| 1) The whole point of the recirc is to keep the turbo spinning freely and retain the compressed charge that you have already "paid for". If the preload in the recirc is causing surge, it defeats the whole purpose of the recirc IMO, especially on a 5 speed. Your concern is a bit misdirected here. The small bit of surge you hear is at the tail end of a bov/GHK'd recirc and will not occur when you are running up through the gears. When you are running the car out, shifting occurs very quickly and a BOV or a GHK'd recirc is going to literally move through a single cycle - it will open briefly during the shift and then close rapidly at the re-opening of the throttle. 2) When I replaced the GHK recirc with a stock recirc, the surge was eliminated, AND the car spooled noticeably better coming off of a shift. (I recognize that the second point is subjective, and may also be skewed by my relief that the noise was gone). So I am asserting that there is a (non-terminal) performance dis-benefit to living with surge. I can see there being a small benefit to maintaining higher spool rates with a non-modified recirc - it will likely open more than a GHK'd recirc and allow more volume to vent, thus producing less backpressure on the compressor and draining less spool during shifts. I'm not so sure that this would be to a level that could be perceived by human senses though given how rapidly the variables are changing during the course of an upshift event. I'm not questioning your abilities of observation by saying this - from my perspective I can only say that there either really is a notable difference or a degree of placebo may be influencing your perception. The only way to definitively call this would be to strap a car on the dyno, run her up through the gears to simulate a pull, and datalog the boost response for both setups. Next time I have the opportunity to do this I would be happy to perform the testing at my shop. Speaking to your perception of the original topic of "surge" though, as I said in the first paragraph reply above, it appears you are mixing the flutter/surge phenomenon into what occurs between high-speed upshifts. Again, this flutter/surge does not occur in this case.
3) The noise. Here there is a trade-off - Goose Honk or Flutter. The noise is quite likely specific to a cars mod list, sound-proofing, age, etc. Goose Honk or Flutter is a matter of personal preference. In my case, the flutter is way more annoying than the goose honk. I respect the preferences each owner has to their vehicles and it does require specific tailoring in a setup to meet their expectations. Sometimes, more often than not, this results in a compromise of some sort. Deal with flutter or deal with honk? Unfortunately it is very difficult to remove both - the honk is easy to get rid of but the flutter can only be removed by using a non-modified recirc - all BOVs are going to cause flutter - to varying degrees depending on how well they are adjusted. Too loose and you'll have issues with the engine smoothly settling down to an idle - too tight and you'll increase the flutter/surge. I know that with some time spent to develop a different recirculation valve you could eliminate both. In fact, you might want to look into some of the aftermarket BOVs which use a recirculating configuration. One main reason the stock recircs honk is because of the valve design - its valve face has a rubber sealing pad that creates a leak free seal to the valve seat, and that rubber seal is probably the biggest contributor to the honk; the honk is a result of the undamped valve/stem/diaphragm/spring assembly going through a mechanical resonance under certain flow and pressure conditions. It is very likely that using any of the larger aftermarket BOV units that incorporate a recirculation configuration would eliminate this problem - the larger valve body will tend to have more mass, a larger control diaphragm, and a much lower mechanical resonant frequency. Which gives me an idea for a BOV/recirc. Instead of using a piston or valve type configuration, if one were to take from a throttlebody and build a recirc using a butterfly valve instead, it would without a doubt eliminate the honk and eliminate the flutter/surge. The butterfly shaft would be controlled by a simple linear pneumatic actuator just like a wastegate actuator. This type of recirculation valve could only work as a recirculation valve - it would not be capable of meeting the requirements for a BOV though. But it would satisfy your preferences. =) And finally, to satisfy my curiosity once and for all, I will remove the stock recircs, block the pipes, and see what happens. If the flutter noise returns, then it clearly must be surge. If is eliminated, it clearly must have been from the recirc valve. I will post results. I think that you would be wasting your time by doing this for only the sake of seeing if it will produce that foh-foh-foh-foh surging sound. I assure you, it will do this. There are two different reasons for the noises we hear - one of them being the flutter of the bov and the other coming from compressor surge. The occurrence of either of these two conditions are going to vary from car to car where different components are used; charge pipe size, bov/recirc/ghk'd recircs, recirculating bovs, etc, turbochargers; and it will also depend on how a given BOV's preload is configured. I'll spend a little time to model a new recirculation valve based on the design I mentioned above. Unfortunately a standard throttlebody isn't going to work though - you cannot allow the butterfly plate to jam tight into the TB bore and expect it to smoothly re-open - it will bind if you do this, and in order for it to fully seal and not bind will require the bore to have "seats" that the butterfly plate will come to rest on as it closes. Possibly a new product could come out of this if there is enough interest....
Thanks for adding some additional clarity.
Enthusiasts soon understand each other. --W. Irving.
Are you an enthusiast? If you are out to describe the truth, leave elegance to the
tailor.
Albert Einstein
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