Question on vacuum
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Senior User
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From: Newport News USA
Question on vacuum
I recently posted about installing the '91 H.O. cam in my 302, and how I've been fairly disappointed in it. I've read that vacuum should be approximately 25in or so. With this new cam, I'm only getting about 19-20, and if I hit the gas, the vacuum begins to plummet, all the way down to "0" if I hit it hard enough. I don't really have any "problems" with the truck running, but I was wondering if this may be the reason for such poor performance??? If it is, would going with the MAF conversioin route be the best "fix", and/or would sticking a Powerdyne supercharger on there help, or just add to the problem?
Really curious about this....
TIA
Luke
Really curious about this....
TIA
Luke
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Joined: May 2003
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From: San Diego
Question on vacuum
Normal vacuum for the 302 is 20in, or just a hair under. If you pop the throttle valve open, there will momentarily be no vacuum at all.
Sounds like it's working just fine.
Regards, Robert.
Sounds like it's working just fine.
Regards, Robert.
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From: Newport News USA
Question on vacuum
Is the vacuum supposed to maintain a constant ~20in, even when just cruising? I've noticed the fluctuating vacuum only just recently when I installed the vacuum gauge, and when cruising it seems to hold more around ~10. Is that expected and normal?
Post Fiend
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From: Memphis, TN
Question on vacuum
At cruise, the gauge shows engine load - not idle quality. As you open the throttle plates, the vacuum drops because more air is rushing in, but the weight of the truck is keeping the engine from revving up like it does when parked. If you're coasting, it should read the same as idle. If you're in backlash (engine braking), it'll read higher. If you're accelerating, it'll read lower.
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From: Newport News USA
Question on vacuum
That's exactly what I thought too. So, to confirm, my low vacuum at cruise speed is something out of whack? And is there anything I can do about it, to rectify the situation? The cam is out of a '91 HO motor, and other than the MAF, there is "almost" no difference between that motor, and mine. Correct? Same heads, just different intake. Could that be where my problem is?
Thanks a bunch ya'll!!
Luke
Thanks a bunch ya'll!!
Luke
Post Fiend
Joined: Nov 2002
Posts: 9,987
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From: Memphis, TN
Question on vacuum
A performance cam will RADICALLY change change the vacuum the engine produces under all conditions, so I can't offer you any advice on anything but a totally stock engine. But low vacuum at cruise doesn't necessarily indicate a problem. My truck is so big, heavy, UNaerodynamic, and has so much rolling resistance that my vacuum gets pretty low when I get rolling. Look at the speed and the vacuum gauge in the lower R corner. The picture was taken with the cruise control on.
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From: Newport News USA
Question on vacuum
Hmmm.... That looks very much like mine. At about 65mph, cruising, and engine turning 2k rpms, my gauge shows ~10-15in.
I just felt it today on the way home, and became more suspicious of it, but when cruising, and with my ~10in of vacuum, it does feel a tiny bit rough. Nothing extremely noticeable, but there all the same.
I'm guessing the recommendation would be to go with a different cam. Sounds good, minus tearing the engine apart again!! LOL
What effect does a supercharger have on vacuum? I'm in the position to get one here very shortly, and am sorely contemplating it.
I just felt it today on the way home, and became more suspicious of it, but when cruising, and with my ~10in of vacuum, it does feel a tiny bit rough. Nothing extremely noticeable, but there all the same.
I'm guessing the recommendation would be to go with a different cam. Sounds good, minus tearing the engine apart again!! LOL
What effect does a supercharger have on vacuum? I'm in the position to get one here very shortly, and am sorely contemplating it.
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Post Fiend
Joined: Nov 2002
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From: Memphis, TN
Question on vacuum
Any thing that pushes more air into the top end ("charges" it, like a turbocharger or supercharger) DECREASES vacuum (naturally) but INCREASES the potential for power because you can mix more gas in if you have more air.
It's also useful to note that any internal combustion engine operates more efficiently at NO vacuum than WITH vacuum. Just FFT...
It's also useful to note that any internal combustion engine operates more efficiently at NO vacuum than WITH vacuum. Just FFT...
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From: Newport News USA
Question on vacuum
Thanks. I guess I should have realized all that because it makes complete sense. By working best at No vacuum, do you mean working best at or as close to 100% volumetric efficeincy? and the same would go for the super/turbo right? They will try to max out, if not exceed, the volumetric eff. Is that correct?
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From: Newport News USA
Question on vacuum
Another question then, regarding the superchargers, vacuum, and MAF:
Why would it be "absolutely necessary for 86-88 SD mustangs to convert to MAF" but not say the same for their similar 5-6psi truck kits?
Is that too vague a question? Or am I just completely missing something?
Why would it be "absolutely necessary for 86-88 SD mustangs to convert to MAF" but not say the same for their similar 5-6psi truck kits?
Is that too vague a question? Or am I just completely missing something?
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Joined: May 2003
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From: San Diego
Question on vacuum
Here's my analysis, speaking as an engineer now.
I just went out and connected the vac guage to the vacuum tree and ran the engine at cruise rpm, about 2600 for me. As the engine is not under load when stationary, and becomes more efficient when at a higher rpm, one would expect the vacuum reading to rise, and it does.
Vacuum at idle is 20", vacuum when stationary at 2600rpm is 23".
When cruising on a flat surface under ambient conditions, at 2600rpm, the vacuum reading will depend upon which gear you are in, and will necessarily be lower due to the fact that the engine is under load by having to move the weight along, which will necessitate opening of the throttle plates, lowering the vacuum. Under further load from forward wind pressure as you shift thru the gears, the throttle plates will have to open even further to maintain 2600 rpm.
Hence, when moving at 70mph, I would expect vacuum readings certainly under 20", and probably in the range of 10" to 14".
Maybe Steve83 could take a drive at 70mph and read his guage for us. I can't quite see it in the pic.
Regards, Robert.
I just went out and connected the vac guage to the vacuum tree and ran the engine at cruise rpm, about 2600 for me. As the engine is not under load when stationary, and becomes more efficient when at a higher rpm, one would expect the vacuum reading to rise, and it does.
Vacuum at idle is 20", vacuum when stationary at 2600rpm is 23".
When cruising on a flat surface under ambient conditions, at 2600rpm, the vacuum reading will depend upon which gear you are in, and will necessarily be lower due to the fact that the engine is under load by having to move the weight along, which will necessitate opening of the throttle plates, lowering the vacuum. Under further load from forward wind pressure as you shift thru the gears, the throttle plates will have to open even further to maintain 2600 rpm.
Hence, when moving at 70mph, I would expect vacuum readings certainly under 20", and probably in the range of 10" to 14".
Maybe Steve83 could take a drive at 70mph and read his guage for us. I can't quite see it in the pic.
Regards, Robert.