I'm not insane!! Better Exhaust = Better Spool
 

Late last week, I mounted up my new (to me) exhaust. Comparison:

Old:
1. Stock Greddy DP.
2. OEM header pipe from triangle flange to cat. (I am NOT going to miss that damn triangle).
3. High-flow metallic cat.
4. Cobalt exhaust system (62mm + resonator + chambered muffler). 62mm is about 2.4".

New:
1. BEGI Greddy replacement DP with SG pipe for the wastegate and 2.5" termination. (thank you Kotomile and FRTFun -- I'm the 3rd owner)
2. 2.5" test pipe.
3. Enthuza 2.5" turbo racer exhaust with Borla straight-through muffler.

I've been retuning the car for the larger exhaust. While tuning EBC, I noticed that spool to 10psi is delayed by about 300RPM compared to the old exhaust. I've also noticed that it takes quite a bit more EBC DC to maintain 10psi. I'm surprised as I was expecting to reach 10psi more quickly, not later.

Here's a theory. It may be that I've increased flow and VE to the point that the turbo is having to work a lot harder to pressurize the intake. Kind of like mounting a larger turbo.

Does that make sense, or am I off my rocker? I'll post up some comparative power numbers tomorrow.

Did you retune?

Those are big changes to your exhaust system for a turbo car, a lot of power free'ed up. You should have seen some significant changes in AFR.

Spark map is the same.

I got the above numbers after burning a VE map modified by an initial round of VE analyze. There were some pretty good changes to the fuel map.

The data came from Open Loop EBC tuning runs that I did this afternoon. Basically, with the EBC solenoid at 100% duty cycle (closed, so no wastegate), the old exhaust would hit 10psi at ~3800RPM while the new exhaust hits 10psi at ~4100RPM.

Same outside temp, same altitude, same conditions, same roadway, same gear, same car weight?

Conditions were very similar. Done in same location. Done in same gear. My car may have picked up some weight due to adding a roll bar -- otherwise should be very similar (i.e., full fuel both runs). It was actually hotter the day I got the better spool, which again, is opposite of my expectations.

Out jogging this AM it occurred to me that I may have changed the behavior of the wastegate. It's the stock Greddy can, and it might be having trouble staying shut now that it has its own exhaust channel. I'll do some experiments with that (getting wastegate-only data and changing preload, etc.) and report back.

Any other thoughts?

Separated gasses downpipe has a lot less pressure on the backside of the flapper and may even induce a slight vacuum effect if the merge is done correctly. I think you are on to something.

You are probably even making a little more power at the same 10psi with less backpressure in the exhaust system.

I sure hope so!!

Power comparison to come (I did baseline Virtual Dyno pulls prior to mounting the exhaust). Just need to get safely retuned and understand the spool situation first.

Can you tighten the wastegate actuator?

Buy my Enthuza 3".

That's what I'm going to experiment with. Basically, log a series of pulls to see if I can improve my spool with more preload while also keeping wastegate-only boost to a reasonable level (~7psi). I've been using wastegate-only operation for trackdays and the car has been completely reliable + matches my current skill level.

Expect to post some data tonight if anything comes of it.

I had problems getting the flapper to seal on the seperated gas DP. Dremel fixed things.

That's interesting. Did you just even out the mating surfaces or create some kind of channel for the WG to sit in?

BTW, picked up this Enthuza and test pipe for $100 from another DFWMiata member. Didn't know yours was for sale.

I had to clearance the divider on the DP, not the turbine housing.

Oh, OK. Yes, I checked that my WG flapper had free movement during install. I had already read multiple tales of woe (both not closing and not opening) regarding the issue. I'll double check that as I mess with the WG this evening.

Mystery Solved
 

Bottom line, I was comparing 3rd gear pulls with the new exhaust to 4th gear pulls with the old exhaust.

However, in discovering the above I did gather a lot of data on my wastegate. Since everybody loves a picture:
https://www.miataturbo.net/attachmen...1&d=1380153429

What we have is a comparison of 4 wastegate-only pulls. MAP on the vertical axis and RPM on the horizontal axis. The pulls are as follows:
1. Original wastegate adjustment with original exhaust (blue line)
2. Original wastegate adjustment with new exhaust (red line)
3. Wastegate tightened 2 full turns with new exhaust (yellow line)
4. Wastegate tightened 4 full turns with new exhaust (green line)

The lines are polynomial data fits. If I had plotted the actual data, it would be a jumbled mess. The curly cues at the line ends are artifacts of the polynomial equation and should be ignored.

Observations:
1. New exhaust spools better by 2-300RPM. Notice how much more vertical the spool line is below the wastegate bends.
2. There was no issue with the wastegate flapper coming off the seat. The lines for all 3 adjustments almost perfectly fall on top of each other before the wastegate bends.
3. I like the tighter adjustment (green line). Faster spool but similar ending boost level as the original exhaust line. Gonna' keep it.

Mystery Solved
 

double post

Great success!

Now we need vids to hear how it sounds

Doesn't sound much different than the old exhaust. I'm surprised by that. I really like the sound -- just a bit "sportier."

Standby for a Virtual Dyno comparison!

Incidentally, it is interesting how you can "tune" your wastegate with this adjustment. The slope prior to the wastegate bend (assuming the flapper is seated) is simply a property of the overall system. Opening up the exhaust makes the slope more vertical.

What the wastegate adjustment really changes is the slope after the wastegate bend. Ideally, that slope should be near horizontal. Notice how the post-WG bend slope of both the old and new exhaust with the original adjustment are nearly parallel (and NOT horizontal). Obviously, this tightening was needed.

VD Comparison
 

As promised:
https://www.miataturbo.net/attachmen...1&d=1380248952

The above compares a pull done on September 9 (old exhaust -- blue line) to one done today (new exhaust -- red line). The days were remarkably similar (both 92°F, baro was 29.96 on the 9th vs. 29.83 today). The only real difference is that I had a lot less heat soak on the 9th (peak MAT of 180°F vs. 213°F today). I messed with the SAE correction values to try to compensate for that -- have no idea whether what I did is valid though.

The new exhaust clearly helped spool. You can see the boost rise much earlier and the torque fatten at lower RPMs. Surprisingly though, it had very little effect at higher RPMs. Not what I expected at all. Pretty disappointed. Perhaps I'm running low enough boost that the old exhaust wasn't actually that restrictive for power?

:dunno:

I don't think I'd want to be at 12.5:1 at 10 psi. That half point of AFR might be hiding a bit of power, but I wouldn't go any leaner than the 12:1 you're at now.