MFI: Sport Muffler lean spikes / flat spot 2-3k rpm

[PatHidden]

Hello all,

I've read through many MFI posts regarding sports mufflers on here and on Pelican.

Pretty much, I am at the point in the CMA process where I am logging AFRs. Currently, while watching live AFRs, I am getting pretty bad lean spikes between 2-3 rpm with minor throttle inputs. I can increase throttle angle and get through the flat spot and after 3k rpm the car behaves normally. I can richen up the main rack to lessen the affects of this flat spot between 2-3k but the rest of the RPM range is then pig rich (like 9 to 11). I can make adjustments where WOT AFRs around 12.5 to 13.5 from 2k to 6.5k, but there is still a very quick lean spot right when I push the throttle to go wide open. However, if I learned anything, just cause AFRs are nice for WOT doesn't mean the car will have good drivability. Normal street driving seems to be the 2k to 4k range at less than full throttle.

So after reading online about sports mufflers, I am thinking my Dansk 2in 2out muffler might be the problem.

However, I came across the post below:

Changing from a stock muffler to a sport muffler or any other exhaust system other than stock on a MFI engine will warrant a close look at the air/fuel ratios under load. What I have done in the past was to document the A/F ratios a various loads with the original stock muffler using a portable A/F ratio meter like the LM-1 or LM-2 by Innovate Motorsports sells. Change on the original muffler to the sport and run some A/F's again. You will most likely see a few lean areas with the sport muffler. Some engines get way leaner than others and this would require an adjustment on the MFI pump to try and correct that. What to adjust depends on where the lean spots are. The best way look at this IMHO would be an engine dyno run with the sport muffler, but now were talking $$ but more accurate data.

Given that post, it is possible to tune around this or maybe my car just falls into the "some engines get way leaner than others" category.


So should I ditch the 2in 2out muffler? OR dig in and start adjusting the governor screws?

If I change the muffler, is a stock 2in 1out the only option? Can I do a 2in 1out sports muffler? (assuming the sports 2in 2out cause the issues and not the sports 2in 1out). I do like the sound of my muffler but if it necessary, I will swap it out. Are there any aggressive sound mufflers that work with MFI?

If I start adjusting the low speed high speeds screws, do those allow for enough range to solve for sports muffler flat spot? I thinking of starting with 3 clicks richer on the low speed screw, then log AFRs. Adjust according to the AFR logs and drivability. This will be a pain and will take plenty of time.

low speed / high speed adjustment



picture of my setup. Stock 911e with bolt on stuff. Nothing crazy.

[tom1394racing]

FWIW…. I have a 3 L twin plug engine with 46 mm PMO‘s in my 73 RSR clone. This engine has a rally muffler that allows me to cap off the twin outlets and run a side outlet with easy interchangeability. What I found is when I run with the twin outlets open, basically open headers, the flat spot is in the 3 to 4000 RPM range right in the middle of the drivability window. If I cap off the twin outlets and use the side outlet, basically a muffled engine, the flat spot moves much lower in the RPM range and is much more satisfying to drive on the street.

[Chilling in VT]

I have been struggling with this for a few years as well. I had a two in, one out Dansk "sport" muffler on my 2.7 mfi engine. When I first put it on, the shop I used had it tuned perfectly. Not flat spots and just smooth power. It ran great for about a year and then I went on a multi day drive with some buddies and had many spirited drives. One day the muffler sounded a bit throatier and the car developed a flat spot between 2700 and 3500 rpm. I could drive through it and it ran great over 3500 rpm. We tried many different tunes, rebuilding the throttle bodies, rebuilding the distributor and cleaning the injectors. Nothing helped. I drove it this way for a bit but was always a bit disappointed.
I too read every post I could about people having this problem and decided to try an early, stock muffler. I found a nice Bischoff one and had it sent my way. I swapped it out and the flat spot immediately went away. All I could think of was that during the multi day, 1500 mile trip, I must have blown out some of the crud out of the sport muffler and it had less resistance, causing the car to run poorly.
I miss the sound of the "sport" muffler but I am very happy with the performance of the stock muffler. I have read that some people have had no issues with running a sport muffler and I am not sure how it works for some and not others. All I can go with is my experience. There are supposedly a few other sport mufflers that will work with mfi (Monty mfi muffler and possibly one of the Smart mufflers). I'd love to hear from someone that is running one of those and see how they work for them.

[myflat6]

I have 2 in/out Dansk sport muffler and have no issues with my 2.5 MFI build. Power is great and smooth - I have the dyno sheets to prove it. However, I believe there have been different iterations of their sport muffler over the years. I believe I have seen different length tailpipes and different diameters? Mine is a bit older version I believe - but can't say for certain. I bought mine from a guy that had it sitting for his build and then ended up not finishing the build and sold stuff off. So many variables at play, especially with modified motors.

The above post has me concerned that maybe I will run into a problem at some point.

[SFE]

Here is my muffler, don't know exactly what it is...

Same flat spot you have...fixed by adding 6 then reduced to 3 clics RICH to the black screw (med range).

But I still have some exhaust pops on very light throttle (like downhill), CMA says impossible to eliminate completely this behavior !

[myflat6]

Here is a photo of mine. The tailpipes measure 178mm long with 50mm openings. Below the Dansk logo it says 92.510DUAL 3415

[PatHidden]

Same flat spot you have...fixed by adding 6 then reduced to 3 clics RICH to the black screw (med range).

That is promising. is your muffler a single tip sports muffler?

This weekend, I will double check the CMA list, and then adjust the med range screws. I will report my findings.

[frederik]

I had an Ansa muffler on my 2.2S, we couldn't get it to run well using the stock MFI pump calibration. The muffler looked just like the stock muffler so it was only after considerable time that we tried a stock one. Flat spot gone, running well. Nothing but stock mufflers for me. I'm particularly a fan of the Porsche Classic muffler, sounds way better than the Dansk offering.

[62S-R-S]

Thermal efficiency and sport mufflers...


https://mechcontent.com/engine-back-pressure/

[PatHidden]

No dice on 4 clicks rich on the mid rpm black screws. I am going to try one more time next weekend and if it doesn't work, I will buy a stock muffler. Partial throttle between 2-3k rpm just feels terrible, you can slowly go through it. Also, if you try to pin it at 2000 rpms, then it lean pops and stumbles for a moment. AFR gauge showing lean spikes.

Given it isn't mentioned in the CMA, does turning the governor screws have the same effect as the idle screw? Left is lean (CCW) and right is rich (CW)?

Thanks all.