Custom Exhaust on 6.9?
 

Hey,

I've been toying with the idea of putting a custom exhaust on my 6.9. I was thinking of a cat-back system using flowmasters (since it it a '79 and not yet smog exempt). That big-block sounds pretty mean with the stock pipes on it, and I think it would really sound great with a Flowmaster system. What concerns me is performance. I understand that all of these cars were dialed in from the factory and I don't want to screw up the back pressure and jeapordize performance, or worse!


Does anyone have any experience doing this or know the possible effects - good or bad? Thanks! :cool:

the only thing I can think of is reducing backpressure is likely to lean out the fuel injection..................that may need correcting. But if you have access to a tuner........one of the guys that do imports with a pair of O2 bungs welded into the pipes as clsoe to the motor as possible (these can be pluged with a screw in plug) you can hook up a wideband sensor and tweek the pump to get the right Air fuel mixture set.

But I bet that baby sounds awesome.

I started out changing my 6.9 in stages and carefully monitored any decline in performance. None yet. First I got what Timevalve calls "engine pipes" which are straight pipes that directly replace the cats. Since you're not smog exempt yet, you might want to skip this. Mine's a 78 and I am free to modify re: emissions. That helped some and didn't adversely affect power; to the contrary, I felt it open up. Later I had the resonator cut out. Big sonic difference and not boomy or unpleasant to my ear at all. I cut the resonator out of my 190 2.6 and had to put it back after a week. I set off every car alarm in the parking garage at work. With no cats and resonator, the sound was good and the power increase was evident on the 6.9. A couple of weeks ago I put in an "X" pipe or crossover or whatever you want to call it. I recommend this and would have done this first if I had known. The exhaust pulses are better balanced and it actually worked to cut the sound down. In fact I read somewhere that a couple of properly placed x-pipes would allow you to run without a muffler at all. When I get a little further along, I might try a flowmaster at the rear, or maybe just try another x-pipe and see what that's like. I wouldn't worry about losing anything, all I have seen so far are gains. The sound suits the car, too. I've done some other stuff to the car recently and it has never run better. Your car is CIS, too and it's all pressure and flow and a mixture adjustment that is mechanical, not electronic, so I don't think a lack of backpressure will alter any of that. Does your car run well otherwise? I took mine on a long trip at Christmas and we ran 90-95 effortlessly. I have got it burning rubber now which it has never done in my 9 years with it. Part of my trouble has been that I have never ridden in one that ran as it did from the factory, so it has only been by other people's desciptions of how their cars ran that has got me changing things and experimenting with various things in the injection system, etc. Hope this encourages you to continue.

Out of curiosity, what are the some of the "other stuff" that you have done to your car to get it burning rubber again? :D I too don't know what the car drove like from the factory (since it was made before I was born :) ) and so who's to say it's not peforming better or worse than factory spec or better in the event that future modifications are made.

The previous owner had the engine completely rebuilt. It has approx 7k on it since then. This is a euro-model which I understand had a little more H.P. but I don't exactly know the details (by driving it, I'd guess around 300). It burns rubber effortlessly and is a blast to drive on a rainy day! I am planning to Dyno it, but haven't got around to it.

Anyway the exhaust is one of a couple of things I want to modify. I'm sure the MBZ engineers did a fantastic job at the factory back in the 70's but I'd bet that exhaust technology has seen some improvements in the last 30 years. I like the idea of the crossover pipe. I had a custom 3" exhaust similar to what you are talking about on a big block Buick ('68 GS 400) years back and LOVED that unbelieveable rumble which came from those pipes- used Flowmaster 2 chambers. I bet that this beast will sound the same.

I've heard that the top end on these is around 150 mph, which I plan to find out for myself. The best I've ever had her was 120+/- mph on a stretch of I-5 in California before my radar detector started to buzz. Oh well, I'm trying to get it on one of the racetracks - the oval at Las Vegas Motor Speedway would be ideal to get an accurate top end attempt, but we'll see. :cool:

I believe the Euro model has 286hp. Mine has the measly 250hp. I drove mine during the LA downpour a few weeks ago and it was rather scary as to how easy it was to break traction on the rear tires from a stop! AS for burning rubber I guess I have never had the confidence to try it since the car shudders a little when under load....do your cars do this or is my tranny slipping?....what's your style? Mash the gas or hold the brakes and rev the engine and then let off?

As for the exhaust I like the crossover idea. Even as it is now, I take the car to a parking garage with a hundred riced cars and it sets off every one of em if you blip the throttle....oh what fun :)

I got mine up to 110mph for a few seconds going uphill at about 4% incline but had to back off because buses don't go 110mph. Perhaps when it has better rubber and track available i'd venture out and see what the top end is.

i dont think you need two or more x pipes (ive never seen or heard of anyone doing that) - their job is to balance out the exhaust gases and let teh exhaust pipes scavange the air as effortlessly as possible - resulting in a more pleasurable sound,better performance....a h pipe might work too i am not sure the fire order of this engine but a x pipe and an h pipe have alternate effects depending on fire order....or shoot even a new style "D" pipe...

i am running a borla performance universal muffler, and i love the sound...it is a 2.5" system with headers....this is not on a 6.9 - but the sound is aggressive without being rash and rude...a very nice rumble and roll - no bee can or 60s muscle car (which i love, but not for my w123) - hadflowmaster 40s - i thought it was too much for it...the borla is more "refined"

for what its worth
jake

id look into headers as that is great for the engine

The biggest increase in performance for me was getting my warm up regulator working right. I had read that the 6.9's strong point was its huge torque and ability to really accelerate from 60 to 100. Mine wasn't doing that. It almost felt like a tire out of balance. I could go fast, but the acceleration from cruising speed to high speed wasn't there, and it seemed like the car was laboring to get it up there and keep it there.
I swapped in a used WUR from a spare engine I bought and it was like having a new car! There was a change in 79 to the plumbing on the vac. lines of the WUR and the vac. hose at the top of the WUR swapped positions with the hose connected to the lower chamber in the WUR. I think this was to provide enrichment at full throttle.
I rebuilt with a kit the "system pressure regulator" and the "pressure compensating valve". These are in the fuel distributor and the kit provides new o-rings and a new closing plug and shims to adjust the control pressure. I don't think this added to any increased acceleration, but it improved cold starts.
I pulled in all the slack from the throttle linkages and now the throttle responds with any movement in the pedal; it's not like you have to move it an inch to get things started.
I'm not saying I can leave 30 yard rubber stripes, but I can mash the pedal from a standstill with no brake torquing and get some pretty serious spin, not that I am advocating this, of course. Yes I am. It's great.
I have been cleaning all the fuel system parts by disassembly where I can and using Lucas FI cleaner in the gas tank. My fuel system was pretty dirty after 27 years without any serious efforts at cleaning.
I read on the m-100 sites that a bit of ATF in the gas tank, like maybe a 1/4 quart per tank every other fillup would help lube and clean the internals of the fuel distributor, etc., and there is an added benefit of raising knock resistance. I bumped the timing up a few degrees, too.
I am nowhere near Euro 6.9 performance territory, but it's running better than ever right now.

With reference to the exhaust business, I wasn't saying that an extra x-pipe would further improve scavenging, one's enough for that; I was saying that some of the old hot rodders can put more than one crossover in and eliminate the need for a muffler. I don't know how restrictive the stock muffler is, but I am guessing it does offer some back pressure and eliminating it would free up the exhaust that much more.

I also pulled in some slack from the transmission pressure rod. This adjustment is exceptionally sensitive and small adjustments make large differences. It holds the lower gear longer and shifts a bit later when you get it just right.

The top speed on a 6.9 is 140-145 MPH. The three-speed tranny is a huge limiter, as are the aerodynamics, which are akin to a brick. It's a lot of sled to move at high speed.

The acceleration of the 6.9 from 60-120 is really indescribable, even with the 250-HP US models. THe European cars with 36 extra HP are not significantly faster, though some people swear they are.

I had my 6.9 up to 125-130 MPH a couple of times, and though they can go that speed pretty easily, they tend to lift a bit and don't feel too firmly planted on the road at that speed despite being a heavy car. It's the aerodynamics, again.

I would think a crossover exhaust would be an excellent idea. ANother tip would be to disconnect the smog pump, as that robs a bit of HP and is an expensive devil to replace, cost-wise. Different people have different ideas when it comes to the exhaust; I found that just goign with a stainless steel Timevalve system was an excellent choice. I doubt that exhaust mods are going to provide much real-world HP difference above stock.

The best things that could be done to get more speed out of a 6.9 would be to replace the transmission with a four or five-speed unit, do a better exhaust, and probably to alter the rear-end gearing for more acceleration.

On the drag strip, the best I could do with the 6.9 was 16.1 seconds; by comparison my 6.3 could do 15.07-15.2 (stock) and the 500E can do (stock) 14.0-14.1. With nitrous oxide, the 500E is down in the 12.9 second range. It is also capable of 185 MPH and feels firmly planted to the road at speeds above 150 MPH.

In the 80-100 MPH range, a 6.9 is a really nice car, very smooth.

Cheers,
Gerry

as far as wind drag ... the SLR uses about 150 horses just to beat the wind at speed....200mph

wow - thats crazy
jake

I got my 450SEL 4.5 up to 115mph today and it felt so so so firm down on the road. Good enough to try and get around cracks and potholes on the highway. It all depends on suspension set up and that 10mph more may make the car act way different. I did this a few times but when the tire pressure is up to spec and running 215s, its comfortable at speed. I used to have balding 195 and it was a scary ride even at 90. Tires make a huge difference too.

Peter

6.9 fans, first, are you aware of a Benz site dedicated to your M100?? Find the one that ends with cc.
Now your pipes, yes, I've heard the theory about performance and pipe size on M100s. My very best performing 6.3 uses stainless Tymevalves. As another very knowledgable fellow said, tune it for that exhaust system. Borlas are slightly bigger, I still haven't used them but only because we now make our own systems, all stainless in two cases including headers but the majority use regular steel headers but stainless crossovers and pipes. We use several different mufflers from Tymevalves to now a new devlopment for us, stainless steel glass packs. The best sound by far, the glass packs, no comparison. Performance??? Don't know yet. The air is too cool to judge the recent Glass packs on the Green Hornet to our Tymevalve system mounted on the Silver Bullet. It feels good but only a time slip proves the horsepower. One thing for sure, regular steel pipes and crossovers are a total waste of funds if you plan on driving the 6.9 for 6 years or more. That's the expected life.
On those cross overs; find some photos of a 6.3 system. Note how close to the headers, the 6.3's crosovers are mounted. The closer the better and thats proven. Here in Oregon, we have now found a pro shop that bends the stainless steel making the under the 6.3 view much cleaner than our original stainless cross overs.

500e 185MPH
 

You expect us to believe that? 185 MPH? that's 300kmh.
I've done 245 and it's right scary. That was in a V6 w210 and they had better suspension than an airmatic w211, or any 211 for that matter.
W211 is limited to 250kph

And it wouldn't do 12.9 on the 1/4 mile either. CL65 does 11.8 and it has twice the HP, bigger tyres to put down and only weighs 300kg more.
Anyone agree?

Gerry's E500 is Nitrous Oxide equipped. Why couldn't his car run in the high 12's? A stock 211 E55 with 469 hp can run in the low 12's so why couldn't Gerry's E500 with a 150 shot of NOS run in the 12's? Stock the E500 has around 320 hp add 150 hp with NOS and you have 470 hp. The CL65 does not have twice the hp at 609 hp. It would need around 940 to have double what Gerry's car has. Besides, the CL 65 is traction limited. I've seen the 65 series run mid 11's on stock tires. I'd imagine low 11's with slicks is very possible for the CL65.