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Blow through VAM set-up. V1.1.0

By Jon Moller

Written 2/15/2008

Introduction

The blow through VAM set-up, which will be referred to as BTV, is a great way to stabilize AFR’s in a ‘close to maxed’ set-up. The VAM is nothing more than a mechanical air meter, telling the ECU how much air the engine is getting. The internal spring is adjustable, (well get to that), allowing for a pretty wide range of tuning modes. It is possible to tune the VAM in its stock location and achieve the same or close to the same results as setting up in the BTV configuration, but you still leave out one benefit. Airflow is relative to pressure. Air at ambient pressure can only flow through the same space at a certain rate, the only increase being velocity. As velocity increases, so does vacuum, and thus there is a certain point where loss can be measured at the VAM inlet. This loss can be seen on a mostly stock motor.


Section one: the reasons, and the tune.

The cure this, (or patch it as some will state) the VAM can be set up in the blow through configuration. This means that the VAM will be after the turbo, but still before the Throttle body, or TB. I recommend that the BTV be set up after the intercooler (IC) as well. This allows the charge air to be cooled before going into the VAM. The VAM has an air temp sensor built in, which tells the ECU the temp of the incoming air. A colder measurement is better, and the temperature coming out of the IC is more accurate of an intake measurement than before it anyways. This way, the actual temp of the air going into the engine is what the VAM actually reads.

Now, back to the airflow. By placing the VAM behind the turbo, the air going through it is pressurized. Pressurized air occupies less volume, allowing for more molecules of air per real cubic inch. this creates more overall flow through the small little meter. (Look inside it sometime, its not very big). Now, when you increase volume and pressure, you will max out the door inside the VAM faster. This door is what conveys flow to the ECU. Since you are in essence increasing AIR FLOW, you must also increase FUEL FLOW. Well get to that later. With the VAM set up this way, you fix two problems. First, the Turbo is allowed to draw in air from a high flow cold air intake, with little to no restriction. This increases spool up, and makes for a happier, more linear boost reading. Second, the max overall flow of air from the intake to the tailpipe will increase, because the biggest restriction WON’T be the VAM.

Now, on to fuel flow. When you increase airflow to the engine, you must also increase fuel flow. To do this, you must remove the VAM cover (the small plastic cap). Inside you will find a sweeping resistor, that moves with the VAM door. To adjust fuel flow, simply loosen the springs mounting screw, and move the spring to a higher tension. This will be done entirely on trial and error. Some find that 11 spaces do the trick, others find they need 20+. Your engine mods and turbo will dictate. What you are actually doing is adjusting the overall tension, so that full boost at WOT, at 6000 RPMS has the door all the way open, but at no other time. At WOT, full boost, 4500 RPMS, the door should NOT be fully open. Basically you want the VAM to ONLY max out when the turbo, engine, and load is also maxed out. This way, the fuel increase will be equal to load and boost increase.

Now, overall, this will cause a lean condition. The door will be more resistant to opening, thus telling the ECU not to add fuel. As RPM’s and load increase, the ECU will add fuel, but not enough. To compensate, you will need an adjustable fuel pressure regulator (AFPR). Brand and style are up to you, it just needs to be vacuum referenced, and be adjustable. You will also need a wideband; again brand is up to you. Your goal here is to adjust base fuel pressure so that you have a constant AFR all the way from low load to high load. This is tricky, and not really fully attainable. You will always have rich/lean spikes, and unless you acquire a tuner of some sort, your pretty much stuck with them. However, a good street tune, and a good WOT tune is achievable with this method, assuming your willing to put in the time required to get it just right.

Section 2: Set up of BTV: how, why, and what’s easiest.

We’ve been over they why, and how to make it work, now how is it set up? First know that VAM was never meant to have boost in it. Since its not built for boost, it must be modified. There are lots of ways to do this, but understanding what usually breaks will help you decide. There are three issues with the BTV set up with a stock VAM. One, there's not really any lip to hold on the intercooler couplers. Second, the bottom of the VAM has been known to split from the top, busting the casing, and ruining the VAM. Third, the black cover needs to be easily removable for tuning, but must also hold boost.

For the first issue, I went to an exhaust shop, and had them make me some step down couplers, shown below. I used some metal epoxy (VAM is aluminium, step downs are steel) and some metal screws to hold them onto the VAM. Now I can weld a bead around the lip of the step downs, so the couplers will stay on easier.

http://i180.photobucket.com/albums/x7/jm.../BTVAM.jpg

Here you see the metal step downs. Cost me like $10 at my local muffler shop, and he custom fit them to the VAM inlet/outlet.

For the second issue, (the bottom blowing apart) there's lots of ways to keep the bottom on; screws, epoxy, brackets, the list goes on. My favorite? Worm clamps! This kills two birds, it holds the bottom together, AND the top on the VAM under boost, and is easily removable.

http://i180.photobucket.com/albums/x7/jm...andvam.jpg

You can see the clamp here. It holds the cover and the bottom together, preventing the VAM from splitting. You can also see the screws holding the step downs onto the VAM.

Also pictured is my by pass valve (BPV), being used as a blow off valve (BOV). I use it this way, because since the BOV is before the air meter, I don’t lose any metered air. There is no need to re-route the air back into the system. Here’s a shot of the bung I welded on to the step down for the BOV.

http://i180.photobucket.com/albums/x7/jm...OVbung.jpg

Now, you are ready to set this system up on your own TF. If you already have a front mount intercooler (FMIC) installed, this could cost as little as $50. If you have the AFPR, and some extra couplers, even less. The results are worth the work, (I dropped almost a full second of ET with it), and the mod is reversible.

Also note in the picture below how my piping is routed. It was done this way for three reasons. First, the charged air coming from the intercooler only has to make one 90* turn, instead of as much as three when set up conventionally. This brings up number two, with this set up, there is no need for a rotated upper. It’s added cost, and not really necessary. The third reason, the VAM can be placed after the IC, as recommended. You also don’t have to lengthen the wires for the VAM plug.

http://i180.photobucket.com/albums/x7/jm...lldone.jpg

I hope this has been helpful, and please feel free to critique it in any way. I will make revisions as necessary.

Thanks!

-J0N
yeah, when i was 'forcefully removed' from the board a yr ago, i was told that i 'contribute little'...unfortunately, that was VERY true.

im doing my best to change that, and some other things along the way.

thanks for the kind words.

-J0N


P.S-clarification time. in the write up it said "pressurized air has more volume". thats actually incorrect. it should read "pressurized air occupies LESS volume, allowing for more cubic feet of air to pass through the same space."

sorry for the boo boo.

-J
Quote:Originally posted by Sluggo:
Jon, I skimmed it over and it looks like you covered the subject very well. It makes me wonder how far you could go without a standalone or tuner, of course for a race only application. If you could get the VAM to open at the same rate as RPMs and use bigger injectors WOT would be tuned leaving idle and drivability a bit rough. Or am I missing something here?
Quote:Originally posted by jmthunderbirdturbo:
[b] Your goal here is to adjust base fuel pressure so that you have a constant AFR all the way from low load to high load. This is tricky, and not really fully attainable. You will always have rich/lean spikes, and unless you acquire a tuner of some sort, your pretty much stuck with them. However, a good street tune, and a good WOT tune is achievable with this method, assuming your willing to put in the time required to get it just right.
It appears that you need 2 tunes. 1 for regular driving and another for WOT pulls. I would think this is only after crossing the barrier of the stock injectors though. [/b]
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well, not necessarily two different tunes, just a compromise between them. my cars a track car, so the tune leans toward WOT. if its a DD, you tune accordingly, for off boost, off idle AFR's.

see, upping spring tension to compensate for more airflow(cause of a ported head, bigger turbo, etc etc)will go as far as necessary to get you the WOT AFR your looking for. but at a certain point, the spring is so tense, that it almost refuses to open at all until it sees boost, even at higher RPM's. this means your AFRs will suck until the turbo spools. at this point, you need a MAF, or something other than a VAM, cause anything but WOT will be lean, and undriveable. you have to make compromises with this mod, but for all of $50 to make the switch, its not a bad way to go.

-J0N
What I'm curious about is a track only car. Do you think you could run a 45# or so injector and tune it for WOT and still be able to start it and drive it around the pits? It would obviously be rich at idle with normal fuel pressures, but how rich?
yea, you could very well do just that. theres two kinds of injectors that are available from other fords that will do the job. i dont remember which two(i think its the 46's and the 52's), but the key to those is that they are ELECTRICALLY the same inside as 35's. that way the La3 wont know the difference, and it will still compensate for rich/lean the best it can, with you helping on the VAM spring and AFPR. this is my next mod, BTW. i have a set of 46's, and when i send them to cruzin to get them rebuilt, ill have them rebuild them with 35pph pintles so they will fit in the rail correctly.

ill let yall know how that works.

-J0N
it should be posted in the FAQ/tech articles section soon. i sent a 'word' document of it to Pete, so it should be ready anytime. but id be honored to see it elsewhere too!

-EDIT-changes made

-J0N
ok cool, ill add it as I redo the site.
Where are the best places to find the tubing? Would it be possible to do this with a volvo IC or would it not even be worth it?
well, im personally against an intercooler that has plastic endtanks, but others have good results, so whatever. ebay intercoolers are more than adequate for 99% of us, diesel truck takeoffs (isuzu NPR for example) are also a good choice. Ebay is where i got the tubing pictured in the writeup. it was $60, and it came with (6) 2 ft long pieces of 2.75" aluminum tubing, with a 90* mandrel bend on one end. the look like big 'L's. it also came with (10) 4 ply couplers, and (20) T bolt clamps. just keep looking on ebay to catch the deals. the best kits are the ones that have bends made into each pipe(like the one i got)cause you can cut/fit each pipe to your needs, and get a custom fit for next to nothing. piecing my kit together with the exact pieces i used would cost $300. dont be afraid to customize your setup, and have some fun with it.

and i cant stress enough the importance of running the pipes from the turbo to the DRIVER side first, then from the passenger to the throttle body. theres less bends, less piping(i measured), and it has lass boost loss(about 1lb per 10lb of boost from what ive seen.


-J0N
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