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JRSC loses boost toward redline

 
Old 10-15-2018, 07:51 PM
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Default JRSC loses boost toward redline

Is it normal for a JRSC/m45 supercharger to lose boost as the engine approaches redline? This is on a 92 1.6L with stock pulleys and no intercooler. As you can see in the log below, the boost drops from 7.8 psi at ~4300 rpm to 5.7 psi at ~5800 rpm. I have checked for boost leaks and belt slip and cant find anything obvious. Any ideas? Is it just normal to get a boost spike when you floor it on a positive displacement blower?
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Old 10-15-2018, 08:44 PM
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PD SCs interact inversely with engine VE, the "boost" is a function of both the engine efficiency and the SC efficiency. It is not unusual for the pressure ratio to drop with increasing RPM.

confirm that your bypass and bypass arm are in good working order, and that you have a good check valve in the idle line.
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Old 10-15-2018, 08:45 PM
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The M45 is a TINY charger but it's got enough "guts" to make 10PSI at 7 grand.

What you are seeing is...IMHO.....without question, belt slip
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Old 10-16-2018, 10:51 AM
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I too would side with belt slip, and/or bypass leak. But a leak would present itself everywhere, not just up top.
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Old 10-16-2018, 11:30 AM
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That is exactly what my datalogs looked like right before I shredded a belt due to belt slip.
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Old 10-16-2018, 11:39 AM
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Ok, consensus seems to be belt slip. The belt is a fairly new gates with no signs of wear or cracking. Guess I'll try tightening it some more tonight (although it seems pretty damn tight). Has anyone tried belt dressing or similar to help with slip?
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Old 10-16-2018, 11:51 AM
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yes. useless. helps for all of 5 min
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Old 10-16-2018, 01:10 PM
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The belts on these things slip until just before they are tight enough to snap the balancer off.

with that much boost you donít have stock pulleys... you probably have a smaller nose pulley.. which causes more slip due to less belt wrap, and causing more parasitic loss from the supercharger.

fyi - with stock pulleys theses things barely push 6psi
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Old 10-16-2018, 01:39 PM
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That's what I was thinking too stealth, nearly 8 psi seems high. I have the rounded nose sebring charger, which does not accept smaller pulleys. I suppose it could be a pulley from the 1.8 model. I guess I'll have to measure it.
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Old 10-16-2018, 02:13 PM
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Stupid question but, could one use like 400 grit sandpaper on the pulley to roughen it up, so the belt gets more grip, without damaging the belt? (supplied it's not slipping on the rougher surface).
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Old 10-16-2018, 06:58 PM
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Well, I took the belt off, cleaned the pulleys with carb cleaner and put it back on much tighter than before. No change, still peaks over 7 psi at 3-4k, then drops toward 5 psi at redline. Sure doesn't seem like it could be belt slippage. I also checked the pulley size and its the stock 72.5mm. At this point I'm beginning to think its just the change in volumetric efficiency of the engine and blower across the rev range. Feel free to say I'm wrong, I would love a couple extra psi at redline.
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Old 10-16-2018, 08:14 PM
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What do the ribs on your pulleys look like? Are they aluminum or steel?

Aluminum Supercharger Pulleys are WORTHLESS.

Once they wear much at all they do not function properly and will slip like mad.
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Old 10-16-2018, 08:16 PM
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Steel pulley. Can anyone confirm that their m45 does not do this on a 1.6??
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Old 10-16-2018, 09:43 PM
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I can confirm that a healthy m45 on a healthy 1.6 will not achieve more than 5 psi up top with stock pulleys at 5280ft. An all-of-it m45 setup on that same platform an ambient pressure barely hits 10 psi anywhere.
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Old 10-17-2018, 09:36 AM
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My M45 made a fairly consistent amount of boost from 3k to 6k, the the pressure went up past that a bit (loss of VE in the 1.6 heads due to bad flow at high rpm).

I am susupecting you have a cam off a tooth, causing poor VE in the midrange. With psoitive displacement superchargers, they move X amount of air per X revolutions (plus minus a bit for efficencies and other things). So how much pressure ends up in the IM is based entierly on how much air the engine takes in per rev. Lower VE (less air per rev) = lower power = higher boost pressure (for the same engine and supercharger RPMs).

Mine made just under 5psi with stock pullies and 5.5 up at redline. With a 150mm crank pulley it made 8.8 and 9.5ish at redline. With 150mm crank and 62.5 nose, it did 10.5 and just over 11 at redline.
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Old 10-18-2018, 03:12 AM
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https://lightweightmiata.com/jrsc/sc/

This site might help you with some visual comparisons. I'd tend to agree with x_25's theory. In my experience:
1. Belt slip didn't just drop to a slightly lower psi. Instead, once the belt slips, it completely lost it and past 5.5k rpm would just tumble all the way back down to 0psi if I continued to floor it.

2. If you truly are on stock pulleys you won't see the 7psi you're seeing midrange, more like 6 if you're lucky. Check the pics above to confirm if you have a 150mm crank pulley overlay installed. That would normally bring you to approx 8psi at sea level (with stock sc pulley).

3. I generally noticed VE increased psi at the upper rpm range, not lowered. Same thing happened on my MP62.
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Old 10-18-2018, 08:54 AM
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I'm certain both pulleys are stock. I just installed a new oem crank pulley because my damper was going out, and my sc nose pulley measures 72.5mm (stock for 1.6 kits). From what I've read 7 psi is not too abnormal for stock pulleys on a 1.6. I actually was hitting 8 psi this morning given the cold weather we have here.
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Old 10-18-2018, 03:43 PM
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have you done a timing belt?

edit: ahh "just installed new oem crank pulley", so the timing belt came off yes?
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Old 10-18-2018, 03:47 PM
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yes, timing belt changed about 20k ago, still appears in good shape as I just checked it recently
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Old 10-18-2018, 11:37 PM
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"In good shape" meaning the TB belt was inspected for proper cam positioning as well? it's been a while, but I think it's supposed to be 19 "teeth" between the I/E cam gears.

There was a post I read on another board where the belt had jumped a tooth on the exhaust cam (18). Once he fixed it, he "lost" psi, but overall power was restored. He looked into it because he said the car the power was dying off around 5k on a dyno session.

I always thought being off by a tooth would make the car run horribly, but perhaps it depends which cam was off.
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