Vacuum @ full open throttle
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Vacuum @ full open throttle
Hit the dyno few months ago. My car is lt4 383 with ported lt4 heads, ported lt4 intake, cc503 cam, 1.6rr, 58mm tb, 32lb bosch III, kn filter cut lid, ported maf, melrose longtubes, race cat's, muffler eliminators.
The problem I'm having is that I have 1.2in-hg vacuum at manifold under full throttle from 3500 to 6500 rpm! ftw? Throttle blades was full open, and the vacuum was confirmed multiple times with meters. Digital and analog.
Anyone had this problem before? I'm doing custom cai now and going to order smooth couplers.
The problem I'm having is that I have 1.2in-hg vacuum at manifold under full throttle from 3500 to 6500 rpm! ftw? Throttle blades was full open, and the vacuum was confirmed multiple times with meters. Digital and analog.
Anyone had this problem before? I'm doing custom cai now and going to order smooth couplers.
Last edited by Zoui; Mar 10, 2012 at 03:40 PM.
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From: Anaheim CA
I took this wonderful intro to engineering class at UCLA in 1964, of which part was called "flow systems". Any time you have ANY kind of transition from perfect laminar flow there will be a difference in pressure. It will ALWAYS be less on the back side of a change in the system, like behind a throttle body.
There needs to be SOME vacuum. That's what sucks the air in. Without forced air induction you need a vacuum to get the air into the engine.
The vacuum gauge is at atmospheric pressure and uses that as a reference. What you're seeing is the difference between atmospheric pressure and the pressure inside the plenum. "Vacuum" is actually a misnomer. It's just lower pressure than atmospheric.
One thing you could do is measure the pressure drop across all the individual components in the system: air filter, pipes, MAF, throttle body, etc. That will tell you where the most restriction is and will point at the part to change/modify that would give you the most benefit.
There needs to be SOME vacuum. That's what sucks the air in. Without forced air induction you need a vacuum to get the air into the engine.
The vacuum gauge is at atmospheric pressure and uses that as a reference. What you're seeing is the difference between atmospheric pressure and the pressure inside the plenum. "Vacuum" is actually a misnomer. It's just lower pressure than atmospheric.
One thing you could do is measure the pressure drop across all the individual components in the system: air filter, pipes, MAF, throttle body, etc. That will tell you where the most restriction is and will point at the part to change/modify that would give you the most benefit.
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One thing you could do is measure the pressure drop across all the individual components in the system: air filter, pipes, MAF, throttle body, etc. That will tell you where the most restriction is and will point at the part to change/modify that would give you the most benefit.
Le Mans Master
Joined: Dec 2002
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From: Seattle Area WA
Keep us posted on what you find and how your upgrades work out. 1.2inHg should be in the range that you should realize some benefits when you remove the restriction from what I have read. Will you be going back to the same dyno? What did you put down now?
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Yep, going back to same dyno. Dyno is rototest, witch seems to give you lower numbers than some other dynos, did only 380rwhp. Few years ago it did 370rwhp @ dynocom dyno with stock heads, stock exhaust, stock headers, stock injectors
Melting Slicks
Joined: Dec 2008
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From: Mystic CT
I took this wonderful intro to engineering class at UCLA in 1964, of which part was called "flow systems". Any time you have ANY kind of transition from perfect laminar flow there will be a difference in pressure. It will ALWAYS be less on the back side of a change in the system, like behind a throttle body.
There needs to be SOME vacuum. That's what sucks the air in. Without forced air induction you need a vacuum to get the air into the engine.
The vacuum gauge is at atmospheric pressure and uses that as a reference. What you're seeing is the difference between atmospheric pressure and the pressure inside the plenum. "Vacuum" is actually a misnomer. It's just lower pressure than atmospheric.
One thing you could do is measure the pressure drop across all the individual components in the system: air filter, pipes, MAF, throttle body, etc. That will tell you where the most restriction is and will point at the part to change/modify that would give you the most benefit.
There needs to be SOME vacuum. That's what sucks the air in. Without forced air induction you need a vacuum to get the air into the engine.
The vacuum gauge is at atmospheric pressure and uses that as a reference. What you're seeing is the difference between atmospheric pressure and the pressure inside the plenum. "Vacuum" is actually a misnomer. It's just lower pressure than atmospheric.
One thing you could do is measure the pressure drop across all the individual components in the system: air filter, pipes, MAF, throttle body, etc. That will tell you where the most restriction is and will point at the part to change/modify that would give you the most benefit.
Le Mans Master
Joined: Dec 2002
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From: Seattle Area WA
Race Director
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You may beat your brains out trying to find that little bit of airflow. The system is not flawless you will see SOME loss due to intake restrictions.
Certainly address the air intake system. If you're still running a factory lid with a paper filter, start there. If you've kicked it up to a K&N with an open lid or a SLP claw then there really isn't gonna be much you can do.
You'll never get a perfect atmospheric pressure, you can just get close. Honestly as close as you are, I don't think you're gonna get it. Now I have seen cars on the dyno that only pulled like 90KPA due to severe intake restrictions.
Certainly address the air intake system. If you're still running a factory lid with a paper filter, start there. If you've kicked it up to a K&N with an open lid or a SLP claw then there really isn't gonna be much you can do.
You'll never get a perfect atmospheric pressure, you can just get close. Honestly as close as you are, I don't think you're gonna get it. Now I have seen cars on the dyno that only pulled like 90KPA due to severe intake restrictions.
Le Mans Master
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From: The Motor City
others have made good suggestions, but you fail to give location. If living in a higher altitude that can be the cause. I would start by looking at map values with car off, at wot should be the same.
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Yeah, I may not get the perfect airflow due the design, but after I'm done, the intake system will be at least better than it was. Going back for stock maf instead of that ported one, polishing the tb, adding smooth couplers and the most important, going for ram air! You all know the flip-tie mod for c5? well, no one has ever done it to c4 cos is hard to do, but that's just how I like it. Filter is going to sit in this hole, enough fresh air you think? 
And I live in Helsinki Finland
And I live in Helsinki Finland
Last edited by Zoui; Mar 13, 2012 at 01:47 PM.
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Ok, I'm confused. You're polishing the TB and you're adding smooth couplers but you're going to go back to a stock MAF instead of a ported one?
I've heard of people doing what you are doing with the air filter, but I would be afraid of the sheer volume of trash and / or moisture it could suck up from there. It may never be a problem, but it seems like it could be. I'd hate to suck up a slug of water just for the possibility of getting 1 or 2 hp.
I've heard of people doing what you are doing with the air filter, but I would be afraid of the sheer volume of trash and / or moisture it could suck up from there. It may never be a problem, but it seems like it could be. I'd hate to suck up a slug of water just for the possibility of getting 1 or 2 hp.
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From: WI
Yeah not sure about that direction, the ported MAF sounds like more flow. There is no water problem though. Yes there is more junk stuck in the filter.
The ram air effect will be very minor but worth it if you spend a lot of time at very high speeds. Not to many do!
Getting air from there will be turning the air different directions like the stock box does. I started making a C5 air bridge and intake to fit. The C4 air filter housing turns the air several times and is very narrow across the radiator. I tried to do a test drag racing last year and made a video. At my HP level it looked like it came up to 5 or so in the upper RPM. Here it is:
The C5 air bridge project is working well so far but needs finishing and a retest. I'll make a new video with that on soon.
The ram air effect will be very minor but worth it if you spend a lot of time at very high speeds. Not to many do!
Getting air from there will be turning the air different directions like the stock box does. I started making a C5 air bridge and intake to fit. The C4 air filter housing turns the air several times and is very narrow across the radiator. I tried to do a test drag racing last year and made a video. At my HP level it looked like it came up to 5 or so in the upper RPM. Here it is:
The C5 air bridge project is working well so far but needs finishing and a retest. I'll make a new video with that on soon.
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Ok, I'm confused. You're polishing the TB and you're adding smooth couplers but you're going to go back to a stock MAF instead of a ported one?
I've heard of people doing what you are doing with the air filter, but I would be afraid of the sheer volume of trash and / or moisture it could suck up from there. It may never be a problem, but it seems like it could be. I'd hate to suck up a slug of water just for the possibility of getting 1 or 2 hp.
I've heard of people doing what you are doing with the air filter, but I would be afraid of the sheer volume of trash and / or moisture it could suck up from there. It may never be a problem, but it seems like it could be. I'd hate to suck up a slug of water just for the possibility of getting 1 or 2 hp.
Based on what I've have seen on this forum, there are guys pushing way over 400rwhp with stock maf. And that ported maf may not be as accurate as stock. I don't think that trash or water will be a problem, it's not a big deal to check/clean the filter every few months, and driving thru 10" deep puddle with a corvette that has 4" ground clearance, would be just stupidity
