CLICK HERE TO VIEW TOPIC IN THE FORUM

custom exhaust system
Joe745iWhenever I park my car after boosting it up a couple of times, the Catalytic Conv. starts smoking profusely. I mean this sucker smokes like a chimney over every square inch (it is pretty funny looking)......and I don't lose one drop of oil over 3000 miles. So I am in the process of replacing the exhaust system, but not being an engineer, or even a good mechanic for that matter, I don't know if going too big will not let the car run good (i.e. lack of pressure or something). My feasible choices are twin 2", 2.25", or 2.5". My goals are to achieve the best possible gas mileage while not limiting the performance of the vehicle with respect to any modifications(Team 745 stuff) Any suggestions? Joe Mangano 1985 745i 120K
M545iJoe, the turbo cars have sufficient backpressure in the exhaust because of the turbo itself, so everything beyond the turbo needs as less restriction as possible, also the heat from the turbo needs to be evacuated as fast as possible through the exhaust, so the wider you go after the turbo the better with respect to the sound level of course. I have two 2,2" straight pipes from the wastegate/turbo joint up to the rear where I have half of the end muffler from the 745i (remember I drive a E28 5 series on the track only). Although the sound with the muffler on very acceptable is changes it when I only use straigt pipes, then it is a real loud. Regards, M545i
cliffojoe, i run a 3" pipe from off the back of the turbo through to the rear of the car. a small resonator sits in the middle and a larger muffler at the rear. as mentioned by m545i, a greater internal area is available with twin 2,2" or 2,5"pipes. less is best as far as restriction in the exhaust after the turbine, as it can cause reversion,ie- cant get burnt gas out, cant get petrol/air in. thats why cars with small turbos go hard down low and then run out of puff, as the small exit path effectively strangles the motor.(generally speaking)
tritterbrewHi, I've been in the process of upgrading my '84 745i and I just tried to run up to the full pressure of the unmodified wastegate and I found that one time I got to 10 psi boost and yesterday my car wouldn't go above 8 psi boost. I know that the chip and fuel mixture settings I've got support the 12 psi of the wastegate spring. I'm thinking that my problem may be restriction downstream of the turbo. My car ran very rich for a while when I got it and I think the cat might be plugged. If I do need a new cat, what do you recommend? It sounds like I should look at bigger pipes if I need to replace things anyway? Of course, this is my road car so whatever I do it needs to be street legal and pass VA emissions. What is the best way to determine a blocked cat? If I find that the cat is fine than I'll came back for advice on looking at the turbo and wastegate. Thanks mucho, Ted R. '84 745i Fraulein
cliffothe cat can become blocked over a period of time as the honeycomb structure inside breaks down or becomes clogged. can you take it out to have a look? they loose their effectiveness over a period of time so you may have to get another one to stay legal.
Joe745iThe car should pass emissions without a cat on it(it could still pass with a bad cat on it). Another way to tell if a cat is bad is to lay underneath it (with it cool) and bang on the bottom of it with the the bottom of your fist/palm and if you hear little particles rattling around, it is bad. I am in the similiar boat a you tritter, my cat is going but I it would be difficult just to replace the cat and resonator because that would require pipe bending and stuff anyway; so I am gonna get the whole kit and cubudle (or whatever). I am going to buy a flowmaster I think with 2- 2" inlet and outlet pipes. It is 17inches long, stainless, and around 100 bucks (for a cat). Then I am going to run twin 2.5's all the way into the muffler. I think that 2.25" would be a perfect combination of HP increase and quietness, but I don't want to limit myself when it comes down to the difference between 400 hp and 500hp. Tuoma stated that 15psi with stock exhaust (approx. 2") is 400 hp but with 2.5" was 450hp. Joe PS. without a cat your car will boost up quicker (much), achieve more power, and obtain better gas mileage than with a cat. If it is the difference from 24 mpg to 27 mpg, the cat on my car is going to come off.
tritterbrewJoe, Thanks for the update. Please let me know if you do go ahead with this approach. Do you do this work yourself, or do you take it somewhere? Looks like a replacement cat from bavauto is about $550 and I suspect that I also may need a new muffler... If there are alternatives to putting in OEM, saving money and getting more performance than I'm all for it! Any and all advice is most appreciated. I'll bang on the bottom and report back. Cheers, Ted R. '84 745i "Fraulein"
Jon 86 745 MSI have a complete 745 stock exhaust system (no Cat) with a really new looking muffler. I would sell it Quite reasonable if anyone is interested. Email me to discuss further. Jon Jacobs 86 745i (Die Rakete) 16 psi+ Boost with Team WWW.745i.com Performance Parts
Joe745iLet me elaborate just a little. Banging on the Cat is just one way to tell if it is bad. I know mine is bad because I can feel excessive backpressure (and hesitation) right before the turbo boosts and I believe that if the cat weren't there, it would be seamless acceleration (or seamless transition to boost) I need a lot of throttle input right now, but know that I shouldn't. The cat looks really old and rusty. And the cat smokes when I boost up a lot. Those are the reasons why I believe I need a new cat. A freeflow cat is actually less restrictive than a stock muffler, so with a half-descent cat, flow should be adequate. Look for places that sell "universal cats" or maybe the company name was Universal Catalytic Conv. Then I found the flowmasters for 100 bucks from "proexhaust.com" or AAple mufflers, or A-1parts.com, and then there is discountconverter.com. I am not going to do this myself because to replace the Cat on my car, I would need welding and bending equipment. See, some cats have bends in them, some don't, etc. My point is that it would be difficult to find an exact match which means there will need to be some fabrication. The cat on my car now is about 36" long with a bend in the middle; hence, I couldn't find an exact match if I wanted to so fabrication is eminent. The people who are doing my whole system make exhausts for all Mallet Corvettes as well as the headers for sprint or midget cars (one of those series). They Are going to flange in the cat on either side (with laser cut flanges) so it can be removed easily and replaced with strait pipes. They are also going to fabricate a muffler for me. They say that the most restrictive part of the exhaust is the muffler, so running 2.5" pipes into a stock muffler wouldn't be too smart. I think Ansa or Bosal may make performance mufflers for the 745 though.
cliffoi know of some who put a section of exhaust pipe through the middle of the cat. this leaves the outside looking stock, while removing the blocking caused by the cat's deterioration. improves the exhaust note too!
Jon 86 745 MSJoe, Sometimes it pays NOT to do something yourself. Good luck on your quest for a new cat. Becareful though on the muffler. Ask John Stein about the noise that a 745 makes without a good muffler. Quite loud. Jon Jacobs 86 745i (Die Rakete) 16 psi+ Boost with Team WWW.745i.com Performance Parts
tritterbrewJoe, How much is this exhaust setup going to run you? Thanks, Ted R. '84 745i "Fraulein"
Joe745iWith a custom muffler and flanges with stainless everything is going to be around $1200. I know I could find a place cheaper, but this guy only uses good stainless and does the best work I could hope for. He already made me and my friend some larger intercooler pipes, and they turned out perfect!!! And Jon, I am concerned about the muffler as well. I think I am going to either have him make me one (they ask how loud you want it), or try what Matt G. did by boring out the stock muffler. When searching the archives of bimmer.org, I saw that he went to 2.25 and said that there was enough room in the muffler for twin 2.5's......barely. Joe 1985 745i
cellguruHey Joe are you doing the flow from the wastegate separate? I have been reading Corky Bell's book (thanx Cliff) and he seems to think that is desirable. Wadda ya think?
Joe745iThere are two pipes that make up the dual exhaust on these turbo cars after the manifold. The huge downpipe from the turbo, and the downpipe from the wastegate. If you look while under your car, you can tell that they are made of "hard core" steel until just before the cat (you can see the weld on my car). The tubes up until that weld should last longer than the car itself, I would imagine. It is much stronger than the rest of the exhaust pipes. I was planning on having a 2 1/2" stainless pipe flanged into each of those downpipes right where that hardcore material ends. Now you made me revisit the predicament on whether or not the downpipe from the wastegate should be made bigger or not. It was argued to me that it is not necessary because of the function of the wastegate, but I would love to hear what you guys think. I am only going to do this once (with this particular car), so it should be done right. I am not sure, given the nature of the wastegate, whether a larger pipe bolted to it would increase performance. Any insight? Joe Mangano
cliffofrom what i have read, venting the wastegate with its own pipe straight out to the atmosphere is the best way. that means there is no impedance to the exhaust gas flow at the junction where both would normally meet. the downside to this is the LOUD noise made by the wastegate dumping air, even more so if the car is running increased boost with the w/gate constantly working to regulate the boost. looking at the exhaust/wastegate pipe joint though,its a very smooth melding of the two. the engineers at bmw are no dummies. they gained alot of experience running cars in the old Group 5 class,(single+twin turbo 3.5 csl's, with up to 700hp.) 700hp! now theres a thought!. clifton
Afshin-SwitzerlandCliffo, That was 950 HP in race condition and 1000 HP on Qualifing/Trainings from BMW ///M1 AC/Schnitzer Gr.5 Racing team .... Yeah buddy, we could keep on dreaming ... !!! Regards, Afshin. 1982 745i
cliffodamn afshin, that means even more boost needed for us guys. i have read articles where the race cars floors were GLOWING red from the heat. i suppose that with that much power its a wonder it didnt sag in the middle.
Afshin-Switzerland...) LOL.
Joe745iCliffo - I plan on bolting a Turbo By-Pass Valve that will vent pressurized air between the turbine and intake manifold back into the pre-turbo side of the intake(but after the AFM). What you described sounds most like a blow-off valve, which dumps the pressurize air into the atmosphere. On cars with AFM's, you are supposed to route the air back into the intake because the AFM has already metered the air and if you vent it to atmosphere, supposedly the computer will not know (although there are pressure sensors, right?) The pre-1983.5 models (sharknose) came from the factory with a by-pass valve, not blow-off. How it works is when you are under boost and there is 6, 10 or even 15 psi of boost running from the turbo into the manifold and you lift off the throttle, the butterfly valve on the throttle body slams shut and that compressed air has nowhere to go and puts horrible stress on the compressor. The only reason these cars last as long as they do is because of the low boost pressures. If you elevate the boost, you need a bypass valve if you expect your turbo to last. I believe John Stein is learning this the hard way (although 200,000+ miles could have meant time for a new turbo anyway). But I am still researching whether or not I should make the exhaust downpipe from the wastegate larger. Joe PS Is that what you meant by venting air, or did you mean something else with respect to the wastegate? (like over-boosting and a pop-off valve) PSII. Check out the quote in my profile Cliff/Afshin Edited by - Joe745i on May 31 2002 14:39:23
cliffojoe,how ya going. the dump or bypass valve is a great idea, especially for manual cars, because as you pointed out, at gear change with the throttle slamming shut, the pressure wave going back to the turbine wheel is enough to stall the compressor, so a valve that dumps that excess to the atmosphere helps when you get back on the gas. when,the factory w/gate,according to the factory manuals, starts cracking open at 2-3 lbs boost,that flow can affect the exhaust if the join betweeen the two is too close/too sharp.in a 'european car' magazine article a few years back,a porsche guy with some 800 flywheel hp put his w/gate air through a pipe straight out to atmosphere. said it sounded like an F-16,as the w/gate worked to hold his boost pressure level.does your w/gate, like mine, operate when you let the throttle slam shut,especially after high loading?