overboost w/ a supercharger??

[Gibson99]

I'm not really sure how, but i am apparently getting overboost on my 98 with a trd blower (and smaller pulley)! seems most of the time if i keep it below about 4000rpm or so, even when floored i'm only seeing 0.5bar, which is exactly what it should be. however, if i run it up towards redline, i get what feels like misfiring and a big lack of power, and my greddy peak-memory gauge shows me 0.7bar! this should not be possible for two reasons: a) there's no wastegate, and b) the compressor is belt driven. that said, it's easily repeatable. any theories on how this could be happening?

my check engine light IS on, but the only DTC i have is for EGR insufficient. that's because i don't have an egr system right now. I do plan to add it back on, but i don't think it will have any effect on this overboost condition.

[Luckynumber5]

If you have something restricting airflow out of the engine the supercharger will boost stack resulting in way more boost than is normally possible.

An example is an 05 Grand Prix GTP where the guy ran way to small a pulley with too few mods, he melted his catalytic converter and it clogged his exhaust. The damn thing would hit 20 psi on a roots blower (thats insane!)

[ASG14]

I have NO idea what your talking about. You mean you get a boost spike?

What is overboost?????

.5 and .7 bar is what in PSI?

What size pulley are you running?

[MyFreeCamry]

Originally Posted by ASG14

I have NO idea what your talking about. You mean you get a boost spike?

What is overboost?????

.5 and .7 bar is what in PSI?

What size pulley are you running?

1 BAR = 14.7 PSI (atmospheric pressure)
.1 BAR = 1.47 PSI
.2BAR = 2.94 PSI
.3BAR = 4.41 PSI
.4BAR = 5.88 PSI
.5 BAR = 7.35 PSI
.6 BAR = 8.82 PSI
.7 BAR = 10.29 PSI
.8 BAR = 11.76 PSI
.9 BAR = 12.96 PSI

If he is running 7psi pulley and hitting 10psi then he does have a slight overboost. On a supercharger that is a little much.
How long has this been happening? Just recently or always? Where are you getting the vacuum source for your gauge at also?

[Eye8Pussies]

Originally Posted by Luckynumber5

If you have something restricting airflow out of the engine the supercharger will boost stack resulting in way more boost than is normally possible.

An example is an 05 Grand Prix GTP where the guy ran way to small a pulley with too few mods, he melted his catalytic converter and it clogged his exhaust. The damn thing would hit 20 psi on a roots blower (thats insane!)

^ what he said

the factors of boost on a roots supercharger are related to rpm and load. given that rpm is at max (so that the blower is spinning as fast as it can go) it is then dependent on load and backpressure. this will also presume the fact that you're under full load (i.e. maybe a slight incline, or at the very least, flat ground and redline)

to "build" boost, counter to say, a turbo setup, we need backpressure, whereas a turbo wants to expel the exhaust as fast as possible. therefore, a car with a clogged exhaust system (like lucky said, a cat- though his case would be extreme) the supercharger will not only boost faster, but it will also allow it to "over boost" due to increased load through extra backpressure. Also, how can you tell if your cat is clogged? at idle when you blip the throttle or hold it down, can you hear your supercharger whine? if you can, then that means there's load on it at idle and it's making boost, which there shouldn't be. at idle, your supercharger should not whine.

As a more personal experience, after the manual swap, although rpms can climb higher with shorter gears and lighter flywheel, etc. boost does not climb as fast due to less load than an auto tranny. similarly, on the number of occasions (unintentionally) when I ran an open exhaust after the y-pipe, performance was only about even to a full exhaust despite the theoretical benefits of being able to dump exhaust. the motor would rev like a honda- ridiculously fast, but there was very little boost unless I was in a long gear such as 4th. no load.


so long story short....start looking at your exhaust first. and then look elsewhere. though, your case would be pretty extreme as well.

are you also still on stock headers and y-pipe?

[Gibson99]

awesome (and quick!) replies, folks!

Originally Posted by Eye8Pussies

^ what he said

the factors of boost on a roots supercharger are related to rpm and load. given that rpm is at max (so that the blower is spinning as fast as it can go) it is then dependent on load and backpressure. this will also presume the fact that you're under full load (i.e. maybe a slight incline, or at the very least, flat ground and redline)

my "test track" is pretty much straight and flat, with at most a 5 degree incline. i can't really reproduce it in first due to lack of traction. i can't floor it in second either because of traction, but it's easier to keep it right at the edge of tire slip in second. in third it's much easier to reproduce, since i only get wheelspin if i slam the 2-3 shift, not to mention the increased load on the engine in 3rd. I am typically NOT looking at my gauges at that speed though, so i'm not sure if it's a momentary spike or if it jumps up there and stays there till i lift. of course once i lift i'm instantly in vacuum so i wouldn't see anything anyway. oh, i've tried it with the a/c turned off too just to see... it made no difference.

to "build" boost, counter to say, a turbo setup, we need backpressure, whereas a turbo wants to expel the exhaust as fast as possible. therefore, a car with a clogged exhaust system (like lucky said, a cat- though his case would be extreme) the supercharger will not only boost faster, but it will also allow it to "over boost" due to increased load through extra backpressure. Also, how can you tell if your cat is clogged? at idle when you blip the throttle or hold it down, can you hear your supercharger whine? if you can, then that means there's load on it at idle and it's making boost, which there shouldn't be. at idle, your supercharger should not whine.

i can pretty much guarantee that my cat is not clogged. and even if it was, it cannot possibly be a restriction due to its current location (my workbench). s/c only has the usual small rattling noise at idle, and only whines momentarily when i blip the throttle. the bypass valve is operating properly when i manually blip the throttle under the hood. My boost gauge's sensor is connected to the port on top of the s/c itself.

so long story short....start looking at your exhaust first. and then look elsewhere. though, your case would be pretty extreme as well.

are you also still on stock headers and y-pipe?

I'm pretty sure this is not exhaust related. I have JP headers and Y-pipe (therefore no cat, though i AM going to add a high-flow unit), and a greddy evo ii cat-back. Last time I was under the car, I didn't see any damage other than minor scratches on the piping - no big dents, so i really doubt there's any restriction in there unless the muffler or one of the resonators has come apart and I just can't tell. since it's all welded together or has bends beyond the flanges, it won't be easy to inspect. I guess I could take off the cat-back (at least the mid-section) temporarily for a test, but around here, that would probably get me a ticket faster than you can say "oh crap."

btw here's the list of mods i think could possibly have even a remote effect on this problem:
- injen CAI, modified by me to have the breather hose connect AFTER the MAF so it can't spray oil on it anymore (the relocation did help a bit)
- 7psi pulley
- dynamic tensioner
- gatorback belt
- sc300 injectors
- safc
- jp headers & y-pipe
- no cat (MIL ON - catalyst below efficiency code)
- blocked-off egr system (MIL ON - egr insufficient code)
- greddy evo-ii cat-back
- all stock internals including cams and cam timing
- magnacor wires
- plasma booster
- fidanza flywheel
- denso ik22 plugs (still look good, no abnormal wear or deposits)
- replaced pcv valve a month ago
- currently running on one knock sensor w/ little jumper wire at ECU

DTCs: there are only two (listed above) and they are directly related to equipment that was intentionally removed.
- no knock sensor codes
- no misfire codes
- no lean/rich fuel trim codes
- no maf codes (except when i left it unplugged and tried to start by mistake)

someone asked how long it's been doing this - well, it's been doing it since i bought the car from Tiem, already modded, about a year ago. Before he sold it to me, it sat in a garage for at least a year, driven maybe 300-400 miles over the course of that year, just to keep it from rusting away. The only performance related mods i've done since were to change the suspension and to relocate the breather hose connection on the intake pipe.

when i first got it, it would start to cut out at about 3500 under full load, but once i discovered the oil on the MAF and cleaned it (w/ QD Electrical Contact Cleaner) and then relocated the breather hose, it moved the "cutout" point to above 4000, and also reduced my EGTs from almost 700C down to 640C with cruise control at 70mph. Changing the PCV valve (still functional but was a little gummed up and new one was only like $3) also seemed to help by a couple hundred rpm, but I could be imagining that... But it never hurts to put a new PCV valve.

With these few changes I've managed to increase my highway fuel economy to just shy of 25 MPG, but top end power is still sorely lacking. I was rather disappointed last week when an e36 M3 and a 350Z vert decided to do a run right in front of me and i could barely even keep up with the Z! I should have destroyed the Z and at LEAST kept up with the M3, but when I'm forced to shift at 4k instead of 6k...

[Eye8Pussies]

hmm...that is very strange then. And yah...a mod list helps for sure...

so wait...what is your real problem right now then? the fact that it boosts so much? or that you're getting misfires?

the misfires are easy to diagnose...there is no way that your ignition system is holding up to that kind of rpm and that kind of boost level. your spark is being blown out.

a bigger question is why you're getting so much boost- up to 10+ psi on your 7psi pulley with not other factors such as load involved. where is your gauge tapped?

[Gibson99]

Originally Posted by Eye8Pussies

hmm...that is very strange then. And yah...a mod list helps for sure...

so wait...what is your real problem right now then? the fact that it boosts so much? or that you're getting misfires?

i think what i'm calling misfires would be more accurately described as the ecu cutting timing to prevent damage. i think it's hearing genuine detonation due to the excessive pressure in the chamber, so it retards the timing and dumps massive amounts of fuel into the chamber (i can see it in the rearview as black/gray smoke). i think that if it was staying steady at 7psi instead of 10 then i wouldn't have this issue.

the misfires are easy to diagnose...there is no way that your ignition system is holding up to that kind of rpm and that kind of boost level. your spark is being blown out.

you almost make it sound like i'm revving to 10k rpm, not just 5! but now that you mention it, i didn't check the gap when i had the plugs out - i was just looking for discoloration, physical damage, oil deposits, etc. the gap might be too big. that's easy enough to check.

a bigger question is why you're getting so much boost- up to 10+ psi on your 7psi pulley with not other factors such as load involved. where is your gauge tapped?

indeed - i think this is the root cause of my problem here. i honestly believe that if it was not boosting so high, everything would be running properly. the gauge is tapped into the same place as everyone else: top driver side of the supercharger.

[5SlowFE]

Originally Posted by Eye8Pussies

...getting so much boost- up to 10+ psi on your 7psi pulley with not other factors such as load involved. where is your gauge tapped?

Shouldn't that be impossible? The supercharger is a belt-driven positive-displacement type. Therefore, it's output will always be the same ( at a given rpm ) regardless of load. Increasing load serves to reduce rpm, so boost decreases, not increases as it can with a turbo. That's why roots superchargers are not waste gated.

OP, have you changed the diameter of the crank pulley? If yes, this could be your problem because the boost/pulley ratio for the s/c is calculated with the stock pulley, and will no longer be accurate if the pulley diameter isn't stock.

Just my $.02. Do I deserve for missing something obvious?

[Gibson99]

Originally Posted by 5SlowFE

Shouldn't that be impossible? The supercharger is a belt-driven positive-displacement type. Therefore, it's output will always be the same ( at a given rpm ) regardless of load. Increasing load serves to reduce rpm, so boost decreases, not increases as it can with a turbo. That's why roots superchargers are not waste gated.

OP, have you changed the diameter of the crank pulley? If yes, this could be your problem because the boost/pulley ratio for the s/c is calculated with the stock pulley, and will no longer be accurate if the pulley diameter isn't stock.

Just my $.02. Do I deserve for missing something obvious?

i thought it was impossible too, which is why i posted. the points brought up about excessive back-pressure do make sense though - the compressor in the s/c is still running but the pressure has nowhere to go so it builds up in the system, creating high numbers. this is how a shop air compressor works - it deliberately has back pressure (sealed tank) which it pumps air into, and you eventually get 100+ psi inside that tank. since a running engine isn't sealed (valves always operating), it can't build THAT much pressure, but it can build more than it was intended to.

my crank pulley is stock. that said, if i HAD changed its size (either over- or under-drive) i would still get consistent boost numbers - they would just be different from whatever someone else w/ the stock crank pulley and same size s/c pulley would get. ie: underdrive crank pulley + 7psi s/c pulley may only yield 6psi. problem is, i'm not getting consistent boost numbers. it's apparently spiking or at least jumping once i start putting a real load on it. maybe if i had better tires (continental 205/65/15 = CRAP) i could put a real load on it in 2nd but right now i can't. *shrug*

[whip_cracker]

Sooooooooo...

In saying this - if you were able to put the stock pulley on... would you still have this problem?

[Gibson99]

Well, probably not, but only because it would be jumping from 4 to 7 psi instead of 7 to 10. Unfortunately I don't have the stock pulley anymore.

That said, the car ran fine for several YEARS with this pulley, so that's why I'm so puzzled on this whole thing.

[Eye8Pussies]

Originally Posted by Gibson99

i thought it was impossible too, which is why i posted. the points brought up about excessive back-pressure do make sense though - the compressor in the s/c is still running but the pressure has nowhere to go so it builds up in the system, creating high numbers. this is how a shop air compressor works - it deliberately has back pressure (sealed tank) which it pumps air into, and you eventually get 100+ psi inside that tank. since a running engine isn't sealed (valves always operating), it can't build THAT much pressure, but it can build more than it was intended to.

my crank pulley is stock. that said, if i HAD changed its size (either over- or under-drive) i would still get consistent boost numbers - they would just be different from whatever someone else w/ the stock crank pulley and same size s/c pulley would get. ie: underdrive crank pulley + 7psi s/c pulley may only yield 6psi. problem is, i'm not getting consistent boost numbers. it's apparently spiking or at least jumping once i start putting a real load on it. maybe if i had better tires (continental 205/65/15 = CRAP) i could put a real load on it in 2nd but right now i can't. *shrug*

*ding ding ding- we have a winner here

But on that note in regards to your question....I wonder if there is something wrong with your exhaust valves and they're getting stuck? I was going to say maybe exhaust cams, but unless those got ground down somehow and made the lift duration even shorter....

Don't know...just throwing ideas out there...

[Gibson99]

Originally Posted by Eye8Pussies

*ding ding ding- we have a winner here

But on that note in regards to your question....I wonder if there is something wrong with your exhaust valves and they're getting stuck? I was going to say maybe exhaust cams, but unless those got ground down somehow and made the lift duration even shorter....

Don't know...just throwing ideas out there...

now that you bring up cams, i remembered something: way back when this car was still under warranty (at around 28-30k), i replaced an exhaust lifter bucket. yes, i was the one who did the labor since i worked at toyota at the time. it had a slight tick that i had a hard time hearing, but once i pulled the lifter bucket, it was very obvious that the top of the valve stem had worn into the bucket itself by over a milimeter... that's a lot of lost lift! i guess i should try listening for that as well. worn lifters could absolutely give me this problem due to insufficient lift.

[Luckynumber5]

Can you monitor your knock levels? Now that I think about it, if your ignition timing is retarded enough it can cause boost stacking.