I have boost fluctuation on a roots supercharged application. This is on a 250cc single cylinder. At first I had no plenum, so I tried building a large 2.25l plenum so see if that would fix the issue. It changed the boost profile but it's still not consistent.
Here is a graph of the two intake pipes, all data is from full throttle and multiple runs. It's quite repeatable but I can't figure out why such large fluctuations (3-5.5 PSI).
I have no idea what's wrong at this point. Need help.
Looks pretty clear that you have intake harmonics happening and they change as the shape and volume of the inlet tract change. What is the displacement of the roots blower, what gear ratio is it running vs. motor rpm?
Graduate of EFI University.
I build, repair, install and tune Megasquirt systems in North Dakota and beyond!
elutionsdesign wrote:Looks pretty clear that you have intake harmonics happening and they change as the shape and volume of the inlet tract change. What is the displacement of the roots blower, what gear ratio is it running vs. motor rpm?
300cc/rev, 26 tooth on the engine, 40 on the blower (roughly 1.5 underdrive).
So the math would be (26/40*300)*2 or 390 cc per engine cycle (4-stroke)? I would think that you'd want a bit more displacement from the blower to keep up, maybe try twice the engine displacement (500 cc per engine cycle).
Graduate of EFI University.
I build, repair, install and tune Megasquirt systems in North Dakota and beyond!
elutionsdesign wrote:So the math would be (26/40*300)*2 or 390 cc per engine cycle (4-stroke)? I would think that you'd want a bit more displacement from the blower to keep up, maybe try twice the engine displacement (500 cc per engine cycle).
I plan on upping the boost eventually, but would like to get everything as sorted as I can at low boost before stressing the engine more. I don't follow "keeping up with the engine", there is positive pressure the entire time so by definition it's keeping up.
Yeah, I see what you mean, I guess my brain is used to turbos. What was the volume of the u-pipe you had on at first? What about reducing the volume of the inlet to a small multiple of engine size?
Graduate of EFI University.
I build, repair, install and tune Megasquirt systems in North Dakota and beyond!
Boost in a roots application will follow the inverse of the engines VE curve, although the percentage swing you are seeing is just a fuzz more than I would expect.
Does PW remain more constant than Boost? Remember, the blower is effectively "grabbing and stuffing" a set volume of air per revolution. If the engine is less effective at inducing volume flow at that speed, boost goes up. If the engine is more effective at inducing volume flow, boost goes down.
wes kiser wrote:Boost in a roots application will follow the inverse of the engines VE curve, although the percentage swing you are seeing is just a fuzz more than I would expect.
Does PW remain more constant than Boost? Remember, the blower is effectively "grabbing and stuffing" a set volume of air per revolution. If the engine is less effective at inducing volume flow at that speed, boost goes up. If the engine is more effective at inducing volume flow, boost goes down.
Yes, PW is more consistent. Here is a screenshot of 5500-10000rpm, WOT, 135kpa(5800 rpm) to 115kpa(9500 rpm) but the PW doesn't drop correspondingly (7.5, 7.8ms respectively). It also feels smooth. It's pulling the hardest around 7-10k RPM but it has the highest psi at 6k rpm. I'm beginning to thing the MAP reading isn't that important for this particular setup. Granted it will mess with the timing curve a little, but the fueling is all done with VEAL.
I finally got the 1:1 ratio pulley in (26 teeth on both engine and supercharger). While it muted the peaks a little, it still follows the same general profile of the other pulley. Also interesting, in the black scatter plot, you can see 3 different groupings. This is from upping the boost for 3 different runs. The MAP profiles follow the same curve. I was also changing the phasing of the supercharger to the engine and noticed no variation in MAP values.
I'm not sure where I'll go from here. I'm thinking I will just up the peak boost to the 6-8 PSI range and call it good. I am only seeing fluctuation on the sensor but can't feel it on the bike and the PW stays consistent. Basically it's an anomaly, but not one that seems to make a difference in how the bike rides.
