legacycentral bbs

Not to derail the thread or anything, but on the subject of PTFB, when I ran 10-11 psi I got PTFB syndrome. I recently lowered it back to stock levels 8-9 psi and now I get half boost at half throttle and full boost at full throttle. Just something I found kind of interesting and weird.

VRG - thats just how i understood it form all of the reading ive done. now that i think of it more it doesnt really make sense but at the same time couldnt it be a posiblity

I don't see any reason it would make a difference. But then again, I don't quite understand why the phenomenon happens at all.

I understand why it happens, I just dont feel like writing four paragraphs right now.

Taking pressure, to regulate the wastegate after the throtle body is incorrect. When the throtle plate is closed, the wastegate will get vacum and stay closed. Unless you've got NO flywheel, it takes a bit for the RPMs to come down. At that time, the turbo will keep spinning until it has built up too much pressure, and stop, thus squeezing hot exhaust through the exhaust turbine, and over heating it, as well as your valves, head/etc.

Short explanation, the cold and hot sides fight eachother inside the turbo. Does that make since?

Heh... well that's all well and good, but I have no intercooler right now... sold the TMIC and have an isuzu diesel radiator that will make a damn fine FMIC mounted where most of my front steel bumper used to be. Actually today I should be connecting it up, as last week I had finally worked the hoses on that will route it around the radiator (hardest part IMHO).

Anyway, are we thinking perhaps there would be less boost spread throughout the gears with a restrictor inside the compressor -> MBC line? Because I definately don't have one of those in right now.

Come to think of it, I kinda see how the restrictor could "solve" that problem like running that line to the manifold could "solve" it. Although with the restrictor at least we're never separating the flow from the throttle plate, so it's the preferrable solution. I wonder if I still have the stock line with that 2mm restriction in it somewhere, that might do the trick. If not, what would you recommend for rigging up a proper restrictor?

so does anyone know why my boost pressure goes up in every gear? Im doing a clutch and a 39 hopfully this week. Im thinking about removing the mbc and just using the stock wastgate to control boost, does anyone know what the vf39's wastgate is set at?

rallitektech wrote:I understand why it happens, I just dont feel like writing four paragraphs right now.

Taking pressure, to regulate the wastegate after the throtle body is incorrect. When the throtle plate is closed, the wastegate will get vacum and stay closed. Unless you've got NO flywheel, it takes a bit for the RPMs to come down. At that time, the turbo will keep spinning until it has built up too much pressure, and stop, thus squeezing hot exhaust through the exhaust turbine, and over heating it, as well as your valves, head/etc.

Short explanation, the cold and hot sides fight eachother inside the turbo. Does that make since?

but thats why you put a small hole in the line so the wastegate doesnt stay closed or open whichever the matter might be

put a small hole in the MBC input line? Isn't that more like making it harder for the wastegate to receive enough pressure to overcome the force of the spring holding it closed...? I don't think that's a good idea either. That's gonna result in the turbo burning the hell out if itself quicker, IMHO.

Anyways, didn't have a chance to find and install a restrictor in the MBC input line, but I did have a chance to completely route and hook up my isuzu diesel radiator as a front-mount intercooler! Even had time to clean the exposed 2.5" exhaust piping (used for FMIC) and clear-coat it so's it won't rust. Had time for four pulls down my friend's street where there are no residences, each time a new hose blew itself off. We'd re-install it, tighten the hell out if it and try to reinforce it somehow, and see which one blew off next. (14 psi blows hoses off pretty readily!)

But then it got dark.

So anyways, got the MBC at 8psi and the BOV at 'really freakin soft' to relieve the pressure ASAP (as my BOV is now in the proper place without the damn Saab 900 IC) and drove home carefully, keeping it in vaccuum only to prevent a roadside incident. Will continue blowing hoses off and reinforcing them (probably have to do them all) during daylight hours. Took a few photos too on my friend's camera, when I get them I'll post some up.

I bet when I find and install that restrictor, the boost spread between gears will be much closer together. Hopefully within 1-2 psi, which is acceptable.

i kept the stock restrictor in mine and it may be the reason why my boost stays steady.

i did find that i was acutaly running more boost than i had previously thought because when i disconected my adjustable air pressure regulator fcd my coost gauge hit 16psi and i hit fuel cut.

if you have a place that sells t bolt clamps id get thoes. all my clamps are worm gear because my pipes are solid enough and dont move enought to blow off, with the exception of one so i invested 6 bucks in 2 t blolt clamps and so far no problems.

you mean the perfectly round clamps usually used for fuel injection hose? Thats not a bad idea. It was tough enough finding worm clamps that clamp down for the pipe size I'm using but I didn't think to ask for t-bolt clamps. Thanks for the idea. That might save me much ghetto-rigging support with mechanics wire, which is how I prevented the Saab 900 IC from blowing its hoses apart.

Kelly - Well, when you can find some time, I'd really appreciate it if you could try a quick explanation of the boost spread phenomenon.

Chris - I just cut a short piece of 1/4" aluminum stock and drilled a 1mm hole in it. The only technical justification I have for it is that "maybe it'll keep boost pressure from making the wastegate fly open so eagerly."

Jason - Where did you keep the stock restrictor?

word.

vrg3 wrote:Kelly - Well, when you can find some time, I'd really appreciate it if you could try a quick explanation of the boost spread phenomenon.

Short version: The turbos get more time time to spool up in the higher gears. Due to the oil bearing, and the weight of the turbines themselves, the turbo might still be getting up to speed by the time you shift in the lower gears. Inertia, compressor efficiancy, wastegate size, and exhaust velocity all come into play as well. The biggest factor is the design of the turbo itself, specifically the difference in size of the two turbines. (exhaust CFMs vs. compressor CFMs)

Hope that kinda gives you a start Vikash. Like I said, if you were to write a paper on how this happens, youd need to run to the store for more paper.

See, that's what I thought too, but then it didn't add up... In low gears, the turbo does reach its peak boost and hold it for quite a while. It's not like it's still rising by the time the shift happens.

Like, I could be rolling down the highway, shift into 4th gear, and mash the pedal, and see 9 psi of boost, held for literally 30 seconds. Then I could shift into 5th gear and do the same, but see 10 psi.

I don't understand why the negative feedback loop of the wastegate doesn't regulate pressure to a constant level. I especially don't understand why it does seem to always reach a steady state, but a not always the same steady state.

I havnt stopped thinking about this yet.

simonasaur wrote:so does anyone know why my boost pressure goes up in every gear? Im doing a clutch and a 39 hopfully this week. Im thinking about removing the mbc and just using the stock wastgate to control boost, does anyone know what the vf39's wastgate is set at?

Your MBC's design or the design of the hoses running to/from it are flawed. That's what causes the pressure to go up in every gear. This is different from boost creep due to inadequate wastegate flow. A good EBC or a proper MBC will get rid of that issue.

I'm using the MRT EBC at the moment and have the same boost in every gear when I set boost at 7psi or 13psi. I have a 3" straight pipe for a turboback, so I'm begging for creep, but don't have any. If I set it to 13psi....I get 13psi even in 1st gear. Full boost before 2000rpm in 3rd/4th/5th.

As far as PTFB goes, I get full boost at 5-10% throttle sometimes, which can be a danger on stock mangement. Sometimes I see A/F ratios of 16:1 or 17:1 when this happens, and I imagine timing is quite advanced as well. I've never heard audible detonation, but I avoid this situation for the most part.

-Mike

Can you explain the flaw in the MBC's design, Mike?

vrg3 wrote:Can you explain the flaw in the MBC's design, Mike?

Definitely not without seeing it. Honestly some look like they'll work fine, but then you install them and they perform poorly, so I can't say I'll know why it doesn't work just by looking at it.

When I made my own MBC I went through a few iterations with different springs before I got it just right. This is a common problem associated with the spring being too heavy, but there might be other things at play here as well.

-Mike

I'm just trying to understand what causes it. You're saying it's things like too heavy a spring? What does having a heavy spring do that causes this to happen?

Take my setup -- I have an auxiliary spring holding the wastegate shut. That's it. No other parts. Why does that system reach a different stable equilibrium in each gear?

Does this still happen with the stock solenoid controlling boost?

No. But in stock form you have a computer controlling boost, not a mechanical system.

My boost controller's made by go fast bits. Hopfully ill get it controlled electronically soon. my rides out of commision right now so it doesnt matter any way right now.

vrg3 wrote:I'm just trying to understand what causes it. You're saying it's things like too heavy a spring? What does having a heavy spring do that causes this to happen?

Take my setup -- I have an auxiliary spring holding the wastegate shut. That's it. No other parts. Why does that system reach a different stable equilibrium in each gear?

You are changing preload and spring rate, but you aren't toying with the amount of airflow to the wastegate actuator. When you stick an MBC in the mix, you're changing air volume to the actuator dramatically by using different line IDs, chamber size in the MBC, etc.

This is what my first hand experience has been. Having stock or SLIGHTLY larger lines to the MBC and to the wastegate actuator works well. Longer and lower rate springs with more preload work better than shorter/higher rate springs with less preload. If the ball size is too big or small, boost control will be erratic. Just try a few sizes until you hit one that works.

-Mike

But air doesn't really flow through the actuator, right? It just has to reach (or leave) it. The actuator's position should depend only on the pressure differential across the diaphragm. Right?

I can understand how the diameter of the hoses and the amount of restriction within the boost controller would affect transient behavior, but I still don't see how they could result in a different steady state.

vrg3 wrote:But air doesn't really flow through the actuator, right? It just has to reach (or leave) it. The actuator's position should depend only on the pressure differential across the diaphragm. Right?

I can understand how the diameter of the hoses and the amount of restriction within the boost controller would affect transient behavior, but I still don't see how they could result in a different steady state.

Right. I said you weren't changing "airflow to the wastegate actuator."

The issue isn't usually at the actuator, it's generally the lines prior to the MBC and the MBC itself. If there is too great a volume of air entering the MBC, it can't regulate airflow as needed. The same goes with too big/small a chamber in the MBC, or too big/small a ball. When changing the ball size, you're basically changing the size of the openning.

Also, airflow is a factor at a diaphram even though pressure is the MAIN factor. For instance, if you slap a big turbo (I'm not talking about a vf30 here ) on a WRX the BOV will leak at a lower pressure than on the stock turbo. We've tested this on multiple cars.

-Mike