what could cause studdering/chugging?

[gofastman]

I was helping my friend tune his car the other night with a 50/50 E85 mixture and we were able to turn the boost up into somewhat uncharted teritory, about 20 or so psi. It was pulling harder than I have ever felt before, but as the car moved to around 4500 RPM there was this strange "chugging" motion, so he let off. I'm thinking it was most likley the clutch (fairly new, stock) or spark blowout, there was only 1 count of raw knock and the IDC's were around 70% if memory serves.
any thoughts?

[Shane@DBPerformance]

What car, motor, turbo, rest of the setup? What are the airfuels? What plugs/gap? Could be way too rich or way too lean. Could be plugs or ignition problem. Could be a boost leak. Could be fuel cut. A car with that much E85 in it will run considerably leaner than on straight pump gas. A lot of people can run 20+psi on normal pump too.

[gofastman]

Originally Posted by ecoli

What car, motor, turbo, rest of the setup? What are the airfuels? What plugs/gap? Could be way too rich or way too lean. Could be plugs or ignition problem. Could be a boost leak. Could be fuel cut. A car with that much E85 in it will run considerably leaner than on straight pump gas. A lot of people can run 20+psi on normal pump too.

Oh yea haha, '91 TSi, e16g, dsmlink, stock wires, 1 step colder plugs, plx 300, aeromotive FPR, Walbro 255.
I know stoich. for ethanal is richer than gas (what, 9:1?) wont the wideband still read 14.7:1 when stoich.?
we were shooting for 11:1 ratio, I think it was slightly leaner
the car was just pressure tested for leaks and checked out.

[Shane@DBPerformance]

You are correct about how the widedand will read. Any idea on plug gap, it might just be that.

[gofastman]

I dont remember the gap, I think they are what came out of the box (NGK coppers) and within the books specs, is ther a better gap to run?
wouldnt a misfire show up as knock? There where no distictive poping sounds or anything, just a pulsing motion

[Thor06]

Are you sure its firing on all cylinders? One of mine was grounded out for a while, it was ok until 4500 and then it would start to run shitty.

[Shane@DBPerformance]

Usually with higher than stock boost, you run a tighter gap. Could be plugs wires also. Yea, often a misfire shows up as knock, but not always. Have you guys increased the timing at all with the E85 yet? Too low of timing can cause misfires also. Adding ethanol or methanol to your setup will cool your combustion chamber temps a lot and it might take more timing to just get it to fire cleanly, but you don't want to go too crazy.