Yeah, I just realized a couple weeks ago that Sonny's is here! I've been meaning to give them a call ever since I realized that - you're right that I definitely couldn't afford them even if they'd touch my engine haha.
Thanks for the suggestion. Unfortunately, looks like Natural Bridge Raceway's drag strip is closed indefinitely/permanently. I might try a bit harder to find another nearby strip, but I haven't had luck in the past.

I'll just start in second and left foot brake the whole run

 

I replaced them 8/17, and I've only put 11,000 miles on it since then. My commute is very short though, and it's much colder than my car likes in the winter, so it's not a lot of healthy run time. I didn't run it a whole lot between the weather warming up and pulling the plugs, so maybe the fouling from the winter just hadn't burned off yet?

There's a lot I don't understand mechanically, but I have a pretty solid grip on the ECM's logic. I've spent many hours reading through the ANHT hac and many forum pages over on Thirdgen to nail that down. If the narrowband reads lean, the ECM gradually increases the INT and eventually the BLM, causing it to add fuel until the narrowband is oscillating around stoich. If the narrowband reads rich, the opposite happens. And fueling at a given point is relatively straightforward.

There is only one narrowband, on the driver's side, and all injectors are given the same PW - this is a batch fire ECM. So, the driver side being rich or lean drives the passenger side in the opposite direction. If the driver side is running richer than the passenger side, the ECM will pull fuel until the driver side is at stoich and the passenger side is lean. If the driver side is running lean, the ECM will bring it back to stoich and cause the passenger side to run rich.

 

That works. I did that on a Dirt bike once. It worked.

 

Alright guys, results are in. I threw an (admittedly sketchy) Autozone pressure tester on it and went for a drive. The gauge was only marked at a few low readings so I had to do some math to figure out the actual pressure using some trig. I wasn't able to quite peg it in 3rd, but I hit somewhere around 7.6 psi. Redline in 2nd was more like 6.8 psi. After I finished and shut off the car, I noticed the gauge wasn't at 0 psi, so it really could be anywhere from 7 to 8 psi, depending on if the gauge was like that to start.

Definitely significant, but maybe not as high as I was expecting/hoping. Basic physics says it's costing about 20hp to push that pressure through. Of course, the pressure will also reduce VE by limiting exhaust evacuation, so maybe... double it? Dunno. Actually, I just realized SuperL98 seems to have nailed guessing the pressure, as he ran his sim at 8psi. He shows a difference of 39HP between 8psi and 0psi. Of course, I'm at 7.5psi and I think a reasonable estimate with a high-flowing catted exhaust is 2psi. That's about 27HP if the sim is right and if it scales linearly with pressure differential. Time to start making some calls to see what it would cost to get some better exhaust on here.

I also noticed the gauge fluttering pretty noticeably, probably around 3 times per second. At idle and cruise the amplitude was pretty high (maybe +/- 2psi), but as load and RPM increased it reduced. Is this just from the uneven firing order, or is it a sign of a problem? I've read a fluttering vacuum gauge can indicate a problem, but my MAP sensor reads rock steady.

I'll throw a video up as soon as it uploads so you can see what I'm talking about.

 

I feel that you are seeing too much pressure. Way too much.

The fluttering needle is probably from the syncopated firing order from one side/bank.

 

I mentioned early in the post that 5 psi cost me 30rwhp on my 355, 8 will be a noticeable restriction.

 

Wow, somehow I missed that 30 rwhp number. Definitely seems like an exhaust upgrade is my next move. Did you happen to measure exhaust pressure with the true duals? Assuming 3" duals drop pressure to 1psi, that would estimate my gain at 30*(7.5-1)/(5-1) = 50rwhp. That's crazy.

Anyone have suggestions for a new exhaust or any idea what it will cost? Is a single 4" an option, or is dual 3" the obvious move?

 

A single 4" is about 12sqin area while 3" duals are about 14sqin. Not a huge difference. However, I suspect the real problem with doing a single 4" is finding a 4" cat and muffler that flow adequately (if they're even available in that size at all). It would also cost you an inch of ground clearance under the diff to run the 4" pipe under it vs dual 3".

My 396 had 3" dual with an X-pipe, 3" metal-substrate cats after the X, and dual 2.5" Corsa mufflers at the end, all done with mandrel bends. It was all custom built by the previous owner/builder (MSR on this forum). It flowed well and sounded glorious.

 

Thanks for the comparison. That's a good point on looking for a 4" cat, although I'd stick with my 2.5" Corsa mufflers. Dual 3" seems like the way to go, just thought I'd ask on the 4". That sounds like a great setup! Did you just have 3" rear crossover pipes that necked down to 2.5" at the mufflers? I'll probably look to have something similar (but, you know, the cheap version ) done.
I was actually concerned about how replacing the rear 2.5" Y with something bigger would affect sound, so I'm glad to have a comparison with the exact same mufflers. One thing I love about the Corsas is that they're not too loud when I don't want them to be. How was it at cruise? I probably don't want mine much louder at WOT, either. I know you got the car like that so don't have a before/after, but any estimate of how much louder it would be?


In case anyone is curious, it's about more than just cross-sectional area - pressure drop in a pipe is proportional to length/diameter * velocity^2. Velocity is inversely proportional to diameter^2, so pressure drop is proportional to length/diameter^5. Ignoring length, units, and many other factors ("proportional to"), two 3" pipes -> 1/3^5 * 1/2 = 0.002, whereas a single 4" pipe -> 1/4^5 = 0.001. This result says that two 3" pipes will have twice the pressure drop of a 4" pipe. This ignores many things, including friction factor (which decreases with velocity), so it's feasible they could compare more favorably.

Not trying to be pedantic, I'm just excited when my engineering knowledge overlaps with cars.

 

Anything above 3 is a problem honestly... I haven't tested mine but any other vehicle I've had its been 1-2 max... and thats on like single 2.75 pipe...

I'm curious, mine goes from 3 to 2.5 to 2.25 dual all the way with an X... I have the tester. I may just thread it in and see what numbers I get quick. That should give you an idea what you should have...

Edit: saw you talking about the map readings... mine reads pretty steady too but a vacuum gauge flutters more... the sensor or the readout seems to have a blend on it to smooth out pulses. Don't get me wrong, its not much anyway. Bad fluttering on exhaust would be a valve problem but even still... you have a cam with overlap. It will flutter.

 

I'd love the data if you felt like hooking it up!

 

Sure thing.

Also, see my add in above. Missed the MAP part.

 

Interesting that the ECM smooths the MAP data. Overlap hadn't crossed my mind as a source. I might still do a leakdown, if only to have a baseline if I'm troubleshooting anything else in the future.

Here's the video. Excuse the shakiness - I think we all know the C4 isn't the smoothest ride. (I didn't take the video - my passenger did.)

 

So your at idle pressure is 1... now pull the gauge and see if the car feels better bung open. The way I was told to check for back pressure is 1.25 max idle and 3 max 2500 rpms... just guessing you're beyond that from exhaust sounds....

 

No, the gauge is way off. When I shut off the car, it still read just over 1psi. I'm kinda concerned that keeping the bung open would torch the underbody pumping out searing hot exhaust gas - it's aimed right for the trans tunnel.

Any chance my cat is just clogged up? 8psi of backpressure seems like a lot on a 300hp engine, even with just a single 3" exhaust.
Edit: Next chance I get I'll do a WOT pull then hit the exhaust with the IR gun to check temperature drop across the cat. If it's causing 8psi of backpressure, there should be a huge temperature drop.

 

 

It was full 3" dual all the way back to the the mufflers. It just necked down to 2.5" at the muffler inlets. I figured that by then the gas had probably cooled enough (i.e. lost some velocity) that the neck-down didn't really create much additional backpressure.

It was kind of loud, honestly. More than I would have predicted for a car that had cats and mufflers. Granted it had no resonator anymore. That said, it was one of the quieter cars at any competition where lots of musclecars gathered. I have friends with recent-year Camaros, Mustangs, and Corvettes that have aftermarket exhausts (some with headers) that were all louder. My car was considerably louder at WOT as it approached peak torque. That was where the "x" seemed to "come on song," and it made a wonderful wail. That was never an issue in normal street driving. What was an issue was drone on the highway at ~70-76mph. For whatever reason, the Corsas either didn't attenuate it at all or they didn't attenuate it enough. Once you get rid of a center resonator and add long-tube headers and such, you've changed enough things that the tuning resonance may move. That's what I assume happened.

 

So the one flaw in that test is that they didn't adjust the carb jetting to have the same a/f for both exhaust systems. This would not happen in any car with a MAF because the engine would directly measure the amount of air going in and adjust fuel accordingly. I'm a little less sure about a speed/density setup, but I believe it would still account for the change in airflow using the MAP sensor. My point is that there would be less actual difference between the bigger and smaller exhaust setups than they found. The other point, of course, is that if a 620hp engine with 3" duals has virtually no power loss compared to open headers (as they found), then 3" dual is more than good enough for any 400hp (or in the OP's case a 300hp) engine.

 

Actually, they state often on that show that they optimize the carb tuning for best horsepower on each run. That's their stated reason for usually running carbs instead of EFI - they're all old skool guys more comfortable with tuning carbs than EFI.

But as you noted, the main reason I posted that link was because if a 600 hp big block gets by just fine with 3" duals, then there shouldn't be an issue with his exhaust.

I still think he needs to address that cold cylinder, though.

 

I just saw that in that particular episode they compared the a/f of the small exhaust to open-header-with-18"-collector-extension and with the small exhaust system it ran considerably richer (jump to 5:55 in the video). And that was the run whose power level they used for the comparison. So for whatever reason, they didn't optimize the jetting for that run, and that a/f difference would have accounted for at least some of the power loss.


Definitely. To drive that point home even further, they had a waaay longer exhaust than a C4 would have, so with a shorter exhaust system and proper a/f adjustment, they probably would have found smaller differences for the 2.5" system too. On a 400hp engine, even a 2.5" system would probably have little loss. But there's no downside other than cost in going with a 3" system, so you might as well go that route if you have the option.

Absolutely.