Boost Plateauing after ~3.5k RPM
 

I recently installed a BPR hot side MP62 kit a couple months ago on my 1.8 NB with the original pulleys that came with the kit. This is the first time driving a supercharged car so I'm not sure if my expectations are accurate but I thought boost should build steadily through the RPM range unless the supercharger is at max (RPM) speed. What I'm experiencing is a rapid climb to about 5PSI at around 3.5k RPM then it just sits there, maybe gets to 6PSI at about 7K. Some crude screen shots below showing the relationship of boost and RPM along with a video to hopefully help make sense of what I'm experiencing. Apologies in advanced for crappy quality, can take a better video if needed.

https://cimg2.ibsrv.net/gimg/www.mia...4ddd487ff9.png
https://cimg3.ibsrv.net/gimg/www.mia...bfedcf12a1.png
https://cimg4.ibsrv.net/gimg/www.mia...6f4afbed18.png


Is this normal behavior? Boost gauge is connected to the nipple at the top/rear of the intake manifold, is that an accurate spot to be pulling pressure readings? My only other thought is that the SC belt is slipping at high RPMs but I'm not entirely sure how to check that. I don't hear any belt squealing or anything but it's possible. Is there a good way to check for that? I do have the auto belt tensioner, should I try a tighter belt maybe to get some more tension?

https://cimg2.ibsrv.net/gimg/www.mia...f2214dba33.jpg

TIA!

I can't read the boost levels in your pictures/video. What pulleys are you currently using?

Belt slip is normally accompanied by belt dust around the pulleys. You can check for that.

Normally, you will see a rise in pressure as the engine rpms increase, until you hit a boost plateau around 3-4k rpm.
You may see another small rise at around 6000rpm, which is caused by intake manifold restriction.

MP62 is a positive displacement SC, you won't see boost rise with rpm like you would on a centrifugal. What size pulleys are you running?

Hola
Yup that's how roots blowers work. I've had both an M45 and MP62's and they deliver most of the boost immediately and creep up from there. With my current pulley combo (130mm/62.5mm on an MP62) I get 9lbs immediately and then it goes up to about 13 before I shift at redline.

Holler when you get bored of that 6-8lbs. ;)

Boost jumps up to 4PSI at 3.5k RPM then slowly climbs to 5-6PSI by 7k RPM. So based on what you're saying I suppose this is normal behavior. I was expecting a consistent climb, but I'm certainly not upset about the instant boost as soon as the bypass valve closes.

Loved reading through your twin charged build thread btw (linked incase anyone wants to check it out)!

Thank you, I see why my assumption is wrong now. I just searched up some rudimentary boost vs RPM graphs for both types and the positive displacement graphs match what I'm experiencing.

100mm crank pulley overlay with a 65mm nose pulley on the supercharger.

Dude! Your Ass or Kitties thread is the reason I sold my M45 kit before installing it and buying a MP62! Thanks for sharing all that info so we could follow you along your build, it was very helpful. 13pissies sounds very enjoyable.

Ohh man. You known, in your thread, I think you mentioned one of the reasons to get the MP62 over the M45 is because you're going to end up spinning the M45 to the moon to get more out of it which in turn will create excessive heat/ware on the supercharger. When I read that, I thought to myself "ahh. I'll be fine with the M45. I won't ACTUALLY want or need more power". I'm so happy I listened to you.

That being said, holler. I have a few supporting mods I need to sort out first before adding anymore boost (I think); intercooler (leaning towards water to air with a reservoir in the trunk and heat exchanger up front), MS 3 Pro ECU (also open to other options, currently using a voodoo piggy back box which is working pretty well), bigger injectors, and upgraded fuel pump (maybe?). Also I'm still on stock suspension, actually... the whole car is pretty much stock besides the supercharger. So lots of general maintenance and go-fast work to do.

Our boost profiles are very similar. My car is a NA8 motor with a MP62 and 110/65 pulleys. I make about 6psi rising to 7.