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wont recover after blipping the gas.

AnotherNewb

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So im running a stock 6bolt motor with a magnus street intake and bc 272/272 cams. Im only seeing 12lb of vacuum at 850rpm idle. Is this right, know cams can lower your vacuum but I didn't think that much. Also, are 1g and 2g o2 sensors the same besides the plug? When I did the 6bolt swap I forgot to swap the sensors out while they were easy to get to so now that the motor is in I was hoping I could just cut and resolder the correct plug. I have an 02 sensor socket but the last/first/only time I have used it, it stripped the hex right off the outside of the sensor.
 
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I'd say you're fine on your vac numbers, considering the big cams. It isn't uncommon for all stock six bolts to be in the -12 to -13 range here in colorado, because of the elevation, and they run just fine.
 

JNR

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What elevation are you at?

You'll get lower vacuum with larger cams, but the 272's aren't really *that* big to be only getting 10-12" (for reference, my chevelle's cam is 234/244 @ .050 flat tappet (big cam) and I even get 12" at idle), but perhaps that is typical with a low idle but wow that seems low considering; Personally, I'd raise the idle up a little to bring the vacuum up slightly (within reason). You guys don't have braking issues though (as in get a couple successive stabs before losing assist)?

If you raise the idle slightly, your vacuum will rise, as will adjusting the timing. But, I wouldn't worry too much about 12" (at sea level), so long as it's a steady reading. What's more important than the amount (provided it's no excessively low) is the behavior of the reading; it's a great diagnosing tool and there's a lot of info on the net, fwiw...
 
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AnotherNewb

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I figured it out. Cas was loose, its a fresh swap with new cams. Bumping the timing around gave me 17-18hg of steady vacuum. Time to break out a timing light, but first I need to deal with a stock regulator being over run on a wally 255. No matter what I do I can't get my o2 sensor to cycle, it reads and works but the car is extreemly rich at idle. Even tried pulling some air on link but still way rich.
 

JNR

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Wow, that's a lot of vacuum for 272's! I mean, it's probably not a bad thing, but surprised, is all. I have 264/264 in #20 and iirc, I have it ~16" (sea level), fwiw.

Just curious how are you getting the reading? Boost/vacuum gauges are decent reference, but for the heck of it, use a diagnostic or a good vacuum gauge with large dial and small increments just to see exactly what you have. Whatever the case, sounds like you fixed the issue, as 12" just seems too low for even those cams (like I said, they are not really that big to suffer such low readings).

[edit: Since they (HKS) don't give cam specs @ .050" lift (which is commonplace here to do an apples to apples comparison), but rather 1 mm, the best I could come up with on a search is the following (shown at .050"):

STOCK ...Stock Turbo Cam......174 / 165.............366"/.343"
HKS264 .......HKS 264.............187 / 186.............392"/.374"
HKS272 .......HKS 272.............195 / 195.............399"/.379"
546/547 .....Web Cams............206 / 206.............400"/.385"

How accurate that is, I'm not sure, but sounds about right if you consider 1mm (.039") lift = 218/216 deg, per HKS card:





Point is, there should be no reason to have such low vacuum using those cams, as they are even smaller than I thought. Granted, a tiny engine like our 2.0 will exhibit a bigger cam behavior with a smaller duration (for instance, even a small V8 would be considered ultra mild with those specs), but even so, I don't see why anything less than 14" or so at sea level would be acceptable, unless something else is wrong, or adjustments are out of wack. But, that's also not knowing how the cam(s) are degree'd (advanced or retarded), as that too will affect it.

Anyway, just an FYI for anybody thinking that super low vacuum is acceptable...
 
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AnotherNewb

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Um... common sense would tell me that showing the computer less air would result in it adding less fuel. I can try pulling fuel too. Another problem is the car dies after blipping the throttle. It wont go back to idle without a little pedal play. Any suggestions? Its on a gm maf and translator so I don't think it would be boost leak related. Also, is anyone running a magnus street manifold? I am and I swear that thing 'hisses' at idle like a vacuum leak. Loudest intake I've ever heard.
 

JNR

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I'm sort of guessing here, but doesn't the ECU use a total volume in its calc and tries to keep a certain ratio, based upon load/rpm, etc.? If that's true, "telling it" there is less air that it would try to fill the total with more fuel? I could be totally wrong on that though and sort of asking myself.

As for the loud manifold, they are pretty thin aluminum, so may just be an issue of it being amplified over the stock, thicker piece?
 

Diego

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Quoting galant1517:
I'd say you're fine on your vac numbers, considering the big cams. It isn't uncommon for all stock six bolts to be in the -12 to -13 range here in colorado, because of the elevation, and they run just fine.



I've seen this when I had 1309 idling, it freaked me out pretty good coming from Iowa at ~16 - ~20 and here its lower.
 
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