2005 E55 AMG. Here are my current mods (only the ones that actually matter in this context): 77mm upper pulley, 168mm crank pulley, 82mm TB, PLM short headers, ported heads with oversize valves, SLR cams.

Short version:

If 168mm+77m = over 17 psi and 168mm+stock(90mm?) = 14.0 psi in second gear (probably slightly higher in higher gears). What size (custom if necessary) supercharger pulley will give me 16 psi (staying safely below 17 psi)?

Long (detailed) version:

Here is my problem: I'm making too much boost. For anyone that doesn't know, the factory ECU ("ME") cannot deal with boost that is much over 17psi: the engineers knew that the car would never make that much so if that much manifold pressure is seen, the ME assumes there must be a problem with the MAP sensor. If 17 psi is exceeded, it results in the ME ignoring the MAP sensor until the key is cycled and basing injection mainly on throttle position and O2 feedback, which results in crazy lean A/F ratios under boost (22+).

I know that most of the conventional wisdom and pulley size boost charts say I should be fine with the 77, 168 combo (especially with headers and ported heads, which should lower the boost), but I'm not. Unless anyone can think of some other reason, I believe this is caused by the SLR cams: they have less valve overlap than the stock AMG cams which (at least in theory) should raise boost because you waste less boost blowing out the exhaust during overlap. I'm surprised it raises it this much, but I can't think of anything else that would account for higher than expected boost (my exhaust is not plugged and I don't have any tapping noises so I assume my lifters are working correctly and my valves are fully opening).

With the 77, 168 combo, I was crossing the 17psi threshold easily, even in first gear, which is a very big problem (as soon as it happens, AFR goes crazy lean and stays that way). I swapped my stock (90mm?) supercharger pulley back on, which has fixed the over-boost issue and results in a peak boost of 14.0psi in second gear (at around 45 degrees F). Boost in higher gears will probably be a bit higher, but I don't have many opportunities to easily try that out where I live. This is also with no belt wrap kit (this is only temporary and I didn't have the right size belt for it), but I don't see much indication of a slipping belt--boost is pretty constant).

I do not want to mess with my crank pulley: it's pinned to the crank (still running true) after an unfortunate incident with a 175mm crank pulley (yes, crank pulleys can come off even if properly installed--the boost was probably pretty high, though, given that it's still over 17psi after stepping down to a 168mm). I am going to have a custom size supercharger pulley made (unless the readily available 83mm turns out to be the right size). I'm aiming for about 16 psi peak boost (maybe slightly higher, but I want to leave enough head-room to make sure I don't exceed 17psi). What size supercharger pulley do you recommend to get there?

Oh, and before people start warning me: I have good cooling (dual heat exchangers and water/meth), bigger injectors, and a looped rail.

Thanks in advance!

 

Why not just install a MAP clamp? Especially if you are just right on the edge of 17psi. Clamp it at 4.6V and call it a day.

https://mbworld.org/forums/w211-amg/615125-ecumaster-piggyback-upp-running.html

 

Thanks for the suggestion!

I have considered that, but based on how easily it exceeds the limit (every time I put my foot down for more than a second, even in first gear), I don't think I'm right on the edge: I'm worried that I may exceed 4.5V by quite a bit in higher gears, which I suspect could cause problems if actual conditions are different from those when dyno tuning. If I was only hitting the limit occasionally in 2nd gear or higher, I would definitely use a MAP clamp.

 

Maybe ask your tuner what he thinks, and look into some of the other thready with guys doing it. I think there are quite a few high boost guys with MAP clamps around here.

 

I did already look into it and I haven't found much so far, but I'll keep looking. The thread you posted before is interesting: I only looked into a MAP clamp that would pass through exact values and just keep the values from going higher than a pre-set value (Split Second VC3-100) but I suppose with a piggy back, I could start cutting back the value as it approached the limit so that it kept going up slightly but never quite reached the limit (this should be almost as good as a wider range MAP sensor, but without requiring changing all the ECU maps to accommodate it). The only piggyback I have experience with is the old SAFCII which only let you set correction factors based on RPM and throttle position, but for this to work, I would need to base the correction on the actual MAP value... I'll have to see if that's possible.

 

 

If it was my set up, I would look into a set long tubes to maximize flow and drop boost. I lost cylinder 8 on my old motor with a 77/168 set up with 550cc injectors and looped rail. It was due to dropping fuel pressure and a partial melted fuel hat. I would upgrade fuel pumps to maintain good fuel pressure with that much boost for insurance. Also keep an eye on that fuel hat for any burn marks.

 

Switch to longtubes and you shouldnt have an overboosting issue. send me a PM, we have XSP for $975.

I would also do the pumps as Das it16 mentioned.