MFI flat spot fixed - it was the freakin' "sport" muffler...

[obrut]

I've been chasing a flat spot in my MFI 2.7 RSR-ish spec engine for some time now. Yesterday we pulled the stock 2-in 1-out muffler off a 2.4T that I recently acquired (BISHOF ?? brand, 911 part no.) and put it on the race car.

The car had a BAD 3000-4000 rpm flat spot which made it impossible to use full throttle in this range (ie. headbutt the steering wheel bad). Now you can snap it open at 3000 rpm in 3rd and it pulls smoothly to redline. It still has a little hesitation at full throttle around 2000 rpm but i put that down to the GE-80 cams. I'm at the track next weekend and will then know if the top end is affected - I suspect it won't from how the car now drives generally.

Just goes to show you shouldn't mess with the original set up too much - particularly with MFI.

We then bolted the 2-in, 1 HUGE outlet sport muffler onto the CIS 2.4T engine - fantastic. Nice raspy note, no flat spotting and it feels like a few more hp.

I am now convinced that MFI engines need back pressure (whereas low comp CIS motors don't).

All in all a great days work.

[campbellcj]

FWIW several of the 2-out sport muffler suppliers explicitly say that these should not be employed for street use on MFI cars. I saw the same notice on the Monty Muffler display at the Ventura GAF today. BTDT myself with an AJ-USA muffler on a 2.2, although I honestly didn't think the flat spot was -that- bad; however, I did ultimately go back to a 1-out stock style for smoothness and noise reasons. As you also mentioned the 2-out style is AWESOME with carbs.

[Cornpanzer]

Thats odd, I have a 2.2 "S" motor with MFI and a twin pipe sport exhaust and it is smooth as silk. If you have a flat spot that bad (head into steering wheel....exagerated of course) then I would think that there is a mixture problem that maybe the smaller exhaust is componsating for.

[Todd Simpson]

Me too...though I never used anything but a sport muffler on my 2.2S. It was mildly flat at lower RPM but I think that's just the nature of the motor.

And the sound above 5K is just ridiculous. I use the same muffler now on a euro 3.2 and it sounds good but not in the same league as the S.

[pwd72s]

FWIW, fellow Early S registry member Chuck Miller & I would both agree. Both of us experienced flat spots with 2 in 2 out mufflers. Flat spots vanished when we went back to stock. The funny aside is that I gave my 2 in 2 out to a guy with a carbed 2.2 911T, and he claimed a performance improvement. That not documented, so I'm suspecting he just enjoyed the sound...

[Cornpanzer]

Well,
I guess I am fortunate because mine works well and I LOVE that raspy sound

[pwd72s]

Originally posted by Cornpanzer
Well,
I guess I am fortunate because mine works well and I LOVE that raspy sound

Yep! You're lucky..."if it ain't broke..."

[obrut]

Just to be clear - the sport muffler I was using was a monty 2-in, 1-out (3" pipe) not a twin centre pipe sport muffler, as some have referred to above. The factory muffler now on the car is a 2-in, 1-out (2" pipe).

I've got a few exhaust ideas I'd like to trial but this will remain the base against which others are compared. I only did a few laps around the block and then about a 10km drove home but the car is transformed. The low end is magic (keep in mind I run GE80 cams) being able to use WOT at 3000 rpm is a godsend for drivability.

[obrut]

ps: the 2.4T engine we put the monty on is a CIS engine - it loves it. It now has a crackle that it never had on my old low comp, weber-ed 2.7 nor my present high comp, MFI 2.7.

[rrcrawford70s]

This debate has been going on for quite some time.

It is TRUE that MFI motors need the backpressure, otherwise you get flat spots and a barely used muffler to sell.

Monty muffler questions arise when people are about to do a muffler change.

Monty does make a muffler for MFI. I have a 2.7 RS Spec motor, with a Monty Muffler, purchased from D. Aase. It runs GREAT, NO flat spots, sounds fantastic.

[obrut]

Ray

I spoke to Monty last week and he does make a MFI-specific muffler that he refers to as a 6-baffle muffler (the extra baffles are to increase the back pressure). That is probably what you have.

I'm not having a go at Monty mufflers per se - all I'm saying is the one that was fitted to my MFI engine did not work (but did bring a 2.4 CIS engine to life).

[jluetjen]

I am now convinced that MFI engines need back pressure (whereas low comp CIS motors don't).

Actually, I don't think that "Backpressure" is the phenonmenon(?) that we're discussing here. If it was it would get stonger and stronger in roughly a linear fashion as the engine is rev'd to red line. I believe that a more accurate description (or mental image) is tuning or harmonics -- as in tuning a piano (but not a fish!). In both situations it's a case of managing the resonances. Adding or removing the SuperTrap plates changes the back-pressure, but not the tuning.

When the muffler and MFI are working correctly, the resonances never quite line up (in fact the best muffler is out-of-tune with the engine) and so for the most part the exhaust goes out the pipe as intended. When you put on the "Sport muffler", I believe that there is a point in the rev range just before the engine comes "on-cam" where the engine (specifically the valve timing) and exhaust are in perfect tune which results in a pressure wave arriving at the exhaust port while the exhaust valve is still open -- the result is that the exhaust is pushed back into the cylinder and contaminating the charge. In the worst case, this occurs during overlap (late in the exhaust valve's open cycle) which can result in the charge being pushed back into the intake tract. I believe that this is the what causes full-race engines on carbs to spit-back in the 3000 - 4000 RPM range when using full-race cams. In any case, the issue goes away at higher rev's because the intake and exhaust charges develop enough inertia to overwhelm the affects of the resonance. This wouldn't be an issue with engines using CIS cams since there is little or no overlap to speak of.

What to do? One: go back to the stock muffler. Alternatively playing around with the header primary length might also have an impact. When I had a FF, I had a set of short (1 or 2 inch) extensions that you could add to header primaries to change the tuning. Adjust the cam timing may also make a change. Finally change the shape of the exhaust system, for example the Phase 9 style exhaust may help -- or may not. I wouldn't be surprised if using stepped headers or having a header primary larger then the exhaust port would also help.

Just some ideas.

[obrut]

John

Thanks - that is a great explanation. I have a few ideas for a tunable system but at present am happy with finally having a well running car. I don't want to do much with the present heat exchangers as I need to retain heat for some cold weather rallies.

My mechanic today fitted a stock muffler to a real 2.7RS with the same problem (and similar muffler) my car had - with the same excellent results. That exhaust trialled is a loner and the owner has now ordered one of the Monty 6-baffle mufflers (we're in Australia and so is Monty). I'll let you know how it goes once it arrives.

[obrut]

ps: your explanation would also fit with how some MFI engines run well/smoothly with really free flowing sport exhausts

[obrut]

Just got the car back from the dyno with the (god knows how old)standard Bischof muffler - 201.4 rwhp!!! (up from 186 with the Monty). A/F curves much smoother also, no flat spots.

The only other change to the engine was Nology leads and matching plugs. I'm pretty happy wiith this power in a single plug 2.7.

[tangerine911e]

Sweet.

[leung]

hi i have a 73 E, am thinking of a monty dual in single out. anybody has any experience?

[Ho Hum 74]

Strange. I have a 2.7 RS spec MFI motor and I have a "home made" sport muffler that I've used for almost 2 years now. I did not do the work but I was told it was a stock muffler which was gutted and had the single outlet welded shut. Then two outlets were welded on like the original sport muffler. My motor pulls like a horse all the way to red-line, no flat spots and superb throttle response. It is running rich so that probably helps but I don't think that my muffler is hurting my performance. I think it's all about the frequency and the length of the outlets to match the harmonics to create a back-pressure pulse. If you get it right it should be fine whether you're using one outlet or two.

T

[oldE]

John,
If I'm not mistaken, the best 'tuned' systems take advantage of the returning shock wave of the exhaust charge and tailor the length of the header so the returning exhaust pulse has JUST reflected off the still closed exhaust valve and is rebounding back down the header as the valve opens. In this way a low(er) pressure area is created at the port, at that instant, which helps extract the spent charge from the cylinder. Because of the changing velocities of exhaust gasses exiting the cylinder as one goes up through the rev range and the corresponding unchanging shock wave timing, the exhaust headers are in effect 'tuned' for a particular RPM range. If your cams are not matched with the particular header, you might get one (or two) 'power bands' and a dead spot before or after due to the mis-matched harmonics. The cooler charge in the intake system makes things like Varioram effective by altering the length of the intake runners as the RPM and throttle opening changes to take advantage of the pulse waves of the intake charge to 'supercharge' the cylinder. Great fun.
Les

[speeder]

Glad to hear about that solution, and 200+ rwhp is fantastic for that motor!

It has actually been documented for years, (decades?), that the quickest way to make an MFI motor slow is to install an aftermarket "sport muffler", either go w/ stock or factory sport muff. I wonder how many millions of hours and dollars have been spent by people trying to get the flat spots tuned out of that set-up, (MFI + Monty/Bursch/Dansk/etc...).