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I'm a college student that is turbocharging their '99 Miata on a budget with a goal of 200-240whp. I've got a 2560r and an eBay manifold off of this forum. I've got experience welding and with the help of my grandfather, I believe we can make our own exhaust. Here's the thing: we've got plenty of tight radius 2.0" pipes already laying around and another 24 ft of straight piping from his own dual exhaust V8 projects. Needless enough to say, we have enough to make my entire exhaust 2.0" for free, but from what I'm reading some people are saying there's a "maximum" flow ~160hp of 2.0" tubing and that I should be using 2.5" or 3.0" at least. I understand that turbos are affected by pressure differentials and if the pressure differential behind the turbine is significant it would greatly affect the turbo. So in other words I wouldn't be able to crank up the boost past the maximum flow point, reportedly 160hp, correct?
The other problem is that I already have a turbo outlet flange that is 2.0", two 2.0" V-band flanges, and a 2.0" flex pipe (and I plan on running no cat obviously) to a 2.5" Roadstersport3 muffler. The only real restriction should be the muffler and the bends and I would think that a 2.5" muffler would be more restrictive than a 2.0" bend.
If 2.0" would be a restriction, how much better would it be if I only made the downpipe out of 2.0" and stepped the midpipe up to 2.5"? What if I only made the first downward 90 degrees bend 2.0" then immediately stepped up to 2.5" as soon as possible?
Restrictive enough that I would recommend selling the RS3 locally for ~200$ and buy 3.0" diameter exhaust piping instead of using existing 2" exhaust piping. It will spool much sooner (don't have data on how much faster but it is substantial)
Well, part of the problem is that the difficulty of the project gets much greater if I do a 3.0" bend. The downpipe flange I've got right now proves that I cannot have a larger outlet on the flange without it being an oval shape. I'm afraid that it would take more than one attempt to get that oval bend right (wasted $) plus possibly modifying the rear shelf.
Here's my current thought process concerning the downpipe:
2.0": easy and free, but may significantly affect spool and flow capacity.
2.5": easy but cost money, flows well enough but maybe leaves 10% spool on the table
3.0": hard and costs money, would need a new muffler too, but flows the best
The work gets a lot harder after 2.5" but doesn't really net me any more power, just how soon the power comes on. Because of that it's hard to justify 3.0". Plus if I am being honest, I want a mildly quiet exhaust and love the way the RS3 sounds. I would use that muffler even if it costs me 200rpm to hit max psi. It would save me the fab work of putting in another muffler too. I have an electronic boost controller paired with an MS2 which should help some with the spool time. I'm more asking about if a 2.0" downpipe is going to interfere with my power goals and if I could possible get away with the mid-pipe being 2.0".
Its a bit unusual, but you could run 2 inch out of the turbo, and 2 inch out of the wastegate and connect that to a X pipe connection that combines both sources.
Then run a further pair of 2 inch pipes side by side out to the back. Two 2 inch pipes will flow the same as one 2.8 inch pipe which would be about right at your power level.
Once its spooled up at maybe half your total rev range, there is very little extra flow out of the turbine beyond that.
Just about all of the extra exhaust flow goes through the waste gate after its reached full boost.
That sounds like a really complicated way to achieve very little. The wastegate should be fully closed until just before reaching target boost, that spool time is a huge advantage of big exhaust.
OP, where won't you're downpipe flange fit if it were 3"? There are hundreds of people that fit round 3" exhausts. It'll be worth it.
You need 2 1/2" for your power goals. It will work OK below 250 hp.
But...
If you have to purchase material anyways.
Going 3" from the start will make it easier to hit your goal and you will never wonder "is exhaust restriction the reason I cannot make the power I want."
It will suffice for any realist power goal you might want later.
If 250 is lame 2 years from now you will already have the most difficult fabrication job done...
Save that 2" stuff for another project. It might work for the intercooler tubing as well.
If you're doing the effort of a custom exhaust, I say do it right or do it twice - 3" and a good muffler. However, with your goals what I understand of your situation, what you have will likely work but i can see 2" being a bottleneck towards 240whp
Just remember that you're in there doing the labor and you will have to do it again if and when it doesn't meet power goals - present or future.
That sounds like a really complicated way to achieve very little. The wastegate should be fully closed until just before reaching target boost, that spool time is a huge advantage of big exhaust.
OP, where won't you're downpipe flange fit if it were 3"? There are hundreds of people that fit round 3" exhausts. It'll be worth it.
I got the ATP Turbo 2" flange. It has a 2.0" hole more directly over the turbine outlet and the hole comes withing 1/4" outside edge of the flange. I was wanting to avoid having to make an oval shaped connection that is necessary for one of These. I considered one of these but it would add extra length to something already hard to fit.
Originally Posted by WigglingWaffles
If you're doing the effort of a custom exhaust, I say do it right or do it twice - 3" and a good muffler. However, with your goals what I understand of your situation, what you have will likely work but i can see 2" being a bottleneck towards 240whp
Just remember that you're in there doing the labor and you will have to do it again if and when it doesn't meet power goals - present or future.
Yeah, the worst thing I want to do is get all of this done and only be able to make 160whp just to have to make the exhaust again. If someone would buy this RS3 Muffler I might go to 3.0" after the downpipe. Tomorrow when it's light out I'll take some pictures and post a listing.
Probably the easiest way to get a really tight smooth bend right after the turbo would be a 2.5 inch ninety degree weld-el heated red hot and gently hammered into the required oval shape.
That would get you past the shelf, and the thicker pipe will be much easier to weld onto the turbo flange.
The outside diameter of that will be 2.875 inches, and a three inch pipe could go over that for the rest of the system.
I wouldn't spend the time and effort making a 2" exhaust. That said, if the car already has an exhaust on it, just connect to that with what you have and send it. I am running a TD04L-13t on a 1.6 and, just to get up and running, I grabbed a local $10 used test pipe (melted my cat learning to tune, oops...) And hooked up my 2.5" downpipe to whatever ebay cat back is on the car from the PO. For uh "testing only" (I say, two years later). Works fine. I am making ~200hp to the wheels on 10ish psi give or take.
Some day I will build a proper exhaust for this thing.
I've yet to pull the trigger again, but I'm back with another plan and I need you all to tell me if it's dumb or not...
Originally Posted by Warpspeed
Its a bit unusual, but you could run 2 inch out of the turbo, and 2 inch out of the wastegate and connect that to a X pipe connection that combines both sources.
Then run a further pair of 2 inch pipes side by side out to the back. Two 2 inch pipes will flow the same as one 2.8 inch pipe which would be about right at your power level.
Once its spooled up at maybe half your total rev range, there is very little extra flow out of the turbine beyond that.
Just about all of the extra exhaust flow goes through the waste gate after its reached full boost.
Similarly to said above, what if I got a divorced port flange, like This, then dump wastegate to the atmosphere and run 2" piping after one of these for the turbine outlet? In my head, I could see it still limiting the spool-up almost identically to a 2" combined outlet, but there would be more potential for power because the wastegate gases being separate. Idk, just another thought I had and if you all think it's dumb I'll continue on. I'd want to be around at least 200whp but 240 would be cool too lol
I've done that and none of them said anything about 2", I even used google and did "site:miataturbo.net" searches. As for the Begi one, I've looked into it already I'm thinking it's turbine outlet is 2-1/4" then it reconnects the two into a 2.5" exhaust, and you're supposed to use something 2.5" or bigger the rest of the way. So, in other words, the Begi is completely different than what I'm asking about since it recombines the wastegate gases and collects into a 2.5" pipe. In fact, find me a thread with the turbine outlet going into a 2" exhaust while the wastegate is dumped separately and I'll venmo you $10 lmfao.
Now, what I'm asking is that with a 2.0" turbine outlet, wastegate dump, and a full 2.0" exhaust back to a 2.5" RS3 muffler would I be limited to under my 240 whp max goal? I've been using the search feature and the closest thing I've come to finding an answer for my question was someone saying they had customers make 450 with a 2-1/4" turbine outlet and a 1.5" wastegate outlet (don't know if collected into the exhaust or separate) which is very similar thing I would be using. That however doesn't give any insight to spool times though, which something I'm asking about. I'm also asking about how much more exhaust flows through the turbine at 180hp vs 240hp. If it is only marginal and the majority does go out of the wastegate after peak boost is achieved, as another user suggested, then in my mind a 2" exhaust would be fine to achieve 240whp. But if it takes more and more exhaust flow out the turbine to keep the turbo at the correct psi at the higher RPMs then maybe 2.0" wouldn't be sufficient. All I'm looking for is someone with some experience to confirm my logic. And yes maybe spool-up wouldn't be the best but it's a dang 2560R with an electronic wastegate controller. In the worst-case scenario I'm thinking 3500rpms to hit wastegate, would that be an accurate guess? I looked through the spool data thread for 2554s and 2560s with varying downpipes and it seems with a normal 2.5" exhaust I could expect to hit wastegate around 3000rpms.
Look at it this way.
If your engine peaks out at something like 240 at the wheels at 7,000 (or whatever).
And it reaches full boost at 3,500 that might be very roughly 120 at the wheels with the wastegate just beginning to open.
Will the exhaust after the turbine, flow enough for 120Hp at that point ? A two inch full muffled system should do it pretty easily, and the difference in spool will be absolutely minimal.
Remember, exhaust back pressure rises square law with flow. Back pressure will be one quarter of what it would be if you tried to feed 240Hp worth of exhaust through that same two inch system.
A two inch system would choke pretty badly at 240Hp, but at 120Hp is only going to have one quarter the back pressure.
As long as back pressure after the turbine is maybe one tenth of the pressure driving the turbine, its going to have almost no effect. Going much bigger just makes more noise.
So its going to spool o/k, but it would very quickly choke after that, if two inch was all that was there. Basically a weakening mid range and no top end power.
Your turbine discharge from memory is 42mm, so 50mm (about 42% greater flow area) is not unreasonable for a turbine only dump pipe.
The question then is how to treat the wasegate discharge? That opens only after the turbine driving pressure is quite high at full boost. So it does not need a huge flow area to control well.
From the point of reaching full boost, all the excess exhaust flows through the wasegate, and it could easily be as much as flows through the turbine at flat out full power.
Quite a few possibilities, but if you are going to combine it with the turbine flow, you will probably need either a second two inch pipe, or maybe a three inch pipe to prevent choking the turbine.
There are advantages to keeping the wastegate discharge separate, such as not needing to muffle it (as much) as the turbine outlet which definitely needs some muffling. It really all depends on the use of the car and your own feelings about noise and practicality.
I have a 2560r on my car and I had a 2.25" exhaust with a 2.5" downpipe and it spooled up around 4000rpm.
I then installed a full 3" exhaust, a larger fuel pump with a return style fuel system, and LS coils. I never drove it until I had it tuned. It now spools up around 3250rpm.
Look at it this way.
If your engine peaks out at something like 240 at the wheels at 7,000 (or whatever).
And it reaches full boost at 3,500 that might be very roughly 120 at the wheels with the wastegate just beginning to open.
Will the exhaust after the turbine, flow enough for 120Hp at that point ? A two inch full muffled system should do it pretty easily, and the difference in spool will be absolutely minimal.
Remember, exhaust back pressure rises square law with flow. Back pressure will be one quarter of what it would be if you tried to feed 240Hp worth of exhaust through that same two inch system.
A two inch system would choke pretty badly at 240Hp, but at 120Hp is only going to have one quarter the back pressure.
As long as back pressure after the turbine is maybe one tenth of the pressure driving the turbine, its going to have almost no effect. Going much bigger just makes more noise.
So its going to spool o/k, but it would very quickly choke after that, if two inch was all that was there. Basically a weakening mid range and no top end power.
Your turbine discharge from memory is 42mm, so 50mm (about 42% greater flow area) is not unreasonable for a turbine only dump pipe.
The question then is how to treat the wasegate discharge? That opens only after the turbine driving pressure is quite high at full boost. So it does not need a huge flow area to control well.
From the point of reaching full boost, all the excess exhaust flows through the wasegate, and it could easily be as much as flows through the turbine at flat out full power.
Quite a few possibilities, but if you are going to combine it with the turbine flow, you will probably need either a second two inch pipe, or maybe a three inch pipe to prevent choking the turbine.
There are advantages to keeping the wastegate discharge separate, such as not needing to muffle it (as much) as the turbine outlet which definitely needs some muffling. It really all depends on the use of the car and your own feelings about noise and practicality.
So the route I ended up deciding to go is 2.5" for the downpipe and then 2" for the rest to the muffler. The budget didn't allow for me to make the entire exhaust 2.5" right now or I would. The downpipe materials weren't more than $60 from Ace Race, so obviously that's worth it. My logic is that the turbine outlet is 42mm (you're correct) and the wastegate hole is 1", for a combined area of 3.6pi or less than what a 2" inch pipe would have. The 2.5" elbow should help with the pressure differential some and if it can flow out of the turbo while hot and expanded the later down the line I would think 2" would be ok for the time being.
Originally Posted by fjs0001
I have a 2560r on my car and I had a 2.25" exhaust with a 2.5" downpipe and it spooled up around 4000rpm.
I then installed a full 3" exhaust, a larger fuel pump with a return style fuel system, and LS coils. I never drove it until I had it tuned. It now spools up around 3250rpm.
Good to hear. What kind of power did you make on 2.25" and gains going to 3"?
Good to hear. What kind of power did you make on 2.25" and gains going to 3"?
It's putting down 275whp and 263ft-lbs of torque now. I'm not sure what it was before, but it's a completely different car. I attribute most of that to the tune, but spooling quicker definitely helps. I asked for a conservative tune, so it might have more in it.
11-20-2020, 05:45 PM
How restrictive would a 2.0" exhaust really be?
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