During part throttle cruising is really when a low vacuum valve is need so blower temps/iat2 are in check. I would have a low vacuum valve with cams in the 5-7in of vacuum range (really anything under 12in HG). Remove the spring add a threaded end cap(thread sealer) and plumb vacuum line to a Boost source so the valve will close under boost. At idle it will function like normal. Your original valve might have not been opening at idle with those cams you have.
Low pressure spring on boost bypass valve on 2.9 whipple
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During part throttle cruising is really when a low vacuum valve is need so blower temps/iat2 are in check. I would have a low vacuum valve with cams in the 5-7in of vacuum range (really anything under 12in HG). Remove the spring add a threaded end cap(thread sealer) and plumb vacuum line to a Boost source so the valve will close under boost. At idle it will function like normal. Your original valve might have not been opening at idle with those cams you have.
I understand what you are saying, but even with the stock spring its going to do the same thing isn't it? Mine only does it when the vacuum is at or near 0, and boost is at zero. Right at that transition. That transition will be there no matter what spring is in there. Ill let you know how I make out.
Maybe we have slightly different issues? Driveability on mine right now is probably an 8.5/10, and with my old setup with the stock spring was probably a 4/10. And my current setup has more overlap in the valve timing.
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A valve that only closed in boost would make my problem already worse than it is. See response below for further elaboration.
My issue is when you're pressing on the gas pedal accelerating, vacuum decreases from say 10 Hg, 9, 8, 7 etc. right at 3 ~ 4 inches of vacuum when the bypass valve closes the car jerks or surges.
On the stock spring the valve closes around 11 inches of vacuum and the jerk or surge is much milder.
My theory is that the bypass valve should close when the engine's around 10 ~ 11 inches of vacuum so that the blower doesn't have much of any air to surge into the engine. Hence avoiding that spike. That way as you roll from 10 inches vacuum towards 0 the blower is already engaged so to speak with the valve closed and as it gets a greater volume of air to compress and move into the engine it'll do so in a linear smooth way.
I think when the valve stays open all the way till 4 inches of vacuum the rotors already have to much air in them so the valve closes and the blower comes on real strong with that sudden surge of air. Hence the surge or jerking motion felt in the car.
Ok, I see what you are saying. Like I said earlier, i actually was out playing with mine today since I had the day off. I specifically tried to make it stumble like we are talking about. It only does it at that transition right at 0 vac, 0 boost. If you come up with something let me know, and if mine is able to be tuned out I'll keep you posted.
OK cool. Ours do sound slightly different. I'm actually about to go play some with mine and pay specific attention to what vacuum does on pull off. I'm starting to wonder if the reason my original cams pulled off so horrible was the stock bypass valve flapping between closed & open as vacuum fluctuated on pull off. It would make sense because I tried removing the blower belt and it pulled off 100 times better. I figured that's just the characteristics of the blower on there. It never occurred to me at the time the bypass valve was probably flapping as I was pulling off.
You're touching the gas, loading up the engine etc. Going to be watching my vacuum gauge very closely while deciding if I want to tear this blower back apart and go back to the stock bypass valve again or keep messing with this current setup.
You're touching the gas, loading up the engine etc. Going to be watching my vacuum gauge very closely while deciding if I want to tear this blower back apart and go back to the stock bypass valve again or keep messing with this current setup.
OK so I hooked up a larger vacuum gauge that was easier to read and with more resolution in the car so I could watch exactly what the vacuum was doing while driving around.
Of interest I noticed the car only has 8 inches of vacuum at warm idle.
After watching closely how the vacuum responds in normal driving around, pulling off etc. I can clearly see going back to the stock bypass valve is not a solution for me. I also see how a lot of my pulling off problems are caused by the bypass valve opening / closing. It causes the blower to surge and the whole car bucks and becomes a pain to drive.
After pondering this for a while I decided to try a restrictor to reduce the flow to the valve so it would close slowly and not have such an instant impact and surge. I pulled the connector off the bypass valve and removed the elbow and joining piece. I then squeezed a 0.022 nitrous jet in it like this. The smallest I had laying around.
Pushed the other end into the hose
Squeezed the 90 back over and the open end you see there then pressed back onto the valve.
Took it for a drive and wow night and day difference. That drastically slowed the valve operation and the seat of the pants surging was gone. I could still see small lean spikes on the wideband so it's not perfect but dang it was good. I was real impressed with how much it fixed drivability.
Then I was testing just how fast it could close the valve such as when you nail the gas. 2nd gear rolls no real difference. 3rd gear around 4k rpms and then suddenly hit the gas and I could definitely tell a lag. It's like the power rolled in smoothly how a turbo car would but with just a half second lag. It took the impressive punch out that just sends the car into sideways fish tails. I guess great for traction because it just rockets in a straight line but was not enjoying that split second lag.
So I pulled the 0.022 jet and swapped it for a 0.030. I could tell the surge was ever so slightly pronounced but still very mild and livable. The 3rd gear punch still goes in a straight line but doesn't feel delayed. Feels instant just the initial instant hit is not has hard. Again probably good for racing but I really like the "fun to drive" factor of the car too.
So right now I'm pretty happy with how drivable and fun the car is again. I'd like to further improve on this idea though. I'm thinking of using a boost bypass valve and bypassing the restrictor nozzle when you go WOT. That way I could go back to the 0.022 or even smaller and really smooth out normal driving but when you nail the gas a bypass valve or solenoid could open up allowing the blower's bypass valve to snap closed instantly like before.
So it's not 100% cured but progress. More to come.
Of interest I noticed the car only has 8 inches of vacuum at warm idle.
After watching closely how the vacuum responds in normal driving around, pulling off etc. I can clearly see going back to the stock bypass valve is not a solution for me. I also see how a lot of my pulling off problems are caused by the bypass valve opening / closing. It causes the blower to surge and the whole car bucks and becomes a pain to drive.
After pondering this for a while I decided to try a restrictor to reduce the flow to the valve so it would close slowly and not have such an instant impact and surge. I pulled the connector off the bypass valve and removed the elbow and joining piece. I then squeezed a 0.022 nitrous jet in it like this. The smallest I had laying around.
Pushed the other end into the hose
Squeezed the 90 back over and the open end you see there then pressed back onto the valve.
Took it for a drive and wow night and day difference. That drastically slowed the valve operation and the seat of the pants surging was gone. I could still see small lean spikes on the wideband so it's not perfect but dang it was good. I was real impressed with how much it fixed drivability.
Then I was testing just how fast it could close the valve such as when you nail the gas. 2nd gear rolls no real difference. 3rd gear around 4k rpms and then suddenly hit the gas and I could definitely tell a lag. It's like the power rolled in smoothly how a turbo car would but with just a half second lag. It took the impressive punch out that just sends the car into sideways fish tails. I guess great for traction because it just rockets in a straight line but was not enjoying that split second lag.
So I pulled the 0.022 jet and swapped it for a 0.030. I could tell the surge was ever so slightly pronounced but still very mild and livable. The 3rd gear punch still goes in a straight line but doesn't feel delayed. Feels instant just the initial instant hit is not has hard. Again probably good for racing but I really like the "fun to drive" factor of the car too.
So right now I'm pretty happy with how drivable and fun the car is again. I'd like to further improve on this idea though. I'm thinking of using a boost bypass valve and bypassing the restrictor nozzle when you go WOT. That way I could go back to the 0.022 or even smaller and really smooth out normal driving but when you nail the gas a bypass valve or solenoid could open up allowing the blower's bypass valve to snap closed instantly like before.
So it's not 100% cured but progress. More to come.
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very interesting Malcolm. im glad you found a (temp.) solution, keep up the good work.
Yes right at 4 inches where the valve closes.
So, I was able to get to my tuner and try and dial out my slight hesitation/stumble at the zero vac/boost point. We were not able to get it much better with the pumpshot/accelerator pump feature. So we logged it and discovered that at the point where it does it the MAF count goes from like 180ish and shoots to around 300 and then back down. That is what is causing my stumble he said. So I need to research if this is a fixable issue or not.
That is exactly my issue. I think we're just explaining it slightly different. Right when the valve closes you get a burst of air from the blower. It's viewed as a lean spike on the wideband and also observed as a spike on the MAF meter. In your case from 180 to 300 and back.
This is what I cured with the nitrous nozzle in the vacuum / boost line going to the bypass valve. It slowed it way down so the valve closes slowly and you won't see near the same spike.
I received my mini little boost controller valve which I'll be setting up this week to bypass the nitrous nozzle when you stab the throttle aggressively creating a zero lag or delay on the valve when trying to race or drive fast.
Ok yes definitely keep us posted on that. I may try a nittous jet or something in the vacuum line. I'm just a little concerned/confused because its definitely at the 0 boost/vacuum point which tells me no matter what spring is in there its gonna do it.
Interesting. Have you tested your bypass valve with a hand operated vacuum pump of sorts to verify where it moves? If it's real close to 0 it may possibly appear it's at 0 when driving? I'm not sure. It almost has to be right when that valve is closing.
Yes I did. 6" of vacuum was the magic number.
When I get back I'm actually gonna try unhooking the vacuum line all together. That way the valve will be shut no matter what. If it still does the hiccup, its unrelated to the bypass and more than likely a cam overlap issue or a MAF issue.
Great idea. Another trick you could try is removing the blower belt. That's how I found my bypass valve issue by pure accident. Snapped a lower on the street and drove home with lower pulley and blower not spinning and discovered hey all my drivability problems just went away. wtf just happened here? lol
Lol I hear ya! Sometimes the oddest things are discovered by accident.
Bringing this back,
Malcolm I know you dont have the 2.9 Whipple anymore but what was the end solution with this experiment.
Did you leave the .030 jet in there or end up doing something different?
I noticed with my new 2.9 whipple and stage 3 comp cams the idle is better with blower belt removed. Curious what you ended up doing..
Thanks,
Malcolm I know you dont have the 2.9 Whipple anymore but what was the end solution with this experiment.
Did you leave the .030 jet in there or end up doing something different?
I noticed with my new 2.9 whipple and stage 3 comp cams the idle is better with blower belt removed. Curious what you ended up doing..
Thanks,
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