So I have put around 1200 miles on my Marlin M3 after sorting out the cooling system ( overheating may be part a symptom of what I am going to describe so keep reading!) apologies for the huge post by the way.

I have the s50b32 twin vanos m3 engine which should put out around 320bhp (more like 310 -15) and I have never really felt the Marlin feels like a m3 powered car, it is bloody quick but I don’t feel a massive vanos kick and the engine lacks Torque... it should rip my face off in a kit car and light the wheels up in most gears... it doesn’t. The engine seems to have a massive flat spot when pulling from low RPMs- a bit of coughing and spluttering

I went for a rolling road and my suspicions were confirmed I only made 260bhp @ 6785 rpm at the flywheel and 219 lb/ft of torque @ 5890 full torque on these engines should be 3500 rpm. As you can see on the results below the torque curve is all over the place. The engine basically flatlined 800 rpm before the Rev limiter and they powered off when it wouldn’t rev any higher. Hopefully the pictures can be seen below:




There was no data logging or afr meter plugged in during the dyno session as it was just a power run.

Anyway moving on I have tested the vanos and the bmw test equipment say it is functioning correctly and no fault codes are coming up. I have also checked all sensors and changed the tps with no improvement. I have smoke tested the intake and found no leaks either.


On a punt I took the car to a local bmw specialist who have knowledge of this engine, he took one look at my exhaust and said there’s your problem, it’s no where near big enough. He suggested the m3 requires at least 2x 60mm Diameter exhaust pipes. My set up was made purely to get me through Iva and has both headers going into A single 200 cell 2” cat and then one 57mm pipe to the back box see pics below








He then took me over to the fab shop who said they could make what I need in the correct size for the power output with a defect cat for £600.


Could I lose effectively 60bhp with my system or is this some one telling me what I want to hear for a quick sale?

Is there any test I could do to confirm that the exhaust is not flowing enough?

Appreciate any help at all from any one that understands dyno results and tuning. I am confident the engine is mechanically and electrically sound

Is that exhaust 6:1 ? If so that's not going to work that well from the outset.

You really need something along these lines

6000rpm
Your Primary Tube Length is 27.19
Your Primary Tube Diameter is 1.70 inches
Your Collector Diameter is 3.37 inches   Two Collectors

On my DB7, supercharged 3.2 straight six, not miles away I have 6:3:1

Primaries 42mm, secondaries 48mm into 3" single pipe, I played around with the secondary length to get the torque further up even so the 3:1 join is neat the rear of the gearbox.

Before this set up I had 2 x 3:1 with twin pipes out the back, 1 5/8" primaries into 2 x 2" out to the back, this was not as good.

It will then be killing the car, by the amount you show .... It's possible.

It does look restrictive that set up tbh,especially the CAT.

I wouldn't necessarily say it's just down to pipe size,but some of those bends/joints don't look to be flowing nicely.

Ref pipe size when I had a custom exhaust made at a BMW specialist that was a Longlife dealer and I questioned him if 2.5inch pipe would be large enough his reply was 'BMW M5's use 2.5 inch diameter pipe so you'll be fine !

I'd say you're doing well to get that power through a 2 1/4" pipe even without the 2" cat pipes.

There's various calculators online. This is just the first I found...

http://www.exhaustvideos.com/faq/how-to-calculate-muffler-size-pipe-diameter/

Also, 600 sounds like a fair price. It’s a cheap mod for 60 bhp

I think the specialist is spot on and you could lose that power especially on an engine like that. I think they used the same engine in the Mpower Z3 but it couldn't produce the same power as the M3 due to not being able to make the exhaust the same length.
I think £600 for a new system is a really good price too.

Thanks for the replies. Looking at the exhaust in more detail I can see it’s a pretty poor design... also the lambdas for each bank are very close . I would be surprised if the ecu could determine the difference between each bank.


I think running the exhaust out the side of the car is part of the issue as it is such a torturous route to run the pipe work in a very tight space. I’m looking to see if I could fit a standard m3 exhaust with rear tail pipes ... would need slight modification. I could then keep my current set up as the MOT exhaust pipe!

I know 600 is a good price, just slightly too expensive at the moment.

Big problem is the person that made the exhaust cut the flanges off my stock manifolds do connecting up to them may be difficult

I'd agree that 2" is too small. I run 2.5" all the way including the cat on my M52B28 engined '7', and yes it does pull your face off
However, I'd only expect the bore size to strangle it at the upper end of the rev range but you say it's lacking punch low down as well. Maybe that is the bad design of the manifolds? Either way it loos like you need a rethink. I presume you don't have room for an M3 manifold?

The manifolds are the stock m3 manifolds however the ends have been butchered (flanges cut off and the two banks of 3 joined together with a y piece) here are some better photos









I’m going to do a backpressure test on the exhaust to see what kind of restriction is being caused...

[Edited on 25/7/18 by mcramsay]

[Edited on 25/7/18 by mcramsay]

Weird, I posted reply to this earlier and it's disappeared?

Apart from the undersized pipes I suspect the main problem is the fact you have joined the two manifolds together via a very short Y, creating a 6:2:1 with extremely short secondaries. Secondary lengths are very important to engine performance, and since BMW designed this system with effectively infinite secondaries (i.e. they don't join up in the M3) then it's not surprising that it has hurt torque and power.

Thanks for the reply. I’m going to get an aftermarket m3 exhaust and modify it to fit my car, with the exhaust exiting at the rear of the car. This gets round the problem of having no room to run the side pipe. Which I hate, it’s too loud in one ear, you get covered in exhaust fumes and I keep burning my legs on the car.


Been looking for an excuse to run a rear exit exhaust so it looks like I have found it.