Boost at minimal throttle - normal?

[fullboogie]

We're chasing a hesitation at throttle tip-in, both from a dead stop and at gear changes, when the car is dead cold. Going half throttle from a dead stop when it's cold will even make it nose over and then recover. There doesn't appear to be any issue once the car is warmed up. I've noticed that it requires only the slightest amount of throttle before boost starts registering on the stock gauge - maybe as little as 1/8 or 1/4 throttle and it'll show a few pounds to 5 pounds of boost. Let's leave MAF transfer tune issues off the table, because the tuner is a credible and experienced guy, and we've discussed this and ruled it out.

Three questions then -

1) Is it normal to show an onset of boost at such slight throttle input? I remember it took about 1/2 throttle or more to see anything with my Lightning. I'm running a TVS with 2.4" pulley, if that matters.

2) Can this issue be caused by a misadjusted bypass valve? To be honest, when the TVS arrived I threw it on the car without bothering to check the adjustment.

3) If it is the bypass valve, what is the correct adjustment procedure?

Any thoughts are much appreciated.

 

[PistolWhip]

I can't answer all your questions, but my car doesn't show any boost on the gauge at less than half throttle. You really have to want it to get it but if you mash it from a dead stop its almost instantly pegging the gauge.
If your seeing boost at low RPM part throttle or cruising speeds I'd say its conceivable that the bypass is either not adjusted properly, not hooked up to the right line or broken.

 

[PistolWhip]

Oh and check your plugs out. If its fine when it's warm, I would think it's unlikely to be anything electronically controlled. That really only leaves a few things unless temp is just coincidental.

 

[19COBRA93]

I have to get reasonably deep in the throttle to register boost as well, for both the factory gauge, and aftermarket.

 

[fullboogie]

Thanks Pistol. I can move my foot just a tad in any gear and I'm getting 5 lbs of boost. Just a smidge more, and I'm at 10 lbs. All at very minimal throttle input. I think what's happening is that when cold, the engine is briefly running lean at throttle tip-in due to the instant low rpm boost. That's what I'm trying to figure out - whether it's normal. Sounds like it's not. I'm going to check out all the connections and hoses tonight, but I'd still like to know what I should see on the actuator arm.

Is there supposed to be preload? .030" gap? What's normal?

 

[tktlwyr]

Throttle position and the onset of boost can be controlled by the tune. Most tuners change this and is one reason why a tune on these cars make them feel so much more snappy compared to the stock tune.

 

[1320 Junkie]

Throttle position and the onset of boost can be controlled by the tune. Most tuners change this and is one reason why a tune on these cars make them feel so much more snappy compared to the stock tune.

^^^^^this

 

[BrokenCool]

I had the same issues with a tuner who worked on my pump gas tune. I sent it out to Willie Figuerao and he clean up the tune now it drives so much better.

I was embarrassed to drive the car when I had those issues. Car sounded like it was going WOT just leaving a light or gas station etc.

 

[Stri8ed]

Throttle position and the onset of boost can be controlled by the tune. Most tuners change this and is one reason why a tune on these cars make them feel so much more snappy compared to the stock tune.

^^^^^this

Thats very interesting. I thought boost was only controlled through mechanical means. Do you know what a tuner does to change when the onset of boost starts?

Sorry to hijack fullboogie.

 

[fullboogie]

Everything I've ever read is that boost is solely controlled by the mechanical bypass valve, and nothing else. If you look at the vacuum hose routing to and from the bypass valve, it's pretty simple - one line from the blower, one line from the CAI. There are no electronics or anything else in the circuit.

Odd thing is that I took off the CAI/TB/elbow last night and tested the bypass valve - it's functioning perfectly, both holding vacuum and closing all the way. Using a borescope (its' tight in there with everything assembled!) I verified that the bypass valve is pulling the valve completely open when the engine is running. That rules out a mechanical issue. I also don't see any way to "adjust" the valve.

I'm going to strap a vacuum/boost gauge to the windshield and see if the mechanical gauge tracks with the dash gauge. If not, I don't know what's wrong with it. If it does, I guess I have super blower or something. I tend to think that it is seeing boost that early, because my WB immediately goes rich when the dash gauge shows boost.

 

[tktlwyr]

Thats very interesting. I thought boost was only controlled through mechanical means. Do you know what a tuner does to change when the onset of boost starts?

Throttle body opens quicker boost comes on sooner. I'm not sure exactly what the table is you modify (I'm not a tuner, but I did sleep at a Holiday Inn Express last night). This is how Ford limits the power for the first 5 miles on the GT500.

Everything I've ever read is that boost is solely controlled by the mechanical bypass valve, and nothing else. If you look at the vacuum hose routing to and from the bypass valve, it's pretty simple - one line from the blower, one line from the CAI. There are no electronics or anything else in the circuit.

The supercharger bypass opens when you have vacuum (idle) and closes under boost. It's mainly just to give the pressure somewhere to escape when you let off of the throttle and prevent rotor damage. It does have an effect on boost, but the main player here is the throttle body.

FWIW, an improperly adjusted bypass can cause a stumble. I know because I chased this a few years ago (my bypass was loose when my blower shipped from Whipple). Have someone operate the throttle and check the operation under the hood to make sure it opens under vacuum and closes when you goose the throttle.

 

[fullboogie]

How exactly does one adjust the bypass valve - I've been asking this since I posted this thread. Looking at the mechanics of the valve assembly, there is no way to adjust it. Yours was loose, which is a different issue.

 

[fullboogie]

Throttle body opens quicker boost comes on sooner.

By the way, that is a very interesting take on it. I hadn't thought of this yet. I'm going to put the vacuum/boost gauge on it tomorrow and report back.

 

[Catmonkey]

By pass valve should snap shut with no vacuum. The by-pass works solely off of engine vacuum, there is no solenoid, so unless you have a vacuum leak in the by-pass actuator, I'm thinking tune.

 

[fullboogie]

By pass valve should snap shut with no vacuum. The by-pass works solely off of engine vacuum, there is no solenoid, so unless you have a vacuum leak in the by-pass actuator, I'm thinking tune.

Valve works exactly as you describe. I really hope it's not the tune - I've spent a lot of money and a shit load of time logging and requesting tune revisions. I'm going to be real disappointed if I have to start over with another tuner.

 

[Poppacapp]

Have 13' TVS on mine with 65mm TB. I can slightly feel some minor surging when accellerating lightly in the gears.. like in neighborhood etc. Looking at my boost gauge(autometer) light throttle wants to build a couple pounds of boost, but it drops off quick... that's probably what I am feeling. Not sure if it is the same problem you are having.

 

[fullboogie]

^^^ Sounds exactly like what I am experiencing. We have the same blower and TB - what pulley do you have? I'm starting to wonder if the bigger blower, bigger TB, and high blower rpm is causing a rapid onset of boost that is difficult to tune for. I'm on the 5th revision of my tune. It's getting better, but it's not there yet.

Is it worse when the engine is dead cold?

 

[Poppacapp]

^^^ Sounds exactly like what I am experiencing. We have the same blower and TB - what pulley do you have? I'm starting to wonder if the bigger blower, bigger TB, and high blower rpm is causing a rapid onset of boost that is difficult to tune for. I'm on the 5th revision of my tune. It's getting better, but it's not there yet.

Is it worse when the engine is dead cold?

I have the 2.67" pulley that comes with the 13' TVS. It does seem to be a little more prevalent when cold. I am at 15psi right now.

 

[fullboogie]

Well, mystery solved. I just got back from an extensive drive with an aftermarket boost gauge. Boost tracks exactly the same between the mechanical gauge and the dash gauge, which really surprises me. I guess the combination of blower, pulley, and throttle body really causes this thing to get into boost very quickly. I'm going to have to live with boost at 1/4 throttle. The tuning issues are undoubtedly related, but at this point it's only when cold.

Poppacapp - thanks for confirming this with me. It's not something I've read about anywhere, so I wanted to get to the bottom of it.