedule schedule. For the 550, though, a method to keep the valves open all the time may be more palatable. So here is how you do it. If you disconnect the long hose from the vacuum accumulator to the solenoid, and connect it directly to the T fitting going to the two pneumatic tailpipe valves, you will have full time vacuum to the bypass valves and they will be open whenever the engine is running. Since you have now bypassed the solenoid valve, cap the intake and outlet openings, or connect a loop of vacuum hose between the solenoid inlet and outlet to prevent crud from being ingested into the solenoid valve in case a later owner wants to go back to OEM operation.

Rich Man's Solution

If money is no problem, and ~$600 is pretty mild by Ferari standards, you can choose to have your 550/575's valves open whenever you want. Capristo has a system that allows you to select when you want your bypass valves to open and close. Their solenoid connects directly into the original bypass solenoid connector, the OEM solenoid then connects to the Capristo solenoid connector, and a remote allows you to select whether the valves are open or closed. Perfect solution, right? Well, for those of you who are Type A personalities (I gave up caffein and am now only a B+), Capristo also has a system that allows you to choose your own opening and closing schedule for a bit over $1K.

Questions?

I doubt there will be any, since this concept is so simple even I can understand it, but let me know and we will go through the details.

Taz
Terry Phillips

[priorj]

curious as to other peoples experience, but my 575 oem valves are not open at idle (warm or cold). i recently had straight pipes installed by well respected shop and i asked them to check out my valves cause they were closed at idle and they claimed they never saw 575 valves open at idle.

[KonaDog06]

2002, closed at idle.

Bill
FL

[tazandjan]

Very interesting. Will double check mine, but pretty sure they are open. My car sounds like a trawler at idle. There is only one solenoid, so that cannot be the difference, but it could be in the Motronic DME. The section on the bypass valves was a tech bulletin, not in the original WSM. The Motronic DMEs were replaced at Assembly No 49203, 53134, 52556, and 55769.

Taz
Terry Phillips

[tazandjan]

Looks like I was just wrong and misinterpretted the tech data, not the first time that has happened. Mine are closed at idle, too, and I guess the HGTC exhaust was loud enough on two tailpipes to fool me. No idea why they did not say closed Idle-2720 rpm instead of 1000-2720 rpm, when the car idles at 750-800 rpm. Who knows? Will make sure when I write an article for the permanent collection, that it has got it correct.

Taz
Terry Phillips