An Ugly Duckling Downpipe?
 

So I’m grabbing some of the last parts for my basic T25 turbo build on my NA 1.8. I just went to pick up a tacotaco manifold from a local (doing it on a budget and took the chance on the manifold), and as I was leaving, they said, “hey want to take this downpipe off my hands for free?” And of course I said yes.

Now that I’m home, I’ve removed the heat wrap that was on it and realized the whole thing was crush bent + the area behind the wastegate wasn’t designed well for flow. I’m a student on a budget who doesn’t own a welder (May have limited access to one though), so if this won’t be too restrictive, I may still use it. How much of a power loss would y’all expect with this downpipe if I chopped it behind the bung and fabbed 2.5in mandrel bend pipes the rest of the way to my 2.5in catback? Let me know if there’s other variables skipping my mind that need to be taken into account.

Dont ever expect to go over 250whp so my hope is the power loss wouldn’t be too significant from reduced flow.

Thanks for educating a noob:x:

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looks to be 2.5" piping?

crush bent is not optimal, but you already have it, so just send it.

no one will tell you exactly how many hp are lost, i would guess almost none, since you said a T25 turbo, which maxes around 230hp any way, so im sure this will be able to max out the turbo.

Not that it changes your point hardly, but I'm running a T25g which essentially sits between a T25 and T28. Should be able to squeeze a bit more HP out with a little more lag. But that's correct, piping will be 2.5 from turbo back. Regardless, that's the feedback I'm looking for. Just wanting to know if its worth still fabbing my own downpipe or even just refabing this one a little bit for better flow behind the wastegate.

It's not the power you lose, it's the spool and midrange torque. It will push the boost curve and torque curve several hundred RPM down if you swap to a 2.5" mandrel DP, and probably close to 1000rpm if you do a 3" DP.

Ahhh, that makes a lot of sense now that I think about it. Less backpressure = faster spool = shifted curve. Would the backpressure still be reduced that significantly by running a 3" downpipe to 2.5" exhaust instead of full 2.5"? While reading up on this stuff, saw Keith from FM say that full 2.5" was tested to flow relatively close to 3" under 250whp.

With that wastegate clearance, youre probably gonna have a hard time keeping boost creep under control. If i were you, which im not; i would integrate that port into the rest of the downpipe.
But, besides that theres no reason this wouldnt work so long as it fits. 3" is optimal but not a requirement

You have it. Install it and see how it works. It is far from the worst thing I have seen, they actually put some reasonable work into that.

This is my answer. At least for the time being I would just run it. Then start talking to local exhaust shops and get estimates for someone to get fab you up a proper down pipe.

Thanks for the responses! I was really thinking there would be more of a downside to running it but I think I’ll I’ll give it a shot. Won’t be putting the turbo system in untill this summer when I have the time so I may get to redo the wasegate area before then. Also as a side note, a friend of mine who races Spec Miata said he could connect me with a guy at our local track who would help me out with welding for the price of materials. Should have clarified that, but this was mainly a question of it was worth the time/effort/materials to make a new DP.

The downpipe is far more important than the catback. Pressure drops as the exhaust gases cool. What might be 1000F at the leading edge of the downpipe could be half that by the time it hits the cat. It will still make a big difference.

FM does lots of testing that is skewed by their location (altitude). The effect of the larger DP will be far less at altitude because there's less airflow passing through the system and less exhaust restriction as well. At sea level, it matters.

Run your crush-bent DP, you have it and it fits, but don't do anything that would hamstring you to it. Plan to replace it as the first thing you upgrade on the car once your basic setup is up and running.

I assure you that with his downpipe design he will not have any issues at all with boost control.

Alright then, thanks for the pointers!

Planing on running the DP I have to get it started then swapping over to a 3” mandrel DP once I have everything else sorted. Was planning on building everything modular anyways so I wouldn’t run into any issues.

Might be some time into summer but I’ll post the results of the change here once it happens if anyone ever needs the info.