VTA or Recirculate?
 

I'm running a Link so don't have to worry about a MAF. Currently I'm running VTA with a small filter on my BOV, all is good. Or is it?

When I was running with a MAF (EMB) I originally ran VTA but had horrible lurching while lifting under part throttle. The theory goes that as the metered air was being vented the AFR would go rich.

Now with a MAP based setup this shouldn't be a concern right? Since the MAP is measuring the pressure in the manifold, and the loss of air is before the throttle body, so the MAP doesn't give a shit. At least this is my assumption, please correct me if I'm wrong.

Does the release of air prior to the TB have no effect on the way the ECU meters air?

Both FM and BEGI recirculate, as do OE turbo setups. So why shouldn't I? To be honest putting the filter on the end of the BOV is easier than the extra plumbing. Would recirculating provide for better drivability?

Someone needs to school me on the pros and cons using a MAP based ECU! :)

VTA for anything without a MAF, unless you want the stealth factor.
The MAP doesn't recognize any air before the butterfly, it could care less whether you are venting or recirc'ing. Recirculating is neccasary if you are using a MAF, you already know the reason. BEGi and FM do it for the quietness I guess, there is no benefit of one over the other if you are MAFless other than the plumbing.

Does the butterfly on the TB seal completely? How quickly does the BOV vent compared to the closing of the TB?

It doesn't seal completely, not all the time, but that is not the important part.
It is the pressure in front of the TB that gets vented/recirculated, once you lift and the TB closes (partially or full) and the manifold doesn't see the positive pressure anymore (that is why you feed the MAP sensor of the maniflod) and then there is vacuum in the manifold.
This is why you feed the BOV a vac/boost line of the manifld as well, when in boost, it keeps the valve closed, but when in vacuum the BOV opens since the spring now gets help from the vacuum to open up and vent/recirc.

Also, once you lift, it takes the manifold a split second to go from positive to negative pressure (well depending on the efficiency of the design).
It should be nearly instantanious.
Hope I make sense, and if I'm wrong someone please correct me.

VTA BOV on my car opens up very quickly when the TB closes. The gauge pressure on one side of the BOV piston goes negative quickly (post TB vacuum) and the pressure on the other side goes positive very quickly (dead head pressure from the compressor). The sudden pressure differential across the piston moves it quickly.

As said here and elsewhere, simply put, with MAP, (like Megasquirt) you can go VTA or recirc. VTA is cheaper and easier to plumb, recirc is quieter. With MAF you need to go recirc.

I don't quite agree with this. There is a reason all OEM's and serious motorsport cars (like rally for example) recirculate the air. The reason is that lag between shifts is all but eliminated when the air is fed back pre-turbo, meaning that the pressurized air helps keep compressor speed up when you lift.

agreed.....there is a difference between atmospheric and recirculating....with a map based system its not as drastic as a maf system, but recirc is ALWAYS better for the car

How applicable is this with a 4" intake tube off the compressor?

I have no idea, nor any data, but my gut feeling is that the proximity & orientation of the return line to the compressor is much more important. Here's how I am doing mine (that's a stock subaru part coming off the compressor intake) :

http://img235.imageshack.us/img235/217/dsc00466yo6.jpg

I agree. Though I have not rigorously tested this, I doubt that recirculation appreciably contributes to maintaining spool. This would take an appreciable buildup of pressure upstream of the turbo and/or a lot of velocity from the recirculation flow. With large intake plumbing (low restriction), and the relatively low amount of flow coming from the recirculation valve, I don't see that happening really.

Of course it probably helps a teeny tiny bit, but is it enough to argue for recirculation? If there is hard evidence that it makes a significant difference I am honestly curious and like to hear it.

I would say if you are running a particularly short intake tube pre-turbo it also would help less. I mean what are we suggesting here that somehow we are cramming more air into the turbo inlet? that we can ram charge our turbo?

I love how stealthy my bov is, barely hear it, and now with the quieter big turbo I can barely tell the car is boosted.

Dont be a ricer, RECIR!! :D

+1

VTA for sure. Depending on how you drive you can either have it vent loud, vent quiet, or not at all.

All turbo drag cars which make the most power VTA, they have no intake on the turbo.

Another point, if pressurized air is escaping through a piston, would it not slow down if it was fed out by a hose following the piston, kind of like a downpipe on a turbo slows the the exhaust gases? Just curious here, I am still going VTA regardless, roice4lyFe FTW!!!

The stock FC RX-7's recirculate, but I've had a few that others have built, and whenever they vented to air, I didn't notice any bogging (they ran a MAF).

Also, keep in mind the job of the blow off valve is to relieve pressure on your turbo so you don't have this huge surge of pressure in your inlet side when the throttle slams shut. If you don't have a big enough BOV, it could cause some damage to the turbo (maybe, possibly... depending how bad it is).

Similarly, if your piping to recirculate the air isn't large enough to flow enough air to relieve the pressure on the turbo, you could get that same effect and possibly damage your turbo.

awesome:

http://www.boostedmiata.com/new_miat...inished008.jpg

ugly:

http://www.boostedmiata.com/turboins...egidone004.JPG

May God help you if that is just normal ducting, if that thing gets a tiny slit your idle will be fugly, big enough and your car will die. :crx: That is of course if that map is still operating as a MAF:)

the AFM hath no flapper within. It hath been reincarnated as a MAP housing.

pipefather = houdini ;)