Is 3" exhaust with cat better than 2.5" with TP?
 

I will be visiting FM on my drive down to Texas for Miata World II. They have the 3" exhaust system available for the NA now (NB system can be seen at http://www.flyinmiata.com/index.php...number=06-39200) and it's my intention to upgrade from my 2.5" FM exhaust and test pipe. However, due to space constraints, FM needed to design a new 3" catalytic converter for this system and as a result, no off-the-shelf 3" test pipe will bolt up anymore.

My question is whether the change (which is not inexpensive) is going to be worth it. Will the restriction from the cat negate the benefit from the bigger exhaust/downpipe?

For reference, notes on my engine build can be found in this thread: https://www.miataturbo.net/forum/t31237/

Thanks everyone.

With the details in your signature alone, yes, the change will be worth it.

(Especially if you are able resale any (all) of the old exhaust...)

- L

Did you see the Enthuza group buy? I'm sure he can make you what you need. https://www.miataturbo.net/forum/t34748/

I know he can make it so you re-use your 2.5" test pipe, I just ordered that very setup.

No.

The cat is by far almost always the biggest restriction in a straight-through system.

In a 2.5" system with a 3" cat with reducers, 40-50% of the total pressure loss was in the cat. Actual measurements.

I'll be in Dallas that weekend. Buy the 3" and I'll take you to little mexico and they'll make you a test-pipe for like $30.

Yes, the change will be worth it.

I might just take you up on that, Hustler. Thanks. Send me a PM with a way to get in touch with you, if you don't mind.

I'll have to say, GO with the Enthuza. I loved mine. And you can get the reasonator and all for less than the FM. And it sounds great. Not to loud either.

Go with the 3" silverline reasonator and a 3" Magnaflow. And the welding is wounderful, and quaility is perfection.

Great guy at Enthuza

I've considered it, but my concern is getting it to fit with my full Butterfly Brace. Space between the chassis and brace is tight, even with a 2.5" system, so I'm afraid a "stock"-design 3" will interfere. FM tested their 3" with the BB brace, so I'm pretty sure it will fit properly.

jason
 

I would give Jason at enthuza a call, he knows his stuff talk to him tell him what you have, if nothing else you get to talk to one of the nicest vendors i have ever dealt with in my entire life.

Well you do have a point there.

He sells exhaust to everyone. He would know if it would work or not.

But either exhaust is a good choice.

I would wait until there is a test pipe. Why get the 3" with a cat. It really defeats the purpose. Then get it and throw it on..

OR go to a fab shop and have them make you one for around the same price. maybe less if you know somebody..

Results from 3" install
 

I thought I'd share the results of my install. FM added the 3" exhaust, downpipe and metallic-core cat on my way down to Texas. It's a beautifully-made piece, all polished stainless steel, perfect mandrel bends and a classy tip (it's a shame that it's mostly out of sight beneath the car). Surprisingly, it's slightly quieter (although not quiet) than the 2.5" system that preceded it. It has a wonderful, deep mellow tone at idle and low rpm, plus an addictive burble on over-run. It's also very quiet at highway cruising speeds, at least until you get on it.

What about results? On the way back, I stopped at FM again and we did some dyno tuning. Unfortunately I was having some heat-related fuel pressure problems after several back-to-back dyno runs and we were limited to 17 psi (Hydra pressure) compared to 19 psi last fall. Here is a synopsis (all corrected numbers, but referencing psig as measured by the Hydra and not the boost gauge):

Before (19 psi):
420 hp @ 7,063 rpm
345 lb-ft @ 5,860 rpm

After (17 psi):
396 hp @ 6,893 rpm
357 lb-ft @ 4,672 rpm

So, I gained a peak of 12 lb-ft at almost 1,200 rpm earlier, running 2 psi less boost. Pretty good results and a clear answer to my earlier question. I guess I'll be nice to the environment and leave the cat in. It also means that I don't have to hold my breath when I start the car first thing in the morning anymore.

Very nice gain, picked up some low end and will run cooler at less psi!

Nice car and nice setup although your numbers seem low for what's been done to the car. Any plans to turn it up and see what it will do? Do you live at a similar altitude to FM and that's why you had them tune it there or have you had the car on a dyno other than the one at FM for tuning?

Thanks JL. The numbers are lower than I've been hoping for, but we've hit a number of snags when trying to turn up the boost.

1) The LS1 coils get unhappy with the dwell numbers that work well at lower boost. This situation varies (heat related, perhaps?) and is causing us to tear our hair out.

2) Fuel pressure has been dropping off at high rpm. I have the FM fuel rail and 1:1 regulator (with integral FP gauge) and this is the history, with each stage being preceeded by dropping fuel pressure after several dyno runs:
a) Thought that Pierberg pump (from previous system) was not able to draw the current it needed. Added relay. This appeared to work and got me the numbers in my sig. from last fall.
b) Thought that stock fuel pump is giving up the ghost after 16 or so years. Replaced with 255 lph pump.
c) Thought Pierberg, which was left in-line as a back-up, might be impeding flow. Removed it.
d) Thought that wiring to stock fuel pump was not able to flow enough current. Added relay.
e) Ran out of time and had to call it a (second) day.

Next step, I think, is to replace fuel system with -6 lines from tank to regulator and replace regulator (just in case). Won't happen for a while though. If anyone has any other suggestions or ideas, I'll be happy to entertain them as I think the car is good for 400+ hp (uncorrected) or close to 500 hp corrected. At least until there's a good intake manifold available, coupled with cams.

To answer your other questions, all dyno tuning was done on FM's Rototest dyno. They are at a tick less than 5,000' and I live at a tick over 2,000'. Unfortunately, there is no-one I would trust to tune my car where I live.

How's the car run down at 2000' or so? The reason I ask is that when I lived/raced at 5000' I would use a completely different tune racing than when I was at a town a couple hundred miles away that was at 2000'.

It would be interesting to see how it does on a dyno in your home area using the fuel that is locally available to you. Also, what kind of numbers does it put down off the meth? That might be a better indicator of your fuel situation. Are you using the hydramist as more of a safety measure or is the tune very aggressive and taking advantage of it in every way possible.

Like I said before, I like your car and what you have done with it. I have been using you as kind of a benchmark for my own setup. That's one reason why I ask about the numbers not using meth, because I was told my own fuel system wouldn't do what I have been asking it to do.

I would say that it runs about the same, except that spool is considerably better (i.e. drivability is fine and it's damn quick). Peak boost changed a little with my MBC, but a turn or so fixed that pretty quickly. It was tuned on 91 octane Colorado gas ("squirrel piss") and I enjoy 94 octane here, so there is a nice safety margin with my tune. Undoubtedly I could tune more to the ragged edge locally, but I think the potential returns are outweighed by potential issues caused by eliminating this safety margin.

As you know, the Hydra works hand-in-fist with the Hydramist, so it will change maps (although not peak boost, on account of the MBC) depending on whether water/meth is flowing or not. Jeremy at FM did a couple of 15 psi or so runs with the WI turned off and the torque curve is reduced proportionately to reflect the more conservative timing numbers used.

One interesting thing that came out of the dyno work is that playing with the WI duty cycle measurably affects the power curve. Too much water and power drops as the flame front is quenched. Too little and knock starts to show up. This shows the benefit of a mapped WI system compared to "dumb" ones that merely turn on or off at a pre-set boost level.

To answer your main question: Higher boost with 91 octane required the WI to be functioning (at least it does at 5,000'). Here at home, I can run the same boost level using 94 octane sans WI, suffering only the slight drop due to the more conservative timing. I normally leave the WI on all the time to enjoy the slight bump in power and also for the safety margin. A tank lasts a long time and is very inexpensive to fill (distilled water and a bottle of Heet - totalling 2 litres or so).

Again, I could tune to the ragged edge but let's face it, the car is plenty fast enough already. Another 5-10% may allow me to play with Z06's, but I'm fairly happy picking on M5's with what I have.

I want to spawn off this thead and change the topic (rather than make a new thread) since my question is so relative....

I have full custom 2.5" exhaust I just made last weekend. I was thinking since I have no cat, would it be advisable to install a cutout just behind my flex pipe (essentially under the driver's seat)??? I have a 2.5" cutout too, thats what got me thinking about it. I mean, I just made this 2.5" because my DP came w/ the car but I've hacked the hell out of it, it's a v-band welded to a 2.5" 90* bend.

Had to cut both o2 bungs off and relocate them. PITA, but it's done now.

So, 2.5" w/ no cat and some kinda knockoff ebay muffer, or 2.5" w/ cutout left open?

(obviously, I realize there will be lots of noise w/ the cutout open)

Oh, PS:

If I install the cutout, how far down stream of the WBo2 should it go so to not mess up any readings? I'm thinking of putting the cutout just in front of the driver's rear tire and weld an oval pipe to the cutout and have it exiting in front of the driver's rear tire. I wanted it exiting on the passenger side so I didn't have so much noise in my ear but... getting under the torque arm would be impossible.