N/A Sufficient Exhaust Size/Diameter

[f0xy]

Hoping someone may be able to give a second opinion on this setup, as to whether the exhaust sizing is sufficient based on the engine.

The current setup consists of: 3.0L 6 Cylinder, 10.2:1 Compression, 244/9.7mm Intake Cam, 228/9mm Exhaust Cam, Single Plenum + TB, Equal Length Exhaust Manifolds which feed into a Single 2.5" to the back. Running on standalone this made 265hp/260lbft.

This is going to change to: 3.0L 6 Cylinder, 10.6:1 Compression, 252/10.3mm Intake Cam, 244/10.2mm Exhaust Cam, 45mm ITB, Same Equal Length Exhaust Manifolds.....

Is the 2.5" system going to be enough for the changes... or more so what are the suggested N/A limits/restrictions for a 2.5" system? Its a relatively simple task to increase the size of the system if required... but if its not required I won't bother.

Cheers

[f0xy]

Inline__engine said:

ive seen 6cyl engines make just over 300bhp on a 2.5" system without cats or much muffling. a 3" may make an extra 5-10 hp or so i wouldnt count on more than that if at all. if you have cats, resonators and mufflers then a 3" system is probably going to be more beneficial as these components in the 3" size will probably introduce less back pressure than the smaller equivalents. if you preserve the exhaust tuned lengths the only downside to a 3" may be a little more noise and cost to change.

your engine sounds like a M5x engine with schrick cams, possibly the jenvey ITB? i presume its street car? in which case id lean towards keeping what you have until youve rolling roaded it after the mods

on youtube there is an "Engine Masters" episode 9 with a 600hp ish V8 they compare two engines 2x2.5" and 2x3" piping. since you are roughly half the hp through 1 pipe you can draw some decent conclusions from it

Edited by Inline__engine on Friday 22 July 03:34

Edited by Inline__engine on Friday 22 July 03:35

Thanks for the detailed reply. Yep, its an M5x based engine (combining M50/2/4) and yes 45mm Jenveys. It is not a street car, track only. There are no cats, its simply a 2.5" system with two silencers. I will probably change the collector for the two banks to 3" (from 2.5") and go from there. It will be having dyno time either way and map changed accordingly.

E-bmw said:

Sorry to hijack this thread slightly, just a quickie.

I have just read the link & at the bottom it implies that if changes are done to the exhaust a recal (I assume remap) would be beneficial.

My 328 was remapped a couple of years ago, since I have gone for BBTB & an M3 full system exhaust.

I never had the car re-mapped again after these thinking that the MAF would effectively re-calibrate the system to take account of the changes.

Would it be beneficial to have the mapping re-visited again, and are there significant gains to be made afterwards again?

I don't think you would notice any significant difference - when I first ran the above 3.0 engine in original spec, it ran 250hp/250lbft on a VERY mildly tweaked standard ECU, the MAF did most of the compensation as it has enough range to make up for the increased displacement (in your case bigger throttle body etc). When I ditched the MAF and moved to MAP on standalone, and properly mapped it, the figures went straight up to 265hp/260lbft

[f0xy]

I remained with the 2.5" to see what the result was - mapped this weekend and came out with a healthy 290hp/265lbft. It was still making power at 7300rpm but we can't really rev it any higher with the standard M54 crank. I think 3" would be beneficial if it would rev to say 8k, but with the current setup I think 2.5" has worked as expected.

[f0xy]

We're only comparing from previous builds to be honest (all on the same RR), and numbers really mean nothing, its all about how it drives.

I don't think the cost of a 3" system for a few HP in this RPM range is worth it at all - it would be a different matter if it could rev higher or had bigger/hotter cams. These builds were simply to see what was possible with BMW only internals, i.e nothing aftermarket. The slight bump in CR was noticed when mapping as we could get a few more degrees timing in there vs the previous engine at the same points.

Its all an unknown really considering most people just go out and pay silly money for a ropey high mile M3 engine. Here are a couple of the runs vs old on top of each other from part way through mapping.

Edited by f0xy on Tuesday 13th September 21:45