.so MY friend has been racing cars his whole life and knows them very well from driver's perspective, but almost never works on his own car so his mechanical application knowledge is limited on the engine side of things. On the other end of the spectrum, I am an Mechanical engineering student and have taken all of my fluid mechanics, heat transfer, and engine cycle courses, but I also have not had much hand on experience with modifying or tuning engines (except for with a couple 600cc na motorcycle engines (no turbos tho)).
Anyway our conversation quickly broke down into generalities and became mostly a theoretical debate. The issue we got stuck on is this: what exactly makes a t-5's car suck so bad in low rpms in its stock configuration?
The only certain logical explanation I could come up is that the engine simply that the does not flow air well through the entire combustion system until the turbo spools up and forces it through. While this conclusion is obvious and widely known (for our cars especially), What I really want to know is
What exactly the the most the single biggest contributing factor to the t-5 engine's air flow limitation in low rpms?
Is it the relatively long length of intake piping? (causing a delay between the time the throttle plate opens and when the boosted or ambient air pressure pressure is able to react to the low manifold pressure)
Is it the intake manifold?(air restriction, making the engine suck harder and the turbo push harder to flow air.)
The Head? (stock head + intake/exhaust valves don't flow well..low volumetric efficiency?)
the Turbo itself? (I already know this is a huge limitation on our cars, but why? Is the it the 15g's compressor design or it's exhaust housing that flows and spools poorly)
The last step is obvious, stock exhaust is pretty damn restrictive.
I had never heard this perspective before, but my fiend used to race stock Evos (td05 turbo) and said that his mechanics would actually turn DOWN the his boost (via mbc) in order to help throttle response and low-end power. This was a bit opposite to my intuition hearing it at first because I had never thought turning down boost would help anything. But I suppose it can make sense theoretically; if you lower boost settings, then the back-pressure between the engine and the turbo is lowered because the gas is not having to do work on the turbine wheel (since its being diverted through the waste-gate i assume?). But how much does this concept actually apply to our cars? whatever the case, I am pretty damn certain either way that I would not want go go this route to help reduce turbo lag on my car simply because then I would be chopping off the top end of the power band.
I then brought up the possibility of putting on a slightly larger turbo on the car (16t) or upgraded the 15g's compressor wheel might help. I have read on here that that these mods give more power, but know not exactly how. I can only reason that an upgraded turbo would not require as much exhaust energy to flow a given amount of air, therefore reducing back-pressure, spool-time, etc). my friend said theres no way would a bigger turbo ever help throttle response. (this is where I kinda stopped, realizing I'd reached the limits of his knowledge on how turbos work),
So what do you guys think?
The reason this all came up in the first place is that my down-pipe got a hole torn in it recently. This has started me down the slippery slope of debating what road to go down replacing to replace it. Whether to just go ahead and get an entire new 2.5" exhaust setup Ive wanted for so long (expensive)... or just replace the stock dp and upgrade the stock turbo and/or engine components (MBC, WG spring, n/a cams etc)..or maybe grab a even grab a used 16t (which would also get choked if i left the exhaust stock size). I only have a little more to spend on my car since I just spend a quite a bit on stage 0 stuff (tires, suspension bushings, steering rack, engine torque mounts, plugs, wires, k&n,vacuum stuff, etc). . so I'm trying to stretch it out.
I have maybe $500 to spend at most, a 2 car garage, and all the tools I could possible need (except for a decent torque wrench
) so I can do all work myself for the most part. I even have access to a machine shop for free fabrication if needed.What do you guys think? Also please correct me where my logic on things might be off. Its only recently that I have finally really began working on my car and learning EXACTLY how it works in relation to the mechanical theory I have learned in class.
