could one of you technical guys talk to me about surge tanks. i see pastrana has one on his car...basically an enlarged section about 2 feet long right before the fmic piping goes into the throttle body. whats the advantage?
I would guess that it works like a cap for a sub. More air available when you need it. I guess the last thing you want is to out suck what your turbo blows. But then, why not just get a bigger turbo?
Is it actually called a surge tank? The increase in cross sectional area will slow down the pressure differential created from the throttle plate rapidly closing. So in theory, the surge tank will make the turbo surge less in the end. There are other things to consider like head loss(friction in layman's terms) from the rate of change in area of the charge piping among other things. Apparently they feel its advantageous for their setup.
I was just thinking about modeling the flow in cosmos or ansys. wish i had more cfd modelling experience
I'm pretty curious now. Considering their limited mass flow and their use of antilag, maybe its a worthwhile thing to use to possibly extend the life of the turbo. :dunno:
yea I'm trying to go through possible uses too based on all the fluid flow i've forgotten :laugh: would like to see a picture of the transition between diameters and the placing in relation to the tb. i've seen pics before but my memory is crappy
i wish i could scan it in for you guys but i cant right now. basiically the pipe goes .. throttle body> 2.5 inch pipe> 2.5 to ~3.5 enlarger>full 3.5inch pipe for about 2 feet>reducer from 3.5 to 2.5>into the fmic
there is a typical piping reducer between diameters. its definitely a noticeable change..maybe its more like 4"
Its not a huge square box or something like that is what you're saying. My guess is to cut down on surge at the turbo itself. but its just a guess.
my guess is it has something to do with the antilag - if the throttle is closed but the turbo is still providing boost the expansion in cross section drops pressure so as you say, cuts down on compressor surge
Thats the only thing that makes sense to me. It doesn't drop the pressure per say, it just slows down that pressure wave that bounces back an forth between the turbo and the throttle plate. That psss pss ps you hear as surge is that wave bouncing back and forth, or a large EERRRRR at higher flow rates before the BOV opens. A higher pressures, that differential moves faster. I hear it in my car at higher pressures, and i think I need a lighter spring for my bov. I've got the -9, but I think I need the lighest one, -7. I only see 14inHG at idle due to my cams, and I think I could get away with the lightest spring.
I was chatting with my roommate and he also brought up that at the transition from the large pipe to the smaller one the velocity will also increase quite a bit. Whether or not its wnds up with more velocity than when using a straight pipe is beyond me.
My guess would be for a couple reasons. The major one being packaging, they are able to get a larger volume of air "waiting" in supply for the motor since there are such large Throttle position changes, so this is a way to kind of simulate a larger plenum without taking up the room since you can see al the stuff they have in that area already. It has to likely decrease right at the end again for regulation purposes, and is smaller to begin with due to ease of packaging again. Could be some benefit with velocity of the air having it neck down, except the transition doesn't look to smooth. Ben
That is also in a WRC car, not the us rally cars. Travis's Team subaru car is running the stock intake and box. I will have to look when I get home, I have some cross section pics of that pipe. There is some custom work in there to smoothen things out. Its not just a larger pipe attached to 2 small pipes. Russ
I don't see why they would want a larger plenum since these motors operate in a lower rpm range than say a street/drag car dur to the restrictor. I'm under the understanding that a larger plenum is good for high rpm. Another one of the guys here whos taken both of the fluids classes said the larger chamber will also smooth out turbulence from the IC. It could *possibly* have less turbulence than with a straight pipe all the way. My guess is still with cutting down the surge from the antilag and radical changes in throttle constantly present in a rally car. Maybe we should email SWRT and they'll give us the low down :laugh:
My guess is there's a nice intake "trumpet" or whatever you call those things inside the throttle-body end of that big pipe.
