I just put a bypass valve on my 93 5.0 mustang intake pipe from the vr 4 paxton to the throttle body, works good but I hit the throttle it expels the the air but the car starts surging then and keeps venting when surging. Is this because I just have it as a blow off valve and not a recirculating valve?
Are you trying to use the valve to regulate boost (like a waste gate) or to control compressor surge (like a blow off valve)?
If you are using it to control boost (like a waste gate) you may be experiencing the results of a metered air leak. The mass air flow sensor sees a certain volume of air, the pcm/ecm uses that value to calculate load and selects the map for fuel trim and injector pulse width based on that. If you have it vented to atmosphere the calculated load is going to be wrong because the volume of air being metered is higher than the volume of air the engine is actually taking in. That miscalculation could be the source of your surge. The cause would be that your valve is just doing too much venting. The cause of the excessive venting could be the spring rate is wrong and it is venting too soon or the reference point is too far down stream (it should be as close to origin of boost as possible in order to react to the pressure rise or fall as quickly as possible and in front of the throttle blade) causing the valve to not close when it should and instead react to the surge. If you need to control the boost to a lower level or rate for reasons like traction you could change your pulley ratio to limit your max boost to ensure you will never have to vent off too much and then use a progressive boost controller to tailor the curve so you wont be venting more than absolutely necessary. There is no way to not effect the MAF at all with the valve venting unless the MAF is after the valve, so the goal is to minimize the impact.
If you are using it to control compressor surge (like a blow off valve on a turbo system) it should not be venting unless it sees manifold vacuum. So as long as you have boost it should be closed (not venting) and when the throttle blade closes blocking the boost going into the engine and the manifold has vacuum it should vent out the boost that would have otherwise been forced back into the compressor (causing compressor surge). Your vacuum source needs to be directly at the manifold under/past the throttle blade. If it is then your spring may be the wrong rate, switch to a spring rate that will make the valve react only to vacuum and not just a drop in boost pressure.
Either way it sounds like a simple adjustment or the reference is coming from the wrong place more so than a wrong part or bad plumbing.
Last edited by 78f100347; 12-27-2016 at 08:52 PM. Reason: error
I'm just trying to reduce compressor surge I don't want to have to rebuild the compressor. Should I hook a hose up and recirculate it back into the power pipe?
It shouldn't expel air when you hit the throttle. Let off, yeah. The Valve should be before the throttle body and the bypass should go after your mass air but before the SC intake. Yeah the power pipe. All the bypass does is make sure you don't have any metered air issues.
Though I could be completely wrong. I've had turbo cars before but these old paxtons are my first time with a SC. Came on a 93 Cobra. How do you like the VR4? One was offered to me but my setup is a SN92 with no FMU. It does have 30# injectors and a good tune. Those 30's wont cut it will they?
Duke no you dont need to recirculate it. When the throttle is closed the MAF reading is basically irrelevant. If you are running it as a blow off valve to control compressor surge and make the spiffy boosted noises you do need to make sure it is not opening while you are under boost. The boost pressure of an SN series is low in comparison to other superchargers and turbo set ups so the spring may be reacting to the low boost pressure as it would to a boost drop and therefore the valve opens. You want to have your reference at the intake manifold after the throttle blade. and you want your BOV to open when it senses vacuum not just a boost drop. It should never be open with the throttle open.
Winchester73, your injectors are fine. I have no clue how the insides go together but I do know an SN92 is only moving like 750 cfm so you likely aren't going to tap out 19 lb. much less 30 lb. My buddy had an sn93 on his lightning which is 351cid, gt40 heads and intake from the factory. We bumped the fuel pressure up and he already had an fmu but I don't know how much of a difference it made. He never ran out of injector, A/F gauge usually showed fat. Now he has VR4, that moves like 1000 cfm. I have his old SN93.
What I meant was I was upgrading SN92 to a VR4. Apparently kits are going to be available again. I was thinking my 30# injectors would not support the VR4 upgrade.
I meant it expels air when I let off the the throttle, when it is under boost it doesn't vent. But when I let off the throttle it blows off like it should but then it starts surging. Why??
I'm also running 42# injectors with this setup and 9lbs of boost out of my vr4 paxton I have paper work that they will put out 1400 cfm of air flow. A sn with a h.p impeller 850 cfm
So VR4 is 1400 cfm...cool. I may want to do that conversion also at some point, I knew the boost close to doubled but didn't realize the cfm was that high, but I don't know a whole lot about the SN chargers at all, except what I have gathered on the interweb which ain't much. Duke, after you let off the throttle and the engine returns to an idle speed it surges correct? At this point is the BOV also hanging open or is it surging pressure too? Do you have access to your data stream to see if your MAF g/s value is jumping as it surges? Also is your valve adjustable or is it just straight spring resistance controlled? What brand is it? I have used Vortec valves and with a radical cam it will keep opening and closing at the factory adjustment. I know on a speed density system and it makes the car super lazy, I imagine MAF would surge a good bit.
Duke, would you? I have a 45K mile 93 Cobra I bought that came with an SN92 and some bolt ons. I'm about to lay some money down on a new VR4 upgrade and was hoping you could answer a few questions I had.Originally Posted by Duke
I don't have access to the data stream on the maf. It is a cheap b.o.v but it has different Springs I can put in, it is not hanging open but it just keeps surging. Maybe I should recirculate it back into the power pipe after the m.a.f?
What would you like to know about the vr 4? I can try to help out.
If you did recirculate it, it would have to be plumbed into the air intake in front of the charger but after the MAF to be still metered air and relieve surge....but then the spiffy boosty noises wouldn't be very loud and that's just not cool. Without pulling any data or knowing if its going rich or lean its kind of hard to say what it is doing exactly but lets try to work with the gauge info. What RPM range is it surging, just idle or anytime you let off the throttle (like going from 75% throttle to pass down to say steady 30% throttle to cruise)? Do you notice any surging on your boost vacuum gauge as the engine surges? Is just the engine surging or do you hear/feel the valve surging too?
If you have no data or gauges to work off of I would say if its like an ebay JDM fanboy turbo Honda BOV (not knocking it, I totally ran before because they're cheap and once its adjusted it will do the job and they tend to be obnoxiously loud which is cool points) you will need to change the spring...... use the heaviest spring and see what happens then the lightest spring. If that doesn't work I would consider maybe trying to run a t with a vent and a check valve on your reference like the vent for a map sensor on the speed density system on Ford trucks have to do when you go past 1 bar on boost.
Yeah it came from ebay and it has no name. But it did come with three Springs. It was cheap I should have known better! I took it off and the car ran fine again. I got a pretty big cam in the car it idles fine without it!
I'll try the heaviest spring.
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